't& i

PHASE CONVERSION 227- |
two voltages e'o and e'2 impressed upon the induction motor much $
more nearly equal, than would be the case with the use of the t
phase converter in shunt to the induction motor. '»<
Series connection of the induction phase converter, to the in- /|
duction motor supplied by it, thus automatically tends to regu- ';'
late for equality of the two-phase voltages, e'0 and e'2, of the indue- ;|
tion motor. Quadrature position of these two-phase voltages
can be closely maintained by a series transformer between iQ and 4
itj as stated above. ^
It is thereby possible to secure practically full polyphase motor 5
output from an induction motor operated from single-phase sup- ff'
ply through a series-phase converter, while with parallel connec- fi
tion of the phase converter, the dropping quadrature voltage ^
more or less decreases the induction motor output. For this "I,
reason, for uses where maximum output, and especially maximum f |
torque at low speed and in acceleration is required, as in rail- ;?•'
reading, the use of the phase converter in series connections to },/*
the motor is indicated. - ;f
Synchronous Phase Converter and Single-phase Generation f
134. While a small amount of single-phase power can be taken
from a three-phase or in general a polyphase system without dis-
turbing the system, a large amount of single-phase power results
in unbalancing of the three-phase voltages and impairment of
the generator output.
With balanced load, the impedance voltages, e' = iz, of a three-
phase system are balanced three-phase voltages, and their effect
can be eliminated by inserting a three-phase voltage into the
system by three-phase potential regulator or by increasing the
generator field excitation. The impedance voltages of a single-
phase load, however, are single-phase voltages, and thus, com-
bined with the three-phase system voltage, give an unbalanced
three-phase system. That is, in general, the loaded phase drops
in voltage, and one of the unloaded phases rises, the other also
drops, and this the more, the greater the impedance in the circuit
between the generated three-phase voltage and the single-phase
load. Large single-phase load taken from a three-phase trans-
mission line—as for instance by a supply station of a single-phase
electric railway—thus may cause an unbalancing of the trans-
mission-line voltage sufficient to make it useless.
A single-phase system of voltage, e} may be considered as com-
bination of two balanced three-phase systems of opposite phase