Electrical Power Systems Technology

Power Control 415

are start/stop control or the forward/reverse/stop control of electrical

motors. Contacts are moved as the handle of the controller is turned to

provide machine control.

Pilot Lights for Switches

Pilot lights usually operate in conjunction with switches. Motor-con-

trol devices often require that some visual indication of the operating con-

dition of the motor be provided. Pilot lights of various types are used to

provide such a visual indication. They usually indicate either an on or off

condition. For instance, a pilot light could be wired in parallel with a mo-

tor to indicate when it is on. Some types of pilot lights are:

1. Full-voltage Across-the-line Lights—These are relatively inexpensive
   lights, but they do not last as long as other types.


2. ransformer-operated LightsT —These use a relatively low voltage to ac-
   tivate the lamp, but they require the expense of an additional trans-
   former, which ordinarily reduces the operating voltage to 6 volts.


3. Resistor-type Lights—These lights use a series resistor to reduce the
   voltage across the lamp.


4. Illuminated Pushbutton—In these, the functions of the pushbutton
   and the pilot-light device are combined into one item of equipment,
   which reduces the mounting-space requirement.

CONTROL EQUIPMENT FOR ELECTRIC MOTORS

There are several types of electromechanical equipment used for the

control of electric-motor power. The selection of power-control equipment

will affect the efficiency of the power-system operation and the perfor-

mance of the machinery. It is very important to use the proper type of

equipment for each power-control application. This section will concen-

trate on the types of equipment used for motor control.

Motor-starting Control

A motor-starting device is a type of power control used to accelerate

a motor from a “stopped” condition to its normal operating speed. There

are many variations in motor starter design, the simplest being a manu-

ally operated on/off switch connected in series with one or more power

lines. This type of starter is used only for smaller motors that do not draw