12

ELECTRICAL APPARATUS

frequency of low motor speeds, thus meeting with a high resist-
ance. With increasing motor speed and thus decreasing
secondary frequency, the current penetrates deeper into the bar,
until at full speed it passes practically uniformly throughout
the entire bar, in a circuit of low resistance—but somewhat
increased reactance.
The deep-bar construction, the eddy-current starting device
and the double squirrel-cage construction thus are very similar
in the motor-performance curves, and the double squirrel cage,
which usually is the most economical arrangement, thus will be
discussed more fully in Chapter II.
II. CONSTANT-SPEED OPERATION
8. The standard induction motor is essentially a constant-speed
motor, that is, its speed is practically constant for all loads,
decreasing slightly with increasing load, from synchronism at
no-load. It thus has the same speed characteristics as the direct-
current shunt motor, and in principle is a shunt motor.
In the direct-current shunt motor, .the speed may be changed
by: resistance in the armature, resistance in the field, change of
the voltage supply to the armature by a multivolt supply circuit,
as a three-wire system, etc.
In the induction motor, the speed can be reduced by inserting
resistance into the armature or secondary, just as in the direct-
current shunt motor, and involving the same disadvantages:
the reduction of speed by armature resistance takes place at a
sacrifice of efficiency, and at the lower speed produced by arma-
ture resistance, the power input is the same as it would be with
the same motor torque at full speed, while the power output is
reduced by the reduced speed. That is, speed reduction by
armature resistance lowers the efficiency in proportion to the
lowering of speed. The foremost disadvantage of speed control
by armature resistance is, however, that the motor ceases to be
a constant-speed motor, and the speed varies with the load:
with a given value of armature resistance, if the load and with it
the armature current drops to one-half, the speed reduction of
the motor, from full speed, also decreases to one-half, that is,
the motor speeds up, and if the load comes off, the motor runs
up to practically full speed. Inversely, if the load increases, the
speed slows down proportional to the load.
With considerable resistance in the armature, the induction