CHAPTER VI
INDUCTION-MOTOR REGULATION AND STABILITY
1. VOLTAGE REGULATION AND OUTPUT
79. Load and speed curves of induction motors are usually
calculated and plotted for constant-supply voltage at the motor
terminals. In practice, however, this condition usually is only
approximately fulfilled, and due to the drop of voltage in the
step-down transformers feeding the motor, in the secondary and
the primary supply lines, etc., the voltage at the motor terminals
drops more or less with increase of load. Thus, if the voltage
at the primary terminals of the motor transformer is constant,
and such as to give the rated motor voltage at full-load, at no-
load the voltage at the motor terminals is higher, but at overload
lower by the voltage drop in the internal impedance of the trans-
formers. If the voltage is kept constant in. the center of distri-
bution, the drop of voltage in the line adds itself to the imped-
ance drop in the transformers, and the motor supply voltage
thus varies still more between no-load and overload.
With a drop of voltage in the supply circuit between the point
of constant potential and the motor terminals, assuming the cir-
cuit such as to give the rated motor voltage at full-load, the
voltage at no-load and thus the exciting current is higher, the
voltage at overload and thus the maximum output and maximum
torque of the motor, and also the motor impedance current, that
is, current consumed by the motor at standstill, and thereby the
starting torque of the motor, are lower than on a constant-poten-
tial supply. Hereby then the margin of overload capacity of the
motor is reduced, and the characteristic constant of the motor,
or the ratio of exciting current to short-circuit current, is in-
creased, that is, the motor • characteristic made inferior to that
given at constant voltage supply, the more so the higher the
voltage drop in the supply circuit.
Assuming then a three-phase motor having, the following con-
stants: primary exciting admittance, Y = 0.01 — 0.1 j] primary
self-inductive impedance, Z<> = 0.1 + 0.3 j] secondary self-induc-
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