Full text of "Magnetic Amplifiers - Principles and Applications"

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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

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

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.

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-

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

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'

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

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

Oi =

(I)

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)

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).

MAGNETIC AMPLIFIERS

15000

12500

o 10000

Z 7500

°- 5000
<n

i2500

B-H CURVE

^INSTEP

5 10 15 20 25

H AMPERE -TURNS /INCH

Fig. 20. B-H curve for iron.

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

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

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)

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

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

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

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

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

Unsaturated

High

Low

Transition

High to Low

Low to High

Saturated

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

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

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

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

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^

r?*=

■T

r ,
1 1
1 <
1 '
1 '

-4--

.____.(

-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.

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).

DIRECT
CURRENT
SOURCE

MAGNETIC

CORE WITH

WINDINGS

ALTERNATING

CURRENT

SOURCE

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

(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

Xl = INDUCTIVE REACTANCE

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.

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

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.

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?

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

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

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

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.

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

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

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

(

■^

1 '
i '

Fig. 46. Variation of series-opposing arrangement.

:d-c

I
1

y-msmmi

-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?

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

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

-=-

ffff

inn !

ffff

Vfff

ffff

'\'\'\'

^.^r^.

\\v

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.

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

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|

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?

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

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

LOAD
WINDING

TRIM

WINDING

BIAS
WINDING

///, //,

\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) .

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

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

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

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

SOLUTION:

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

k1c = Nl

xIl =

I| =

600MA; THEREFORE,

16.6 TURNS

= 2,000

, THEREFORE,

Irtr

= 5MA

.005*

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

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.

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

+ 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.

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

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.

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

ALTER POSITION OF CORE

ALTER NUMBER OF TURNS

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 ^

MECHANICALLY
ACTUATED

\ ?

ELECTRIC
SOURCE

ELECTRICALLY ACTUATED

1 rtAD

ELECTRICALLY
ACTUATED

SWIT*' ■

urt

\^ 1

i ^

ELECTRIC

SOURCE

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

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

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

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.

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

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

CHOPPERS

AND
HEATERS

MIXERS

WIRE
EXTRUDERS

>. I 1 INSULATION

yi_J ELEMENTS

SOLVENT BATHS
(DECREASING)

FINISHED WIRE
1

COATING MACHINE

THIN LAYER OF

MAGNESIA

STRIP HEATERS
DRYING MAGNESIA

U/IMniKlf^

IJA^LUkir-

TE^,

FINISHED CORE

IMPREGNATION

POTTING

TESTING MACHINE

FOR MAGNETIC

PROPERTIES

FINAL TEST

MANDREL

STACKS, FINISHES

SIZES, CORE

ANNEALING FURNACE

IN HYDROGEN
ATMOSPHERE 1200" C

FINAL
PRODUCT

Fig. 74. Consfrucfion of a magnefic amplifier.

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.

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

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

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.

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

A-C
SOURCE

o o °

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

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

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

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

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

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

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

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.

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

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

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.

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

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

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

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

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

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

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

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.

MAGNETIC AMPLIFIERS

' DEMOD

NUCLEAR REACTOR

SWITCH

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-

MAGNETIC AMPLIFIERS

SYSTEM APPLICATIONS

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.

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.

/
/
/

/
/

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

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

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

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