230 ELEMENTS OF ELECTRICAL ENGINEERING

A six-phase converter is usually fed from a three-phase system
by three transformers or one three-phase transformer. These
transformers can either have each one secondary coil only of

E
twice the star or Y voltage, = — => which connects with its two

terminals two collector rings leading to two opposite points of
the armature, or each of the step-down transformers contains
two independent secondary coils, and each of the two sets of
secondary coils is connected in three-phase delta or Y, but the
one set of coils reversed with regard to each other, thus giving
two three-phase systems which join to a six-phase system.

The different transformer connections then are distinguished
as "diametrical," "double delta" and "double Y."

For further arrangements of six-phase transformation, see
"Theory and Calculation of Alternating-current Phenomena/'
fourth edition, Chapter XXXVI.

The table below gives, with the direct-current voltage and
direct current as unit, the alternating voltages and currents of
the different converters.

•3 i
* «
a

i

2 -s
3 «

11
n
Ij
J
a
3
A
i

« £

H
1
1
Ip,
1
*3
a

CQ ™
£•<•§
H
fe
bo
H
^

1
1
1
1
1
1
1

Volts between collector ring and neutral point.

-0.354
-0.354
2\/2 = 0.354
2\/2 -0.354
2\/2 -0.354
2V2 -0.354
2>/2 -0.354

1
•N/3
VI"

1

sin. —*

Volts between adjacent collector rings.
1 0
«»0 707
2>/2 — 0 612
2^2*
— 0 612

= 0 354
0 183
n V2

Amperes per line ......

v/2
V^2 \^
__
1 \^
V2 3

2\/2 n

1.0
-1.414
-0.817
-0.943
-0.707
-0.472
0.236

•V/2

Amperes between ad-j aceat lines ..........

\/2
^|
2y/2 3\/3

_.

, IT

-1.414
-0.817
-0.545
^-0.5
-0.472
0.455

These currents give only the power component of alternating
current corresponding to the direct-current output. Added
thereto is the current required to supply the losses in the machine,
that is, to rotate it, and the wattless component if a phase dis-
placement is produced in the converter.