Forward / reverse circuit
Two contactors run a motor in either direction; reverse swaps two phases, and an interlock prevents both closing at once.
What it does
A forward/reverse circuit runs a three-phase motor in either direction — for hoists, doors, conveyors, and machines that need to go both ways.
How it works
Reversing a three-phase motor just needs two of its phases swapped. So there are two contactors: one connects the phases straight through (forward), the other crosses two of them (reverse).
Crucially, both contactors must never close at the same time — that would short two phases together. An interlock prevents it: each contactor's normally-closed auxiliary contact is wired in the other's coil circuit, so energising one positively blocks the other. A mechanical interlock often backs this up.
Why it matters
Switching straight from forward to reverse while the motor is still spinning (plugging) causes a huge current surge — circuits usually require a stop, or a delay, between directions. The interlock is a safety feature and must never be defeated.
Safety first
Never bypass the forward/reverse interlock — both contactors closing shorts two phases. Reversing gear often drives crushing loads; secure the area.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Motor goes one way but won't go the other (e.g. down but not up)
A reversing drive works in one direction only. One command (say, down) runs fine; the other (up) does nothing, or just hums/trips. Common on hoists, doors, and conveyors.
Forward/reverse interlock locking out both directions
Neither direction will run — the interlock that stops both contactors closing together appears to be holding everything off, so no movement at all.
Reversing starter trips when changing direction
The drive runs each direction on its own but trips protection when you change from forward to reverse (or vice versa), often if the changeover is too quick.
Related definitions
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Interlocks
Logic that prevents an unsafe or impossible combination of states — like two contactors closing together.
Start/stop circuit (seal-in)
A momentary start button that latches a contactor on, held by its own auxiliary contact until stop is pressed.