Why is electrical interlocking between forward and reverse contactors mandatory?
If the forward and reverse contactors pull in at the same time, the motor supply phases are short circuited to each other; the result is welded contacts, blown fuses and a fire risk. To prevent this, the normally closed auxiliary contact of each contactor is wired in series with the coil circuit of the other, and a mechanical interlock is added as well.
In a reversing circuit two contactors determine the direction of rotation by swapping the motor phases. If both pull in at the same time, two phases are connected directly together and a phase-to-phase short circuit occurs.
The protection has two layers:
- Electrical interlocking: the normally closed auxiliary contact of the forward contactor is wired in series with the coil circuit of the reverse contactor, and the NC contact of the reverse contactor in series with the forward coil circuit. While one is pulled in, the coil of the other cannot be energised.
- Mechanical interlocking: the mechanical interlock accessory fitted between the contactors physically prevents both from closing at the same time even if a contact has welded.
This measure is even more important when a wireless remote control is used: the receiver's relay contacts are independent of one another and it is not right to rely on one being interlocked with the other in software. Always build the interlocking in hardware in the power circuit.
In addition, leaving a short delay for the motor to stop before changing direction reduces both mechanical and electrical stress. The limit switches must also be wired in series with the same coil circuit, so that the limit is protected independently of the radio link.
Full article: Wireless remote control installation: pairing, relays, safety