142 ELECTRICAL APPARATUS
with compounded alternators, and absent where the motor
terminal voltage remains constant without any control by prac-
tically unlimited generator capacity and absence of voltage drop
between generator and motor.
Comparing the stability coefficient, k8, of the motor load and
the stability coefficient, /bo, of the entire system under the assumed
conditions of operation of Fig. 54, it is seen that the former
intersects the zero line very steeply, that is, the stability remains
high until very close to the maximum torque point, and the motor
thus can be loaded up close to its maximum torque without
impairment of stability. The curve, fco, however, intersects the
zero*line under a sharp angle, that is, long before the limit of
stability is reached in this case the stability of the system has
dropped so close to zero that the motor may drop out of step by
some momentary pulsation. Thus, in the case of instability due
to the regulation of the system, the maximum output point, as
found by test, is not definite and sharply defined, but the stability
gradually decreases to zero, and during this decrease the motor
drops out at some point. Experimentally the difference between
the dropping out by approach to the limits of stability of the
motor proper and that of the system of supply is very marked
by the indefiniteness of the latter.
In testing induction motors it thus is necessary to guard
against this phenomenon by raising the voltage beyond normal.
before every increase of load, and then gradually decrease the
voltages again to normal.
A serious reduction of the overload capacity of the motor, due
to the regulation of the system, obviously occurs only at very
high impedance of the supply circuit; with moderate impedance
the curve, kr is much lower, and the intersection between fcr and
ka occurs still on the steep part of fc,, and the output thus is not
materially decreased, but merely the stability somewhat reduced
when approaching maximum output.
This phenomenon of the impairment of stability of the induc-
tion motor by the regulation of the supply voltage is of prac-
tical importance, as similar phenomena occur in many instances.
Thus, with synchronous motors and converters the regulation
of the supply system exerts a similar effect on the overload
capacity, and reduces the maximum output so that the motor
drops out of step, or starts surging, due to the approach to the
stability limit of the entire system. In this case, with syn-