THEORY AND CALCULATION OF
ELECTRICAL APPARATUS

CHAPTER I •
SPEED CONTROL OF INDUCTION MOTORS
I. STARTING AND ACCELERATION
1. Speed control of induction motors deals with two problems:
to produce a high torque over a wide range of speed down to
standstill, for starting and acceleration; and to produce an
approximately constant speed for a wide range of load, for
constant-speed operation.
In its characteristics, the induction motor is a shunt motor,
that is, it runs at approximately constant speed for all loads,
and this speed is synchronism at no-load. At speeds below full
speed, and at standstill, the torque of the motor is low and the
current high, that is, the starting-torque efficiency and especially
the apparent starting-torque efficiency are low.
Where starting with considerable load, and without excessive
current, is necessary, the induction motor thus requires'the use
of a resistance in the armature or secondary, just as the direct-
current shunt motor, and this resistance must be a rheostat,
that is, variable, so as to have maximum resistance in starting,
and gradually, or at least in a number of successive steps, cut
out the resistance during acceleration.
This, however, requires a wound secondary, and the squirrel-
cage type of rotor, which is the simplest, most reliable and there-
fore most generally used, is not adapted for the use of a start-
ing rheostat. With the squirrel-cage type of induction motor,
starting thus is usually done—and always with large motors-~-
by lowering the impressed volt,age by autotransformer, often
in a number of successive steps. This reduces the starting
current, but correspondingly reduces the starting torque, as it
does not change the apparent starting-torque efficiency.
The higher the rotor resistance, the greater is the starting
torque, and the less, therefore, the starting current required for
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