INDUCTION-MOTOR REGULATION 141
at no-load, down to about 0.01, and remains constant at this
latter value, over a very wide range.
The resultant stability coefficient, or stability coefficient of the
system of motor and supply, 7c0 = k8 + kr, as shown in Fig. 54,
thus drops from very high values at light-load down to zero at
the load at which the curves, k8 and kr, in Fig. 54 intersect, or
at 5800 kw., and there become negative; that is, the motor drops
out of step, although still far below its maximum torque point,
as indicated by the arrows in Fig. 54.
Thus, at constant voltage maintained at the motor terminals
by some regulating mechanism which is slower in its action than
the retardation of a motor-speed change by its mechanical
momentum, the motor behaves up to 5800 watts output in
exactly the same manner as if its terminals were connected
directly to an unlimited source of constant voltage supply, but
at this point, where the slip is only 7 per cent, in the present
instance, the motor suddenly drops out of step without previous
warning, and comes to a standstill, while at inherently constant
terminal voltage the motor would continue to operate up to
7000 watts output, and drop out of step at 8250 synchronous
watts torque at 16 per cent. slip.
By this phenomenon the maximum torque of the motor thus
is reduced from 8250 to 6300 synchronous watts, or by nearly
25 per cent.
87. If the voltage regulation of the supply system is more
rapid than the speed change of the motor as retarded by the
momentum of motor and load, the regulation coefficient of the
system as regards to the motor obviously is zero, and the motor
thus gives the normal maximum output and torque. If the
regulation of the supply voltage, that is, the recovery of the
terminal voltage of the motor with a change of current, occurs at
about the same rate as the speed of the motor can change with
a change of load, then the maximum output as limited by the
stability coefficient of the system is intermediate betjveen the
minimum value of 6300 synchronous watts and its normal value
of 8250 synchronous watts. The more rapid the recovery of
the voltage and the larger the momentum of motor and load,
the less is the motor output impaired by this phenomenon of
instability. Thus, the loss of stability is greatest with hand
regulation, less with automatic control by potential regulator,
the more so the more rapidly the regulator works; it is very little