CHAPTER IX
SYNCHRONOUS INDUCTION MOTOR
97. The typical induction motor consists of one or a number
of 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 at 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 .dentical in effect 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 maximum