Page 134 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

Source: ELECTRICAL EQUIPMENT HANDBOOK

CHAPTER 7
SPEED CONTROL OF

INDUCTION MOTORS

Until the advent of solid-state drives, induction motors were not used in many applications
requiring speed control. The normal operating range of an induction motor is within less
than 5 percent slip. At larger slip, the efficiency of the motor will drop significantly because
the rotor copper losses are directly proportional to the slip of the motor (P RCL sP ). The
AG
speed of an induction motor can be controlled by varying the synchronous speed or the slip for
a given load. The synchronous speed can be varied by changing the electrical frequency or the
number of poles. The slip can be changed by varying the rotor resistance or terminal voltage.

SPEED CONTROL BY CHANGING THE
LINE FREQUENCY

The rate of rotation of the stator magnetic field depends on the electrical frequency. The no-
load point on the torque-speed curve changes with the frequency (Fig. 7.1). The base speed
is the synchronous speed at rated conditions.
The speed of the motor can be adjusted by using variable frequency control. A variable
frequency induction motor drive can control the speed from 5 percent of the base load to
twice the base speed.
There are limits on the voltage and torque as the frequency is varied to ensure safe
operation. When the speed is being reduced below the base speed, the terminal voltage to
the stator should be decreased linearly with decreasing stator frequency. This process is
called derating. If the motor is not derated, the steel in the core will saturate and large
magnetization current will flow in the machine.
The flux in the core of an induction motor is given by Faraday’s law:

d
υ(t) N
dt
Solving for the flux gives
1

υ(t) dt
N
1

V sin tdt
N M
V M
cos t
N

7.1
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.