Operational Power Control Systems 443
changes the phase sequence applied to the motor. A control circuit for
three-phase induction-motor reversing is shown in Figure 16-17.
When the forward pushbutton is pressed, the forward coil will en-
ergize and close the F contacts. The three-phase voltage is applied from
L1 to T1, L2 to T2, and L3 to T3, to cause the motor to operate. The stop
pushbutton de-energizes the forward coil. When the reverse pushbutton
is pressed, the reverse coil is energized, and the R contacts will close. The
voltage is then applied from L1 to T3, L2 to T2, and L3 to T1. This action
reverses the L1 and L3 connections to the motor, and causes the motor to
rotate in the reverse direction.Starting Protection
Overload protection was discussed previously. Protection of expen-
sive electric motors is necessary to extend the lifetime of these machines.
The cost of overload protection is small compared to the cost of replacing
large electric motors. Motors present unique problems for protection since
their starting currents are much higher than their running currents dur-
ing normal operation. Solid-state overload protection and microprocessor-
based monitoring systems are now available to provide motor protection.Figure 16-16. Control circuit for the forward and reverse operation of a DC shunt
motor