PHASE CONVERSION 229
The purpose of the phase balancer, thus, is the elimination of
the voltage unbalancing due to single-phase load, and its capacity
must be that of the single-phase impedance volt-amperes. It
obviously can not equalize the load on the phases, but the flow
of power of the system remains unbalanced by the single-phase
load.
135. The capacity of large synchronous generators is essentially
determined by the heating of the armature coils. Increased load
on one phase, therefore, is not neutralized by lesser load on the
other phases, in its limitation of output by heating of the arma-
ture coils of the generators.
The most serious effect of unbalanced load on the generator is
that due to the pulsating armature reaction. With balanced
polyphase load, the armature reaction is constant in intensity
and in direction, with regards to the field. With single-phase
load, however, the armature reaction is pulsating between zero
and twice its average value, thus may cause a double-frequency
pulsation of magnetic flux, which, extending through the field
circuit, may give rise to losses and heating by eddy currents in
the iron, etc. With the slow-speed multipolar engine-driven
alternators of old, due to the large number of poles and low per-
ipheral speed, the arnpere-turns armature reaction per pole i
amounted to a few thousand only, thus were not sufficient to |
cause serious pulsation in the magnetic-field circuit. With the
large high-speed turbo-alternators of today, of very few poles,
and to a somewhat lesser extent also with the larger high-speed
machines driven by high-head waterwheels, the armature reac-
tion per pole amounts to very many thousands of ampere-turns.
Section and length of the field magnetic circuit are very large. •
Even a moderate pulsation of armature reaction, due to the uri-
balancing of the flow of power by single-phase load, then, may
cause very large losses in the field structure, and by the resultant
heating seriously reduce the output of the machine.
It then becomes necessary either to balance the load between .
the phases, and so produce the constant armature reaction of
balanced polyphase load, or to eliminate the fluctuation of the
armature reaction. The latter is done by the use of an effective
squirrel-cage short-circuit winding in the pole faces. The double-
frequency pulsation of armature reaction induces double-fre-
quency currents in the squirrel cage—just as in the single-phase
induction motor—and these induced currents demagnetize, when