SYNCHRONOUS INDUCTION MOTOR 167
and zero values of the primary circuit, but gives a definite torque
if they are displaced therefrom. This torque may be positive
or negative according to the phase displacement between ad-
mittance and primary circuit; that is, the lag or lead of the
maximum admittance with regard to the primary maximum.
Hence aa induction motor with single-armature circuit at syn-
chronism acts either as motor or as alternating-current generator
according to the relative position of the armature circuit with
respect to the primary circuit. Thus it can be called a syn-
chronous induction motor or synchronous induction generator,
since it is an induction machine giving torque at synchronism.
Power-factor and apparent efficiency of the synchronous in-
duction motor as reaction machine are very low. Hence it is
of practical application only in cases where a small amount of
power is required at synchronous rotation, and continuous current
for field excitation is not available.
The current produced in the armature of the synchronous
induction motor is of double the frequency impressed upon the
primary.
Below and above synchronism the ordinary induction motor,
or induction generator, torque is superimposed upon the syn-
chronous-induction machine torque. Since with the frequency
of slip the relative position of primary and of secondary coil
changes, the synchronous-induction machine torque alternates
periodically with the frequency of slip. That is, upon the con-
stant positive or negative torque below or above synchronism
an alternating torque of the frequency of slip is superimposed,
and thus the resultant torque pulsating with a positive mean
value below, a negative mean value above, synchronism.
When started from rest, a synchronous induction motor will
accelerate like an ordinary single-phase induction motor, but
not only approach synchronism, as the latter does, but run up
to complete synchronism under load. When approaching syn-
chronism it makes definite beats with the frequency of slip, which
disappear when synchronism is reached.