Wiring 21 — Forward / Reverse Motor Starter

Forward / Reverse Motor Starter

Wire paired forward and reverse contactors. The forward contactor preserves phase order; the reverse contactor swaps L1 and L3 before the common overload. A common Stop/overload chain and separate relay NC contacts provide electrical protection and cross-interlocking in addition to the mechanical interlock expected between contactors.

The device in the real world

Gate openers, hoists, mixers and reversing conveyors need a motor that can run in either direction. A three-phase motor reverses when two of its phases are exchanged, so the panel uses two contactors, one for each direction, with two of their output poles crossed. The two contactors must never be closed together, so the control circuit adds a second layer of protection, using an auxiliary contact of each direction's relay to block the other.

Why it is wired this way

Reversing any two phases reverses a three-phase motor. Both contactors must never close together: the power poles would create a phase-to-phase short. Mechanical and electrical interlocks are deliberately redundant.

Worked example

Follow a forward command. Pressing the forward selector sends 24 V through the reverse relay's normally closed contact to KM-F, which closes and feeds the motor phases in their normal order. If somebody now tries to select reverse, the reverse command has to pass the forward relay's normally closed contact, which is open while forward runs, so KM-R cannot close. Only after stopping does the other direction become available. In the power wiring, KM-R is identical to KM-F except that its T1 and T3 outputs land on the overload's L3 and L1 positions, which exchange two phases.

Common mistakes

• Crossing the phases on the supply side of KM-R instead of between its outputs and the overload relay, so forward and reverse differ in the wrong place.
• Crossing the wrong pair inside KM-R, which gives forward rotation instead of reverse.
• Wiring each interlock contact to its own coil instead of the opposite one, so it does nothing useful.
• Leaving the shared stop and overload chain out of one of the directions.

What you can now do

You can wire a reversing starter, explain why swapping two phases reverses rotation, and place electrical interlocks so both directions cannot close together.

Commissioning standard

Before the first run, prove with a meter, panel isolated, that no combination of the two contactors can connect the phases together.

Training material. Follow your site procedures, local electrical code and the manufacturer's instructions. Lockout/tagout and a qualified person are required for real equipment.