Reverse-Forward Motor Starter Wiring Diagram

#electronics #diy #tutorial

A three-phase induction motor reverses direction by swapping any two of its three supply phases. Swap L1 and L3 (keeping L2 in the center) and the rotating magnetic field in the stator reverses -- the rotor follows. In practice, two contactors handle this: one connects the phases in the forward order (L1-L2-L3 to T1-T2-T3), and the other connects them in the reversed order (L3-L2-L1 to T1-T2-T3). The circuit that controls when each contactor is allowed to close -- and ensures the two never close simultaneously -- is the reverse-forward starter.

Why the Interlock Is Not Optional

If both contactors close at the same time, L1 and L3 are connected directly together through the closed contacts. That is a bolted phase-to-phase short circuit. The result is a fault current limited only by cable and busbar impedance -- potentially tens of thousands of amps. Contactors weld shut. Busbars vaporize. That is why the interlock is the non-negotiable center of the design.

Components Required

KMF (Forward contactor) -- closes to connect L1→T1, L2→T2, L3→T3
KMR (Reverse contactor) -- closes to connect L3→T1, L2→T2, L1→T3
Overload relay (F1) -- thermal/magnetic protection in series with motor supply
Three pushbuttons: FWD (NO), REV (NO), STOP (NC)
Mechanical interlock accessory -- physical lever between KMF and KMR frames
Short-circuit protection -- MCB or fuses upstream of contactors
Three-phase supply (L1, L2, L3)

Power Circuit Wiring

Both contactors share the same three-phase supply at their line terminals. The distinction is in how they cross the connections at the load side.

Forward contactor KMF:

L1 → KMF L1 → KMF T1 → T1 (motor terminal U)
L2 → KMF L2 → KMF T2 → T2 (motor terminal V)
L3 → KMF L3 → KMF T3 → T3 (motor terminal W)

Reverse contactor KMR:

L3 → KMR L1 → KMR T1 → T1 (motor terminal U)
L2 → KMR L2 → KMR T2 → T2 (motor terminal V)
L1 → KMR L3 → KMR T3 → T3 (motor terminal W)

In practice, KMR's line-side connections are crossed: L1 feeds KMR's L3 terminal, and L3 feeds KMR's L1 terminal. L2 remains in the center position. The two-phase swap reverses the phase sequence presented to the motor, and the motor rotates in the opposite direction.

The overload relay connects in series on the output side, between the T1/T2/T3 terminals of both contactors and the motor terminals. One overload relay handles both directions because the current magnitude does not change with direction -- only the phase order does.

Safety Note

The three-phase supply is present at the line terminals of both contactors at all times. With 400V or 480V between phases and the cabinet energized, working inside is potentially fatal. De-energize and lock out/tag out the upstream isolation device before opening the panel. Verify all three phases are dead with a calibrated voltage tester before touching any bus connection. A bolted phase-to-phase fault through inadvertent contact can occur in an instant.

Control Circuit Wiring

The control circuit is where the logic lives. A 120VAC control circuit (or 24VDC from a control transformer) is most common in industrial practice.

Terminal Sequence (120VAC example):

L1 control (hot) → STOP pushbutton (NC contact, terminals 1 and 2)
STOP terminal 2 → junction node A (this is the common point before FWD and REV branches)
Node A → FWD pushbutton (NO, terminals 3 and 4) → KMR NC auxiliary (interlock, terminals 21/22 of KMR) → KMF coil A1
KMF NO auxiliary (13/14) bridges across FWD pushbutton -- seal-in for forward run
Node A → REV pushbutton (NO, terminals 5 and 6) → KMF NC auxiliary (interlock, terminals 21/22 of KMF) → KMR coil A1
KMR NO auxiliary (13/14) bridges across REV pushbutton -- seal-in for reverse run
Both coil A2 returns → overload relay NC contact (95/96) → L2 control (neutral)

Sequence of Operation

Starting forward:

Press FWD. Current flows through STOP(NC) → FWD(NO) → KMR's NC interlock contact → KMF coil A1.
KMF energizes: main contacts close (L1-L2-L3 to motor in forward order). KMF's NO auxiliary seals in. KMF's NC auxiliary (21/22) opens, breaking the coil path to KMR.
Release FWD -- KMF remains energized through seal-in contact.

Stopping:

Press STOP. NC contact opens. KMF coil A1 drops out. Motor coasts to a stop (or brakes if a braking resistor or dynamic braking circuit is included).
All contacts return to normal state.

Starting reverse:

Press REV. Current flows through STOP(NC) → REV(NO) → KMF's NC interlock contact → KMR coil A1.
KMR energizes: main contacts close (L3-L2-L1 to motor -- reversed phase order). KMR NO auxiliary seals in. KMR NC auxiliary opens, breaking the path to KMF.

Attempting to reverse without stopping:
If the motor is running forward and the operator presses REV without first pressing STOP: the REV pushbutton signal reaches KMF's NC interlock contact -- which is already open because KMF is energized. KMR cannot energize. The operator must press STOP first to de-energize KMF, which closes the KMF NC interlock, and then press REV.

This design is intentional. Plugging (direct reversal without stopping) generates severe mechanical shock and typically 3-5 times rated motor current during the reversal transient. Some applications (overhead cranes, machine tool feeds) require plugging and use special control schemes with current limiting or dynamic braking to manage it.

Electrical and Mechanical Interlock Wiring

The electrical interlock -- each contactor's NC auxiliary in the other's coil circuit -- is already described above. It is effective as long as the auxiliary contacts are wired correctly and the contacts themselves are in good condition.

The mechanical interlock is an add-on accessory (Schneider LAD9R10, ABB CAM interlock kit, Siemens mechanical interlock bar, etc.) that mounts between KMF and KMR. It couples the movable armatures of the two contactors. When one armature moves to the closed position, the mechanical link prevents the other armature from moving at all.

Both interlocks must be present. Either one alone is insufficient. An electrical interlock with a welded contact on one contactor will not prevent both from being closed simultaneously. A mechanical interlock with incorrect electrical wiring can fail at switch-on due to race conditions in the control circuit.

Designing the Starter in CircuitDiagramMaker

The reverse-forward starter has enough connections that a well-drawn schematic saves real time at commissioning. In CircuitDiagramMaker, draw the power circuit first with both contactors side by side -- label the phase-crossing at KMR clearly. Then draw the control ladder below with the FWD and REV branches in parallel, the interlock contacts clearly shown in each branch, and the overload relay NC in the common A2 return leg. Running a logic simulation confirms that neither coil can be energized simultaneously.

Create Your Own Reverse-Forward Starter Diagram

Draw KMF and KMR side by side in the power section; show the L1/L3 phase swap at KMR's line terminals
Place the overload relay downstream of both contactors on the common motor supply conductors
In the control ladder, show STOP NC at the top of both branches
Draw the FWD branch: FWD(NO) → KMR NC interlock → KMF coil, with KMF NO seal-in across FWD
Draw the REV branch: REV(NO) → KMF NC interlock → KMR coil, with KMR NO seal-in across REV

Create your own reverse-forward motor starter diagram -- free

Key Takeaways

Direction reversal is achieved by swapping any two of the three supply phases -- conventionally L1 and L3 are swapped while L2 remains centered.
Two contactors are required: KMF for forward (L1-L2-L3 to T1-T2-T3) and KMR for reverse (L3-L2-L1 to T1-T2-T3).
Simultaneous closure of KMF and KMR creates a bolted phase-to-phase short circuit -- double interlocking (electrical + mechanical) is mandatory.
Electrical interlock: each contactor's NC auxiliary (21/22) is wired in series with the other's coil circuit.
Mechanical interlock: an add-on accessory that physically prevents both contactors from closing at the same time.
The standard control circuit requires STOP before reversing -- direct plugging requires additional current limiting.
One overload relay covers both directions because FLC does not change with rotation direction.

Originally published at https://circuitdiagrammaker.app/blog/reverse-forward-motor-starter-diagram.