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Full text of "Magnetic Amplifiers - Principles and Applications"

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magnetic amplifiers 



magnetic amplifiers 

principles and applications 



by PAUL MALI 

Instructor in Electrical and Mathematical Technology, 

and Director of Education and Training, 

Electric Boat Div., General Dynamics Corp. 




John F. Rider Publisher, Inc., NEW YORK 



Copyright August 1960 by John F. Rider Publisher, Inc. 



All rights reserved. This book or any parts thereof may 

not be reproduced in any form or in any language 

without permission of the publisher. 



Library of Congress Catalog Number 60-12440 
Printed in the United States of America 



PREFACE 

CUSTOMISED 
61268562 



This book was written because of a need to make available fundamental 
concepts of magnetic amplifiers. It is intended primarily for technical 
aides, electronic technicians, electrical draftsmen, electricians, and stu- 
dents, interested in a fundamental knowledge of the operation and appli- 
cations of magnetic amplifiers. It also serves as a review for electrical 
engineers who have long been away from the electrical field. Engineers 
and designers are given a quick introduction to the language and circuits 
associated with magnetic amplifiers, before attempting the more intricate 
concepts and difficult circuits. 

The basic principles and laws governing the operation and uses of mag- 
netic amplifiers are presented along with application in diverse industrial 
systems. Extensive mathematics and detailed circuitry have been limited 
for a simpler and more fundamental presentation, to be easily assimilated. 

The author wishes to gratefully acknowledge the efforts of David Anderson, 
staff assistant to the senior vice president of operations of General Dynam- 
ics Corp., New York, who has made the publishing of this book possible. 
Thomas Pugarelli of the State Technical Institute, Hartford, Conn., must 
be mentioned for recognition in submitting valuable comments and criti- 
cisms. Additionally, the author wishes to acknowledge the valuable assist- 
ance of the following people of the Electric Boat Div. of General Dynamics 
Corp., William Sardelli, Donald Bowers, Frank Kohanski and Dianne M. 
Donch. 

The author wishes to thank the organizations who have granted permis- 
sion to make reference to their systems in the section on applications. 
These are: Westinghouse Electric Corp., Minneapolis-Honeywell Regulator 
Co., and The Louis Allis Co. 

Recognition should also be given to my wife, Mary, for her unceasing 
efforts in keeping two well-meaning children from making unplanned 
"physical" revisions of this manuscript. 

Paul Mali 
August, 1960 
Groton, Conn. 



CONTENTS 



Introduction 1 

Magnetism 3 

Electromagnetism 11 

Magnetic Circuits 23 

The Saturable Reactor 26 

Self-Saturating Types 34 

Three-Legged Core Magnetic Amplifiers 39 

Compensating Magnetic Amplifiers 42 

Polarized Magnetic Amplifiers 46 

Amplifier Gain 50 

Feedback 53 

General Uses and Construction 56 

Maintenance and Troubleshooting 68 

System Applications 72 

Glossary 96 

Index 99 



INTRODUCTION 



1 



Magnetic amplifiers were developed as early as 1885 in the United States. 
At that time they were known as saturable reactors and were used prima- 
rily in electrical machinery and in theater lighting (Fig. 1). 



CONTROL OF 
ELECTRICAL 
MACHINERY 



ELECTRIC 
MOTOR 



CONTROL OF 
MOTOR SPEED 




• DIMMING 

THEATER 

LIGHTS 



Fig. 1 . Early saturable reactors were used for control of motor speed and 
dimming theater lights. 

Overshadowed by the advent of vacuum tubes, the uses of magnetic ampli- 
fiers were limited until World War II. During the Battle of Britain, when 
the Germans were firing long-range rockets at major British cities, a 
group of British engineers and scientists pieced together the shattered 
fragments of one of these rockets that failed to explode. One component 
of this rocket was a magnetic amplifier, used to regulate the frequency 
output of an a-c generator. The_X£markable discovery, was that this com- 
ponent, of only a few pounds, had no vacuum-tubes , or moving parts. 
Investigation later showed that this device w£is one of the results of a 
German develop^ient prograoi started early in World War II. 

It is interesting to note that the Germans published very few internal re- 
ports on magnetic amplifier development, either during or following the 

war. No basic theoretical disclosures were made by German scientists, 

since their development was conducted more on a trial-and-error basis 
(Figs. 2 and 3). 



MAGNETIC AMPLIFIERS 




Fig. 2. The German navy used the magnetic amplifier in fire control systems. 

The fact that the Germans introduced the device into fields previously 
dominated by electron tubes is considered by many to be as great a con- 
tribution as the technical development of the device. 

Although Germany is unquestionably responsible Jpr the rebirth of the 
magnetic amplifier, other countries, especially the United States, held a 
considerably greater number of patents on it. The efforts of Sweden, 
England, and Japan were also considerable. It was during World War II 
that the U. S. Navy started to exploit the device for purposes other than 
power regulators. 

The increased research and development contracts throughout the country 
today reveal an increased demand in the use of this remarkable device 

MAGNETIC AIWPLIFIERS WERE USED... 



,.T0 LEVEL AIRCRAFT 



...TO PITCH AIRCRAFT 



.TO ROLL AIRCRAFT 




Fig. 3. The German air force used the magnetic amplifier in control systems for aircraft. 



MAGNETISM 3 

in many electrical systems. It can be truly said in considering its potential 
worth and use in the electrical field, that the magnetic amplifier is still 
growing to full maturity. 



MAGNETISM 

An understanding of the operating principles of magnetic amplifiers re- 
quires a knowledge of magnetism, electromagnetism, and magnetic circuits. 
Thorough mastering of, these basic concepts will give the reader an easy 
understanding of the operation of magnetic amplifiers and their use in 
electrical systems. 

Nature of Magnetism 

The ancients were fanjiliar with a natural stone that would attract bits 
of iron. It was a form qf iron ore, now known as magnetite, and the power 
of attraction possessed Iby it was called magnetism.. It was later learned that 
other material (such a* nickel, cobalt, and iron alloys) could possess this 
peculiar property by an artificial means of using an electric current. Over 
the years, our knowledge of the exact nature of magnetism has improved; 
but it is still incomplete. Laws concerning magnetic properties of materials 
were observed and learned as these materials were used under practical 
conditions. Today, the use of magnetism and the laws of its behavior are 
vital and necessary parts of the field of electricity. . 

Types of Magnetic Mpterial 

Any material which cain be made to possess this peculiar property of hold- 
ing bits of iron is called a magnetic material, and exhibits the properties 
of a magnet. This magnet is said to possess polarity by virtue of its two 
poles — a North Pole (North-seeking) on one end, and a South Pole 
(South-seeking); on the other end. 

The amount of magnetism possessed by different materials varies. If a 
material can bej strongly magnetized (possessing a great deal of magnet- 
ism) it is classified as ferromagnetic. If a material can only be slightly 
magnetized (possessing a small amount of magnetism) it is classified as 
paramagnetic. l|f a material cannot be magnetized (possesses no magnet- 
ism) it is classified as non-magnetic (Fig. 4). Most magnetic materials 
are paramagnetic, such as chromium, aluminum, and zinc. Very-few mag- 
netic materials are ferromagnetic, such as iron, steel, nickel, and their 
alloys. 

Magnets may tje classified as permanent magnets, temporary magnets, or 
electromagnets.i A hard steel bar (when magnetized) becomes a tempo- 



MAGNETIC AMPLIFIERS 



STRONG 
MAGNETISM 



WEAK 
MAGNETISM 



NO 
MAGNETISM 







FERROMAGNETIC 
MATERIALS 



PARAMAGNETIC 
MATERIALS 

■J" . '"a^. 



NON-MAGNETIC 
MATERIALS 






Fig. 4. Magnetized and nonmagnetic materials. 

rary magnet, since it loses its magnetism within a short period of time. 
Any magnetic material whose magnetism can be strengthened or weakened 
by an electric current is called an electromagnet. 

Modern Theory of Magnetism 

All materials are composed of atoms whose structure resembles our own 
solar system. Electrons revolve around a nucleus in well-defined paths, 
as planets revolve around the sun. These paths or orbits are many, each 
containing electrons not only revolving around the nucleus, but also spin- 
ning on their own axes. Because of these movements, tiny magnetic fields 
exist between the electron and the nucleus, as well as around the electron, 
with North and South Poles established. 

The modern theory of magnetism is that no matter how many electrons 
there are in each ring or orbit, the direction of the spins (clockwise or 
counterclockwise) varies, so that within one atom some of the electrons 



REVOLVES 



SPINS 



ELECTRON 




Fig. 5. Electron revolving around nucleus and spinning on its axis. 



MAGNETISM 



are spinning in a clockwise direction, and the remaining spin, counter- 
clockwise (Fig. 5). This would indicate that an' atom may contain an 
excess of spins in a certain direction or a deficiency of spins or a balanced 
number of spins. Even though each electron has a tiny magnetic field with 
a North and a South Pole, a collection or group of these electrons spinning 
in the same direction, would add their fields to create a greater magnetic 
field. Materials having atoms whose number of electron spins are unbal- 
anced, are considered to be magnetic materials. Those materials having 
a balanced number of spins are nonmagnetic. 

If the tiny magnetic field of a spinning electron lines up (alignment of 
poles) with other fields within the atom, and this one atom aligns itself 
with other atoms to form a group, this aligned group of atoms is called 
a domain. 

The domains of magnetic materials are considered as tiny magnets with 
direction (North and South Poles). These domains are arranged in a 
haphazard or random fashion within the material when it is demagnetized. 
An illustration of this is shown in Fig. 6, with the arrows representing and 



Fig. 6. Domams in an un- 
mognetized material. 




showing the direction of the domain. When an external force is capable 
of causing these tiny magnets to align or orient themselves so that they 
are pointing in the same direction, the magnetic material is said to be 
magnetized (Fig. 7A). 

Fig. 7B shows all the domains oriented in one direction. Any attempt by 
an external force to improve on this alignment would be useless since all 
the domains are already aligned and pointing in one direction. The mag- 
netic material is said to be saturated, and it would have its maximum 
piagnetic strength. Magnetic materials require varying amounts and dif- 
ferent types of external means to cause maximum alignment of these 
domains. This also means that the degree of magnetic strength of mate- 
rials can be varied by varying the alignment or orientation of these 
domains which act as tiny magnets. 



MAGNETIC AMPLIFIERS 




(A) 



Fig, 7A. Domains in a slightly magnetized material. 



3k. ». jto- k> 

>• ^ jfc- »> 

jb. >. j^ ^ 

*■ »• ^ ^ 

♦- >■ »■ ^ 



(B) 

Fig. 7B, Domains in a strongly mognetized moteriol. 



Alignment of 
domains 

Haphazard and 

randomly aligned 
Partially aligned 



Degree of Degree of 

magnetization saturation 

No magnetism Unsaturated 



Minimum Slightly saturated 

magnetism 

Maximum alignment Maximum Saturated 

magnetism 



Lines of Force (Magnetic Flux) 

Iron filings sprinkled on a piece of cardboard placed on top of a magnet, 
arrange themselves in a definite pattern (Fig. 8), This pattern indicates 
the influence the magnet exerts in the space surrounding the magnet. This 
influence is attributed to what is called lines of force, since a definite force 
is exerted on objects placed in it. This influencing area surrounding the 
magnet is called a magnetic field. 

The production of these lines of force in a magnet is attributed to the 
collective effects of the lines of force of the tiny domain magnets within 
the structure of magnetic materials. The lines of force within a magnetic 



MAGNETISM 



Fig, 8. Magnel 
farce around a 



ines of 
magnef. 




field is often rej'erred to as lines of flux, or simply flux. They are often 
designated by the symbol "(^". The unit of flux is one line and is called 
the maxwell. Th^; flux per unit area, or the number of maxwells, is known 
as the flux density and is designated "B". The unit of flux density, or one 
maxwell per sqiiare centimeter, is called gauss. Therefore, by definition: 



Flux density (B) = 



_ Total flux (<^) 



Area (A) 



Example : Evaluate the flux density of a toroid having a cross-sectional 
area of 3 square centimeters and a total flux of 6^,000 Jine^ (maxwells) . 
Solution: A toroid is a doughnut-shaped magnetic material with a circular 
cross-section (Fig. 9). 

CROSS 

W(* SECTIONAL 

AREA 



B =_0 = 6.000 LINES = 2,000 LINES OR GAUSS 
A 3CM2 CM^ 



TOROID 
Fig. 9. A toroid is a doughnut-shaped material with a circulor cross-section. 




Properties of Lines of Force or Flux Lines 

Flux lines, or lines of force, are continuous, and always form closed loops. 
They can be cut or broken by moving objects, but they immediately mend 
or complete their loops after the objects pass through the field; they are 
never left open or broken. 

Flux lines have direction, as shown by the arrows in Fig. 10. They emerge 
from the North Pole, go through the air, and enter the South Pole to re- 
turn through the magnet. 



MAGNETIC AMPLIFIERS 




Fig. 10. Magnetic field around bar magnet. 



Flux lines never cross each other. They can be bunched, crowded, or 
thinned out over a large area. When they are bunched or crowded, each 
flux line has a repelling effect upon its neighboring flux line. This tends 
to keep them separated from one another. The concentration of flux lines 
in and around the poles is greater than away from the poles. Flux density, 
therefore, is greatest in the region of the poles, and least in the regions 
away from the poles. 

A flux line has tension. It can be stretched or constricted along the direc- 
tion of the lines of force. This tension is somewhat like a stretched rubber 
band that tends to become as short as possible. 

Flux lines pass through all materials. There is no insulator for magnetic 
lines of force. Since materials vary in their composition they offer different 
amounts of resistence. Hence, flux lines tend to take the path of least 
resistance, preferring iron or steel to air or gas. 



Laws of Magnetism 

If the North Pole of a magnet is placed within the magnetic field of the 
North Pole of another magnet, or if the South Pole is placed within the 
magnetic field of another South Pole, forces of repulsion will exist. If the 
North Pole of a magnet is placed within the magnetic field of a South Pole, 
or if a South Pole is placed within the magnetic field of another North 
Pole, a mutual attraction between poles will exist. 

The amount of attractive or repulsive force that these poles exert depends 
upon the strength of the magnet's field and the distance or location of the 
objects being affected. 



MAGNETISM 



FORCE OF REPULSION 

OR ATTRACTION 

IN DYNES 



MAGNETIC POLE 
WEIGHT 
I GRAM 



I CM. 



Fig. 11. The dyne is a unit of force. 



Xz 







Fig. J 2. Pattern of magnetic flux lines. 



10 MAGNETIC AMPLIFIERS 

„ ,„, Pole strength (Mi) X Pole strength (M2) 

r orce ( r ) -— ; 

(Distance between poles)^ or (d'^) 

Force (F) equals attractive or repulsive force between poles expressed in 
units of dynes. Mj M2 equal the unit of magnetic poles expressing the 
strength of the magnet; and defined as one which, when placed 1 cm from 
an equal and similar pole in vacuum or air, repels it with a force of 1 dyne. 

What a Dyne Is 

The dyne is the unit of foTce in the Metric System, and when it acts on 
a weight of 1 gm, this force causes the weight to accelerate at the rate of 
1 cm/sec/sec. The pole moves from position Xi to position Xj at a rate 
of 1 cm /sec in the time interval of 1 sec. (See Fig. 11.) 

Fig. 12 illustrates the pattern the magnetic flux lines take for two different 
conditions: Unlike poles attracting and like poles repelling. Example: Two 
North-pointing poles of strength 10 and 20 units, respectively, are 5 centi- 
meters apart in air. Determine the type of force between the poles, the 
direction of the force, and the magnitude of the force. For the solution, 
see Fig. 13. (Note: The nature of the medium that contains the lines of 
flux plays an important role in the computation. It will be described in a 
later section on permeability.) 

FORCE OF ^ 

"* REPULSION 



FORCE =_MjMz = JCIX20 =^00 = 50 DYNES 
~W ~? 4 

Fig. 13. Solution to example given. 



Review Questions 



1. What has been accomplished in the field of electricity as a result of our understand- 
ing of the laws of magnetism.' 

2. Explain the reasons for different degrees of magnetic strength possessed by different 
materials. 

3. If a coil is to be wound around a toroid core to produce 12,600 lines within the 
core with a flux density of 6,300 lines per square centimeter, what is the circum- 
ference of the coil? 

4. Two magnetic poles, North and South, are facing each other 20 centimeters apart 
with strengths of 15 and 25 units respectively. Determine the type of force between 
the poles, direction and magnitude of the force. 



ELECTROMAGNETISM 1 1 

Magnetic Field Around a Conductor 

If a straight vertical conductor carrying an electric current pierces a card- 
board (Fig. 14), the plane of the cardboard will contain lines of flux which 
illustrate the principle: Current-carrying conductors produce magnetic 
fields around conductors. 



Fig. 14. Concentric field 
around a conductor. 




To determine the direction of a magnetic field around a conductor, grasp 
the conductor with the left hand, pointing the thumb in the direction of 
current flow. The fingers will then point in the direction of the magnetic 
field. 

When a conductor is bent to form a loop (Fig. 15) the closed flux loops 
are no longer circular. They become more crowded in the space inside 
the loop of wire, and less crowded in the space outside the loop of wire. 
Accordingly, the intensity of the magnetic field within the loop has in- 
creased since there is a denser field within the loop. When several loops 
of a conductor are placed together to form a coil (Fig. 16), the intensity 
of the magnetic field is multiplied by the number of turns of wire. The 
value of the field intensity at any point for two turns would be twice that 
for a single loop; for three turns, three times that for a single loop. A 
large number of turns of the conductor produces a large magnetic field. 

Fig. 16 shows how each individual loop produces a field and contributes 
to the overall magnetic field of the coil. In the upper conductors of the 
coil, the current flows toward the reader (dot) and produces a clockwise 
magnetic field. Since all upper conductors produce magnetic fields in the 
same direction, the individual fields combine to form a field in one direc- 



12 



MAGNETIC AMPLIFIERS 




Fig. 15. Magnetic field around current-carrying loop. 

tion. In lower conductors of the coil, the current flows away from the 
reader (cross) and produces a counterclockwise magnetic field. Each indi- 
vidual conductor field — since it is in the same direction — combines to 
form a continuous field. This coil of wire, due to looping many conductors 
in helix fashion, creates an overall magnetic field similar to that of a bar 
magnet with North and South Poles (Fig. 17). 



WWCtlOH OF CURRENT 




Fig. 16. Cross-sectional-view of a coil carrying a current. 



ELECTROMAGNETISM 



13 



When a piece of soft iron is inserted within the coil, the intense magnetic 
field (instead of passing through air) passes through a material of less 
resistance. This increases the number of lines of flux formed to close the 
circuit between the North and South Poles. 




MAGNETIC FIELD PRODUCED BY A 
CURRENT FLOWING THROUGH A COIL 






// 
; \ 



^»«fc^h^B>^.a w 



-?-r' 



y 






MAGNETIC FIELD PRODUCED 
BY A BAR MAGNETIC 



Fig. 17. Magnetic fields. 

Magnetomotive Force iff) 

To cause ffux to flow in a particular material, a force is necessary. It is 
called magnetomotive force. It is a measure of the strength of a source 
which produces lines of force or lines of flux. Magnetomotive force is often 
referred to as the ampere-turns of a coil, since it is the product of the 
number of turns of the coil and the current flowing through the coil. The 
symbol used for this force is mmf. 




5^=NI = 500x0.25 = 125 AMPERE- TURNS 
Fig. 18. Solution for example. 



Force (<gr) = Ampere (l) X Number of turns of coil (N) 

Example: Determine the mmf of a coil wound on a toroid, when 0.25 
ampere is flowing through the coil of 500 turns. Solution: Fig. 18. 



14 MAGNETIC AMPLIFIERS 

Magnetizing Force or Magnetic Intensity (H) 

If we increase the amount of current flowing through a coil, we find that 
the intensity of the field is increased proportionately. Therefore, magnetic 
field intensity (H) is related to an electric current for any given set of 
conditions. The field intensity in the air on the inside of a coil can be 
found by using the following equation: 




Where: N equals the total number of turns; I equals the current flowing 
through the coil; 1 equals the length of the coil in centimeters; 1.26 equals 
the constant of the coil. The field intensity (H) can be considered as the 
force tending to produce magnetic flux in each unit length of a magnetic 
circuit, and is often referred to as the magnetizing force. Substituting 
magnetomotive force in the above mentioned equation: 



H = y or F = HI 



Pernaeability - 



The measure of the magnetic conductivity of a substance is known as its 

permeability, and is indicated by the Greek letter ft. It indicates the rela- 

tive eas_e by which a material will permit magnetic fluxJq_pass.jJiXQugh it. 

By definition, the permeability"'oFany magnetic material can be expressed 

as: 

„ , ... , , Flux density (B) 

rermeability (a) = r^ r^ p^ 7777 

' ^ Magnetizmg force (H) 

Permeability compares the relative ease of a substance in conducting flux 
lines with air (air being standard) . For example, a given coil with a known 
current produces 1,000 lines of flux when air is its core. With a change 
in core material, the coil will produce, 6,000 lines of flux. This indicates 
that the core material is capable of conducting magnetic flux six times as 
great as air. (Its permeability is 6.) Air is used as a reference medium 
and has a permeability of 1. Ferromagnetic materials have high perme- 
abilities; paramagnetic materials have low permeabilities. 

Reluctance 

Relu ctance is the resistance o r opposition a material offers lines of flu x in 
a portion or entire magnetic circuit. Ihe amount of reluctance depends 
upon the type of material and the physical dimensions of the material. 

Reluctance (rT?) ~ Length (1) 



Permeability (jx) X Area (A) 



ELECTROMAGNETISM 



15 



If we substitute jn = -rp in the above equation and simplify: 
H 



Oi = 



(I) 



BA 



Substituting magnetomotive force 3^ = HI and Flux (f) = BA, we have 

The reluctance of a material would depend upon the ratio of magnetic 
potential drop (mmf) to the flux in any part of a magnetic circuit. 
Example: A certain toroid has a winding of 300 turns. When this winding 
carries a current of 5 amps, 50,000 maxwells of flux are produced. Deter- 
mine the permeability of the toroid core if the mean length is 12 cm and 
the cross-sectional area is 0.5 cm^ (Fig. 19). Reluctance must first be 
determined: 



Reluctance (Oi) = 



mmf jfT) ^ N X I ^ 300 X 5 _ 1500 
Flux (cj)) ~ (j) ~ 50,000 ' 50,000 

Rearranging (R = — -r- to solve for permeability (jj,) 



= .03 



Permeability (fi) = 



Length (1) 



12 



I = 5a 



Reluctance {J^ X Area (A) (.03) (.5; 
FLUX = 50,000 LINES 



= 800 




CROSS-SECTIONAL 
AREA=.5cm2 



MEAN 
LENGTH /=l2cm 

Fig. 19. Solution for example 



Magnetization (B-H) Curves 

The amount of flux lines (B) in a magnetic material depends upon the 
magnetic conducting ability of the material {fjb) and the amount of mag- 
netizing force (H). The effect of any change in magnetizing force on the 
flux density within the material can be observed by a B-H curve or mag- 
netization curve of the material (Fig. 20). 



16 



MAGNETIC AMPLIFIERS 



15000 

12500 

o 10000 

Z 7500 

UJ 

°- 5000 
<n 

i2500 

_) 

















" 








y 


B-H CURVE 




















\ 












\ 












^INSTEP 











5 10 15 20 25 

H AMPERE -TURNS /INCH 

Fig. 20. B-H curve for iron. 



30 



The amount of magnetizing force is indicated horizontally (abscissa) 
from zero to higher values, and the flux density is indicated vertically 
(ordinate) from zero to higher values. It can be seen from the curve that 
at very low values of magnetizing force (0-2) the flux density increases 
at a very low rate (up to instep). This is due to the initial inertia pos- 
sessed by the domains within the material, and the energy required to 
overcome this inertia. In the region of 2-4 ampere- turns per inch, the 
curve is almost linear, indicating that the domains are now responding 
by orienting in a particular direction at a nearly linear rate. Beyond 5 
ampere-turns per inch, the rate of increase of flux drops off until a point 
is finally reached — no matter how great the magnetizing force — the 
amount of flux lines the magnetic material is capable of holding, has 
reached a maximum. The magnetic material is said to have maximum 
magnetism or be saturated. The domains of the material have completed 
their alignment and are all pointing in the same direction. 



UNSATURATED 
-REGION-*^ 



TRANSITION 
REGION 



SATURATED REGION 




Fig. 21. Regions of the magnetization curve. 



ELECTROMAGNETISM 



17 



The magnetization curve differs in shape and characteristics with different 
materials. Some would have large slopes, others small slopes. Some would 
saturate with a small amount of magnetizing force, others would require 
a large amount of magnetizing force to saturate the material. A magneti- 
zation curve would depend upon the following. (1) Type of material: 
Cast iron, wrought iron, steel, or alloy of these with other materials; (2) 
Degree of purity: The amount of non-metals or other impurities within 
the material; (3) Heat treatment: Preparation of the material in anneal- 
ing processes, temperature of heat, and length of time at that temperature ; 
(4) Degree of radiation exposure: The intensity of neutron flux on gamma 
rays has an influence on certain materials; (5) Previous magnetic history: 
Whether or not the material has been subjected to a high degree of mag- 
netization in the past. ^ 

The magnetization curve can be divided into three operating regions as 
in Fig. 21: Region A (unsaturated region), increasing values of magnet- 
izing force (H) causes flux density (B) to increase very rapidly; Region 
B (transition region), core tending toward saturation despite increasing 
values of magnetizing force (H), flux density (B) decreases slowly; 
Region C (saturation region), large values of magnetizing force (H) 
result in little or no flux density change (B) within the core. 




HARD STEEL 




SOFT IRON 



B B 

Fig. 22. Hysteresis loops for soft iron and steel. 



Hysteresis 

Hysteresis is a Greek word meaning to lag. Hysteresis in a magnetic mate- 
rial means the magnetic flux of lines of force lag behind the magnetizing 
force that causes them. When a magnetic material is subjected to an in- 
creasing magnetizing force until the saturation point is reached, and then 
the magnetizing force is decreased to zero and established in the opposite 
direction until the saturation point is again reached (and if the magnetiz- 
ing force is again decreased to zero and again increased until the cycle is 



18 



MAGNETIC AMPLIFIERS 



completed) , the relations between flux density and field intensity for all 
parts of the cycle may be represented by a curve (Fig. 22) called the hys- 
teresis loop. Note that the lines of force lag behind the magnetizing force 
that cause them. The hysteresis loop for any given material (Fig. 23) rep- 



SMALL 
COERCIVE FORCE (OD) 
REQUIRED TO ELIMINATE 
LARGE RESIDUAL 
MAGNETISM (OC) 



B 



V 



LARGE 
COERCIVE FORCE 
(OD) REQUIRED 
TO ELIMINATE 
SMALL RESIDUAL 
MAGNETISM (OC) 




LARGE 
COERCIVE FORCE 
(OD) REQUIRED 
TO ELIMINATE 
LARGE RESIDUAL 
MAGNETISM (OC) 



Fig. 23. Types of hysteresis loops for different magnetic materials. 



resents the flux density variation produced by one complete cycle of alter- 
nating current. The characteristics of these loops are as follows. ( 1 ) Points 
a and a^ : Positive saturation levels with increasing positive magnetizing 
force. (2) Points e and e^ : Negative saturation levels with increasing 
negative magnetizing force. (3) OC and OC: Positive residual magnet- 
ism in the cores. The magnetic material has the ability of retaining its 
magnetism despite the removal of the magnetizing force. (Note that hard 
steel is capable of retaining a larger amount of flux density than soft iron. ) 
(4) od and od^ : Coercive force. This is the amount of magnetizing force 
necessary to reduce the residual magnetism to zero, or the amount of 
force necessary to clear the material of all its flux. (5) of and oP : Nega- 



ELECTROMAGNETISM 



19 



live residual magnetism produced by the negative cycle of an alternating 
current. (6) Areas within the loop: Represent the power losses per cycle 
of magnetization of the iron core. 

Electromagnetic Induction 

Michael Faraday, about 1831, experimented and tested the effect of a 
magnetic field upon a conductor moving through the field. He also tried 
the opposite condition of moving a field through a stationary conductor. 
By these methods, he determined that an electromotive force was induced 
in the conductor. The current caused to flow in the conductor by the in- 
duced emf is called an induced current. The process in which an emf is 
induced in a conductor when it is cut by a magnetic field, is called 
electromagnetic induction. The factors affecting the amount of emf are: 
(1) Number of turns of the conductor when made into a coil; (2) Speed 
by which the conductor cuts the field or the field cuts the conductor on a 
per unit time basis; (3) Strength of the magnetic field; (4) The angle 
or physical relation between magnetic field and conductor. 









< 




FIELD COLLAPSES 



FIELD 
MAXIMUM 
Fig. 24. Changes in the magnetic field from an alternating current. 



All of these factors are brought together in Faraday's famous Laws of 
Induction: "Whenever the number of hnes of flux threading through a 
coil is changed, an emf is induced in that coil. The amount of emf induced 
is proportional to the rate at which the number of lines of force through 



20 



MAGNETIC AMPLIFIERS 



the coil is changing." If current i in a coil is a steady -state current (does 
not change), a field is produced which is also steady-state. However, when 
the alternating i changes in magnitude with time (represented as di/dt), 
such as in a-c current when it alternates directions 60 times per second, the 
magnetic field <j) builds up, expands in a positive direction, then collapses ; 
builds up, expands in a negative direction, then collapses, and does this 
60 times per second (Fig. 24) . 

Faraday's Laws, may be expressed in the equations following: 

(1) e = L -t- and (2) e = N ^ 
^ ' dt dt 

Where N equals Number of turns of wire; L equals Inductance in henries; 
e equals induced back voltage emf ; d<^/dt equals change in flux with time; 
di/dt equals change in current with time. 

Setting equations 1 and 2 equal to each other and solving for L, we have : 

, di -, drf) , 

L -T- = N -r- and 



A 
dt 



dt 



L di = N d<i and L = N ^ 

di 



The constant of the coil (L) is called inductance, and is a function of the 
number of turns of the coil and the rate of change of magnetic flux with 
respect to current. It is also that property of a circuit which causes an 
emf to be induced as a change in circuit current occurs. 



TRANSITION 
REGION 



SATURATED REGION 




Fig, 25. Magnelization effects on inductance. 

Induction and the Magnetization Curve 

As described earlier, the magnetization curve is a graphic representation 
of the relationship between magnetizing force (H) and flux density (B) in 
a magnetic material. Also described was the effect of operating in various 



ELECTRO/AAGNETISM 21 

regions of the curve with changes in magnetizing force on nonsaturation. 
Operation in various parts of the magnetization curve also has a signifi- 
cant effect on the inductance of a coil wound around the magnetic core 
material. 

Since inductance has been defined as L equals N d(/)/di, and since mag- 
netizing force (H) is related to current (H equals N [di] for changing 
values), the magnetization curve coordinate system can be changed (Fig. 
25). Region A (unsaturated region): Small change in current (di) in 
region A results in large change in flux (dc/)), therefore, the inductance 
of a coil in this region of operation is extremely high. The denominator 
of L equals d{/)/di is small, therefore, L is very large. This high inductance 
or resistance results in a very large coil impedance, creating a large oppo- 
sition to current flow. Region B (transition region) : The knee of the 
magnetization curve is the region between the unsaturated and saturated 
core. It represents the area where the core tends toward saturation or 
tends toward desaturation. Inductance, inductive reactance, and imped- 
ance are undergoing changes from values of maximum to minimum, or 
from minimum to maximum. Region C (saturated region) : Large change 
in current (di) in region C results in a small change in flux (dcjb). There- 
fore, the inductance of a coil in this region of operation is very small. The 
denominator of L equals N d(/)/di is large. Therefore, inductance reactance 
results in a very small coil impedance, creating a very small opposition to 
current flow. 

Summary of Characteristics 





° Saturation 






Current Flow 


Region 


of Core 


Inductance (L) 


Impedance (Z) 


in Coil 


A 


Unsaturated 


High 


High 


Low 


B 


Transition 


High to Low 


High to Low 


Low to High 


C 


Saturated 


Low 


Low 


High 


Review Questions 









1. How does each individual loop in a coil produce a field that contribiites to the 
overall magnetic field of the coil? 

2. Explain the similarities and the differences between magnetic fields produced by 
coils, with that of bar magnets. 

3. The mmf of a coil wound on a toroid is 250 ampere-turns when .75 amperes is 
flowing through the coil. Determine the magnetic intensity (H) of a coil wound 
on a toroid when .75 amperes is flowing through a coil ol length 3.25 centimeters 
and 333 turns. 

4. Give a statement of definition of permeability as well as mathematical expression 
of this definition. 



22 MAGNETIC AMPLIFIERS 

5- Derive an expression relating permeability (n) with Reluctance ( (R ). Is this an 
inverse relationship? 

6. Explain how the slope of the magnetization curve, when changed, affects the 
saturated and unsaturated regions of a core. 

7. Draw a hysteresis loop when the coercive force is approximately twice that of 
the force created by residual magnetism. Explain the cycle of operation and the 
relationships between the two forces. 

8. Slcetch an alternating sine-wave and explain the changes in magnetic field created 
by this wave in one cycle. 

9. What is the inductance of a coil with 10 turns when the field collapses from 
2,000 lines to 1,500 lines and when current changes from 5.5 amps to .13 amps.' 

10. Plot a curve showing the relationship of the cross-sectional area of a toroid core 
to its mean length, when the material under consideration has a reluctance of .03 
and a permeability of 600. 



MAGNETIC CIRCUITS 



23 



A magnetic circuit is a path of low reluctance to the flow of flux lines as 
produced by a magnetizing force. The three basic types of simple magnetic 
circuits (Fig. 26) are: Air core, iron core, and iron core with air gap. To 
gain a better understanding of magnetic circuits as used in magnetic 




AIR CORE 











i 






1 
1 

1 
1 


< 




> 


k 


"- 




1 


1 







IRON CORE 




IRON CORE WITH AIR 
GAP 



Fig. 26. Three basic types of simple magnetic circuits. 

amplifiers, an analogy or comparison is made between electric and mag- 
netic circuits. This comparison shows the similarities between them so 
that an understanding of electric circuits forms a basis for an easy under- 
standing of magnetic circuits. 



Comparison 



Parameters of 
electric circuits 
Electric current (I) 
Resistance (R) 
Voltage (E) 
Ohm's Law (I = E/R) 
Resistivity (p) 
Parameters of resistance 

R = p (L/A) 



Analogous parameters 
of magnetic circuits 
Magnetic flux ((h) 
Reluctance ( 5) ) 
Magnetomotive force ( ^5") 
Magnetic law W) = t9"/5) 
Permeability (p.) 
Parameters of reluctance 

a" = iVnA 



Where L equals Length of core; A equals Area of core 



24 



MAGNETIC AMPLIFIERS 



In making an analogy between electric and magnetic circuits (Fig. 27) 
for similarities, note that these two circuits differ in some respects: Flux 
is not strictly analogous to current, since current is rate of flow while flux 
is more nearly a state or condition of the medium established; and a 



CHARACTERISTICS OF AN 
ELECTRIC CIRCUIT 



-I 



E -=- 

+ 



CHARACTERISTICS OF A 
MAGNETIC CIRCUIT 




1. CURRENT FLOWS ONLY IN A 
COMPLETE CIRCUIT LOOP 

2. VOLTAGE IS NECESSARY FOR 
CURRENT TO FLOW. 

3. RESISTANCE OFFERS OPPOSI- 
TION TO CURRENT FLOW. 

4. PARAMETERS OF RESISTANCE, 
SUCH AS LENGTH, RESISTIVITY, 
AND AREA, EFFECT CURRENT 
AND VOLTAGE IN CIRCUIT 



1. MAGNETIC FLUX WHEN 
ANALOGOUS CREATED FORMS COMPLETE 

LOOP 

2. MAGNETOMOTIVE FORCE IS 
ANALOGOUS NECESSARY FOR FLUX LINES 

TO EXIST. 

3. RELUCTANCE OFFERS OPPOSI- 
ANALOGOUS j\q^ jq FLUX LINES. 

4. PARAMETERS OF RELUCTANCE, 
SUCH AS LENGTH, PERME- 

ANALOGOUS ABILITY, AND AREA. EFFECT 

FLUX AND MAGNETOMOTIVE 
FORCE IN A CIRCUIT 



Fig. 27. Circuit analogy. 

magnetic circuit can never be entirely opened since a magnetic field must 
exist at all times in the vicinity of a magnet while an electric circuit can 
be easily opened (Fig. 28). 



Review Questions 

1. Explain the important characteristics of a magnetic circuit. 

2. Describe how reluctance varies in the three simple types of magnetic circuits. 

3. If reluctance in magnetic circuits is analogous to resistance in electric circuits, show 
how the total reluctance of three magnetic materials in parallel is equal to; 



Gi T = 

4. In what ways do magnetic circuits difter with electric circuits? 

5. Draw a sketch of three magnetic loops and show how you would increase the 
reluctance of one loop to a greater value than the other two. 



MAGNETIC CIRCUITS 



25 




TOROID CORE 



RECTANGULAR CORE 




CONCENTRATED MMF ON A CORE 

m. mM 



DISTRIBUTED MMF ON A CORE 



SINGLE MAGNETIC MATERIAL 




MATERIAL MATERIAL 

B A — * 



^ 



3=3 



L- --!— : j 



MATERIAL 
^- B 



RELUCTANCE CHANGE 



SIMPLE MAGNETIC CIRCUIT 



RELUCTANCE CHANGE 





o 6 



MAGNETIC CIRCUIT ■ MORE THAN ONE WINDING 





rF^ 


1 


r?*= 


■T 




r , 
1 1 
1 < 
1 ' 
1 ' 


-4-- 


l: 


i 

1 




.____.( 


L 


-J 




MATERIAL 
« B 



COMBINED RECTANGULAR CORES 



MAGNET C C RCU TS 



COMBINED ' E CORES 




MAGNETIC CIRCUIT VARIABLE RELUCTANCE AND AIR GAP 



Fig. 28. Types of magnetic circuits. 



26 



MAGNETIC AMPLIFIERS 



THE SATURABLE REACTOR 

The saturable reactor is the device from which the magnetic amplifier has 
been fundamentally developed. Consequently, a thorough understanding 
of the saturable reactor is a must before one is able to fully understand 
the magnetic amplifier. 

The saturable reactor consists of three essential elements: Direct current 
source, magnetic core with windings, and alternating current source (Fig. 
29). 



o 




G) 




G) 




DIRECT 
CURRENT 
SOURCE 




MAGNETIC 

CORE WITH 

WINDINGS 




ALTERNATING 

CURRENT 

SOURCE 




L 

A 
D 









Fig. 29. Three essential elements of the saturable reactor. 

The Basic Saturable Reactor 

These three elements, when connected together, form the essentials of the 
saturable reactor. Its operation is based upon the following principle: 
The flow of current from a coil wound on a magnetic core can be made 
to vary by varying the saturation of the core. Following sections explain 
more fully this important principle upon which not only saturable reactors 
operate, but also magnetic amplifiers. 

Simple Saturable Reactor 

The simple saturable reactor (Fig. 30) consists of two electric circuits 
connected by a magnetic circuit, so that the operating characteristics of 
any one circuit effects the operation of all the interconnected circuits. In 
Fig. 30, d-c current in the control loop flows through windings Ni which 
establish the magnetomotive force cTc or ampere turns of the control wind- 
ing. Current flows through this winding and sets up a d-c flux (in one 
direction) in the magnetic circuit loop. 

Since an a-c source is connected in the load loop, a-c current flows through 
winding N2, establishing the magnetomotive force (^Tl or ampere-turns) 
of load winding N2. Since this current is alternating, the flux set up in 
the magnetic circuit loop is constantly changing in magnitude and direc- 
tion. 

Within the magnetic core there now exist two types of flux: (1) (jia-e, the 
flux created by d-c current which is constant in magnitude and constant 
in direction. This means the field builds up and remains steady-state. 



THE SATURABLE REACTOR 



27 



(2) (^a-c, the flux created by a-c current which is changing in magnitude 
and changing in direction. This means the field builds up to a maximum 
in one direction, collapses, and builds up to a maximum in the opposite 
direction. ^ 



RHEOSTAT 




-1^1' 



LOAD 
LOOP 



LOAD 



ELECTRIC 
CIRCUIT 



CONTROL \ V 
WINDING 



■ — -* v^ 



■<5^^ 



LOAD 
WINDING 



Fig. 30. Simple saturable reactor. 



The a-c flux tends to saturate and then desaturate the core because of its 
cyclical operation. This results in a changing inductive reactance in the 
load winding. The d-c flux, according to its strength, aids or opposes the 
a-c flux in its saturate or desaturate effects in the core. Hence, the d-c flux 
tends to control the a-c flux controlling the reactance of the load winding. 
The use of separate windings on a single core has distinct advantages. 
Load winding N2 consists of comparatively few turns of heavy wire be- 
cause of large current requirements of different loads. Control winding 
Ni consists of many turns of fine wire. Since magnetomotive force depends 
upon the number of ampere-turns, a small current in the control winding 
produces a magnetomotive force equal to that of the load winding. Usually, 
d-c in the order of milliamperes controls a-c in the order of amperes. 

The following describes the steps in the operation and control of the sim- 
ple saturable reactor: 1. Zero d-c control current in the control loop — A. 
Since only a-c current is flowing through the load windings, an extremely 
high inductive reactance (Xl) is present in the load windings. This is 
due to the high inductance (L) of the load windings and the action of 
the varying magnetic field produced by the a-c. B. Extremely high induc- 
tive reactance in this winding results in a high impedance (Z) which 
limits the flow of a-c current to a low value. This high reactance also 



28 MAGNETIC AMPLIFIERS 

causes a large voltage drop across the load windings in series with the 
load, limiting the current supplied to the load. C. Since current is limited 
to a low value to the load, minimum power is transferred to the load since 
power is a function of current. 2. Increase d-c control current in control 
loop. A. D-c current creates a flux ((ft/a-c) which, when superimposed on 
the a-c flux (<^a-c); collectively saturates the core. B. Since the core is 
near saturation or fully saturated (core unable to hold any more flux 
lines), the inductive reactance is greatly reduced. This is due to the fact 
that no additional changing flux can be held by the core. C. With reduced 
inductive reactance the impedance of the load windings is greatly reduced. 
D. Large a-c currents are now permitted to flow through the load. E. This 
results in maximum power transfer to the load. 3. Decrease d-c control 
current in control loop. A. With less d-c current flowing there is less total 
flux in the core and the core desaturates. This results in the core's ability 
to support once again the changing flux ((/)a-c), creating a high inductive 
reactance, and resulting in increased impedance in the load winding. B. 
Minimum power transfer results since current to the load is greatly 
reduced. 

This operation of the saturable reactor is shown graphically in Fig. 31. 
Curve A represents the variation of inductive reactance (Xr,) or imped- 
ance (Z) with changing d-c control current. Curve B represents the varia- 
tion of current (I) and power (P) to load, with changing d-c control 
current. 

These curves mean: When the d-c is zero, coil impedance is maximum, 
and output current or power is minimum; and when the d-c increases to 
maximum, coil impedance decreases to minimum, and output current and 
power increase to maximum. 

Basically, the principle of operation of the simple saturable reactor can 
be stated in two parts: As magnetic coie_ Mtu.rates, cujxenl to load in- 
creases; as magnetic core desaturates, current to loa^dgcxeases. 

Operating Characteristics of the Simple Saturable Reactor 

Using the simple saturable reactor in Fig. 30, the following information 
describes operating characteristics. 

Linearity and distortion — The amount of d-c flowing in the control loop 
together with the number of turns in the winding determine the magneto- 
motive force (3fc) of the control winding. Since magnetomotive force is 
equal to the magnetomotive force of the load winding (gfj,), operating 
point A on the magnetization curve is established. This pomt will change 
up or down depending on the amount of control current flowing. As can 
be seen in Fig. 32, when the operating point falls in the linear region the 
output is relatively undistorted. Consequently, the sinusoidal input, both 



THE SATURABLE REACTOR 



29 







Xl = INDUCTIVE REACTANCE 


1 


Z = IMPEDANCE 


A-C '^ 
LOAD 
OUTPUT 


I = CURRENT 
P = POWER 


\ 


CURVE A / 




W(XuORZ) A. 




\. /CURVE B 




N. y^ (lORP) 









Fig. 31. Input to output relationships. 



positive and negative halves, will be amplified with very little distortion. 
As the operating point is moved to B, the non-linear portion of the curve, 
by increasing the mmf, one-half the output waveform (Fig. 33) is dis- 
torted. This operation falls primarily in the transient region of the magnet- 
ization curve. The distortion occurs because the positive and negative half 



^trS" 



OUTPUT 
UNDISTORTED 




Fig. 32. Output wave when operating in linear region. 

cycles of the input to the load winding operate on the nonlinear and linear 
portions of the magnetization curve. This means that the core is under- 
going first a tendency to saturate, then a tendency to desaturate. The out- 
put results in a waveform in which the positive half cycle is distorted and 
the negative half cycle is undistorted. 



30 



MAGNETIC AMPLIFIERS 



OUTPUT 




Fig. 33. Distorted output waveform. 

If the operating point is moved to C (Fig. 34) by a large increase in 
magnetomotive force, a highly distorted output waveform results. This is 
due largely to very small alterations in flux which result in a small induct- 
ance. At this operating point, saturation occurs for both positive and nega- 
tive half cycles, resulting in a highly distorted waveform. 



NONLINEAR REGION 
C 




INPUT 
Fig. 34. Highly distorted output waveform. 



Amplification — Another important operating characteristic o'f the simple 
saturable reactor is amplification. Fig. 35 shows the amplification of a 



THE SATURABLE REACTOR 



31 



common sine-wave input for a particular operating point in the linear 
region of the magnetization curve, as fixed by the magnetomotive force 
of the control windings. The magnetization curve of the core material 




Fig. 35 



INPUT 
Amplification in a simple saturable reactor. 



determines the degree of amplification. Different types of core materials 
have different magnetization curves. More particularly, the slopes of these 
curves differ and affect the amount of amplified output. 



OUTPUT 




LOW 
AMPLIFICATION 



MEDIUM 
AMPLIFICATION 



HIGH 
AMPLIFICATION 



Fig. 36. Amplification versus magnetization curves. 



32 



MAGNETIC AMPLIFIERS 



Fig. 36 shows the effects on amplification of operating with varying slopes 
of different core material. The core material having the greatest slope is 
capable of high amplification, while the core material with least slope has 
a lower amplification. 

A-C OUTPUT 



NEGATIVE 
OUTPUT REGION 



_CONTROL 
REGION ' 



POSITIVE 
OUTPUT REGION 




+ D-C 



CONTROL AMPERE -TURNS 

Fig. 37. Transfer characteristic curve. 



Control or transfer characteristic — The control or transfer characteristic 
(Fig. 37) of a simple saturable reactor is a functional plot of output or 
load current versus control ampere-turns for various loads at rated voltage 
and frequency. 

As the d-c increases either negatively or positively, saturation occurs with- 
in the core, and inductance (L) decreases, resulting in an increase in a-c 
output. When a ^ zero d-c i nput signal exists in the control loop, a very 
definite low-level outgut signal exists in the load loop, and is called quies- 
cent current . Illkis^dHe Jo,lhsJa£tj£k.iHSnJte JTOP^^ doesjipL-ejsist 

mthe.loiurj«i«di»gS^ Consequently,, sp^ passes through, To re- 

duce the quiescent current to zero, external circuitry is required (this will 
be explained later) . 



/ 



THE SATURABLE REACTOR 33 



Review Questions 



1. What are the basic elements of the saturable reactor and what principle governs 
its operation? 

2. Name and describe in what way other electrical devices use this same principle. 

3. How does impedance vary in a saturable reactor as the powet output increases to 
a maximum? 

4. Describe the current variarions to a load as a magnetic core undergoes saturation. 

5. What value of d-c current is required in the control winding of 500 turns of a 
saturable reactor when the magnetomotive force changes from 350 ampere-turns 
to 800 ampere-turns? 

6. What is the effect on amplification when the slope of the magnetization curve 
is doubled? 

7. Why does quiescent current exist in a saturable reactor? 



34 



MAGNETIC AMPLIFIERS 



SELF-SATURATING TYPES 

The magnetic amplifier is a combination saturable reactor and rectifying 
diode, as shown in Fig. 38. Its operation is similar to saturable reactors, 
explained previously. The diode, connected in series with the load wind- 
ings, improves its operating characteristics so that it is described as the 
self-saturation type of magnetic amplifier (half-wave). Fig. 39 shows a 
full-wave self-saturating magnetic amplifier. 



LOAD 




HALF -WAVE 



Fig. 38. Self-saturatrng magnetic amplifier. 



The solution of four basic operational problems of the saturable reactor 
which placed a serious limitation on its use, spurred the development of 
the magnetic amplifier. These operational difficulties are: (1) Saturate 
and desaturate cycles — Every half cycle that saturates the core is followed 
by an equal half cycle that desaturates the core. (2) Transformer action — 
Voltages are induced in the control winding by the load winding. (3) 
Quiescent current — A definite a-c output exists in the load winding even 
though there is a zero d-c control current in the control winding. (4) 
Polarization — An increase in d-c current, either positive or negative, 
results in an increased output. 

The development of the magnetic amplifier from the simple saturable 
reactor necessitated solving each of these difficulties and has resulted in 
its unlimited use in electric circuits. Following sections describe in detail 
how each difficulty has been solved in the current magnetic amplifier. 



SELF-SATURATING TYPES 



35 



RHEOSTAT 

I — vyv 



-=-En-c 




)^~M-H 




EESffiEH 



-^ — \^- \^ 



LOAD 



Fig. 39. Full-wave self-saturating magnetic amplifier. 



Self-Saturating Magnetic Amplifier 

When a-c flows in the load windings of a magnetic amplifier, a magnetic 
field is created within the core that reverses its direction during each half 
cycle of the supply voltage. During the first half of this alternating voltage, 
the resultant flux builds up and then collapses in one direction. During 
the second half cycle, it builds up and collapses in the opposite direction. 
Magnetic flux created in the first half cycle opposes the magnetic flux 
created in the second half cycle. This means that any attempt to saturate 
thecore with magnetic flux during the first half cycle is accompanied by 
an equal attempt to desaturate the core during the second half. Conse- 
quently, at a desirable operating level, the core is subjected to saturating 
and desaturating fluxes due to the alternating nature of the supply voltage. 
Less gain or amplification results. 



RESISTANCE 




-REVERSE 
VOLTAGE 



FORWARD 
VOLTAGE 



Fig. 40. Diode resistance versus applied voltage. 



36 MAGNETIC AMPLIFIERS 

Elimination of the desaturating half cycle is achieved by adding a recti- 
fying diode in the load loop. This diode blocks the desaturating half cycle 
due to its inherent characteristics of unilateral conductivity (ability of a 
device to conduct current in one direction). The ability of the diode to 
conduct current in one direction is due to the low ohmic resistance offered 
in one direction, and the extremely high ohmic resistance offered in the 
opposite direction (Fig. 40). 

When the polarity of the alternating load voltage biases the diode in the 
forward voltage direction (direction of low ohmic resistance), the diode 
conducts current to the core to create flux in the direction of saturating 
the core (Fig. 41). 

FORWARD VOLTAGE - DIODE CONDUCTS 

■« CURRENT (ELECTRON) FLOW 



>f- 



DIRECTION OF LOW RESISTANCE 



REVERSE VOLTAGE - DIODE DOES NOT CONDUCT 

*■ CURRENT (ELECTRON) FLOW 



>h 



DIRECTION OF HIGH RESISTANCE 



Fig. 41. A diode is a device which conducts current in one direction. 

When the polarity of the alternating load voltage biases the diode in the 
reverse voltage direction (direction of high ohmic resistance), the diode 
does not conduct current to the core, but blocks or isolates the second 
half cycle which tends to desaturate the core. With the output current in 
only one direction, the reactor tends to be self-saturating. This also re- 
duces the amount of magnetomotive force (i^c) required of the control 
winding which is normally used to bring about saturation of the core. 

The addition of a rectifier in the output winding changes the relationship 
between control and load current from the transfer curve, already des- 
cribed, to the transfer curve of Fig. 42, and represents the transfer char- 
acteristics of a self-saturating type of magnetic amplifier. 



SELF-SATURATING TYPES 



37 



The characteristics of the transfer curve are as follows: (1) Full output 
region — Operation of the magnetic amplifier in this region results in 
maximum current output due to full saturation of the core. Load current 
is limited only by the resistance of the rectifier and load windings. (2) 
Control region — Operation of the magnetic amplifier in this region results 
in a current output which is limited. The core is unsaturated, which devel- 
ops a variable impedance in the coil that acts to control the output current. 



CUTOFF 
REGION 



FULL OUTPUT_ 
REGION 




^D< if" +D-C 

CONTROL AMPERE -TURNS 
Fig. 42. Transfer characteristic of self-saturating magnetic amplifier. 



(3) Cutoff region — Operation of the magnetic amplifier in this region 
results in output current being cutoff except for the small amount of 
quiescent current flowing. This quiescent current can be eliminated with 
the aid of external circuitry. This will be explained later. (4) Polarity — 
The transfer curve of the self-saturating type magnetic amplifier shows 
that the amplifier output is sensitive to the polarity of the control current. 
That is, as the control current increases positively, the output increases. 
As the control current increases negatively, the output decreases. 

The rectifier diode has become an integral part of the magnetic amplifier 
because of its important role in the operation of the core. Both forward 
and reverse resistance have an important bearing on the operating charac- 
teristics of the inagnetic amplifier. 

The forward resistance tends to reduce the useful output voltage. It also 
causes heat losses (PR) within the rectifier. This is an undesirable power 
loss since it tends to heat again the rectifier which produces adverse effects. 



38 



MAGNETIC AMPLIFIERS 



The reverse resistance is important to the operation of magnetic amplifiers 
for the following reasons: (1) If the ratio of reverse to forward resistance 
is low, the amplification or gain will be considerably reduced (desatura- 
tion cycle is partially blocked). (2) Power loss caused by reverse current 
causes the temperature of the rectifier to increase, (3) Reverse resistance 
current reduces the d-c output. 



A=PERFECT RECTIFIER 




-''B=STANDARD 
/ RECTIFIER 

,' C = HIGH LEAKAGE 
^-- RECTIFIER 



D = RECTIFIER REMOVED 



Fig. 43. Effects of rectifier leakage. 



Fig. 43 shows the effects of rectifier leakage on the transfer curve during 
the second half cycle. It is during this cycle that the positive saturation is 
reduced. If the rectifier does not completely block this half of the cycle, 
leakage results and increased positive saturation occurs. Note how the 
transfer curve is changed with increasing rectifier leakage during the 
second half cycle. 



Review Questions 

1. Define what is a magnetic amplifier. 

2. What four operational difficulties of the saturable reactor had to be solved before 
the magnetic amplifier could undergo greater acceptance? 

3. In what way does the diode in the load loop eliminate the desaturating half-cycle? 

4. Describe in what way the diode in the load loop changes the transfer characteristic 
of a magnetic amplifier. 

5. For purposes of switching, in which region of the transfer characteristic would you 
expect the point of operation? 

6. What are the efiects of a low ratio of reverse to forward resistance of the rectifier 
diode? 



THREE-LEGGED CORE MAGNETIC AMPLIFIERS 



39 



Transformer action between two windings on the same core (Fig. 44), 
such as a toroid, creates a serious problem. With a large a-c flowing in the 
N2 windings, a high voltage is induced in Ni due to normal transformer 
step-up action — a function of the turns ratio. This induced voltage causes 
a-c to flow in the control loop, disrupting d-c control action. In fact, the 
high voltage induced in many turns of fine wire of a very small conductor 



■AA/V 



D-c := 




Fig. 44. Transformer action of two windings. 

size, will exceed the dielectric strength and maximum voltage require- 
ments. This results in immediate breakdown of winding Ni. Additionally, 
the sensitivity and response requirements of the load demands the control 
loop be free of any interferring actions. 




Fig. 45. Series-opposing arrangement. 



Several methods have been suggested and used to nullify transformer 
action: (1) A highly reactive filter choke in series with the control loop 
has been used. Most of the induced a-c voltage appears across this choke 
so that very little a-c flows in the control loop. This method is not suitable, 
even though it frees the control loop of any interferring action of a-c. A 
high voltage is still induced in the control winding, eventually breaking 
down the wire insulation. (2) Another method connects a second core 



40 



MAGNETIC AMPLIFIERS 



for a series-opposing arrangement as shown in Fig. 45. This arrangement 
separates the single magnetic loop into two magnetic loops, with the load 
and control windings connected in series. Coils Ni and N3 are wound so 
that induced voltages in Ni and N3 are 180° out of phase with each other 
and therefore cancel. This is an improvement on the method using a 
reactive filter. However, the induced high voltages stress the coils unduely 
to impose a serious limitation (Fig. 46). (3) The most effective method 





























LOAD 

AAA 


r-AA 








V V V 


e:d-c 






















N 


( 


■^ 


r 










^ 


^ 




J 




(O 




L 








[^ 






1 ' 
i ' 


1) 


1 




C 

c 


3 


3 













































Fig. 46. Variation of series-opposing arrangement. 




:d-c 



I 
1 



I 

t 



y-msmmi 



I 



\ 



-y-fmm---' 



LOAD 



Fig. 47. Three-legged core arrangement to eliminate transformer action. 



THREE-LEGGED CORE MAGNETIC AMPLIFIERS 41 

of eliminating transformer action is shown in Fig. 47. A three-legged core 
arrangement with the control winding in the center leg and the load wind- 
ing separated into two windings is mounted on the outer legs of the core. 

The construction of the outer core legs is made larger (less reluctance) 
than the middle leg (greater reluctance). This tends to confine all the a-c 
flux in the outer periphery of the core with very little entering the center 
core (flux taking the path of least reluctance). Flux that does not enter 
the center leg is cancelled, due to the directional efFects of d-c and a-c 
fluxes. 

This method of construction and winding connection eliminates the high 
voltage induced in the control winding, thereby freeing the control loop 
of an action or effect by the load loop. This arrangement also maintains 
the sensitivity and response demanded in critical electrical systems. 

Review Questions 

1. Explain transformer action between two windings of a toroid core . 

2. What are the methods used to eliminate transformer action, and which is the 
most feasible? 

3. In a three-legged core arrangement, how many magnetic loops exist in the core? 



42 



MAGNETIC AMPLIFIERS 
COMPENSATING MAGNETIC AMPLIFIERS 



The quiescent current that exists in the output of a magnetic amplifier 
poses a serious problem, particularly if cascading of amplifiers is desirable. 
This current exists on the output because of the inability of the coil to 
possess impedance large enough to prevent the flow of current. Conse- 
quently, a definite output exists despite a zero d-c control current in the 
control loop. 



I— A/VV 



-=-D-C 



Txnxr 



rrmrr 



■^ — c»- 




LOAD 



Fig. 48. Magnetic amplifier with rectifier bridge in output. 

Fig. 48 represents a schematic diagram of a magnetic amplifier with a 
rectifier bridge. To determine the amount of quiescent current flowing in 
this circuit, the simplified Fig. 49 is used. Assume: (1) The inductance 
of coils N2 and N3 total 200 henries. (2) The a-c source is 110 volts, 60 
cycles. (3) Impedance of the bridge is 4640 ohms. 

IIOV 




Fig. 49. Calculating quiescent current. 



COMPENSATING MAGNETIC AMPLIFIERS 



43 



Then: (1) Inductance reactance (Xl) of loop equals 27r fL equals 2 X 
TT X 60 X 200 equals 75,360 ohms. (2) Total impedance of loop equals 
75,360 ohms-coils plus 4640 ohms-bridge equals 80,000 ohms. (3) Quies- 
cent current (I) equals E/Z equals 110/80,000 equals 0.00134 equals 
1.34 ma. 



A-C INPUT 




-( 1 




< 


rA/W 












LO 


-=- 






ffff 




ffff 


inn ! 
















ffff 


Vfff 


ffff 


















- 






'\'\'\' 


^.^r^. 


\\v 






W 




w — 




w- 


-• 



1st stage 2nd stage 3rd STAGE 

Fig. 50. Cascaded magnetic amplifiers. 

This level of quiescent current becomes significantly large if the output 
of one amplifier is used to control another in a cascade arrangement (Fig. 
50). In a cascade circuit, the overall gain is the final product of all the 




LOAD 



Fig. 51. Compensated magnetic amplifier. 



44 



MAGNETIC AMPLIFIERS 



stage gains. If the current gain for the second and third stages is 100 each, 
then the quiescent current output of the first stage (1.34 ma) would be 
increased by 100 X 100 X 1.34 ma, or 13.4 amperes. 

If more stages were added, this current would increase proportionately 
and become large enough to saturate the cores in the latter stages. This 
would mean an undesirable output in the load loop despite the zero d-c 
in it. 

With this quiescent current flowing, added amplifiers are useless. To cor- 
rect for quiescent current, compensated magnetic amplifiers are used. 
Compensated amplifiers are designed to eliminate quiescent current and 
to make cascading of magnetic amplifiers more practical. The use of an 
inductive reactor in Fig. 51 eliminates quiescent current. The use of this 
reactor shifts the transfer characteristic as in Fig. 52. 



OUTPUT 



QUIESCENT 
CURRENT 



WITHOUT REACTOR 



WITH REACTOR 




-H T +H 

Fig. 52. Effect of adding compensating reactor. 



The use of an inductor to eliminate quiescent current is explained as fol- 
lows (Fig. 53) : (1) Inductive reactance Xl2 + 3 is the total reactance of 
the load windings. (2) Inductive reactance Xl 4 is the reactance of the 
compensating reactor. (3) When no current (d-c) is applied to Ni, the 
inductive reactance of Xl 4 is designed in value to equal Xl 2 + 3- (4) Since 
these reactors are equal, the total voltage, Vt (Vi + V2) divides equally 
across the reactors so that Vi equals V2. (5) At the source Vt equals 
V3 + V4 and because the tap is at the center, V3 equals V4. (6) Vi + V2 
equals Vt; and V3 + V4 equals Vt; and Vi equals V2; and Vg equals V4. 
Then Vi equals V4; and V2 equals V3. (7) No potential difference exists 
across the load. Hence, there will be no conduction or quiescent current 
flowing. (8) When control current flows, Xl 2+3 decreases. V2 is now less 
than Vi, and conduction takes place since a difference in potential exists. 



THREE-LEGGED CORE MAGNETIC AMPLIFIERS 



45 



Thus, the compensation reactor eliminates quiescent current when control 
curent is zero, but still allows a-c loop current to flow when control cur- 
rent is applied (Fig. 53). 



"■Lz+a 



Xl4 



iN4 V| 



1 



IT 






p 



Fig. 53. Schematic diagram of a compensated magnetic amplifier. 



Review Questions 

1. What is meant by the term "cascade"? 

2. What is the quiescent current of a magnetic amplifier with three stages cascaded, 
containing the following values for each stage: 

Inductance of Load Coils = 75 henries 
Impedance of Bridge = 3750 ohms 
Gain of each stage = 10 
Source of a-c =110 volts, 60 cycles 

3- In what way does an inductor eliminate quiescent current? 

4. What change occurs with the transfer characteristic, with the addition of an 
inductor in the load loop? 

5. Is there a maximum number of magnetic amplifier stages that can be practically 
connected together? Why? 



46 



MAGNETIC AMPLIFIERS 



POLARIZED MAGNETIC AMPLIFIERS 

Circuits shown previously for self-saturating magnetic amplifiers were 
non-polarized since they could not discriminate between positive and nega- 
tive d-c control current in the control winding. The self-saturated types 
have been somewhat polarized by the introduction of the rectifier element 
in the output of the load winding. Polarization is achieved in magnetic 
amplifiers by use of an additional winding in the center leg of the core, 
called the bias winding (Fig. 54). 




LOAD 



Fig. 54. Compensated magnetic amplifier with bios. 



The effects of the bias winding shift the transfer curve (Fig. 55) so that 
the output current of the amplifier responds differentially to positive and 
negative control currents. Zero d-c in the control loop results in zero output 
in the load loop and increasing positive d-c current will produce an in- 
crease in the output loop. 

The direct current flowing in the bias winding introduces a flux into the 
core with fixed magnitude and direction. This level of flux aids the con- 
trol winding flux in bringing the core to different degrees of saturation. 
Hence, the amount of magnetizing force (ampere-turns) of the control 
winding is considerably reduced. The degree of biasing, shifting of trans- 
fer curve to the right or to the left, can be changed by altering the amount 
of direct current flowing in the bias winding. 



POLARIZED MAGNETIC AMPLIFIERS 



47 



OUTPUT 




CURVE SHIFTED DUE TO 
BIASING 



+H — 

Fig, 55. Result of bias winding. 



Magnetic Amplifiers With Special Windings 

The specialized requirements of various electrical and electronic applica- 
tions has demanded that the magnetic amplifier be highly flexible in its 



CONTROL 
WINDING 



///////, 



SPECIAL 
BIAS WINDINGS 



JJi/i' JJ'i. 



■mam. 



CONTROL 

WINDING 

O 



LOAD 
WINDING 

r 



TRIM 

WINDING 





BIAS 
WINDING 



L 



///, //, 



i 



\crrrrrr 



\J \^ \J \J \J V Vi^ 



LOAD 
WINDING 



Fig. 56. Special windings. 



use. These applications and their diverse systems vary radically with one 

another as to operating characteristics they require. The magnetic ampli- 
fier can be made highly flexible with the use of special windings (Fig. 56) . 



48 



MAGNETIC AMPLIFIERS 



It is not possible to cover all these special windings since they not only 
vary in types and names, but also vary from application to application. 
These windings, often called special bias windings or trim windings, are 
used to achieve a particular operation for a particular application (Fig. 
57). 

SUMMARY OF CHANGES OR IMPROVEMENTS TO PRODUCE THE COMPENSATED 
SELF- SATURATED MAGNETIC AMPLIFIER WITH BIAS 



DIFFICULTIES OF 
SATURABLE REACTOR 



I. TRANSFORMER 
ACTION 



IMPROVEMENTS 



RESULTS 



I. SEPARATE A-C I. THREE-LEGGED 

FROM D-C FLUXES CORE MAGNETIC 

AMPLIFIER 



TRANSFER 
CURVE 

OUT 



2. SATURATE AND 
DE -SATURATE 
HALF CYCLES 



3. QUIESCENT 
CURRENT 



2. DIODE IN LOAD 
WINDING 



3. INDUCTOR IN 
LOAD LOOP 



4.N0N-P0LAR1ZAT10N 4. BIAS WINDING 



2. SELF-SATURATED 
TYPES MAGNETIC 
AMPLIFIER 



3. COMPENSATED 
SELF-SATURATED 
MAGNETIC 
AMPLIFIER 



4. COMPENSATED 
SELF-SATURATED 
MAGNETIC 
AMPLIFIER WITH 
BIAS 



OUT 



+ 

OUT 



+ 

OUT 



Fig. 57. Saturable reactor to magnetic amplifier. 



Variations in Transfer Curve 

The following curves (Fig. 58) illustrate the effects on the transfer curve 
due to changes in load resistance, supply voltage, and frequency. 



Review Questions 

1. What is meant by the term "polarization"? 

2. How is a magnetic amplifier polarized? 

3. In a non-polarized magnetic amplifier, what is the output current with increasing 
negative d-c in the control loop? 

4. What are the effects of the bias winding on the transfer characteristic? 



POLARIZED MAGNETIC AMPLIFIERS 



49 



5. What effect does the bias winding have on the magnetizing force of the control 
winding? 

6. What great advantage does the addition of special windings give a magnetic 
amplifier? 

7. How many magnetic loops exist in a three-legged core magnetic amplifier with 
load windings, control windings and two special bias windings in the center leg? 



OUTPUT 



LOAD VARIATION 




SUPPLY VOLTAGE 
VARIATION 



ABOVE RATED LOAD 
RATED LOAD 

BELOW RATED LOAD 



ABOVE RATED VOLTAGE 

RATED VOLTAGE 
BELOW RATED VOLTAGE 



SUPPLY FREQUENCY 
VARIATION 




ABOVE RATED FREQUENCY 
RATED FREQUENCY 
BELOW RATED FREQUENCY 



Fig. 58. Effects oh the transfer curve due to changes in load resistance. 



50 MAGNETIC AMPLIFIERS 

AMPLIFIER GAIN 

The ratio of output voltage, current, or power in an amplifier stage to the 
input voltage, current, or power, respectively, represents the gain of an 
amplifier. Increasing the gain means increasing the output current, volt- 
age, or power being delivered to the next stage or to a load. 

Current Gain 

The current gain of an amplifier is the ratio of output current to input 
current. Current gain ( Aj) equals Load current (Ii) / Control current (To) . 
It was pointed out earlier that when the amplifier operates on the linear 
portion of the magnetization curve, the magnetizing forces (H) of the loop 
and the load loop are equal. Therefore : He equals Hl ; ( NI ) c equals ( NI ) i, ; 
Nplc equals NlIl — Where: He equals Magnetizing force of control loop; 
Hi, equals Magnetizing force of load loop; (NI)c equals Ampere turns of 
control loop; (NI)l equals Ampere turns of load loop; Nc equals number 
of turns in control loop ; Nl equals number of- turns in load loop. 

Rearranging: Nc/Nj, equals Il/Ic- Substituting the basic gain equation: 
Ai equals Il/Ic, we have Aj equals Nc/Nl equals Il/Ic- 

The current gain of a magnetic amplifier is not only a ratio of output 
current to control current but also a ratio of the number of turns in the 
control winding to the number of turns in the load winding (Fig. 59). 

PROBLEM: FIND THE CURRENT FLOWING IN THE LOAD WINDINGS OF AN AMPLIFIER 
WHOSE GAIN IS 200. CONTROL CURRENT IS 100 MA. 

-O 

Il'AiIc 

II = 200 X .1 = 20 AMPERES 




Fig. 59. Sample problem and solution. 

The gain of an amplifier can also be determined graphically by deter- 
mining the slope of the transfer characteristic. From analytical geometry 
we obtain the slope intercept form: M equals Y2 minus Y1/X2 minus Xi; 
Where : M equals slope, Y and X equal coordinates. Translating this form 
into transfer characteristic coordinates, the current gain (Fig. 60) is the 
ratio of the difference in flux density to the difference in control current 
for any two points on the linear portion of the magnetization curve. 

. _ AB _ B2 - Bi 

J\j — — ' — 

Ale -lc2 ~ Icl 



AMPLIFIER GAIN 



51 



PROBLEM: FIND THE GAIN OF AN AMPLIFIER WITH THE 
FOLLOWING TRANSFER CURVE. 

SOLUTION: 



B 


GAUSSES 






1500 
1250 
1000 


_/ 


/ 


note: IN SELECTING 

TWO POINTS, USE 
THE LINEAR POR- 
TION OF THE 
CURVE. 


750 


--/ \ 






500 
25o/ 


1 1 A = 




- *i . 1000-750 250 
-Idc, .20-.10 10 

= 2500 



.10 .20 .30 .40 
CONTROL CURRENT Ij 



Fig. 60. The current gain is the ratio of the difference in flux density to the 
difference in control current. 



Power Gain 

Power gain of an amplifier is the ratio of the output power to the power 
used in the control circuit. Since: Pout equals Il^Rl and Pin equals Ic^Rc 
and Ap equals Pout/Pin equals Il^Rl/Ic^Rc and Ap equals IL^/Idc times 
Rl/Rc equals Ai^ times Ri,/Rdc. Also: Ap equals IlVl/IcVc equals Aj times 
Vl/V(1c. Overall gain of amplifiers in cascade: At equals Ai times A2 
times A3 (Fig. 61). Where: Ap equals power gain, Ic equals current in 
control loop, Re equals resistance of control loop, II equals current in 
load loop, Rl equals resistance of load loop. At equals overall gain of 
amplifiers in cascade. 



PROBLEM: FIND THE POWER GAIN OF A FIRST STAGE AMPLIFIER WITH THE BELOW LISTED 




Nc= 2000 
TURNS 



Vi = 30 V 
Vz = 120 V 
A =600 MA 
MMF = 10 AMPERE-TURNS 









VALUES 


SOLUTION; 






Ap= 


vit 


IcVc 


--I 


SINCE 








No 


k1c = Nl 


xIl = 


10 


I| = 


600MA; THEREFORE, 


N( 


16.6 TURNS 




Nc 


= 2,000 


, THEREFORE, 


Irtr 


= 5MA 






Ap 


.005* 


30 

120 


= 30 



Fig. 61. Power gain is the ratio of the output to the control circuit power. 



52 MAGNETIC AMPLIFIERS 

Review Questions 

1. What is meant by the term "gain" of an amplifier? 

2. What is the current gain of a magnetic amplifier with 50 milliamps control current 
and 1,250 milliamps load current? 

3. Show how changes in slope of the transfer curve affects the gain of a magnetic 
amplifier? 



FEEDBACK 



53 



Feedback in any amplifier is the process of returning a portion of the 
output signal to add to or subtract from the input signal. When the feed- 
back portion of the output signal aids the input signal, the feedback is 
said to be regenerative or positive. When the feedback signal opposes the 
input signal such that the gain is reduced, the feedback is said to be 
degenerative, or negative (Fig. 62). Adding feedback to a magnetic 



INPUT , 




INCREASING 
OUTPUT _ 



INPUT 



POSITIVE 
FEEDBACK 




DECREASING 
MAGNETIC \ OUTPUT, 
CORE 



NEGATIVE 
FEEDBACK 



POSITIVE FEEDBACK^ 



NEGATIVE FEEDBACK^ 



WHEN FLUX OF FEEDBACK AIDS WHEN FLUX OF FEEDBACK OPPOSES 

THE FLUX OF THE INPUT FLUX OF THE INPUT 

Fig. 62. Positive and negative feedback. 



amplifier is accomplished by an additional winding on the core. The curves 
in Fig. 63 show how the transfer characteristic is changed with the use of 
a feedback winding. 

OUTPUT 



EXCESSIVE POSITIVE 
FEEDBACK 



POSITIVE FEEDBACK 

NO FEEDBACK 



NEGATIVE FEEDBACK 




Fig. 63. Effect of feedback on transfer characteristic. 



54 



MAGNETIC AMPLIFIERS 



Negative Feedback 

Since the feedback portion of the output signal reduces the ampere- turns 
of the control winding, the output is proportionately reduced. A. Linearity 
of transfer characteristic is improved. B. Response time is shortened or 
reduced. C. Power gain is reduced (note slope of curve). D. Figure of 
merit is increased. 

Positive Feedback 

Since the feedback portion of the output signal aids or increases the 
ampere-turns of the control winding, the output is proportionately in- 
creased: Linearity becomes poorer; response time increases, gets longer; 
power gain increases; figure of merit decreases. 

With feedback and proper design of other parts of the magnetic amplifier, 
an unusual type of operation is achieved, called bistable operation. This 
means the magnetic amplifier can operate at any one of two levels or two 
states, at positive or negative saturation. 



SWITCHING 
POINT \ 



POSITIVE SATURATION 

7 



+ H 



NEGATIVE SATURATION 



-SWITCHING 
POINT 



Fig. 64. Bistable operation of magnetic amplifier. 



From Fig. 64 the square-appearing hysteresis loop is used for bistable 
operation. For increasing or decreasing values of control current, there 
appears switching points that change the satuation of the core and cause 
the output to change from a low level to a high level or from a high level 
to a low level. This provides the magnetic amplifier with one of two values 
of output, with one value of input. The magnetic amplifier with this type 
of bistable operation is either on or off, with no intermediate output. This 
operates in the same manner as a relay. 

Response and Time Constants 

Response time is defined as the time required for a magnetic amplifier to 



FEEDBACK 55 

reach any specific percentage of its final output value after an instantane- 
ous change in control signal (expressed in seconds or cycles). Time con- 
stant is defined as the time required for a magnetic amplifier to reach 
63% of its final output value, after an instantaneous change in control 
signal. 

Response time and control circuit resistance are inversely proportional. 
Increasing the total resistance of the control loop decreases the response 
time of the amplifier. Decreasing the total resistance of the control loop 
increases the response time of the amplifier. 

The use of a ratio (termed figuTe of merit) is often used in magnetic ampli- 
fiers. It is the power gain (Ap) divided by the response time (T), and 
expressed as power gain per cycle. Figure of merit K equals Ap/T. It shows 
the effects of changing, reducing, or increasing response time. A typical 
value of the figure of merit K (power gain per cycle) is 300-400 for low- 
power amplifiers and 100-200 for high-power amplifiers. 



Review Questions 

1. Define "negative" and "positive" feedback as used in magnetic amplifiers. 

2. With increasing positive feedback, what is the effect on the transfer characteristic? 

3. Is the power gain of a magnetic amplifier reduced with increasing positive feed- 
back? Why? 

4. Explain "bi-stable" operation of a magnetic amplifier. 

5. Which type of feedback improves the linearity of the transfer curve? 

6. What relationship exists between response time and control circuit resistance? 

7. Determine the power gain of a magnetic amplifier with a figure of merit of 5, and 
a response time of one-half second. 



56 



MAGNETIC AMPLIFIERS 



GENERAL USES AND CONSTRUCTION 

Magnetic amplifiers can be connected within electrical equipment in any 
one of five basic functional uses: (1) Amplification — To increase amount 
of current to load; (2) Control — To vary amount of current to a load; 
(3) Switching — To turn current to a load on or off; (4) Memory — To 
store a given current for a period of time to a load; (5) Computation — To 
vary and increase current to a load. 

First Function: Amplification 

The use of the magnetic amplifier for amplification within electric circuits 
is primarily to vary the output (increase or decrease) in accordance with 
variations and conditions of the input. The following describes and com- 
pares other methods and devices available in accomplishing this same 
function. 



GRID 



INPUT 



PLATE . 



AAA — o 



OUTPUT 



CATHODE 



COLLECTOR 




VACUUM TUBES 



TRANSISTORS 



Fig. 65. Adjustable current devices. 



SENSOR 



[J^LOAD^])=. 




Fig. 66. Adjustable voltage system. 



GENERAL USES AND CONSTRUCTION 



57 



Electronic — Adjustable current devices (Fig. 65). The output current 
of these electronic devices increase from a low value to a very high value 
depending on the bias (control d-c voltage) of the input. 

Rotating machinery — Adjustable voltage devices. The Amplidyne (Gen- 
eral Electric trademark) is a sensor or control element which develops the 
variations in the control current which controls the Amplidyne (a specially 
constructed d-c generator for a two-stage amplifier) which in turn controls 
the field windings and voltage generated in a d-c generator. Voltage ampli- 
fication is delivered to the load in accordance to variation in control cur- 
rent pickup by the sensor (Fig. 66). 



r^VVV 





TOROI 

MAGNETIC ") k 

CORE g [ 




LOAD 



Fig. 67. Adjustable inductance system. 

Magnetic — Adjustable inductance devices. The control current varies the 
impedance of the magnetic core which permits more or less current flow 
in output (Fig. 67). 

Second Function: Control 

The use of the magnetic amplifier for control purposes within electric cir- 
cuits is primarily to regulate, within limits (maximum and minimum), 

ALTER RESISTANCE 



A-C 
SOURCE 



RHEOSTAT 




LOAD 



Fig. 68A. Variable resistance method. 



58 



MAGNETIC AMPLIFIERS 



the current being delivered to a load. The following describes and com- 
pares other methods and devices available in accomplishing this same 
function. 

Variable resistance methods — By altering the amount of resistance be- 
tween a source and the load (Fig. 68A). 

Variable thermionic emission methods — By altering the amount of elec- 
tron emission between cathode and plate (Fig. 68B). 

ALTER ELECTRON EMISSION 




SOURCE 



Fig, 688. Variable tiiermionic emission method. 



Variable reactance methods — By altering the amount of inductive react- 
ance between source and load (Fig. 69). 

Third Function: Switching 

The use of the magnetic amplifier for switching purposes within electric 
circuits is primarily to turn on or off the current being delivered to a load, 
or to change connections of a circuit (Figs. 70A and 70B). 

Fourth Function: Memory 

The use of the magnetic amplifier for storage purposes within electric cir- 
cuits is primarily to hold a given state or condition intended for a load 
for a period of time. Fig. 71 describes and compares other methods and 
devices available in accomplishing this same function. 



GENERAL USES AND CONSTRUCTION 



59 



ALTER POSITION OF CORE 




ALTER NUMBER OF TURNS 



1 

A-C 
SOURCE 






LOAD 


' 




lll4f%; 





ALTER MAGNETIC SATURATION 



A-C 
SOURCE 








LOAD 
















D-C 









Fig. 69. Variable reactance methods. 



MAKE AND BREAK PARTS 




1 riAH 


MECHANICALLY ACTUATED 


















SWITCH ^ 


i 




MECHANICALLY 
ACTUATED 













\ ? 


















ELECTRIC 
SOURCE 




ELECTRICALLY ACTUATED 










1 rtAD 


























ELECTRICALLY 
ACTUATED 




SWIT*' ■ 


urt 






a 


\^ 1 








i ^ 


5 
















ELECTRIC 

SOURCE 





Fig. 70A. Moke and break parts mechanically actuated. 



60 



MAGNETIC AMPLIFIERS 
EMIT OR NON-EMIT 



ELECTRICALLY 
ACTUATED 



I — LOAD 



SOURCE 



SATURATED OR DESATURATED 



ELECTRICALLY 
ACTUATED 




SOURCE 



Fig. 70B. Non-make or break of parts. 



ELECTROMECHANICAL DEVICES 



CONTROL 




MOTOR 
DRIVEN 






,.,. 




LOAD 


* 






* 










1 








SOURCE 





MAGNETIC AMPLIFIERS 



CONTROL 



MAGNETIC 
AMPLIFIER 



SOURCE 



CONTROL 




TIMING 




LOAD 




RELAY 








, 


, 








SOURCE 





LOAD 



Fig. 71. Use of the magnetic amplifier for storage purposes within electric circuits. 



GENERAL USES AND CONSTRUCTION 61 

Fifth Function: Computation 

The use of the magnetic amplifier for computation is primarily to solve 
mathematical equations. Integration or summing up is one of the principle 
operations it can solve. Fig. 72 describes other methods in doing this. 



ELECTRONIC 



IN— AAAA 




MECHANICAL 



OUT 



MAGNETIC 



IN.-AAAA 



MAGNETIC 
AMPLIFIER 



* •OUT 



Fig. 72. Computation function. 



Magnetic amplifiers can be used in circuits requiring amplification, con- 
trol, switching, storage, or computation. Other methods and devices are 
available to achieve these same circuit functions. Each method or device 
has advantages and disadvantages in accordance to the way it is applied 
in circuits. Be alert to any indiscriminate comparison of magnetic ampli- 
fiers with other devices and methods. A valid comparison can be made 
only on a point by point evaluation of an advantage or disadvantage with- 
in a circuit. However, even this method becomes very difficult since advan- 
tages and disadvantages can be so drastically altered by the way in which 
these devices are applied. This difficulty is furthered because of the inher- 
ent characteristic differences which give one or the other an advantage 
in one circuit but not in another. The following describes the advantages 
and disadvantages of magnetic amplifiers, relative to other components 
fulfilling the same function. 



62 MAGNETIC AMPLIFIERS 

Summary of Functional uses of Magnetic Amplifiers With Other Devices 





AMPLIFICATION 




Method 


Device 


Principle 


Electronic 


Vacuum tube 


Adjustable voltage 




Transistor 


Adjustable current 


Rotating machines 


Amplidyne 


Adjustable voltage 


Magnetic 


Magnetic amplifier 
CONTROL 


Adjustable 
inductance 


Resistance 


Rheostat 


Alter resistance 


Thermionic emission 


Vacuum tube 


Alter electron 
emission 


Reactance 


Solenoid 


Alter position of 
core 




Auto transformer 


Alter number of 
turns 




Magnetic amplifiei 


Alter magnetic 
saturation 




SWITCHING 




Make and break 


Toggle 


Open and close 


parts 




contacts 




Relay 


Open and close 
contacts 


Non-make and 


Vacuum tube 


Emit or non-emit 


break parts 








Magnetic amplifier 


Saturate or 
desaturate 




MEMORY 




Electromechanical 


Clock 


Actuator delays 




Timing relay 


Actuator delays 


Magnetic 


Magnetic amplifier 
COMPUTATION 


Residual magnetism 


Electronic 


D-c operational 


Adjustable & 




amplifier 


varying current 


Mechanical 


Mechanical 


Summing incre- 




integrator 


mental distances 


Magnetic 


Magnetic amplifier 


Adjustable & 
varying current 



GENERAL USES AND CONSTRUCTION 63 

Advantages of Magnetic Amplifiers 

Stepless control. A. Uniform control, continuously variable over a wide 
range, is made possible without interrupting power in the main circuit. 
B. No make or break parts in adjusting conditions in the system. 

Long life. A. Static devices (no moving parts, contacts, or bearings). 
B. Rugged and simple in construction. C. Can be hermetically sealed to 
resist effects of adverse environmental conditions. D. Periodic maintenance 
eliminated, no tube replacement. E. Life of magnetic amplifier is deter- 
mined by life of associated rectifier diodes — in excess of 20,000 hours. 

High power gain. A. Adjustable gain : Large amounts of power can be con- 
trolled using small amounts of d-c power. Gain equals output/input. B. 
Gain of several million from single-stage amplifier is possible. 

Noiseless control. A. Noiseless control due to operation based on altering 
saturation of core. B. Low hum; no more than transformer of similar 
rating. 

High efficiency. A. No filament heating power is required. B. Low internal 
power loss, since internal impedance is largely reactive and the only resist- 
ance is that of the windings and forward resistance of the rectifiers. C. 
Overall efficiency is usually better than 50% and as high as 80-90% in 
the larger sizes. 

Control remotely. Manual or automatic control can be centrally located, 
or some distance from the reactor. 

Safety. Input and output can be isolated electrically. High potential power 
requirements can be isolated from low potential coil and metering require- 
ments, removing hazards from operating personnel. 

Accuracy. A. Closely controlled predictable currents to permit closely con- 
trolled allowable increments in main power transfer. B. Isolation of power 
currents from control currents improves accuracy. 

Reliability and maintenance (by nature of parts) . A. Can easily meet high- 
shock requirements. High-shock is sudden mechanical stress intended to 
bring about a component failure. It is a requirement for all equipment 
installed for the U. S. Navy and other armed forces. B. Can easily meet 
vibration requirements. Vibration is any periodic mechanical stress. C. 
Can easily meet acceleration requirements. Acceleration is a change in 
the functional aspects of components as a result of rate of velocity changes. 
D. No filaments to burn out as with electron tubes. 

Systems component. Control windings and load windings (Fig. 73) may 
be matched to signal and load impedances, respectively. Maximum power 
transfer occurs when impedance of the load equals that of the source. 



64 



MAGNETIC AMPLIFIERS 




n:^;/ 



LOAD 



LOW IMPEDANCE SOURCES (THERMOCOUPLES, 
PHOTO CELLS, ETC.), FOR CONTROL 



Fig. 73. Load windings may be matched to load impedances. 



Duty cycle and overloading. A. Longer duty cycle than electron tubes for 
handling momentary overloads. Duty cycle equals average power/peak 
power. B. A magnetic amplifier can carry overloads equal to an equiva- 
lent transformer. 

Disadvantages of Magnetic Amplifiers 

Calibration. Not easily calibrated or adjusted, because of the special equip- 
ment required. 

Cost. A. Relative newness and low production rates make first cost high. 
However, the cost of a magnetic amplifier control system over a period 
of time may be lower due to savings in maintenance costs and fewer pro- 
duction stoppages because of breakdowns. B. Nonfamiliarity of use from 
operating personnel makes repair and maintenance more costly. 

Stability. A. Poor stability; stabilizing circuits are generally required with 
added loss in gain. (Stability means maintaining certain characteristics 
constant despite changes in voltage, temperature, etc. High inductance 
loads create instability.) B. Poor line regulation amplified. Small changes 
in a-c supply voltage cause a relatively large change in the output of most 
magnetic amplifier circuits. Additional circuits are needed to minimize 
this. 

Impedance range. Impedance of magnetic amplifiers cannot be increased 
to infinity or decreased to zero. Full output (core saturated) impedance is 
reduced to d-c resistance of core windings. With output set for zero, zero 
d-c control current, and maximum impedance, there is still a small amount 
of a-c output. 



GENERAL USES AND CONSTRUCTION 



65 



Frequency response. A. Because magnetic amplifiers operate from a-c sup- 
ply voltages, they cannot be expected to respond to frequencies higher than 
the supply voltage frequency. An amplifier operated from a 60-cycle line 
should not be expected to amplify control signals with frequencies over 
60 cycles. 

Sensitivity and distortion. A. D-c signals smaller than 1 microwatt cannot 
be satisfactorily handled by present commercially available magnetic 
amplifiers. B. The output of a magnetic amplifier is a highly distorted 
wave form of the supply voltage frequency. Presence of carrier frequency 
and harmonics is a severe handicap in certain applications. 

Construction (Fig. 74) 

Core types. In the design and construction of magnetic cores, it is desirable 
that the following properties be maintained: (1) Minimum hysteresis and 
eddy current losses. This means low resistivity, coercive force, and ability 
to construct core in thin laminations or tapes. (2) High saturation flux 



FERROMAGNETIC 
RAW MATERIAL 



STEEL 



1- w ir 



CUTTING AND SIZING 
STRIPPING MACHINES 



BELT SANDERS 
(DEBURRING) 



BUNCHERS 



PULLING MACHINES 
(DRAWING .ANNEALING, 
AND CLEANING WIRE) 



STRANDERS 






COPPER 

RAW 
MATERIAL 



PULLING MACHINES 



Lc 



CHOPPERS 

AND 
HEATERS 



MIXERS 



WIRE 
EXTRUDERS 



>. I 1 INSULATION 

yi_J ELEMENTS 



SOLVENT BATHS 
(DECREASING) 



T 

FINISHED WIRE 
1 



COATING MACHINE 

THIN LAYER OF 

MAGNESIA 



STRIP HEATERS 
DRYING MAGNESIA 



U/IMniKlf^ 


IJA^LUkir- 






. 


Tr 


.^ 






" 


TE^, 




1 

FINISHED CORE 

\ 














IMPREGNATION 

OR 

POTTING 


TESTING MACHINE 

FOR MAGNETIC 

PROPERTIES 






1 




, 


1 




FINAL TEST 



MANDREL 

STACKS, FINISHES 

SIZES, CORE 



ANNEALING FURNACE 

IN HYDROGEN 
ATMOSPHERE 1200" C 




FINAL 
PRODUCT 



Fig. 74. Consfrucfion of a magnefic amplifier. 



66 



MAGNETIC AMPLIFIERS 



density, for large power-handling capacity for a given weight of core 
material; (3) Hysteresis loop — as nearly rectangular as possible. (4) High 
stability of magnetic characteristics and changing temperatures. 

The more popular types of cores used in magnetic amplifiers are as follows: 
Ring cores (toroids). Several types have been developed (Fig. 75), each 
containing a magnetic characteristic superior to the other in some particu- 
lar installation. 





STACKED RING TOROIDAL CORE 



TAPE WOUND TOROIDAL CORE 



Fig. 75. Ring core toroids. 

Rectangular cores. Rectangular laminations (Fig. 76) having one or more 
legs to hold the windings are frequently used. Many of these are called 
U and. I punchings because of their shape and may contain as many as 
four legs. Economy in stacking time, best arrangement to facilitate dissi- 
pation of heat from coils, and magnetic characteristics are some of the 
factors which determine the shape and type of core to be used. 





TWO-LEGGED 
RECTANGULAR CORE 



THREE-LEGGED 
RECTANGULAR CORE 



Fig. 76. Rectangular laminations. 



Insulation of core. Before the coil is wound on the core, the core is insu- 
lated to meet high-voltage tests between windings and ground. This insu- 
lation is lapped in various ways to meet requirements of high voltage tests 
of no more than 1300 volts between winding and ground. 



GENERAL USES AND CONSTRUCTION 67 

Coil winding. Coils are usually wound by a winding machine on a straight 
arbor on which is placed a piece of heavy insulation to serve as a mechani- 
cal support as well as to provide electrical insulation to the ground. Where 
the smallest possible overall size is wanted, the opening in the center is 
kept to a minimum; hence, the winding is put on by hand. This a slow 
and tedious procedure which makes the unit expensive. Layer insulation, 
coil wrappers or channels, are placed over the coil to hold wire without 
dropping turns and for protection. Coil ends or pigtails are brought out 
for necessary connections. 

Insulation. Proper insulation between turns, layers, and coils, and to core 
and ground, follow normal transformer practice. Proper insulation class 
is observed in accordance with ambient temperatures, to avoid insulation 
breakdown. Class A is limited to 105 °C hot-spot temperature. Class B is 
limited to 130°C hot-spot temperature, Class H is limited to 180° hot-spot 
temperature. 



Review Questions 

1. List the five basic functional uses of magnetic amplifiers. Explain the current 
behavior requirements of each funrtional use. 

2. Compare devices used for control purposes as to methods and principle of 
operation. 

3. What reasons exist in preventing anyone from saying that magnetic amplifier 
devices are best for amplification, control, switching, etc.? 

4. Long life of the magnetic amplifier is attributed to what factors? 

5. What values of efficiency are attainable with magnetic amplifiers? What factors 
reduce this efficiency? What factors increase this efficiency? 

6. What is meant by hi-shock requirements? 

7. What disadvantages exist in the use of magnetic amplifiers and how can they 
be overcome? 

8. What is the basic reason for the construction and development of different 
core types? 

9- Explain the factors which determine the shape and type of core used in magnetic 
amplifiers. 



68 MAGNETIC AMPLIFIERS 

MAINTENANCE AND TROUBLESHOOTING 



A systematic technique must be employed to identify, localize, and remedy 
troubles. Almost without exception, magnetic amplifiers are manufactured 
with extremely reliable components. Long periods of service are to be 
expected with minimum maintenance. However, there are certain basic 
maintenance rules worth following, to ensure continuance of their inherent 
long service life. Since the magnetic amplifier is designed into a system, 
system analysis and maintenance technique is required. The systematic 
approach is as follows: (1) Identify the type of trouble and establish its 
origin in the system; (2) Localize the trouble to a faulty circuit; (3) 
Locate the faulty components within the circuit; (4) Remedy the trouble 
by disassembling, repairing, or replacing the faulty component; (5) Test 
the equipment for proper operation after repair. 

Preventive Maintenance 

Preventive maintenance is the technique of detecting and correcting 
troubles before they occur. Preventive maintenance is easier to practice 
with machinery or something which has moving and wearing parts, than 
with electronic equipment or magnetic amplifiers which do not have mov- 
ing parts. There are some preventive rules, however, which can be used 
for magnetic amplifiers. 

(1) Equipment. Periodic inspection should be made to ensure that the 
equipment is not subjected to atmospheric conditions such as high humid- 
ity, corrosive fumes, vapors of certain chemicals and other damaging con- 
ditions. Periodic inspection and cleaning should also be made to clear any 
foreign matter which might impede free circulation of cooling air around 
heat generating equipment. All equipment should be inspected for clean- 
liness, discoloration, swelling, peeling, corrosion, frayed insulation, etc. 
The chassis must be kept dry and free of dirt, grease, and oil. 

(2) Mechanical. Check all mounting bolts, screws, etc., to be sure that they 
are not working loose due to vibration. In addition, be sure mechanical 
parts requiring movements have freedom of action. 

(3) Saturable reactor. Check periodically to be sure no damage is being 
done to the saturable reactor. They should never be operated over their 
rating. The line voltage must not exceed the rating stipulated by the mag- 
netic amplifier and the load must not consume more power than can be 
delivered safely by the power supply. 

(4) Resistors. Check for excessive discoloration due to overheating. If 
this is evident, test the resistor for proper value. 



MAINTENANCE AND TROUBLESHOOTING 69 

(5) Rectifiers. As rectifiers are used for a period of time, they begin to 
show signs of aging. Aging increases their internal resistance and reduces 
their eiBciency. This results in a decrease in output current and voltage. 
High internal resistance will be noted by abnormally high calibrator set- 
tings when adjusting the line voltage. 

(6) Fuses or circuit breakers. Check all circuit protective devices to be 
sure they are not open. Also check to be certain the protective device will 
function in the event of a short circuit or overload, to protect the com- 
ponent it serves. If a protective device opens twice in succession, do not 
replace it until the cause of failure has been corrected. 

Corrective Maintenance 

The technique of repair or readjustment of components or equipment after 
failure or malfunction involves the following (which must be observed to 
apply properly corrective maintenance). 

(1) A thorough understanding of magnetic amplifier theory and the 
systems of which it is a part. (2) Knowledge of manufacturers' manuals, 
instruction books, and tables is important, to get specific values of the 
components involved (e.g., resistances of windings). (3) To determine 
whether any trouble lies in the amplifier or in other sections of the system, 
the simplest method of determining this is to apply a voltmeter to the out- 
put of the amplifier. If the output of the amplifier is correct (according to 
that specified by the manufacturer), the trouble lies elsewhere, and not in 
the amplifier. (4) When the output of the amplifier is not correct, a step- 
by-step analysis from output to input will determine the location of the 
trouble. 

Troubleshooting 

Troubleshooting methods employed for magnetic amplifiers are like those 
for any similar electrical control device. (See Troubleshooting Chart.) 

(1) Measure the voltages across each winding to be sure each winding is 
energized and performing its function. Control windings, bias windings, 
load windings, trim windings, etc., should be measured. These voltages 
should agree with values specified by the manufacturer. 

(2) If voltages across any windings are not according to specified values, 
the magnetic amplifier should be de-energized and resistance measure- 
ments taken at certain ambient temperatures, and compared with the 
manufacturer's values. 

(3) No attempt should be made to adjust circuits by altering fixed com- 
ponent values. These are carefully designed, taking time constants into 
consideration. Changing values can very likely start the system hunting. 



70 



MAGNETIC AMPLIFIERS 



(4) Approximately 90% of all troubles associated with magnetic ampli- 
fiers are found to be with the rectifiers. Before continuity and voltage 
checks are made on the windings, the rectifiers should be checked. High 
temperatures may damage the rectifiers and shorten their life. Replace 
the rectifier if output current has fallen more than 10% from its daily 
reading, provided output voltage and load are the same. Be sure reduced 
output current is not caused by a loose connection. 



Troubleshooting Chart for Magnetic Amplifiers 



SYMPTOM 

Meter or output 
voltage reads zero 



PROBABLE CAUSE 

Open circuit 

breaker or force 
Loss of input power 

to control or load 

windings 

Open rectifier stack 



Open windings 



REMEDY 

Close breaker or 
replace fuse 

Check circuits or 
circuit compon- 
ents of power 
sources 

Check continuity 
with ohm meter 
replace rectifiers 

Check continuity 
with ohm meter 
replace reactor 



All indicators 
inoperative 



Faulty indicating 

light 
Loss of power 

to light 



Replace bulb 



Check power supply 



Incorrect output 
voltage readings 



Faulty or aged 
components 



Shorted windings 
(control or load) 



Identify faulty cir- 
cuit then identify 
faulty component 
and replace 

Check resistances of 
windings replace 
reactor 



Constant ouput for 
all values of a 
changing input 



Faulty saturable 
reactor 



Replace reactor 



Overheating or 
smoking of 
rectifiers 



Shorted or grounded 
connection in pre- 
amplifier circuits 



Remove shorts and 
grounds replace 
defective com- 
ponents 



MAINTENANCE AND TROUBLESHOOTING 



71 



Faulty rectifier 



Check rectifier 
for aging 



Check input and 

output voltages 

replace if 

necessary 
Note day-by-day 

changes in output 



Sharp decrease in 
rectifier ouput 
voltage 



Shorted rectifier 
stack 



Check forward and 
reverse resistances 
of rectifiers; 
replace 



Overheating or 
smoking of 
resistors 



Shorted or grounded 
connection in 
circuit containing 
resistor 



Check the circuit 
containing resis- 
tor, isolate and 
repair 



Continuous blowing 
of fuses 



Shorted component 
or grounded 
connection in 
amplifier 



Check power supply 
circuit components 



Review Questions 

1. Despite the long service life of magnetic amplifiers, what are the reasons for 
preventive maintenance? 

2. What is the systematic approach to general maintenance of magnetic amplifiers? 

3. What causes a gradual reduction in eflSciency and output current? 

4. Which part of the magnetic amplifier causes the greatest trouble? 

5. Rectifiers are replaced at what reduced values of output current? 

6. What are the probable causes of incorrect output voltage readings? 



72 MAGNETIC AMPLIFIERS 

SYSTEM APPLICATIONS 

Magnetic amplifiers find increasing applications in electrical systems. This 
increased use is largely due to the realization by designers of its versatility, 
flexibility, and reliability. In the applications which follow, CW represents 
control winding, ILW represents input load winding, OLW represents out- 
put load winding, BW represents bias winding, and FBW represents feed- 
back winding. 

Power Control to Electric Heaters 

Electric heating deals with the conversion of electrical energy into heat 
and the distribution and practical use of the heat so produced. The success 
of large installations of electric heating depends not only on the amount 
of heat that could be supplied, but also the control of this heat. Electric 
heating, because it is favorably low cost in certain regions, is being used 
more extensively for industrial applications. The two general methods of 
electric heating are: The electric arc (limited in application), and resist- 
ance heating (wide application). Alternating current is primarily used in 
this type of heater, although direct current can be used. The resistance 
heating method is based on the PR effect, and is related as follows: Q 
equals PRt; Where I equals current in amperes, R equals resistance of 
path in ohms, t equals time in which current is flowing in seconds, and Q 
equals amount of heat produced in btu's. 

From the preceding equation, the current (P) becomes the significant 
term in producing heat. Any control system to be used to control this 
current, must employ a method in which current can be varied. 

The following method describes an automatic control system for a large 
heater installation (200 leva and above) in which heat is not only pro- 
duced, but varied and controlled for short periods of time. 

Fig. 77 shows a two-stage magnetic power amplifier used to control a large 
three-phase saturable reactor. The thermal sensing device generates a signal 
proportional to the temperature or changes in temperature of the heaters, 
and feeds this signal to a controller. This controller compares this actual 
temperature with an ordered temperature so that a signal is generated that 
represents the difference, if any. If there is a difference, this signal is then 
used to actuate a motor-driven powerstat that varies the input control to 
two stages of magnetic power amplifiers. The output of the final stage 
becomes the input control of the saturable reactor that connects and con- 
trols the supply current to the heaters. This current to the heaters is varied 
in accordance to the degree of saturation of the reactor. By this method, 
tremendous amounts of current or power can be released and controlled 
to a bank or system of heaters. 



A- C SOU RCE 

|(ZI~[Z)| 



SYSTEM APPLICATIONS 

TRANSFORMER 



73 




ILW g.A.».4.8 OLW 



ILW 




SATURABLE 



REACTOR 



OLW 



,cw 



OLW CWi MAGNETIC 
AMPLIFIER 
2na STAGE 



HEATER 



° o o| TRANSFORMER 



ACTUAL 




CONTROLLER TEMPERATURE 

Fig. 77. Automatic heoting system. 



Solenoid Valve Control 

A solenoid is an electromagnet having an energizing coil (somewhat cylin- 
drical in shape) which acts on a movable ferromagnetic core or plunger 
positioned in the center of the coil. 

Magnetic amplifiers provide a means of controlling solenoid-activated 
valves (Fig. 78) in piping or hydraulic systems. Manual or automatic con- 



IN- 




SOLENOID 



COIL 



PLUNGER 



DISK 



OUT IN 

VALVE 




ENERGIZED DE" ENERGIZED 

Fig. 78. Controlling solenoid-activated valves. 



74 



MAGNETIC AMPLIFIERS 



trol elements can be used with these systems. A detector, within the 
medium, senses flow, rate of flow pressure, temperature, etc., and gener- 
ates a signal to the controller which is connected in the control loop of the 



m CD 

D 



A-C 
SOURCE 




o o ° 



LJ 



TRANSFORMER 




POTENTIOMETER 



Fig. 79. Control of solenoid actuator valves. 



magnetic amplifier. The magnetic amplifier is biased for switching charac- 
teristics. Hence, the output of the amplifier energizes or de-energizes the 
solenoid valve in accordance to the requirements of the system (Fig. 79V 

Lighting Control 

Television and theater lighting equipment has had a rapid development 
in recent years. Actors' props and settings must be visible, in proportion 
to their importance, from all parts of the stage or auditorium. Naturalism, 
composition, and mood are additional factors to be achieved by artificial 
stage lighting. These lighting requirements demand a high degree of flexi- 
bility in equipment to create brightness, color, distribution of light, and 
direction and location of equipment. Hence, dimmer control equipment for 
lighting is an important part of stage equipment. 

Magnetic amplifiers are a means of controlling lamp brilliancy by pro- 
portional dimming (Fig. 80). Individual or group lighting control is made 
possible by the use of this component. A manually operated powerstat 
varies the d-c current control input to the magnetic amplifier. This control 
current varies the degree of saturation which, in turn, varies the current 
supplied to the lamps. Power in the order of milliwatts is capable of con- 
trolling power in the order of kilowatts. 



SYSTEM APPLICATIONS 




75 

LAMPS 



OLW 






RECTIFIER 



Fig. 80. Light dimming control system. 

Overload Detection System 

A high degree of reliability is required of the components that make up 
the circuits which protect electrical systems. Overloads — either overcur- 
rents or overvoltages — pose important operational considerations in these 
systems. This protection is characterized by an inverse-time relationship, 
where extreme overloads are allowed to exist for only a brief period of 
time or slight overloads are allowed to exist for a longer period of time 
before protective devices are actuated. 



A-C 
SOURCE 



MAIN POWER CONTACTORS 



ILW 




CURRENT 
DETECTORS 



(^L0AD.,]l=3 



Fig. 81. Overload trip device. 

Undervoltage or overvoltage (or current) is sensed as power is delivered 
to a load. This detector senses voltage changes and generates a signal pro- 
portional to these changes to the control winding of a magnetic amplifier. 
The magnetomotive force developed by the control winding delivers an 
output which activates or de- activates the main line contactors, opening 
or closing the circuit (Fig. 81). 



lb MAGNETIC AMPLIFIERS 

Control System for Semiconductor Production 

The wide use of transistors and diodes in electrical equipment and circuits, 
has demanded that the processes used to make these semiconductors, be 
highly efficient in their productive output. Seed crystal growing is presently 
the most common and efficient method of otbaining these crystals. This 
process makes use of a crystal-pulling apparatus shown in Fig. 82. 



INDUCTION 
COIL HEIGHT 



-PULLING ROD 



V 



KZA 



-SEED CRYSTAL 

-CRUCIBLE 
SILICON OR GERMANIUM 



Fig. 82. Crystal-pulling apparatus. 



Germanium or silicon, in order to produce a single crystal, is first melted 
in a crucible — germanium in a carbon crucible silicon inside a carbon 
crucible with a quartz liner. 

A pulling rod, to which the seed is attached, is lowered into the crucible 
and then slowly withdrawn. This seed is a small piece of crystal with the 
proper physical crystalline structure. As the metal cools, it forms a single 
crystalline structure upon the seed. The rate of pull, the size of the seed, 
and the temperature of the melt, determine the size of the crystal. 

The temperatures required for silicon are in the vicinity of 1500° C; for 
germanium, approximately 1000° C. Consequently, the heating process to 
be used must be reliable, adaptible to temperature measurements, and lend 
itself to accuracy in the controls necessary for the pulling operation. 

Fig. 83 illustrates a control system for governing the amount of heat input 
to the melt, using an induction type of heating process. A temperature 
sensor generates a signal proportional to the melt temperature, and is an 



SYSTEM APPLICATIONS 



77 



input signal to the recording potentiometer. The output of the recorder is 
proportional to the deviation between the set-point and measured variable 
(a signal proportional to the ordered temperature and actual temperature) 
and goes to a current proportioning unit. Any deviation from the set- 
point by the controlled variable results in an error signal applied to a rate 
network. (The recorder and the proportioning control unit are means of 
gaining extreme sensitivity.) The output of the proportioning unit is fed 
into a magnetic amplifier where it is amplified and converted. The output 
signal then goes to the control winding of a saturable reactor which de- 
livers controlled r-f power or current to the induction coil. 



FURNACE 



INDUCTION 
COIL 



TEMP 
SENSOR 



R-F 

GENERATOR 

(450 KC) 



A-C 
SOURCE 



ILW 




D 



SATURABLE 



REACTOR 




RECORDING 
POT 



SET POINT 




CURRENT 

PROPORTIONING 

UNIT 



gjg)^ 




MAGNETIC 
lAMPLIFIER 

ILW 



Fig. 83. Control system for semiconductor crystal production. 



Temperature-controlled Ovens 

Temperature-controlled ovens must be able to not only maintain a fixed 
inside temperature in the presence of wide variations in ambient condi- 
tions, but also be able to change from one temperature to another with 
minimum delay. Magnetic amplifiers in the control system (Fig. 84) pro- 
vide a rugged, reliable and accurate regulated system. 

Temperature sensors in the form of a temperature sensing bridge are 
placed within the oven. This bridge is balanced when the oven reaches an 
ordered temperature. Temperature fluctuations cause an unbalanced bridge 



78 



MAGNETIC AMPLIFIERS 



and cause current to flow in the control winding of the first-stage magnetic 
amplifier. The output is then fed into the control winding of another mag- 
netic amplifier, which controls the power supplied ^to the heaters as well 
as the power supplied to the motors. The motor-driven fans and the heaters, 
together with a magnetic amplifier, provide a fast-response temperature 
control system. 



SDS 



A-C SOURCE 



TRANSFORMER 
a RECTIFIER 



ORDERED 
TEMPERATURE 



' ' SWITCH 



• *•• d**oo 



' CONTROLLER 







RELAYS 



Fig. 84. Temperature control system. 



Metering in Electrochemical Systems 

The required accuracy in measuring power on the level of 100,000 kw and 
above (125,000 amps at 800 volts) poses a difficult task for ordinary meas- 
uring instruments. Accurate methods of metering mean accurate methods 
of control. In the aluminum production industry, as well as other chemical 
processes, large amounts of direct current are required. 

A method used requires individual units (rectifiers) paralleled to obtain 
large currents. The metering of a total current on any line is made by a 
method of adding the individual currents. Previously, totalizing shunts 
were used. However, they are difficult to build, install, and are often hazard- 
ous to operators and maintenance personnel. Magnetic amplifiers are used 
not only to meter the pot-line current, but also to expedite the location of 
trouble in individual cells. This magnetic amplifier type of installation 



SYSTEM APPLICATIONS 



79 



uses a multiple- wire tap, one on each bar of a bus. The control wire (one 
per bar) is placed close to the bus but insulated from it, as shown in 
Fig. 85. 

(Note: Pot-lines are a series of furnaces [pots] arranged in a tandum 
[line] so that materials, such as aluminum, as they are processed, go from 
one pot to another. Pot-line current would be the current required in the 
electrochemical process in each pot.) 



TAPS 



VARNISH 



WIRE 





w 






BUS 






Fig. 85. Bus bar wire tap. 

As direct current flows into the bus bar, a proportionate amount is allowed 
to flow through the bus tap points. The millivolt drop between the tap 
points varies with the amount of current in the bus. This millivolt drop 
will (with the millivolt drops of the other bus bars) determine the total 



SENSORS 
(CONTROL WIRES) 



!>■ 



MAGNETIC 
AMPLIFIER 



z>- 



NETWORK 

CONTROL 

LOOP 



A-C 
SOURCE 



METER 



ILW 



RECTIFIER 




Fig. 86. D-c metering of large power lines. 



proportionate amount of current flowing in the system. This is summed in 
a network control loop (Fig. 86) which generates a proportionate signal. 
This signal is then fed into the control winding of a magnetic amplifier, 
and amplified sufficiently to be recorded on a meter. 



80 



MAGNETIC AMPLIFIERS 



Speed Control of D-C Motors 

The following describes an adjustable speed drive for operation from an 
a-c power source. Its basic function is to control drive motor speeds in an 
infinite number of steps over a wide range of speeds. 

Operating speed levels as low as 1/8 ^i base speed, to as high as five times 
the base speed, is possible. Base speed is defined as the speed of a d-c motor 
when rated armature and field voltages are applied. A one-line diagram is 
shown in Fig. 87. 




Fig. 87. Speed control of d-c motors from o-c sources. 

System Theory 

According to d-c motor theory, speed change can be accomplished by 
either changing armature voltage or changing motor field strength. Applied 
voltage (Vt) is equal in magnitude to the counter emf (Ec) generated in 
the inotor armature plus the IR drop in the armature. Vt equals Ec plus IR, 
since E^ equals K(^N. Substituting and rearranging: N equals Vt minus 
IR/K(^; Where Ec equals counter emf, (f) equals flux, N equals speed, and 
K equals constant of the machine construction. 

The preceding equation shows that speed is a function of applied voltage, 
field strength, and armature voltage drop. With a given armature current 



SYSTEM APPLICATIONS 



81 



and field strength {(f)) , the speed is a function of this terminal voltage 
(Vt) . If terminal voltage is increased, speed increases to produce a greater 
emf. Also, if the terminal voltage is decreased, speed decreases for the same 
reason. From the above equation, it is seen that with a constant terminal 
voltage and rated armature current, speed change can be accomplished by 
increasing or decreasing the field flux. In accordance with the following 
expression, motor torque, speed, and horsepower are related. HP equals 
NT/5252; Where N equals speed, T equals torqile, and HP equals horse- 
power. Consequently, varying the armature voltage below base speed results 
in constant torque; varying the field strength above base speed results in 
constant horsepower (Fig. 88). 



APPROX 

ALLOWABLE 

TORQUE 



CONSTANT TORQUE | 


1 ^V^ARYING TORQUE 

ARMATURE ] MOTOr""--^^^^ 
VOLTAGE CONTROL 1 FIELD CONTROL ~- 
/\ 1 ^ 


1 1 



ZERO 
SPEED 



BASE 
SPEED 



MAX 
SPEED 



APPROX 

ALLOWABLE 

HORSEPOWER 




ARMATURE 
■ VOLTAGE CONTROL 



ZERO 
SPEED 



CONSTANT HP 



MOTOR 

FIELD CONTROL 
/s 



BASE 
SPEED 



MAX 
SPEED 



Fig. 88. Motor torque and horsepower curves. 



Speed Change Below Base Speed 

Speed change below base speed is achieved by varying the voltage across 
the d-c drive motor armature. This is accomplished by varying the field 
strength of the d-c generator. Field current is supplied by the power wind- 
ings of the magnetic amplifier. System operation (Fig. 89) is as follows: 

(1) After store button has been depressed, holding coil M is energized 
closing all M contacts. This starts the a-c motor driving the d-c generator. 

(2) The start button at control station is depressed, holding coil IM is 



82 



MAGNETIC AMPLIFIERS 



energized, closing all IM contacts which supply a-c to transformers and 
rectifiers. This results in direct current being available at control poten- 
tiometers A and B. Small currents flow through the power windings of 
the magnetic amplifiers with low-power transfer to each field. (3) Chang- 
ing the control current (potentiometer A) in the control winding varies 
the d-c output from the power windings, which supplies the generator field. 
This results in a variable output voltage from the generator which is ap- 
plied to the drive motor armature resulting in a speed change. 

^h 




START-UP STATION 



START Jr -— r 




D-c GEN] 
FIELD I 



I — vwyvw 




A -OPERATION BELOW BASE SPEED 
B - OPERATION ABOVE BASE SPEED 



i-WSAAAAr-i 



Fig. 89. Diagram of adjustable speed drive. 



SYSTEM APPLICATIONS 83 

Speed Change Above Speed Base 

To effect a change in speed above base speed, the field strength of the 
motor is varied. Operation is as follows: (1) Hold generator output volt- 
age constant by leaving potentiometer A alone. (2) Since d-c is available 
at potentiometer B, varying its output will vary the output of the power 
windings supplying d-c to the motor field. This strengthening and weaken- 
ing of the field changes the motor speed. 

Use of Special Windings 

Power and control windings. These windings function in accordance with 
basic principles of op'eration of saturable reactors as brought out earlier. 

IR compensating winding. Changes in load result in changes in armature 
current and reduced motor speed. Since this is objectionable, a means of 
modifying this effect is accomplished through the use of the IR compen- 
sating winding. As armature current increases, a greater flux is developed 
in the magnetic amplifier, due to the compensating winding which tends 
to saturate the core. This increases the current in the d-c generator field, 
increasing the generator voltage. Hence, changes in motor speed due to 
loads are reduced. 

Antihunt winding. The antihunt winding is connected to the generator field 
through two antihunt capacitors (C). The -circuit operates only during 
changes in speed levels to cushion any abrupt or oscillating changes. This 
is accomplished by designing the capacitors so that they charge when the 
field is increasing and discharge when the field is decreasing. This effect 
of the capacitors is delivered to a winding which introduces opposite satu- 
ration effects to that of the power windings. 

Regulators (General) 

Regulation is the process of keeping constant some condition like speed, 
temperature, voltage, or position, by an electrical or electronic system 
which automatically corrects deviations from a desired output (errors). 
Regulation is, therefore, based on feedback, control is not. Automatic 
electronic regulators can be designed and constructed to maintain a level 
of output for the following conditions: Speed, voltage, current, frequency, 
torque, tension, temperature, position, etc. 

The degree to which the regulator keeps constant these conditions, depends 
upon the accuracy and reliability of the components in the system. The 
magnetic amplifier serves as an excellent means toward system reliability 
and accuracy as illustrated and described in the following systems. 

Speed Regulators 

Speed regulators (Fig. 90) are important where constant speed must be 



84 



MAGNETIC AMPLIFIERS 



maintained, such as in the generation of alternating current (d-c motor 
driving and a-c alternator). Speed variations result not only in poor volt- 
age output, but also in frequency deviations. 



TRANSFORMER 

r54 




»aae □ e®| 



SPEED 

INDICATOR 

TACHOMETER 



COMPARATOR 



REFERENCE 
SPEED 



Fig. 90. Speed regulator. 



Speed deviations on the motor shaft output is sensed with a tachometer 
indicator whose proportional signal is compared with a preset reference 
speed. The output of the comparator is an error signal, proportional to 
the difference in speed. This error signal goes to the control winding of 
a magnetic amplifier whose output supplies the excitation current for the 
motor field. 

Automatic Line Voltage Regulators 

Fluctuating voltages, both high and low, have undesirable effects on utili- 
zation equipment. Low voltage causes lights to dim, motors to slow down 
or pull out of step, electronic equipment to function improperly, etc. High 
voltage causes some equipment to perform unsatisfactorily and other equip- 
ment, such as lights and control devices, to burn out. 

The following describes a voltage regulating system which automatically 
maintains the output voltage of a generator constant, regardless of load 
changes. This system makes use of magnetic amplifiers and consists of the 
following circuits: Start or field flashing circuit, voltage sensing circuit, 
frequency compensation, reactive droop compensation, voltage adjustment. 
All these circuits are intended to maintain the system (either for single 
operation or parallel operation) terminal voltage constant despite load 
variations. 



SYSTEM APPLICATIONS 



85 



Starting (Field Flashing) Circuit (Fig. 91) 

The diesel-generator system and regulating equipment are started by de- 
pressing a master pushbutton. This bushbutton connects a battery power 
source to the generator field winding while the engine comes up to speed. 
The battery supplied exciting current is delivered to the field winding of 
the generator through normally closed contacts of a relay. When the gen- 
erator output voltage becomes sufficient to energize the relay, the battery 
circuit is interrupted, permitting the voltage regulator to assume full auto- 
matic control of the generator field excitation and output voltage. 



CURRENT 
TRANS 



REACTIVE CURRENT 
DROOP TRANS 



MOVER V / 




SELF-SATURATING 

POWER AMP 



SELF -SATURATING 
PREAMP 



Fig. 91. Automatic line voltage regulator. 



86 MAGNETIC AMPLIFIERS 

Voltage Sensing Circuit 

This portion of the circuit receives its signal from the a-c generator output 
bus. The output voltage of this circuit varies in magnitude and polarity 
as its input voltage increases and decreases. This is accomplished by the 
use of reactors and capacitors whose values and connections are such that 
current through these components are equal at only one voltage value. 
This is termed the balance point of two impedances. The operation of the 
voltage regulator depends upon the fact that an increase in voltage above 
the balance point causes an increase in current flow through the series and 
parallel impedance. When the voltage decreases below the balance point, 
the current through the series impedance decreases, and the current through 
the parallel impedance increases. When this unbalance occurs, a current 
flows in the control winding of the premagnetic amplifier stage. The first 
stage and second stage magnetic amplifiers are properly biased and sup- 
plied with power. The output of the second stage causes the generator field 
current to change in the proper direction to bring the a-c voltage back to 
the balance point of the regulator. 

Frequency Compensation Circuit 

Because variations in the load upon the power supply may cause fluctua- 
tions in the speed of the diesel engine, means have been provided to com- 
pensate for the effects of variable output frequency. The negative feedback 
around the preamplifier stage is included to assure stable system operation. 
A portion of the output signal is connected to oppose the input signal and 
tends to reduce wide variations in voltage changes. In case of overspeed of 
the diesel, the over-frequency relay closes, connecting the negative feed- 
back winding. This reduces the voltage to a value which will not damage 
the regulator. Inductors and capacitors are specified with values so that 
frequency changes result in reactance changes that can only upset the net- 
work balance within a specified band (55—65 cycles). 

Reactive Droop Compensation 

When generators are required to operate in parallel, the reactive kva 
carried by each should be in proportion to the generator rating. Division 
of reactive kva between generators can be obtained by causing each regu- 
lator to droop its regulated voltage as the reactive current increases. This 
is accomplished by a reactive current droop transformer, the output of 
which is connected to a resistor in the regulator circuit. The voltage droop 
across the resistor caused by the transformer current gives an indication 
to the regulator of the amount of reactive current supplied by the generator. 

Voltage Adjustment 

The voltage adjusting unit is essentially a variable voltage transformer. 



SYSTEM APPLICATIONS 



87 



connected to provide approximately 20% of the voltage supplied to the 
regulator. With this unit, it is possible to set the generator voltage at any 
value within the 20% range. Voltage adjustment is accomplished by 
changing the balance voltage conditions developed by the L, C, R circuits 
described previously. 

Fig. 92 illustrates another type of voltage regulator for maintaining voltage 
on transmission lines, and makes use of magnetic amplifier control to 
change transformer taps in accordance to load fluctuations. This step type 
regulator is becoming increasingly important. 



LOAD 



-ir 



s- 



transformer! 
bank with i 
a-c variable 
source taps i 



innnr 



PPS9PP 



VOLTAGE 

DROP 

COMPENSATOR 




LINE DROP 



VOLTAGE 
DETECTOR 



MOTOR 
CONTROLLER 



r^'i 




POSITIVE 
FEEDBACK 



TIME-DELAY 
ELEMENT 



Fig. 92. Magnetic amplifier control for load tap changing equipment. 



Step regulators, as the name implies, regulate the voltage in steps. For this 
reason, the equipment must be designed and connected to perform a level 
stepping function due to any changes or deviations in voltage. The mag- 
netic amplifier used in this system has a bistable output (Fig. 93). 

For certain input signal values, the output may have one of two values. 
This type of operation gives the magnetic amplifier the necessary band- 
width and compounding requirements for step regulator control. 

The voltage drop that may exist between source and load is subtracted from 
the output voltage of the source (line drop compensator). This voltage is 
then connected to a detector which generates a proportional signal corres- 
ponding to any deviation in voltage from the required balance voltage. If 



88 



MAGNETIC AMPLIFIERS 



this voltage exceeds the allowable limit, the voltage-regulating magnetic 
amplifier will amplify tfie deviation and send a signal to the time delay 
unit. While the voltage detector measures the deviation in voltage continu- 
ously, no action should take place until this deviation has exceeded the set 
bandwidth. 



"^ 



RAISE 



BANDWIDTH 



7" 



LOWER 



J. / 



-Eo Eo +Eo^ 

Fig. 93. Bistable output of a magnetic amplifier. 



The time delay element provides a means of timing the deviation voltage 
until the end of the timing period. Then, the bistable magnetic amplifier's 
output energizes a small control relay which in turn energizes the motor 
control relays. This starts the motor which changes the necessary taps on 
the transformer. 

Nuclear Reactor Protection Systems 

The term scram refers to the rapid reduction in nuclear reactor power 
level. This is accomplished by inserting the control rods into the reactor to 
bring the reactivity to its lowest state. Fast trip scram circuits are neces- 




Fig, 94. Nuclear reactor protection system. 



SYSTEM APPLICATIONS 



89 



sary in reactor systems in the event of any malfunctions. (See Fig. 94.) 
Three malfunctions are capable of scramming a nuclear reactor under fast 
trip conditions : ( 1 ) Neutron flux scram — The amount of neutron flux 
present in a reactor is an indication of the actual power level. This is also 
proportional to the amount of heat generated. If heat becomes excessive, 
the reactor needs to be scrammed. (2) Reactor period — This is the time 
in seconds in which the reactor power level is increased or decreased by 
the factor "e" or approximately 2.7. This tells how fast the power levels 
are changing. (3) Cooling ratio — This is a measure of the ratio of neutron 
flux to coolant flow which is proportional to the difference in temperature 
between the generating heat source and the coolant removing this heat 
(delta T). This also indicates the large upward temperature transients. 



NUCLEAR 



REACTOR 



HEAT 
EXCHANGER 




FOR COOLING RATIO 



FLOW 
METER 



Fig. 95. Malfunctions are detected by sensors. 



Cooling ratio, reactor period, and neutron flux malfunctions are detected 
by sensors (Fig. 95) . Detected signals go to mixers whose outputs are con- 
nected to one of three control windings of a magnetic amplifier. The duplex 
bridge-type magnetic amplifier with the three control windings is so biased 
that the loss of one input causes the output to fall to zero, initiating a fast- 
trip scram by closing a primary safety solenoid valve. 

Control Rod Position Indicator for Nuclear Reactors 

Various methods of indicating the position of the control rods in nuclear 
reactor control systems are used. The following describes a method de- 
signed to provide remote indication of the displacement (in inches) of 
each control rod from the bottom of the reactor. The overall system can 

be characterized into the following areas: Position indicator coils, induct- 
ance bridge, demodulator, premagnetic amplifier and output magnetic 
amplifier. 



90 



MAGNETIC AMPLIFIERS 



' DEMOD 




NUCLEAR REACTOR 



SWITCH 



S' 



METER 



LIGHT 
INDICATOR 



OLW 



Fig. 96. Indicating system for position of control rod. 



Each of these areas are interdependent, and overall system performance 
depends on the proper operation of each area (Fig. 96). 

System Description 

The position indicator coil is so installed in the control rod drive mecha- 
nism that its inductance is dependent upon the position of the control rod. 
This is accomplished by installing the detector coil in the hollow center of 



CONTROL 
ROD 



CONTROL ROD 

MAGNETIC 
MATERIAL 
CONSTRUCTED 

POSITION OF 'VdHB^ "^^° * 

MAXIMUM <^^tlllrS SCREW 

INDUCTANCE 

POSITION OF 
MINIMUM 

INDUCTANCE ""^-i^-^ DETECTION 

COILS 

Fig. 97. Effects on inductance due to material and position of control rods. 





SYSTEM APPLICATIONS 91 

a screw made of magnetic material and connecting it to the control rod 
(Fig. 97). Vertical movement of the screw and control rod changes the 
amount of magnetic material surrounding the detector coil. When the con- 
trol rod is in the bottom position, the minimum amount of magnetic lead 
screw material surrounds the detector coil and its inductance is at a mini- 
mum. As the screw is raised from the bottom position, more of its mag- 
netic material surrounds the detector coil, causing the inductance to in- 
crease. This inductance is the result of the higher permeability of the 
material surrounding the inductance coil. 

Inductance Bridge 

When the control rod is in the bottom position, the inductance bridge is 
balanced (Fig. 98). Any changes in the rod position result in changes in 
inductance in one arm of the bridge. This unbalanced condition produces 
an output proportional to the position of the control rod. The basic pur- 
pose of the variable resistances in two of the arms of the bridge is to com- 
pensate for the decrease in capacitance of the capacitor with an increase 
in temperature. Once the bridge has become unbalanced, it delivers an a-c 
output which is applied to the demodulator circuit for rectification. A test 
inductor is included for circuit testing. This test is accomplished by insert- 
ing an inductance of equal value to that obtained when the rod is with- 
drawn to the halfway position. 

The basic purpose of a demodulator is to provide rectification of an a-c 
signal. The demodulator circuit receives the a-c output of the inductance 
bridge, rectifies it, and delivers it as a control signal to the preamplifier. 
The demodulator utilizes two pairs of transistors which operate as syn- 
chronous switches. One pair conducts during the positive half circle, and 
the second pair conducts during the negative half cycle producing full- 
wave rectification. An a-c biasing transformer (Tl) alternately turns each 
pair of transistors on and off by applying an a-c voltage between the base 
and collector. In addition to delivering an output to the preamplifier input, 
the demodulator also supplies a signal to an indicating meter-selector 
switch. This is, however, accomplished through a temperature-compensat- 
ing network. A phase shifting network compensates for changes in induct- 
ance from rod position, and changes in resistance of coils due to tempera- 
ture effects. 

The preamplifier is a linear magnetic amplifier whose control signal is fed 
from the demodulator output to produce an output inversely proportional 
to the demodulator output, i.e., d-c output decreases as the control current 
increases. Fixed and adjustable biases provide an output independent of 
line voltage variations. The output magnetic amplifier is biased to produce 
an output directly proportional to the output of the preamplifier. With 
this arrangement, the preamplifier and output amplifier deliver their maxi- 



92 



MAGNETIC AMPLIFIERS 




SYSTEM APPLICATIONS 



93 



mum outputs when the inductance bridge is balanced, or the control rod 
is at the bottom position. This provides fail-safe operation. 

Magnetic Core Shift Registers in Digital Computers 

Registers used in digital computers are data or information storage de- 
vices. Registers can be connected in a circuit to hold (store) a full chain 
of coded binary data with one bit in each register. 

Magnetic cores, acting as magnetic amplifiers, can be connected in cas- 
cade to function as a register. When such a chain (Fig. 99) or cascaded 
cores are used, a shift pulse can shift the entire train of binary information. 



INPUT 




Fig. 99. Cascaded cores function as a storage register. 



By this method, each new bit it transferred into the first element. Then 
the whole train of data is shifted down the line to make room for the next 
bit. This continues until all information is transferred and stored. 

Each element in this chain consists of a magnetic core that can be switched 
or changed from a negative saturation state to a positive saturation state 
(Fig. 100), with a high degree of stability in each state. Changing the 
core from one state to another is accomplished by pulsing the windings in 
such a manner as to reverse the direction of the flux within the core. 




-TL 



SATURATED 
NEGATIVELY 



INCOMING 
PULSE 




SATURATED 
POSITIVELY 



Fig. 100. Magnetic core can be switched from a negative saturation stote 
to a positive saturation state. 



94 



MAGNETIC AMPLIFIERS 



The ability of the core to remain in either state indefinitely is attributed 
to the high degree of retentivity or residual magnetism of the core. Mate- 
rials having a high residual magnetism exhibit a square hysteresis loop, 
as in Fig. 101. 





B 


P 


A 




c 


/ 
/ 
/ 

/ 
/ 


^ 





J 




c 




N D 





Fig. 101. Square hysteresis loop for magnetic cores. 



Properties of Square Hysteresis Loop 

(1) A-B, positive saturation portion of loop. (2) C-D, negative saturation 
portion of loop. (3) B and D switching points at knee of curve. These are 
the points in which positive saturation is changed to negative saturation, 
and vice versa. (4) The change from positive to negative or negative to 
positive is very sharp. (5) Residual magnetism OP (positive) and ON 
(negative) remains relatively the same through each cycle. (6) Magnetic 
core is never in neutral condition, it switches from one state to another. 
(7) Low impedance at saturation levels which result in maximum current 
output are necessary to pulse the next magnetic core in cascade. (8) Ex- 
tremely high impedance between saturation levels prevents any output 
between switching points. 

Core-to-Core Transfer 

The core-to-core transfer of a magnetic core shift register (Fig. 102) de- 
livers the output pulse from one core to the input of the second core, to 
switch this second core from one state of saturation to another. In digital 
computers, the binary digits and 1 are represented in each core element 
as negative and positive saturation. In other words, a core element in a 
negative saturation state is said to be in binary state 0, and a core element 
in a positive saturation state is said to be in a binary state 1. 

From Fig. 102, when a pulse arrives in the write winding of core A, it 
switches the core from negative to positive (0 to 1). The sense winding of 



SYSTEM APPLICATIONS 



95 




SFW= SHIFT WINDING 
WW= WRITE WINDING 
SW= SENSE WINDING 



SFW 



SFW 



Fig. 102. Core-»o-core transfer in a magnetic core shift register. 

core A has induced in it an emf, but with a polarity that is reversely ap- 
plied to the output diode. No conduction occurs between the cores. When 
an addition pluse arrives in the write winding of Core A, nothing happens 
because the core is already saturated positively (state 1). When a pulse 
arrives at the shift winding, the core now changes from a positive to a 
negative state (1 to 0). The sense winding of core A has induced in it 
an emf, but with a polarity that is forwardly applied to the output diode, 
causing conduction between the cores. This conducted pulse is fed into 
the write winding of core B and changes its state from negative to positive 
saturation, and the cycle is repeated. The delay element is necessary to 
prevent the write and shift pulses from coming together in time on any 
given core. By this method of switching saturation states of a magnetic 
core, binary information can be stored or held in a register for an indefi- 
nate period of time. 

To clear or erase a magnetic core shift register, a repeating pulse is ap- 
plied to the shift windings of each core that has been serially connected. 
As each shift pulse occurs, every core in the magnetic shift register which 
is in a 1 state, shifts this 1 to the next core, and so on to the end of the 
register. This action continues until the register is cleared of all binary I's. 

Reviev/ Questions 

1. Compile a list of applications, as explained in the text, under the five functional 
uses of magnetic amplifiers: Control, amplification, switching, memory, com- 
putation. 

2. What factors give magnetic amplifiets increasing applications in electrical systems? 

3. In applying magnetic amplifiers in electrical systems, why is it important that the 
entire system be considered in the faaors of design, operation or maintenance? 



96 GLOSSARY 

Amplifier, Magnetic — A device using saturable reactors either alone or 
in combination with other circuit elements such as rectifiers and resis- 
tors, to obtain control or amplification. 

Bias Windings — Those control windings by which the operating condition 
is translated by an arbitrary amount. 

Control Characteristic — A functional plot of load current versus control 
ampere-turns for various loads and at rated supply voltage and fre- 
quency. 

Control Windings — Those windings by which control magnetomotive 
forces are applied to the core. 

Efficiency — Ratio of output to input. 

Flux — Term used to designate collectively all the magnetic lines of force 
in a region. 

Gain — Relative amplitude of output voltage, current, or power, in an 
amplifier stage or system to the input voltage, current, or power, re- 
spectively, expressed in db. 

Hum — Low audio frequency, equal to, or twice, the power line frequency. 

Impedance — Total opposition that a circuit offers to the flow of a-c cur- 
rent or any other varying current at a particular frequency. The ratio 
of the effective value of the potential difference between the terminals 
to the effective value of current. 

Inductance — Property of a circuit or two neighboring circuits which 
determines how much emf will be induced in one of the circuits by 
a change in current in either of them. 

Linear — A relation such that any change in one of the related quantities 
is accompanied by an exact proportional change in the other. 

Noise Level — The strength of acoustic noise at a particular location. 

Output Windings — Those windings other than feedback associated with 
the load and through which power is delivered to the load. 

Reactance — The opposition in ohms offered to the flow of a-c by induct- 
ance or capacitance in an a-c circuit. It is the component of impedance 
of a circuit not due to resistance. 

Reactor — A device that introduces reactance (inductive or capacitive) 
into a circuit. 

Saturable Reactors (also called saturable-core reactors) — D-c controllable 
reactors, saturable transformers, and transductors. Saturable reactors 



GLOSSARY 97 

are ferromagnetic inductors in which the current versus voltage rela- 
tionship, is adjusted by control magnetomotive forces (mmf) applied 
to the core. This is accomplished by an a-c winding impedance con- 
trolled through the saturation of the core effected by a magnetomotive 
force. 

Saturation — Maximum possible magnetization of a magnetic material. 
Maximum number of lines of flux a magnetic material will hold. Fur- 
ther increases in magnetizing force produces little or no increase in 
flux density. 

Saturation Curve — A magnetization curve for a ferromagnetic material. 

Self-Saturation — Refers to the connection of a half -wave rectifying circuit 
element, in series with the output windings of saturable reactors. Out- 
put current has a d-c component to increase the saturation of the core, 
and results in a reduction in ampere-turn requirements of control 
winding. 

Signal (input and control) — An independent input variable. It is applied 
to the magnetic amplifier as an independent magnetomotive force. 



INDEX 



99 



Acceleration, 10 

Accuracy, 63 

Air core, 23 

Air gap, 23 

Alignment of domains, 6 

Alignment of electrons, 5 

Ampere-turns, 13, 26, 46 

Amplidyne, 57 

Amplification, 29, 30, 56, 62, 96 

Applications, 72 

Bar, magnet, 12 
Bias, 36 

Bias winding, 46, 96 
Bistable operation, 54 

Calibration, 64 

Cascading, 42 

Coercive force, 18, 66 

Computation, 56, 58, 62 

Conductor, current carrying, 11 

Control, amplifier, 56, 57, 62 

Control characteristic, 32, 96 

Control region, 37 

Control winding, 27, 28, 96 

Core materials, 31 

Core types, 65 

Corrective maintenance, 69 

Cost, 64 

Current gain, 50 

Cut-off region, 37 

Degenerative feedback, 53 
Demagnetized, 5 
Desaturate cycle, 34 
Desaturation, 21 
Dielectric strength, 39 
Digital computers, 93 
Diode, 36 . 
Distortion, 28, 65 
Domains, 5, 16 
Duty cycle, 63 
Dyne, 10 

Efficiency, 63, 96 
Electrical machinery, 1 



Electrical systems, 41 
Electromagnetic, 4 
Electron emission, 58 
Electron spins, 4 
Electronic amplification, 57 

Faraday's Laws, 19, 20 

Feedback, 53 

Ferromagnetic, 3, 1'4 

Figure of Merit, 55 

Filter choke, 39 

Flux density, 7, l6 

Flux, properties of, 8, 96 

Force, 10 

Force, attraction and repulsion, 8 

Frequency, 48 

Frequency response, 65 

Full output region, 37 

Fuses, 68 

Gain, 35, 38, 43, 50, 63, 96 
Gauss, 7 

Harmonics, 65 

Heaters, 72 

Helix, 12 

Hermetically sealed, 63 

Hum, 96 

Hysteresis loop, 17, 54, 66, 94 

Impedance, 21, 27, 28, 42, 96 
Impedance range, 64 
Induction, 18, 96 
Inductive reactance, 27 
Inductor, 44 
Instep, 16 
Insulator, 8, 39, 67 
Iron core, 23 

Lighting control, 1, 74 
Linearity, 16, 28, 54, 96 
Lines of flux, 7, 14, 24 
Load resistance, 48 
Load winding, 27, 35 
Long life, 63 

Magnetic amplification, 57, 23, 26 



100 



INDEX 



Magnetic amplifiers: 

compensating types, 42 

definition, 34 

self -saturating type, 34 

three-legged core type, 39 
Magnetic circuit, 15, 23, 26 
Magnetic conductivity, 14 
Magnetic cores, 93 
Magnetic field intensity, 11, 14 
Magnetic fields, 6, 11 
Magnetism, 3 
Magnetite, 3 
Magnetization curve, 15, 20, 

28,31 
Magnetomotive force, 13, 27 
Maintenance, 68 
Maxwell, 7 
Memory, 56, 58, 62 
Metering in electrochemical 

systems, 78 
Negative saturation, 18 
Noiseless control, 63 
Noise level, 96 

Nuclear reactor control rods, 89 
Nuclear reactor protection 

systems, 88 

Output, 42, 53 
Output winding, 96 
Overload detection system, 75 
Overloading, 63 

Paramagnetic, 3, 14 
Permealbility, 14, 23 
Phase, 40 

Polarization, 34, 37, 46 
Positive feedback, 54 
Positive saturation, 18 
Power, 28 

Power control to heaters, 72 
Power gain, 50 
Power regulators, 2 
Preventive maintenance, 68 
Punchings, 66 

Quiescent current, 34, 42 



Radiation exposure, 17 
Reactance, 21, 27, 44, 96 
Reactance methods, 58 
Rectangular cores, 66 
Rectifiers, 68 
Rectifying diode, 36, 37 
Regenerative feedback, 53 
Regulators, 83 
Reliability, 63, 72 
Reluctance, 14, 15,23,41 
Remote control, 63 
Residual magnetism, 18 
Resistance methods, 58 
Resistivity, 23, 65 
Resistors, 68 
Response, 39, 54 
Ring cores, 66 

Safety, 63 
Saturable reactors: 

definition, 96 

history, 1 

maintenance, 68 

principle of operation, 28 

simple type, 26 
Saturation : 

changes, 21 

cycle, 34, 96 

positive or negative, 18 
Self -saturation, 97 
Semi-conductor production, 76 
Sensitivity, 39 
Series-opposing, 40 
Shift registers, 93 
Signal, 97 
Sine-wave, 30 
Slope, 16, 31, 50 
Soft iron, 13 

Solenoid valve control, 73 
Speed control of motors, 80 
Stability, 64 
Stepless control, 63 
Storage, 61 
Supply voltage, 48 
Switching, 56, 58, 62 



INDEX 101 

Temperature, 67 Transient region, 29 

Temperature, control ovens, 77 Transition, 21 

Tension, 8 Trim windings, 69 

Thermionic emission methods, 58 Troubleshooting, 68, 69, 70 

Time constants, 54 

Toroid, 7, 13, 15, 39, 66 Unilateral conductivity, 36 

Transfer characteristic, 32, y^j^^^^ ^^^^^ 28 

Transformer action, 34, 39 Waveform, 29