Electrical Power Systems Technology

Operational Power Control Systems 431

top pushbutton will momentarily close, and the lower pushbutton will

momentarily open. When points 2 and 3 are connected, current will flow

from L1 to L2 through coil F. When coil F is energized, NO contact F will

close, and the NC contact F will open. The “forward” coil will then remain

energized. The reverse pushbuttons cause similar actions of the reversing

circuit. Two control relays are also required in this case.

The circuit of Figure 16-8 is similar in function to the circuit in Figure

16-7. The pushbutton arrangement of this circuit is simpler. When the NO

forward pushbutton is pressed, current will flow through coil F. When the

forward control relay is energized, NO contact F will close, and NC contact

F will open. This action will cause a motor to operate in the forward direc-

tion. When the NC stop pushbutton is pressed, the current through coil F is

interrupted. When the NO reverse pushbutton is pressed, current will flow

through coil R. When the reverse coil is energized, NO contact F will close,

and NC contact F will open. This action will cause a motor to operate in the

reverse direction, until the stop pushbutton is pressed again.

Figure 16-7. An instant forward-reverse-stop pushbutton control circuit

Figure 16-8. A forward-reverse-stop pushbutton control circuit