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CHAPTER IV
INDUCTION MOTOR WITH SECONDARY EXCITATION

38. While in the typical synchronous machine and commu-
tating machine the magnetic field is excited by a direct current,
characteristic of the induction machine is, that the magnetic
field is excited by an alternating current derived from the alter-
nating supply voltage, just as in the alternating-current trans-
former. As the alternating magnetizing current is a wattless
reactive current, thfe result is, that the alternating-current input
into the induction motor is always lagging, the more so, the
larger a part of the total current is given by the magnetizing
current. To secure good' power-factor in an induction motor,
the magnetizing current, that is, the current which produces
the magnetic field flux, must be kept as small as possible. This
means as small an air gap between stator and rotor as mechanic-
ally permissible, and as large a number of primary turns per pole,
that is, as large a pole pitch, as economically permissible.

In motors, in which the speed—compared to the motor out-
put—is not too low, good constants can be secured.. This,
however, is not possible in motors, in which the speed is very
low, that is, the number of poles large compared with the out-
put, and the pole pitch thus must for economical reasons be kept
small—as for instance a 100-hp. 60-cycle motor for 90 revolu-
tions, that is, 80 poles—or where the requirement of an excessive
momentary overload capacity has to be met, etc. In such motors
of necessity the exciting current or current at no-load—which
is practically all magnetizing current—is a very large part of
full-load current, and while fair efficiencies may nevertheless be
secured, power-factor and apparent efficiency necessarily are
very low.

As illustration is shown in Fig. 20 the load curve of a typical
100-hp. 60-cycle 80-polar induction motor (90 revolutions per
minute) of the constants;

Impressed voltage:
Primary exciting admittance:
Primary self-inductive impedance:
Secondary self-inductive impedance:.

60 = 500.
Fo = 0.02 - 0.6 j.
Z0 = 0.1 + 0.3 j.
zl = 0.1 +0.3 y.