Short answer: a reversing pair is two contactors wired so that the second one swaps two of the three phases, and the interlock is what stops them closing together. That interlock has to exist in three places at once — a mechanical link between the two contactors, a normally-closed auxiliary contact of each contactor wired into the other’s coil circuit, and a control scheme that never issues both commands. Take away any one of the three and the failure mode is the same: a phase-to-phase short through two closed contactors, cleared by the upstream breaker, with welded contacts left behind.
We assemble and export these starters as an independent trading company — not a manufacturer, and not an appointed distributor of any brand. What follows is the wiring and the ordering logic we use when a buyer asks for forward-reverse control, at any grid voltage from 110 V to 240 V single-phase or 380–415 V three-phase.
What the power circuit actually does
Forward contactor K1 takes L1, L2, L3 to T1, T2, T3 in order. Reverse contactor K2 takes the same three lines but crosses two of them at its outgoing terminals — typically L1 to T3 and L3 to T1, leaving L2 alone. Two phases swapped is enough to reverse a three-phase motor; swapping all three changes nothing electrically.
The crossing is done with short links between the contactor terminals, not by landing two wires in one terminal. A doubled conductor in a 25 A terminal is a hot spot waiting for a thermal inspection, and it is also the point where a rushed panel gets the phase order wrong without anyone noticing until commissioning.
Three layers of interlock
Each layer covers a different failure, which is why a serious starter uses all three rather than choosing one.
| Layer | What it is | What it prevents | What it cannot prevent |
|---|---|---|---|
| Mechanical | A rocking arm or slider clipped between the two contactors so one armature physically cannot close while the other is closed | Simultaneous closure from any cause, including a stuck coil or a mis-wired control circuit | Nothing about timing — it releases as soon as the first contactor opens, while the arc is still burning |
| Electrical | The normally-closed auxiliary contact of K1 in series with the coil of K2, and K2’s NC in series with K1’s coil | Energising the opposite coil while the first is held closed, or while its armature has not yet dropped out | A welded main contact: the NC auxiliary follows the armature, not the main contacts |
| Control logic | Push buttons and PLC or relay logic that break the opposite command before making the new one; a timer where required | Both commands held at once, or a command that arrives during a fault | Wiring errors between the logic and the coils |
The ordering matters too. In the control circuit the stop button and the opposite contactor’s NC contact both sit in series with the coil, so the coil can only be energised by a path that passes through “not the other one”. Wiring the NC contact in parallel with a self-holding contact, or placing it after the coil, produces a circuit that looks correct on a schematic and does nothing in the panel.
The dead time nobody puts on the drawing
When a contactor opens, the arc does not stop at the instant the coil loses power. The armature releases in a few milliseconds and the arc in each pole extinguishes when the current passes through zero — for a small AC-3 contactor at 400 V that is typically in the region of 10 to 30 ms, and longer at higher currents or on a lower power factor. If the opposite contactor is allowed to close inside that window, the two arcs overlap across the phase swap and the result is a short circuit inside the starter, not at the motor.
That is why reversing starters often include a short delay between “forward opens” and “reverse closes”. Some control schemes use an off-delay timer set around 50 to 100 ms; others rely on the natural drop-out of a large coil plus the mechanical interlock. Manufacturers publish the release time and the recommended minimum interval for their own contactors, and it is worth reading that number instead of assuming it. A 9 A contactor and a 95 A contactor do not release in the same time.
Wiring the control circuit in order
A conventional 220–240 V coil circuit runs as follows. Take one side of the control supply through the control fuse or MCB, then through the emergency stop or thermal overload NC contact, then to the stop button. From the stop button the circuit splits to the forward and reverse push buttons. Each push button feeds its own coil through the opposite contactor’s NC auxiliary contact, and each coil’s own normally-open auxiliary contact is wired in parallel with its push button to hold the coil in after the button is released.
Two practical details are worth writing into the drawing. First, the control supply should be taken from the same side of the isolator as the main circuit or from a separate transformer, not from a point that can be live while the panel is believed dead. Second, the coil circuit should be identifiable at a glance: the same wire number on both sides of a contact that belongs to the same function makes fault-finding at 2 a.m. a different job.
| Symptom at commissioning | Usual cause | Check |
|---|---|---|
| Both contactors chatter when one button is pressed | NC auxiliary taken from the wrong contactor, so each coil breaks the other | Trace which contactor’s NC contact is in which coil circuit |
| Reverse works only after waiting a few seconds | Coil circuit is being held by the mechanical interlock against a residual command, or a timer is set too long | Measure drop-out time and compare with the published release time |
| Forward works, reverse trips the upstream breaker | Phase swap made at the incoming side instead of the outgoing side, shorting two lines through both contactors | Verify the swap is on T1–T2–T3 of K2 only |
| Motor runs the wrong way in one direction only | Swap applied to the wrong phase pair, producing single-phase operation instead of reversal | Check that exactly two phases are crossed, and the third is straight through |
Sizing the pair
Both contactors carry the motor current in one direction only, so each is sized for the motor, not for twice the motor. For a 7.5 kW four-pole motor at 400 V, an AC-3 rating around 16 A is the usual starting point and the thermal overload relay is set to the motor’s nameplate current, not to the contactor rating. Intermittent reversing duty (AC-4) is a different electrical life: plugging and inching a motor wears contacts faster, so a starter that reverses every few seconds should be ordered one frame up rather than at the edge of its AC-3 rating.
Coil voltage is a separate ordering field from the contactor rating, and it has to match the control supply and the frequency. A 220 V 50 Hz coil and a 220 V 60 Hz coil are different part numbers even where the printed voltage looks the same. For markets on 110–127 V the control circuit is usually fed from a small control transformer rather than from a line-to-line voltage, which also keeps the coil order simple across a mixed-voltage project.
What to put on the purchase order
Motor kW and poles, rated current and AC-3 duty; the number of reversing operations expected per hour; coil voltage and frequency; whether the mechanical interlock kit and both NC auxiliary blocks are included; overload relay range and whether it is mounted under the contactor or separately; enclosure type, IP rating and DIN rail depth; destination country and the certificate the destination requires. Say explicitly whether the starter is to be supplied as a wired assembly or as loose components — the two are priced and packed very differently.
FAQ
Is the mechanical interlock enough on its own?
No. It blocks simultaneous closure but releases the instant the first contactor opens, which can be before the arc has extinguished. The electrical interlock across the coils and the dead time in the control scheme cover the gap.
Can a PLC replace the electrical interlock?
It can replace the push buttons, not the NC auxiliary contacts in the coil circuit. Software interlock is a logic decision; the NC contact is a physical one that still works when the output module fails or the program is in the wrong state.
Does a reversing starter need a separate thermal overload for each direction?
No. The motor current flows through one contactor at a time, so a single overload relay in the motor circuit protects both directions. Two relays are sometimes used where the load differs between directions, but that is a process requirement, not a wiring one.
What dead time should be set?
Start from the contactor manufacturer’s published release time and the arcing time at the rated current, then allow a margin: a setting in the region of 50 to 100 ms covers most small and medium AC-3 starters. Verify by test on the actual panel rather than by assumption, because the margin depends on the contactor size and the load.
Related: Star-delta starter wiring with CJX2, NR2 and NTE8 · AC-1 vs AC-3 vs AC-4 contactor duty · 50 Hz and 60 Hz contactor coils · Setting an NR2 thermal overload relay · Motor protection circuit breakers · PC class vs CB class changeover · Cross-reference an existing device · Request a quotation