Three-phase motors can be reversed by changing any two stator leads. Forward–reverse controls also employ interlocking to prevent both the forward and reverse coils from being energized at the same time.
A typical forward–reverse control is shown in the figure.
The dashed lines drawn from the F and R coils to a single line indicate mechanical interlocking. Mechanical interlocks are used to prevent both forward and reverse contactors from being energized at the same time. When one contactor is energized, a mechanism prevents the other from being able to close its contacts even if the coil should be energized. Electrical interlocking is accomplished by using the two normally closed auxiliary contacts connected in series with F and R coils.
Note that the normally closed F contact is connected in series with the R contactor coil, and the normally closed R contact is connected in series with the F contactor coil. When the forward push button is pressed, a circuit is completed through the normally closed R contact to the F coil. When F coil energizes, all F contacts change position.
The three F load contacts close and connect the motor to the power line, causing the motor to run in what is considered the forward direction. The normally open F auxiliary contact connected in parallel with the forward push button closes to maintain the circuit when F push button is released. The normally closed F contact connected in series with R coil opens. This would prevent R coil from energizing if the reverse push button were to be pressed.
Before the motor can be operated in reverse, the stop push button must be pressed to break the circuit to F coil. When F coil de-energizes, all F contacts return to their normal positions. When the reverse push button is pressed, a circuit is completed through the now-closed F auxiliary contact to R coil. When R coil energizes, all R contacts change position. The three R load contacts close and connect the motor to the power line. Note that the connections for L1 and L3 that go to the motor have been reversed. This causes the motor to operate in the reverse direction. Also note that the normally closed R auxiliary contact connected in series with F coil is now open, preventing a circuit from being established to F coil if the forward push button should be pressed.