Three-phase asynchronous AC motors are characterized by simple structure, convenient maintenance, and reliable operation. Compared with other motors of the same capacity, they are lighter, lower in cost, and cheaper, and therefore widely used.
In the forward and reverse control circuit of a three-phase squirrel-cage induction motor , two contactors, KM1 and KM2, can be used to switch the phase sequence of the three-phase power supply to change the motor's rotation direction. However, the two contactors cannot be engaged simultaneously; if they are engaged at the same time, it will cause a short circuit in the power supply, as shown in the figure below.
Figure 1. Motor forward and reverse rotation
When the motor is running in the forward direction, KM1 is energized and engaged. The phase sequence of the motor at this time is L1, L2, L3, which means it is running in the forward direction.
Contactor interlocking forward and reverse control circuit
When the motor is running in reverse, KM2 is energized and engaged. The main contacts of KM2 close and switch the three-phase power supply phase sequence of the motor. At this time, the phase sequence of the motor is L3, L2, L1, which means it is running in reverse.
Figure 2. Forward and reverse control circuit of a three-phase squirrel-cage induction motor
a) No interlocking circuit b) With electrical interlocking circuit c) With double interlocking circuit
Double interlocking forward and reverse control circuit
