Reverse Forward Control Diagram: Interlocked Contactor Circuit for 3-Phase Motors
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A reverse-forward control diagram shows how two interlocked contactors swap two supply phases to reverse a three-phase induction motor's direction of rotation.
Reversing the direction of a three-phase induction motor requires reversing its phase sequence at the motor terminals. This is achieved by swapping any two of the three supply phases. A dedicated reverse-forward (or forward-reverse) control circuit uses two contactors — one for each direction of rotation — wired so that closing one contactor swaps two phases relative to the other.
If contactor F (forward) connects supply phases L1, L2, L3 to motor terminals U, V, W, then contactor R (reverse) must connect supply phases in a swapped arrangement — for example, L3, L2, L1 to motor terminals U, V, W. This swaps phases L1 and L3, reversing the phase sequence and thus the direction of the rotating magnetic field in the stator.
The critical safety requirement is that contactors F and R must never be energised simultaneously. Simultaneous energisation would create a direct phase-to-phase short circuit on the supply, potentially destroying the contactors, blowing fuses, and creating a fire hazard. Two levels of interlocking are used:
Mechanical interlock: A physical linkage between the two contactors' moving parts prevents both armatures from closing at the same time. This is a fail-safe mechanical barrier.
Electrical interlock: An auxiliary normally closed (NC) contact from contactor F is wired in series with the coil circuit of contactor R, and vice versa. When contactor F energises, its NC auxiliary contact opens the coil circuit for contactor R, preventing it from being energised regardless of what control signals are present. This provides a second layer of protection.
The control circuit is typically fed from a reduced-voltage control supply (often 110 V AC or 24 V DC derived from a control transformer) for operator safety. A stop pushbutton (NC) is wired in series with both forward and reverse coil circuits. Separate forward (F) and reverse (R) start pushbuttons (NO) each energise their respective contactor coil.
Overload relay contacts are wired in series with the control circuit to interrupt both coil circuits on motor overload.
All wiring must be carried out by a qualified electrician and must comply with applicable standards including IEC 60947 (contactors and motor starters), NEC Article 430 (motors), BS 7671, or equivalent.
How to wire reverse forward control diagram
- Plan the power circuit wiring Map the supply phases L1, L2, L3 through the main isolator, fuses or MCCB, and into the input terminals of both contactors F and R in parallel. The output terminals of contactor F connect straight through to motor terminals U, V, W. The output of contactor R must swap L1 and L3 — connect R's output T1 to motor U (from L3), T2 to motor V (from L2), T3 to motor W (from L1).
- Wire the overload relay in the power circuit Connect the motor side of both contactors through the three-phase thermal overload relay before the motor terminal box. All three phases must pass through the overload relay for correct protection.
- Design the control circuit supply Derive a control circuit supply from a control transformer (typically 110 V AC secondary, or 24 V DC via rectifier) fused separately. The control circuit should not be derived directly from the high-voltage motor supply without appropriate protection.
- Wire the stop and overload contacts in series Wire the normally closed stop pushbutton in series with the overload relay NC contact. This forms the common series leg through which all coil circuits must pass before reaching their respective forward or reverse coil.
- Wire the forward start and interlock Wire the forward start pushbutton (NO) in series with contactor R's NC auxiliary contact, then to contactor F's coil. A NO hold-in auxiliary contact from contactor F bridges across the forward start pushbutton for self-holding.
- Wire the reverse start and interlock Wire the reverse start pushbutton (NO) in series with contactor F's NC auxiliary contact, then to contactor R's coil. A NO hold-in auxiliary contact from contactor R bridges across the reverse start pushbutton.
- Fit mechanical interlock and test Install the mechanical interlock bar between the two contactors per the manufacturer's instructions. Test control circuit operation on a bench: verify forward energises, reverse does not operate and vice versa. Confirm stop de-energises both. Only then connect the power circuit and test motor rotation.
Specifications
| Phase reversal method | Swap any two of three supply phases (e.g. L1↔L3) |
|---|---|
| Interlock type | Mechanical (physical bar) + electrical (NC auxiliary contacts) — both required |
| Control circuit voltage (typical) | 110 V AC or 24 V DC from control transformer |
| Relevant standards | IEC 60947-4-1, NEC Article 430, BS 7671, AS/NZS 3000 |
| Overload relay type | Three-phase bimetallic thermal or electronic overload relay |
| Auxiliary contact used for interlock | NC (normally closed) from each contactor to opposite coil circuit |
Safety warnings
- Simultaneous energisation of both contactors creates a direct phase-to-phase short circuit. Both mechanical and electrical interlocks are mandatory safety devices — do not omit either. Verify interlock operation before connecting motor power.
- All work on three-phase motor control circuits must be carried out by a qualified electrician. The circuit must comply with IEC 60947, NEC Article 430, BS 7671, AS/NZS 3000, or applicable national standards.
- Isolate and lock out the main isolator before working on any part of the circuit. Verify absence of voltage on all three phases and in the control circuit using a calibrated voltage tester.
- Never attempt to reverse a running motor by direct switching without appropriate protection. If plugging (direct reversal) is intended by design, the contactors and motor must be rated for that duty.
- The thermal overload relay must be set to the motor's nameplate full load current. Incorrect setting will fail to protect the motor against overload and may void equipment warranty.
Tools needed
- Calibrated voltage tester (three-phase capable)
- Multimeter
- Insulated screwdrivers
- Phase rotation tester or rotation meter
- Wire end ferrules and crimping tool
- Cable duct and DIN rail mounting hardware
- Motor current clamp meter
Common mistakes
- Wiring the electrical interlock with NO auxiliary contacts instead of NC — this creates a circuit that energises both contactors simultaneously rather than preventing it.
- Omitting the mechanical interlock and relying on electrical interlock alone — a welded contact in either contactor can defeat a purely electrical interlock.
- Swapping only one phase between forward and reverse contactors instead of two — swapping one phase does not reverse the motor.
- Not verifying motor rotation direction before connecting the mechanical load — running a conveyor, pump, or compressor in the wrong direction can cause immediate mechanical damage.
- Setting the overload relay to the supply voltage rather than motor full load current, leaving the motor unprotected.
Troubleshooting
- Motor runs in the same direction on both forward and reverse
- Cause: Reverse contactor output wiring does not correctly swap two phases Fix: Isolate and verify the phase connections at contactor R output terminals. Confirm that two phases are transposed relative to the forward contactor output. Use a phase rotation meter to verify.
- Forward contactor energises but immediately drops out
- Cause: Overload relay tripped, or NC contact of overload relay not resetting correctly Fix: Check overload relay trip indicator. Reset if tripped and investigate overload cause (mechanical binding, motor fault, phase loss). Verify control circuit continuity.
- Both contactors cannot energise — no motor response on either start pushbutton
- Cause: Open circuit in common series leg: stop pushbutton, overload relay NC contact, or control circuit fuse Fix: With power applied to control circuit only, trace voltage through the series leg. Identify the open point. Verify fuse integrity, overload relay reset, and stop pushbutton contact continuity.
Frequently asked questions
Why must two phases be swapped to reverse a three-phase motor?
A three-phase induction motor's direction of rotation follows the phase sequence of the supply. Swapping any two of the three phase conductors reverses the sequence (e.g. L1-L2-L3 becomes L1-L3-L2), which reverses the direction of the rotating magnetic field in the stator and therefore reverses the rotor's direction of rotation.
What is the difference between mechanical and electrical interlock?
Mechanical interlock is a physical coupling between the two contactors that prevents both from closing simultaneously — it is a hardware fail-safe. Electrical interlock uses NC auxiliary contacts from each contactor wired into the opposite contactor's coil circuit, preventing simultaneous energisation through the control circuit. Best practice requires both types to be used together.
Can the motor be switched directly from forward to reverse without stopping?
Switching directly from forward to reverse is called 'plugging' and applies significant mechanical and electrical stress to the motor, contactors, and driven load. Most applications require the motor to stop fully before reversing. Timing relays or zero-speed switches are used to enforce a pause before the reverse contactor can energise.
What role does the overload relay play in a reverse-forward circuit?
The thermal overload relay monitors motor current in all three phases. Its NC contact is wired in series with both contactor coil circuits. If the motor draws excessive current — due to mechanical overload, phase failure, or stall — the overload trips, opening the contact and de-energising whichever contactor is active, disconnecting the motor from supply.
What is a contactor's auxiliary contact?
An auxiliary contact is a low-current switching contact mechanically linked to the contactor's main armature. NO auxiliary contacts close when the contactor energises; NC auxiliary contacts open when it energises. In reverse-forward circuits, NC auxiliary contacts provide the electrical interlock.
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