Reverse Forward Control Circuit Diagram: Two Interlocked Contactors

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A reverse forward control circuit diagram shows two mechanically and electrically interlocked contactors that swap two phase connections to reverse a three-phase induction motor's direction of rotation.

Reversing a three-phase induction motor requires swapping any two of the three phase connections to the motor terminals. When the phase sequence at the motor terminals changes (e.g. from L1-L2-L3 to L3-L2-L1, which swaps L1 and L3), the rotating magnetic field in the stator reverses direction, and the rotor follows.

The reverse-forward control circuit achieves this using two separate contactors: - Forward contactor (KM1): connects L1→T1, L2→T2, L3→T3 to the motor - Reverse contactor (KM2): connects L3→T1, L2→T2, L1→T3 — swapping L1 and L3

Note: only two phases need to be swapped. L2 (the middle phase) remains connected to T2 in both directions. Swapping L1 and L3 is the simplest and most common implementation.

The critical safety requirement is interlocking: if both contactors were energised simultaneously, they would create a direct three-phase short circuit between L1 and L3 — a bus-bar fault capable of destroying the contactors, the panel, and potentially causing a fire or explosion. Two forms of interlocking are always used together:

1. Electrical interlock: Normally-closed (NC) auxiliary contacts from each contactor are wired in series with the coil circuit of the other. When KM1 is energised, its NC contact opens in the KM2 coil circuit, preventing KM2 from being energised simultaneously — and vice versa.

2. Mechanical interlock: The two contactors are physically coupled with a mechanical interlock bar that physically prevents both from closing simultaneously, regardless of the electrical control circuit state. This is mandatory as a second barrier — electrical interlocks alone can fail due to contact welding.

The control circuit typically uses start/stop pushbuttons: a Forward Start button energises KM1; a Reverse Start button energises KM2. Pressing Reverse when running Forward should first stop the motor (or the circuit enforces a zero-speed interlock via a timing relay or tachometer) before energising the reverse contactor.

How to wire reverse forward control circuit diagram

1. Design the main circuit (power circuit) The main circuit carries the motor's full-load current. Route L1, L2, L3 from the supply through the main circuit breaker or fused isolator, then through the thermal overload relay, and into the common input terminals of both KM1 and KM2. The output terminals of KM1 connect to motor terminals T1, T2, T3 as L1→T1, L2→T2, L3→T3. The output terminals of KM2 connect as L3→T1, L2→T2, L1→T3 (only L1 and L3 are swapped). Connect the motor terminals T1, T2, T3 to the motor via appropriately rated cable.
2. Size the contactors and overload relay Both contactors must be rated to the motor's full-load current and locked-rotor current (both contactors see the same motor current — size them identically). The overload relay is set to the motor full-load current as stated on the motor nameplate. For a reversing application, select contactors with adequate AC3 duty rating for motor starting. The mechanical interlock kit must be specified for the contactor series — it is model-specific.
3. Wire the mechanical interlock Mount the two contactors side by side on the DIN rail with the mechanical interlock bar fitted between them per the contactor manufacturer's instructions. The interlock bar mechanically prevents both contactors from closing simultaneously. Test by manually pressing one contactor closed and confirming the other cannot be manually closed.
4. Wire the electrical interlock in the control circuit In the control circuit (typically 24 V AC or DC, or 230 V AC supplied through a control fuse): wire a normally-closed (NC) auxiliary contact of KM1 in series with the coil circuit of KM2. Wire a NC auxiliary contact of KM2 in series with the coil circuit of KM1. This ensures that when KM1 is energised and held in, its NC contact opens the KM2 coil circuit — and vice versa.
5. Wire the pushbutton control circuit A Stop pushbutton (NC) is wired in series with both forward and reverse start circuits. A Forward Start pushbutton (NO) in parallel with a KM1 hold-in auxiliary contact (NO) forms the forward latching circuit: pressing Forward energises KM1, which seals in via its own auxiliary contact and remains energised until Stop is pressed or the overload trips. A Reverse Start pushbutton (NO) in parallel with a KM2 hold-in contact forms the corresponding reverse latching circuit. Some designs add a button interlock: pressing Reverse when KM1 is energised first de-energises KM1 before energising KM2.
6. Connect the overload relay trip contact to the control circuit The thermal or electronic overload relay has a normally-closed (NC) contact in the control circuit and/or a normally-open (NO) contact for a fault alarm. Wire the NC overload contact in series with both contactor coil circuits so that an overload trip de-energises both contactors immediately regardless of which direction the motor is running.
7. Test the interlocks and direction before connecting the load Before connecting the motor: with main power applied to the control circuit but the motor disconnected or the main isolator open, press Forward — KM1 should energise. Verify KM2 cannot be energised while KM1 is held in. Press Stop. Press Reverse — KM2 should energise. Verify KM1 cannot be energised. Connect the motor; run Forward and verify rotation direction. Run Reverse and verify rotation is opposite. Check motor current against nameplate full-load ampere with a clamp meter.

Specifications

Safety warnings

• This diagram is for reference and educational purposes only. Motor control circuit design, installation, and commissioning must be carried out by a qualified electrical engineer or licensed electrician, in accordance with NEC / NFPA 70 (USA), BS 7671 and the relevant motor control standards (IEC 60947-4-1 for contactors and motor starters), AS/NZS 3000 (Australia/New Zealand), or the applicable local regulations. Motor control panels may require formal design approval and inspection.
• Always isolate the main circuit at the upstream isolator, lock out and tag the isolator, and verify all three phases at the main terminals are dead with a calibrated three-phase voltage tester before working inside the panel. Three-phase voltages (typically 400 V line-to-line in Europe, 480 V in North America) are lethal at very low contact currents.
• NEVER operate the Forward and Reverse contactors simultaneously. Even briefly, simultaneous energisation creates a direct phase-to-phase short circuit. Both mechanical and electrical interlocking are mandatory safety requirements — not optional improvements. The mechanical interlock must be rated for and correctly installed on the specific contactor model.
• Never bypass or temporarily remove the overload relay to 'get the motor running' during commissioning or fault diagnosis. The overload relay is the primary protection against motor winding thermal damage. Running without it risks destroying the motor windings — a far more costly outcome than diagnosing the root cause of the trip.
• Reversing a motor that is still rotating (plugging) causes very high current surges and mechanical shock loads. If the application requires frequent reversal, specify the motor and contactors for plugging duty (indicated by their AC4 duty rating), and consider adding an anti-plugging timing relay or zero-speed detection to enforce a stop before reversal.

Tools needed

• Calibrated three-phase two-pole voltage indicator (for dead verification at panel voltage)
• Clamp-type current meter (for verifying motor running current against nameplate FLA)
• Digital multimeter (for control circuit continuity and coil voltage verification)
• Insulated screwdrivers (flat and cross-head, VDE rated, appropriate for terminal sizes)
• Torque screwdriver or torque wrench (terminals on motor contactors have specified torque values — consult manufacturer datasheet)
• Wire ferrule crimping tool and insulated ferrules for panel wiring
• DIN rail cutter and cable duct tools for panel assembly
• Lockout/tagout kit for isolator
• Phase rotation meter (for verifying L1-L2-L3 sequence and motor rotation direction)

Common mistakes

• Wiring both contactors to apply the same phase sequence to the motor (forgetting to swap L1 and L3 on the reverse contactor), so the motor runs in the same direction regardless of which contactor is energised.
• Installing electrical interlocking only, without the mechanical interlock bar, leaving the circuit vulnerable to simultaneous closure if electrical interlock contacts weld.
• Wiring the NC auxiliary contacts as the hold-in (sealing) contacts instead of as the interlocking contacts, resulting in the circuit de-energising the contactor immediately after energisation (the contactor seals in with NO contacts, not NC).
• Setting the overload relay to the wrong current — setting it significantly above the motor FLA provides inadequate overload protection; setting it below causes nuisance tripping on motor starting current surges.
• Failing to verify motor rotation direction before connecting the driven load — running a pump in reverse, for example, can damage the pump impeller and mechanical seal.
• Using contactors of different models or manufacturers without a compatible mechanical interlock, and attempting to improvise an interlock that does not provide reliable mechanical separation.

Troubleshooting

Forward contactor energises but motor does not turn in reverse when Reverse button is pressed (motor stops on Stop, then Reverse is pressed)

Cause: Electrical interlock NC contact from KM1 is not opening the KM2 coil circuit, or the NC contact has welded closed, or the Reverse pushbutton wiring is open Fix: Isolate the control supply. Measure continuity of the NC auxiliary contact on KM1 with the contactor manually held closed — it should be open. If it reads closed (welded), the contact block requires replacement. Check the Reverse pushbutton for continuity when pressed. Trace the KM2 coil circuit for open connections.

Overload relay trips within seconds of starting in either direction

Cause: Overload relay setting is too low for the motor's starting current (or FLA), motor is mechanically jammed, or one phase of the supply is missing causing single-phase running (higher current in remaining phases) Fix: Measure motor current with a clamp meter during starting and compare to the motor nameplate FLA and locked-rotor current. Verify the overload relay is set to the nameplate FLA. Check that all three phases are present and balanced at the contactor input terminals. Check the driven load for mechanical binding.

Main circuit breaker trips immediately when either contactor is energised

Cause: Short circuit in the motor wiring between the contactor output and the motor, internal motor short circuit, or contactor is wired with both contactors applying the same phases without the phase swap — causing a line-to-line short on the bus Fix: Immediately investigate the wiring before re-energising. Disconnect the motor leads and verify the phase connections on both contactors with a multimeter and the diagram. An incorrectly wired reverse contactor that does not swap phases but instead creates a short between L1 and L3 through the forward contactor bus bars will cause an immediate fault. Verify the reverse contactor connections against the diagram.

Frequently asked questions

Why is it necessary to swap only two phases, not all three, to reverse the motor?

Swapping any two of the three phase connections reverses the phase rotation sequence. Swapping all three simultaneously also reverses the rotation, but swapping just two is mechanically simpler and produces an identical result. The conventional choice is to swap L1 and L3, leaving L2 connected to T2 in both directions, which simplifies the contactor wiring.

Why is mechanical interlocking required in addition to electrical interlocking?

Electrical interlock contacts can weld closed under high current or fault conditions, rendering the electrical interlock ineffective. A contact that has welded shut will not open when de-energised, potentially allowing both contactors to close simultaneously. Mechanical interlocking — a physical linkage between the two contactors — provides a redundant barrier that cannot be defeated by contact welding.

What is the effect of reversing a loaded motor without stopping it first?

Plugging — reversing a running motor — applies the reverse contactor while the motor is still rotating in the forward direction. The effective voltage applied is approximately twice the rated voltage, and the starting current surge can be 10–15 times full-load current. This causes severe mechanical shock to the driven load and thermal stress on the motor windings. A timing relay (anti-plugging relay) or zero-speed switch can prevent reversing until the motor has stopped.

What type of overload relay is used in a reverse-forward circuit?

A standard thermal overload relay or an electronic overload relay is wired in series with the motor supply — in the main circuit, after the contactors. The overload relay protects against sustained overcurrent (overload) in both the forward and reverse directions. A single overload relay covers both directions because the motor current passes through the same overload relay regardless of which contactor is energised.

Can a reverse-forward circuit be used with a single-phase motor?

Yes, but the method differs. Reversing a single-phase capacitor-start motor requires swapping the start winding connections (not swapping two of three phases). The contactor arrangement therefore switches the start winding polarity rather than phase connections. The interlocking principle — preventing both contactors from energising simultaneously — remains equally important.

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