Electrical Power Systems Technology

Operational Power Control Systems 433

ling a single-phase motor is shown in Figure 16-10. Note that a magnetic

contactor relies upon an electromagnetic coil, which energizes when cur-

rent passes through it. The activated coil performs the function of closing

a set of normally open contacts. These contacts are connected in series

with the power input to the motor that is being controlled.

In Figure 16-10, the START pushbutton switch is an NO switch. When

the start switch is pressed, current will flow through the coil of the mag-

netic contactor. This action energizes the coil, and the solenoid of the con-

tactor is drawn inward to close the contacts, which are in series with the

power lines. Once these contacts are closed, current will continue to flow

through the electromagnetic coil through the holding contacts. Current

will continue to flow until the STOP pushbutton switch is pressed. The

stop switch is an NC switch. When it is pressed, the circuit to the electro-

magnetic coil is broken. At this time, current no longer flows through the

coil. The contacts now release and cause the flow of current through the

power lines to be interrupted. Thus, the motor will be turned off. Magnetic

contactor circuits are sometimes referred to as across-the-line starters. The

relay principle is utilized, since a small current through the coil controls a

larger current through a motor.

Types of Starting Systems

Motor starting, particularly for large motors, can play an important

role in the efficient operation of an electrical power system. There are sev-

eral systems used to start electric motors. The motor-starting equipment

that is used is placed between the electrical power source and the motor.

Electric motors draw a larger current from the power lines during starting

Figure 16-10. Magnetic contactor motor control circuit (Schematic diagram)