Synthesize the forward and reverse control circuit of the motor using three AC contactors

In large tamping machines, mixers, pile drivers, cranes, etc. used in urban and rural construction sites, when the motor capacity is large and the forward and reverse switching is performed under heavy load, strong arc sparks are often generated, which can easily cause phase short circuits and often burn branch wires, switches, fuses, automatic air switches, etc. In order to avoid such situations, an AC contactor is cleverly added to the forward and reverse circuit, as shown in the circuit in Figure 1.

Brief description of working principle: When the motor rotates forward, press the forward button SB3, its normally closed contact is disconnected first, cutting off the reverse control circuit, then its normally open contact is closed, connecting the forward control circuit, the forward contactor kM1 is electrically attracted and self-locked, the power contactor KM is also electrically attracted, the motor is connected to the three-phase power supply in the positive sequence, and starts to run in the forward direction. When the forward direction changes to reverse direction, press the reverse button SB2, its normally closed contact is disconnected first, cutting off the forward control circuit, so that the forward contactor KM1 is de-energized and released, the power contactor kM is also released with the power off, then its normally open contact is closed, connecting the reverse control circuit, so that the reverse contactor KM2 is electrically attracted and self-locked, the power contactor kM is also electrically attracted, the motor is connected to the three-phase power supply in the reverse sequence, and starts to run in the reverse direction. It can be seen that when the forward direction is changed, the two contactor main contacts KM1 and kM form a 4-break arc extinguishing circuit, which can effectively extinguish the arc and prevent interphase short circuit. The same is true for reverse to forward.

The author has used a contactor to connect some construction sites, and the practical use has proved that the effect is good, and the above-mentioned faults can be avoided. This method is concise and practical, and it is really a case of learning from one example and applying it to other situations. Why not give it a try?