230 ELECTRICAL APPARATUS
the armature reaction is above, and magnetize when it is below
the average value, and thereby reduce the fluctuation, that is,
approximate a constant armature reaction of constant direction
with regards to* the field—that is, a uniformly rotating magnetic
field with regards to the armature.
However, for this purpose, the m.m.f. of the currents induced
in the squirrel-cage winding must equal that of the armature
winding, that is, the total copper cross-section of the squirrel cage
must be of the same magnitude as the total copper cross-section
of the armature winding. A small squirrel cage, such as is suffi-
cient for starting of synchronous motors and for anti-hunting
purposes, thus is not sufficient in high armature-reaction machines
to take care of unbalanced single-phase load.
A disadvantage of the squirrel-cage field winding, however,
is, that it increases the momentary short-circuit current of
the generator, and retards its dying out, therefore increases the
danger of self-destruction of the machine at short-circuit. In
the first moment after short-circuit, the field poles still carry full
magnetic flux—as the field can not die out instantly. No flux
passes through the armature—except the small flux required to .
produce the resistance drop, ir. Thus practically the total field
flux must be shunted along the air gap, through the narrow sec-
tion between field coils and armature coils. As the squirrel-cage
winding practically bars the flux to cross it, it thereby further
reduces the available flux section and so increases the flux density
and with it the momentary short-circuit current, which gives
the m.m.f. of this flux.
It must also be considered that the reduction of generator out-
put resulting from unequal heating of the armature coils due to
unequal load on the phases is not eliminated by a squirrel-cage
winding, but rather additional heat produced by the currents
in the squirrel-cage conductors.
136. A synchronous machine, just as an induction machine,
may be generator, producing electric power, or motor, receiving
electric power, or phase converter, receiving electric power in
some phase, the motor phase, and generating electric power in
some other phase, the generator phase. In the phase converter,
the total resultant armature reaction is zero, and the armature
reaction pulsates with double frequency between equal positive
and negative values. Such phase converter thus can be used to
produce polyphase power from a single-phase supply. The iu-