CHAPTER IX 
SYNCHRONOUS INDUCTION MOTOR 

97. The typical induction motor consists of one or a number 
df primary circuits acting upon an armature movable thereto, 
which contains a number of closed secondary circuits, displaced 
from each other in space so as to offer a resultant closed secondary 
circuit in any direction and at any position of the armature or 
secondary, with regards to the primary system. In consequence 
thereof the induction motor can be considered as a transformer, 
having to each primary circuit a corresponding secondary cir- 
cuit— a secondary coil, moving out of the field of the primary 
coil,* being replaced by another secondary coil moving into the 
field. 

In such a motor the torque is zero a) synchronism, positive 
below, and negative above, synchronism. 

If, however, the movable armature contains one closed cir- 
cuit only, it offers a closed secondary circuit only in the direc- 
tion of the axis of the armature coil, but no secondary circuit at 
right angles therewith. That is, with the rotation of the arma- 
ture the secondary circuit, corresponding to a primary circuit, 
varies from short-circuit at coincidence of the axis of the arma- 
ture coil with the axis of the primary coil, to open-circuit in 
quadrature therewith, with the periodicity of the armature 
speed. That is, the apparent admittance of the primary circuit 
varies periodically from open-circuit admittance to the short- 
circuited transformer admittance. 

At synchronism such a motor represents an electric circuit of 
an admittance varying with twice the periodicity of the primary 
frequency, since twice per period the axis of the armature coil 
and that of the primary coil coincide. A varying admittance 
is obviously identical in effeel with a varying reluctance, which 
will be discussed in the chapter on reaction machines. That 
is, the induction motor with one closed armature circuit is, at 
synchronism, nothing but a reaction machine, and consequently 
gives zero torque at synchronism if the maxima and minima of 
the periodically varying admittance coincide with the 


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 an 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.