INDUCTION MOTOR

63

The load curves of this motor, as induction motor, calculated
in the customary way, are given in Fig. 22.
Converted into a synchronous motor, it gives the constants:
Synchronous impedance (1):
Z = r + jx =; 0.1 + 10.3 j.
Fig. 23 gives the load characteristics of the motor, with the
power output as abscissae, with the direct-current excitation,
and thereby the counter e.m.f., ey varied with the load, so as to
maintain unity power-factor.
The calculation is made in tabular form, by calculating for
various successive values of the energy current (here also the
total current) iit input, the counter e.m.f., e, by equation (8):
62 = (500 _ ai fl)2 + K)0.61 iV,
the power input, which also is the volt-ampere input, the power-
factor being unity, is:
PO = e&i = 500 i\.
From e follow the losses, by (17), (19) and (20):
in armature resistance: 0.1 i^\
in field resistance: 0.001 e2;
hysteresis loss: 2.5 kw.;
and thus the power output:
P = 500 ii - 2.5 - 0.1 ii* - 0.001 e2
and herefrom the efficiency.
Fig. 23 gives the total current as i> the nominal induced voltage
as e} and the apparent efficiency which here is the true efficiency,
as 7.
As seen, the nominal induced voltage has to be varied very
greatly with the load, indeed, almost proportional thereto. That
is, to maintain unity power-factor in this motor, the field excita-
tion has to be increased almost proportional to the load.
It is interesting to investigate what load characteristics are
given by operating at constant field excitation, that is, constant
nominal induced voltage, e, as this would usually represent the
operating conditions,