442 Electrical Power Systems Technology
Forward and Reverse Operation of Motors
Most types of electrical motors can be made to rotate in either forward
or reverse direction by some simple modifications of their connections.
Ordinarily, motors require two magnetic contactors to accomplish forward
and reverse operation. These contactors are used in conjunction with a set of
three pushbutton switches—FORWARD, REVERSE, and STOP. When the
FORWARD pushbutton switch is depressed, the forward contactor will be
energized. It is deactivated when the STOP pushbutton switch is depressed.
A similar procedure takes place during reverse operation.
DC Motor Reversing
DC motors can have their direction of rotation reversed by chang-
ing either the armature connections, or the field connections to the power
source. In Figure 16-16, a DC shunt-motor-control circuit is shown. When
the forward pushbutton is pressed, the F coil will be energized, causing
the F contacts to close. The armature circuit is then completed from L1
through the lower F contact, up through the armature, through the upper
F contact, and back to L2. Pressing the stop pushbutton will de-energize
the F coil.
The direction of rotation of the motor is reversed when the reverse
pushbutton is pressed. This is due to the change of the current direction
through the armature. Pressing the reverse pushbutton energizes the R
coil and closes the R contacts. The armature current path is then from L1
through the lower R contact, down through the armature, through the up-
per R contact, and back to L2. Pressing the stop button will de-energize the
R coil.
Single-phase Induction Motor Reversing
Single-phase AC induction motors that have start and run windings
can have their direction of rotation reversed by the circuit in Figure 16-18.
If we modify the diagram by replacing the shunt field coils with the run
windings, and the armature with the start windings, directional reversal
of a single-phase AC motor can be accomplished. Single-phase induction
motors are reversed by changing the connections of either the start wind-
ings or the run windings, but not of both at the same time.
Three-phase Induction-Motor Reversing
Three-phase motors can have their direction of rotation reversed
by simply changing the connections of any two power input lines. This