INDUCTION MOTOR

55

tion-motor secondary, the synchronous machine feeds low-fre-
quency exciting currents into the induction machine, and thereby
permits controlling it by using suitable voltage and phase;

If the induction machine has n times as many poles as the
synchronous machine, the frequency of rotation of the synchro-

1 I — s

nous machine is ™- that of the induction machine, or-------How-

n n

ever, the frequency generated by the synchronous machine must
be the frequency of the induction-machine secondary currents,
that is, the frequency of slip s.
Hence:

or:

1

n"+ 1'

that is, the concatenated couple is synchronous, that is, runs at
constant speed at all loads, but not at synchronous speed, but at
constant slip —-T-T-
72> ~"p JL .
4. Concatenation with a low-frequency commutating machine.
If a commutating machine is mounted on the induction-motor
shaft, and connected in series into the induction-motor secondary,
the commutating machine generates an alternating voltage of the
frequency of the currents which excite its field, and if the field
is excited in series or shunt with the armature, in the circuit of
the induction machine secondary, it generates voltage at the
frequency of slip, whatever the latter may be. That is, the
induction motor remains asynchronous, increases in slip with
increase of load.
5. Excitation by a condenser in the secondary circuit of the
induction motor.
As the magnetizing current required by the induction motor is
a reactive, that is, wattless lagging current, it does not require a
generator for its production, but any apparatus consuming lead-
ing, that is, generating lagging currents, ksuch as a condenser, can
be used to supply the magnetizing current.
40. However, condenser, or synchronous or commutating
machine, etc., in the secondary of the induction motor do not
merely give the magnetizing current and thereby permit power-
factor control, but they may, depending on their design or appli-
cation, change the characteristics of the induction machine, as
regards to speed and speed regulation, the capacity, etc.