Reverse-Forward Starter Control Diagram

Reverse-Forward Starter Control Diagram — interactive diagram. Open it in the editor to customise components and wiring.

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A reverse-forward starter control diagram shows two interlocked contactors that swap two of three supply phases to reverse motor rotation, with mechanical and electrical interlocks preventing both contactors from energising simultaneously.

Reversing a three-phase induction motor is straightforward in principle: swapping any two of the three supply phases (for example, L1 and L3) at the motor terminals causes the rotating magnetic field to reverse direction, and the rotor follows. In practice, two separate magnetic contactors — one for forward (KM1) and one for reverse (KM2) — accomplish this phase transposition. The forward contactor connects L1-L2-L3 to motor terminals U-V-W in the normal sequence. The reverse contactor connects L3-L2-L1 to U-V-W, swapping phases L1 and L3.

The critical design challenge is preventing both contactors from closing simultaneously. If KM1 and KM2 were both energised at once, two phases would be short-circuited through the closed main contacts — an arcing fault that destroys both contactors and risks fire. Two independent interlock systems are therefore always used together, never either one alone.

Mechanical interlock: a physical linkage between the two contactor frames ensures that when one contactor pulls in, a lever physically prevents the other from closing. This is often an add-on interlock kit that mounts between the two contactors.

Electrical interlock: each contactor's coil circuit is wired through a normally-closed auxiliary contact belonging to the other contactor. When KM1 is energised its NC auxiliary contact in the KM2 coil circuit opens, preventing KM2 from energising even if its pushbutton is pressed. The same logic applies in reverse for KM2.

The control station has three pushbuttons: stop (NC, common to both coil circuits in series), forward (NO, in KM1 coil branch), and reverse (NO, in KM2 coil branch). Each run button has a hold-in auxiliary contact wired in parallel to provide low-voltage protection and seal-in, identical to a standard 3-wire start-stop circuit. To change direction safely, the operator must press stop first to de-energise the running contactor before pressing the opposite direction button.

How to wire reverse forward starter control diagram

Isolate supply and apply lockout-tagout Switch off and lock out the main circuit breaker. Verify all three phases at the contactor mounting positions are at zero volts using a CAT III voltage tester. Apply LOTO tags before handling any wiring.
Mount and mechanically interlock the two contactors Mount KM1 (forward) and KM2 (reverse) side by side on the DIN rail. Fit the manufacturer-supplied mechanical interlock kit between the two contactor bodies per the kit instructions. Verify by hand: pushing one armature in must prevent the other from moving.
Wire the main power circuit Connect L1, L2, L3 to the line-side terminals of both contactors in parallel (both contactors see the same supply). On the load side: KM1 load terminals connect L1→U, L2→V, L3→W. KM2 load terminals connect L3→U, L2→V, L1→W (L1 and L3 are swapped). Both sets of load terminals join at the overload relay input.
Wire the overload relay and motor Connect the common load-side wiring through the overload relay heater elements to motor terminals U, V, W. Set the overload relay to the motor nameplate FLA.
Wire the coil circuits with electrical interlock Control supply → NC stop button → NC overload relay contact → junction point. From junction: branch to KM1 coil via KM2's NC auxiliary contact (electrical interlock), in series with forward NO pushbutton (with KM1 NO auxiliary in parallel). Separate branch to KM2 coil via KM1's NC auxiliary contact, in series with reverse NO pushbutton (with KM2 NO auxiliary in parallel). Both coil return lines go to the control neutral/common.
Verify interlock function before energising motor Restore control supply only (or use a low-voltage source). Press forward — KM1 should pull in. While KM1 is energised, press reverse — KM2 must not pull in. Press stop. Press reverse — KM2 pulls in. While KM2 is energised, press forward — KM1 must not pull in. Confirm both directions release on stop.
Test under motor load Connect motor. Run forward — verify rotation direction. Stop. Run reverse — verify rotation reverses. Check motor current in both directions with a clamp meter and confirm it does not exceed nameplate FLA under the intended load.

Specifications

Motor reversal method	Phase transposition: swap two of three supply phases (e.g., L1 and L3)
Contactor configuration	Two identical AC-3 contactors (KM1 forward, KM2 reverse) with shared overload relay
Interlock types required	Mechanical interlock kit AND electrical interlock (NC auxiliary contacts cross-wired)
Control circuit type	3-wire with hold-in auxiliary contacts and inherent low-voltage protection
Overload relay quantity	One, common to both directions, wired downstream of both contactors
Typical application	Conveyors, machine tools, hoists, gates, pumps requiring bidirectional operation
Applicable standards	IEC 60947-4-1, NFPA 79, NEC Article 430, IEC 60364-5-55
Direction change procedure	Press stop → wait for motor to come to rest → press opposite direction button

Safety warnings

Simultaneous closure of both contactors causes a phase-to-phase short circuit. Always fit both mechanical AND electrical interlocks — never rely on only one type. Verify interlock function with a functional test before connecting the motor.
Reversing the motor direction under full load causes a transient current spike that can reach several times full-load current. Where the application requires frequent or rapid reversing, consult the motor manufacturer's duty cycle rating and consider a dedicated reversing starter or soft-starter with reversing capability.
Always isolate and verify dead before wiring. Apply lockout-tagout per OSHA 1910.147 or EN ISO 50110. Three-phase voltages are lethal.
The overload relay must be properly set to the motor nameplate FLA. Reversing starters are sometimes installed with the overload bypassed 'temporarily' — this creates an unprotected circuit and is a code violation.
Installation must be performed by or under the supervision of a licensed electrician and must comply with applicable wiring codes: NEC/NFPA 70, BS 7671, IEC 60364, AS/NZS 3000, or local equivalent.

Tools needed

Calibrated CAT III voltage tester
Lockout-tagout equipment
Insulated screwdrivers (flat and cross-head)
Wire stripper and ferrule crimper
Clamp-on ammeter (true-RMS)
Continuity tester or multimeter
Cable labels

Common mistakes

Omitting the mechanical interlock and relying solely on electrical interlock — a contact weld or wiring error can then allow both contactors to close together.
Swapping the wrong phase pair on the reverse contactor, resulting in the motor running the same direction on both contactors.
Wiring the electrical interlock NC contacts in the wrong coil branch, so the interlocks cross-connect incorrectly and neither direction runs.
Connecting both contactors' load-side terminals to separate motors instead of the single common motor — a wiring error when two identical contactors are being installed near each other.
Not pressing stop between direction changes, leading to mechanical shock loading on the motor coupling and driven machinery.
Failing to verify the overload relay trip contact is in series with both coil branches, leaving the motor unprotected in one direction.

Troubleshooting

Motor runs in the same direction regardless of which button is pressed: Cause: Phase swap on the reverse contactor load terminals is incorrect — either no phases were swapped, or the wrong pair was changed Fix: Isolate supply and LOTO. Verify the reverse contactor load wiring: it must swap exactly two of the three phase connections relative to the forward contactor. Correct the wiring, restore supply, and retest rotation.
Both contactors energise simultaneously (interlock failure): Cause: Mechanical interlock not installed or misaligned; electrical interlock NC contacts wired incorrectly or contacts are welded open Fix: Immediately isolate supply. Do not restore until both interlocks are verified. Check the mechanical linkage alignment. Test each NC auxiliary contact for continuity when the opposite contactor is energised from a safe low-voltage source.
Contactor hums loudly but does not fully close: Cause: Coil voltage too low, mechanical interlock binding, or worn shading ring on AC coil Fix: Measure control supply voltage at coil terminals under energised conditions. Verify mechanical interlock is not too tight (should allow free operation). Inspect shading ring on AC contactor poles — replace contactor if ring is broken.
Overload trips in one direction but not the other: Cause: Phase imbalance caused by incorrect phase swap wiring, or one motor terminal connection is loose on one of the contactor's load sides Fix: Measure motor current on all three phases in both directions with a clamp meter. Locate the high-current phase, trace back to the contactor load terminal, and inspect for loose connection or incorrect wiring.

Frequently asked questions

Why must two phases be swapped to reverse a three-phase motor?

The direction of the rotating magnetic field in the stator is determined by the phase sequence of the three supply voltages. Swapping any two phases reverses the sequence (e.g., ABC becomes CBA), which reverses the direction of the rotating field and consequently the direction of rotor rotation.

What is the difference between mechanical and electrical interlock?

Mechanical interlock is a physical linkage that prevents the second contactor's armature from moving when the first is closed. Electrical interlock uses each contactor's NC auxiliary contact in the other's coil circuit to break power to the coil. Both must be used together — either alone is insufficient protection.

Can the operator switch directly from forward to reverse without pressing stop?

In a standard interlocked circuit, pressing reverse while forward is running causes the forward coil circuit to be broken via the electrical interlock before reverse can energise, effectively inserting an automatic stop. However, direct changeover is mechanically harsh and produces high motor currents — pressing stop first is strongly recommended practice.

Is a separate overload relay needed for each direction?

No. One overload relay is wired in the motor supply lines downstream of both contactors' load terminals. Because only one contactor is ever closed at a time, the overload relay monitors the actual motor current regardless of direction.

Why are only two phases swapped, not all three?

Swapping any two phases is sufficient to reverse the phase sequence and therefore the motor direction. Swapping all three phases returns to the original sequence (ABC rearranged to CBA is a reversal; rearranging to BCA is a different shift but still a reversal — however, swapping pairs is simplest to wire and verify).