Forward Reverse Motor Wiring Diagram: Contactors, Interlocking, and Control Circuit
This is a free printable forward reverse diagram: download the diagram as SVG or open it and print to paper or PDF.
Understand forward-reverse motor wiring diagrams including contactor interlocking, push-button control circuits, and phase reversal for three-phase induction motors.
A forward-reverse motor circuit allows a three-phase (or single-phase) induction motor to run in either rotational direction by switching the phase sequence presented to the motor terminals. For a three-phase motor, reversing any two of the three supply phases causes the rotating magnetic field to reverse, and the motor follows.
The circuit requires two contactors: a forward contactor (F) and a reverse contactor (R). Each contactor, when energised, connects the motor to the supply — but the reverse contactor has two of its load poles wired in transposed (swapped) order relative to the forward contactor. A common arrangement is: forward wiring connects L1-L2-L3 to motor terminals T1-T2-T3. Reverse wiring connects L1-L3-L2 to T1-T2-T3 (L2 and L3 are swapped).
The critical safety requirement is interlocking — both contactors must never be energised simultaneously, because doing so would create a three-phase short circuit on the supply. Interlocking is implemented in two ways, which must be used together:
1. Electrical interlocking: a normally-closed (NC) auxiliary contact of the forward contactor is wired in series with the reverse contactor coil circuit. If F is energised its NC contact opens, preventing R from being energised. An NC auxiliary of R is similarly wired in series with F's coil. This is a hardware interlock.
2. Mechanical interlocking: a physical linkage between the two contactor armatures prevents both from closing at the same time. Many manufacturers supply interlocking kits for their contactor ranges.
The control circuit uses three push buttons: STOP (NC contact, breaks the control circuit), FORWARD (NO contact, seals through the forward contactor auxiliary), and REVERSE (NO contact, seals through the reverse contactor auxiliary). Latching (hold-in) is achieved by wiring an NO auxiliary contact of each contactor in parallel with its own start button, so the operator can release the button and the contactor remains energised.
A thermal overload relay protects the motor against sustained overcurrent. It is wired in series with the motor leads and its NC trip contact is wired in the control circuit so a trip de-energises both coils.
All wiring must conform to IEC 60947-4-1 (for IEC contactors) or NEMA ICS 2 (for NEMA starters), and local installation codes.
A ladder diagram for a forward-reverse motor starter uses two contactors — Forward (F) and Reverse (R) — whose main contacts swap two of the three supply phases to change motor rotation direction. The control rung includes mechanical and electrical interlocks between F and R coils so both cannot energise simultaneously, which would create a phase-to-phase short circuit. The forward rung typically reads: Stop NC — Forward PB NO — F coil, with an F auxiliary NO seal-in contact, and an R NC interlock in series. The reverse rung mirrors this arrangement. Draw and annotate your forward-reverse ladder diagram free in the browser-based editor.
How to wire forward reverse diagram
- Size and select contactors and overload relay Determine motor full-load amperage (FLA) from the nameplate. Select contactors with an AC-3 (IEC) or equivalent NEMA rating above the motor FLA. Select a thermal overload relay with an adjustment range that includes the motor FLA. Both contactors should be the same model to simplify interlocking.
- Wire the power (main) circuit Connect the three-phase supply (L1, L2, L3) to the incoming terminals of both contactors in parallel. From the forward contactor outgoing terminals, wire L1-L2-L3 in straight order to the overload relay input (T1-T2-T3). From the reverse contactor outgoing terminals, wire with two phases swapped — for example L1 to T1, L3 to T2, L2 to T3 (L2 and L3 transposed). The overload relay output feeds the motor terminals.
- Install mechanical interlock If an interlock bar or mechanical interlock kit is available for your contactor model, fit it between the two contactors per the manufacturer's instructions. This prevents simultaneous mechanical closure regardless of control circuit signals.
- Wire the control circuit Pick a control voltage (often 110 V AC via a control transformer, or the same 230 V/400 V supply). Wire the STOP button NC contact in series on the control line. From the STOP button, wire to the FORWARD NO push button, then to the forward contactor coil. From the STOP button, also wire to the REVERSE NO push button, then to the reverse contactor coil. Add the NC auxiliary of the forward contactor in series in the reverse coil branch, and the NC auxiliary of the reverse contactor in series in the forward coil branch (electrical interlocking).
- Add hold-in (latching) auxiliary contacts Wire an NO auxiliary contact of the forward contactor in parallel with the FORWARD push button. Wire an NO auxiliary contact of the reverse contactor in parallel with the REVERSE push button. This allows the operator to release the push button and the contactor remains held in by its own auxiliary contact.
- Wire the overload relay trip contact Connect the overload relay's NC auxiliary trip contact in series with the control line, typically in the same series branch as the STOP button. When the overload trips, this NC contact opens and de-energises whichever contactor was picked up, stopping the motor. The overload must be manually reset (or auto-reset, depending on model setting) before the motor can be started again.
- Test the interlocking before connecting the motor With motor leads disconnected, apply control power. Press FORWARD — verify only the forward contactor picks up and the reverse cannot be energised while F is held. Release FORWARD, press STOP. Press REVERSE — verify only R picks up. Then test that pressing both FORWARD and REVERSE simultaneously energises only the first pressed (or neither, if an anti-plug relay is fitted). Connect the motor only after confirming correct operation.
Specifications
| Control circuit voltage (typical) | 110 V AC (IEC) or 120 V AC (NEMA) from control transformer; also 24 V AC/DC in modern PLCdriven systems |
|---|---|
| Contactor utilisation category (squirrel cage motor) | IEC AC-3 |
| Minimum contactor rating above motor FLA | Equal to or greater than FLA; select next standard size up |
| Overload relay setting | Set to motor nameplate full-load amperage (FLA) |
| Phase reversal for reverse direction | Two phases swapped (e.g., L2 and L3 transposed); only two, not all three |
| Mechanical interlock separation | Per manufacturer — typically prevents simultaneous armature travel exceeding 2 mm |
| Control circuit wiring size (typical) | 1.5 mm² (AWG 14) insulated, 600 V rated |
| Main circuit wiring size | Per motor FLA and cable run length, per local wiring code tables |
Safety warnings
- All motor control work must be performed by a licensed electrician in compliance with applicable codes (NEC/NFPA 70, IEC 60364, BS 7671, AS/NZS 3000). Verify compliance with local authority before commissioning.
- Isolate, lock out, and verify dead (using a calibrated voltage tester on all three phases) before working on any motor control panel or wiring. Three-phase voltages are lethal.
- Mechanical and electrical interlocking of forward and reverse contactors is not optional. Simultaneous energisation of both contactors creates a bolted three-phase short circuit capable of causing an arc flash explosion.
- Never attempt to reverse direction without ensuring the motor has fully stopped, unless the application is specifically designed for plug reversal and appropriate protective devices are installed.
- Set the thermal overload relay to the motor's nameplate full-load amperage, not to the supply fuse rating. A relay set too high will not protect the motor.
Tools needed
- Calibrated non-contact and contact voltage tester (CAT III minimum)
- Digital multimeter
- Clamp ammeter
- Insulated screwdrivers
- Wire strippers and ferrule crimping tool
- Cable markers and marker pen
- Lockout/tagout padlock set
- Continuity tester
Common mistakes
- Omitting mechanical interlocking and relying on electrical interlock alone. A welded contact in one contactor will defeat electrical interlocking; mechanical interlock prevents simultaneous closure regardless.
- Swapping three phases in the reverse contactor instead of two, resulting in the reverse direction producing the same rotation as forward (or a different single-phase condition).
- Wiring the overload relay NC contact on the load side of the control circuit rather than the supply side, so a trip does not interrupt contactor coil power correctly.
- Using push buttons with only NO contacts for the STOP button. The STOP button must use an NC contact so a broken wire or disconnected button defaults to STOP, not RUN (fail-safe principle).
- Omitting the hold-in auxiliary contact, so the motor only runs while the push button is physically held down.
- Installing the overload relay current setting at the fuse rating rather than the motor nameplate FLA, providing no overload protection to the motor.
Troubleshooting
- Motor runs in one direction only; reversing produces no movement
- Cause: Reverse contactor coil circuit is open — check electrical interlock wiring, push button contact, or coil itself Fix: With power isolated, use a continuity tester to trace the reverse contactor coil circuit. Verify the NC auxiliary of the forward contactor is functioning (open when F is energised, closed when F is de-energised). Check the reverse push button NO contact.
- Motor trips overload relay immediately on starting in reverse
- Cause: Phase sequence is incorrect in reverse direction causing single-phasing, or overload relay setting is too low Fix: Check voltage on all three motor terminal pairs with a multimeter while attempting to start in reverse. If one phase is missing (reads zero), the reverse wiring has an error. Verify two phases are correctly transposed in the reverse contactor wiring.
- Both contactors close simultaneously, causing fuse operation or supply trip
- Cause: Electrical interlock auxiliary contacts are wired incorrectly, or a contactor auxiliary contact has failed closed Fix: Isolate power immediately. Check that NC auxiliary of F is wired in series with R coil circuit, and NC auxiliary of R is in series with F coil circuit. Test each NC auxiliary contact with a continuity tester — it must be open when the contactor is energised. Replace faulty contactor if auxiliary contact is welded.
- Motor continues to run after pressing STOP
- Cause: STOP button NC contact is faulty (stuck open or bypassed), or hold-in auxiliary contact is wired across the STOP button instead of the start button Fix: Check the STOP button wiring and verify the NC contact opens when pressed. Verify the hold-in auxiliary is in parallel only with the FORWARD or REVERSE push button, not with the STOP button.
- Control circuit hums but contactors do not pick up
- Cause: Control voltage is too low, coil is open-circuit, or control transformer is undersized Fix: Measure control circuit voltage at the coil terminals. If below 85% of rated coil voltage, investigate the control transformer rating and wiring. Test coil resistance with a multimeter; compare to specification. Replace coil or contactor if open-circuit.
Frequently asked questions
How does reversing two phases of a three-phase motor change its direction?
A three-phase induction motor's rotation follows the rotating magnetic field set up by the stator windings. The field rotates in the sequence A-B-C. Swapping any two phases, say B and C, creates the sequence A-C-B, which is the mirror image — the field rotates the opposite way, and the rotor follows it.
What is electrical interlocking and why is it essential?
Electrical interlocking uses a normally-closed auxiliary contact from each contactor wired in series with the other contactor's coil. If the forward contactor is picked up, its NC auxiliary opens and physically prevents the reverse contactor coil from receiving power. This prevents both contactors closing simultaneously, which would short-circuit the supply phases.
Can I reverse direction without stopping first?
Electrically, a plug-reversing circuit can reverse a running motor, but this applies full voltage in the opposing direction to a spinning motor — the effect is equivalent to a starting current spike and substantial mechanical shock to the load. For most mechanical loads, stopping first is required. Plug reversal is only acceptable for applications specifically designed for it (e.g., some hoists and machine tools).
What does the thermal overload relay protect against?
The thermal overload relay protects the motor against sustained overcurrent caused by overloading, stalling, or running on a single phase (phase loss). It does not protect against short circuits — that is the function of the upstream fuses or circuit breaker. Both protections are needed.
What is the difference between IEC and NEMA contactors in a forward-reverse circuit?
IEC contactors (per IEC 60947) are sized by utilisation category (e.g., AC-3 for squirrel cage motors) and tend to be physically smaller for a given rating. NEMA contactors (per NEMA ICS 2) use size designations (00, 0, 1, 2, etc.) and are typically more conservatively rated. The wiring principles for forward-reverse interlocking are the same for both.
How is a forward-reverse starter shown in a ladder diagram?
In a ladder diagram the forward rung energises the F contactor coil through a normally-closed (NC) stop contact, a normally-open (NO) forward pushbutton, and an NC auxiliary contact from the reverse contactor (electrical interlock). A parallel NO F auxiliary contact seals in the run. The reverse rung is identical in structure but substitutes R pushbutton and R coil, with an F NC interlock. This interlocking prevents simultaneous coil energisation and the resulting phase-to-phase short.
Related diagrams
- 3 phase forward and reverse wiring diagram
- 3 phase forward reverse motor control circuit diagram
- 3 phase forward reverse switch wiring diagram
- forward reverse motor control diagram
- forward reverse switch wiring diagram
- reverse forward circuit diagram