RDOL Starter Control Diagram: Reversing Direct-On-Line Starter with Interlocked Contactors
This is a free printable rdol starter control diagram: download the diagram as SVG or open it and print to paper or PDF.
An RDOL starter control diagram shows how two interlocked contactors swap any two motor supply phases to reverse a three-phase induction motor safely and reliably.
A reversing direct-on-line (RDOL) starter achieves motor direction reversal by swapping any two of the three supply phases to the motor terminals. If the forward contactor (KM1) connects L1→U, L2→V, L3→W, then the reverse contactor (KM2) connects L1→W, L2→V, L3→U — the transposition of L1 and L3 reverses the phase rotation and therefore the direction of the rotating magnetic field in the stator.
The critical safety requirement is interlocking: KM1 and KM2 must never close simultaneously. If both contactors were energised at the same time, L1 and L3 would be connected together through the closed main contacts, creating a direct three-phase short circuit. Two forms of interlocking are required by good engineering practice.
Mechanical interlocking uses a direct mechanical linkage between the two contactor armatures — when one contactor is pulled in, a physical bar prevents the other from closing. This is the primary safety layer and is implemented via an interlock module that fits between the two contactor bodies.
Electrical interlocking uses NC auxiliary contacts: KM1's NC auxiliary contact (KM1-b) is wired in series with the KM2 coil circuit, and KM2's NC auxiliary contact (KM2-b) is wired in series with the KM1 coil circuit. If KM1 is energised, KM1-b opens, making it impossible to energise KM2 through the control circuit even if the forward button is released and the reverse button pressed simultaneously.
The control circuit sequence: Forward button (SB-F, NO) → energises KM1 coil → KM1 main contacts close (forward rotation) → KM1 sealing contact holds KM1 energised → KM1-b NC contact opens to block KM2. Stop button (SB-STOP, NC) de-energises both contactors. Reverse button (SB-R, NO) operates identically through the KM2 path.
Directly reversing a running motor (pressing reverse without stopping first) causes a severe current surge and mechanical shock that damages couplings and shaft seals. A time-delay relay or electronic control relay is commonly added to enforce a delay between forward and reverse commands.
How to wire rdol starter control diagram
- Select and mount the two contactors and overload relay Choose two identical contactors of the same size and manufacture — this ensures the mechanical interlock module fits correctly. Mount them on a DIN rail or back-plate side by side per the interlock module manufacturer's instruction. Fit the mechanical interlock between the two contactor bodies and verify it operates correctly — pressing KM1 in manually should prevent KM2 from closing and vice versa.
- Wire the main power circuit Connect three-phase supply (L1, L2, L3) to both contactors' input terminals in parallel — L1 to KM1-L1 and KM2-L1, L2 to KM1-L2 and KM2-L2, L3 to KM1-L3 and KM2-L3. Wire KM1 outputs straight through: KM1-T1→U, KM1-T2→V, KM1-T3→W. Wire KM2 outputs with L1 and L3 swapped: KM2-T1→W, KM2-T2→V, KM2-T3→U. Route both sets of output terminals through the overload relay to the motor.
- Wire the overload relay into the main circuit Feed both contactor output cable sets through the overload relay main current terminals in the correct order. The overload relay must sample current on all three phases going to the motor — ensure no phase is bypassed. Connect the overload relay output to the motor terminal box (U, V, W).
- Wire the electrical interlocking in the control circuit Add a KM1 NC auxiliary contact (KM1-b) in series with the KM2 coil circuit path, upstream of the KM2 coil. Add a KM2 NC auxiliary contact (KM2-b) in series with the KM1 coil circuit path, upstream of the KM1 coil. Test: with KM1 energised, the KM2 coil circuit must be broken by KM1-b and vice versa.
- Wire the control pushbuttons Wire stop button SB-STOP (NC) in series with both the forward and reverse circuits. Forward button SB-F (NO) connects to KM1 coil via the NC overload relay contact (95–96), KM2-b interlock, and stop button. Reverse button SB-R (NO) connects to KM2 coil via the same overload relay NC contact, KM1-b interlock, and stop button. Add sealing (hold-in) contacts KM1-a parallel to SB-F and KM2-a parallel to SB-R.
- Add time-delay interlocking if required For applications where plugging reversal must be prevented, add an on-delay timer relay (TON) in the control circuit. The timer starts when the stop button is pressed; only after its delay (typically 2 to 10 seconds, long enough for the motor to coast to a stop) does it permit the reverse contactor to be energised.
- Commission and verify With all covers fitted and personal protective equipment in place, apply power and test the forward direction with a brief jog — confirm correct rotation direction. Press stop and verify the motor decelerates. Test reverse and confirm the motor now rotates in the opposite direction. Verify the stop button stops the motor from both forward and reverse states. Check running current on all three phases with a clamp meter and compare to motor nameplate FLA.
Specifications
| Phase transposition for reversal | Any two of L1, L2, L3 swapped (commonly L1 ↔ L3) |
|---|---|
| Contactor duty category | AC-3 (squirrel-cage induction motor — IEC 60947-4-1) |
| Mechanical interlock | Mandatory; prevents simultaneous closure of KM1 and KM2 |
| Electrical interlocking contact type | NC auxiliary contact of opposing contactor in series with coil |
| Typical control circuit voltage | 24 V AC/DC, 110 V AC, or 230 V AC (per installation design) |
| Anti-plugging delay (recommended) | 2 s – 10 s on-delay timer after stop before reverse permitted |
| Governing standard | IEC 60947-4-1 (contactors and motor starters) |
Safety warnings
- All installation and commissioning work must be performed by a licensed electrician in accordance with IEC 60364, NEC/NFPA 70, BS 7671, or AS/NZS 3000. Lockout/tagout (LOTO) all energy sources before any wiring work — three-phase supplies are lethal.
- The mechanical interlock is a mandatory safety device, not optional. Do not operate an RDOL starter that has a mechanical interlock module missing, damaged, or incorrectly fitted. A phase-to-phase short circuit from simultaneous contactor closure releases enormous arc energy and creates a serious arc-flash and fire hazard.
- Arc-flash hazard: during commissioning and fault-finding with power energised, maintain appropriate arc-flash safe work distance and wear arc-rated PPE (face shield, gloves, arc-flash suit as required by the arc-flash risk assessment) in accordance with NFPA 70E or equivalent.
- Never attempt direct reversal of a motor driving a pump or compressor while the load is spinning — the resulting current surge and hydraulic shock can rupture piping and destroy mechanical seals. Ensure the motor and load have fully stopped before engaging the reverse direction.
Tools needed
- Insulated screwdrivers, 1000 V rated (flathead and Pozidriv/Phillips)
- Digital multimeter, CAT III 1000 V minimum (voltage, resistance, continuity)
- Approved voltage indicator (non-contact, for safe isolation verification)
- Lockout/tagout (LOTO) kit with padlocks and safety hasp
- Current clamp meter (to verify three-phase motor current during commissioning)
- Torque screwdriver or wrench (terminal tightening to manufacturer specification)
- Phase rotation tester (to verify supply phase sequence before connection)
Common mistakes
- Not transposing the correct phases on the reverse contactor: both L1→L1 and L2→L2 and L3→L3 connected to both contactors identically will produce the same direction regardless of which contactor closes. Exactly two phases must be swapped on the reverse contactor wiring.
- Using different contactor sizes for KM1 and KM2: the mechanical interlock module is designed to fit two identical contactors of the same frame. Mixing sizes means the mechanical interlock cannot be fitted, removing the primary safety layer.
- Omitting the sealing (hold-in) auxiliary contact on each contactor: without the sealing contact in parallel with each start button, the motor runs only while the button is held. The sealing contact is essential for maintained operation.
- Wiring both contactors' outputs to the same set of overload relay input terminals: the overload relay must see the actual motor current. If both contactors share inputs, the overload relay sees double-current during any reversal transient and may nuisance-trip.
- Pressing forward and reverse simultaneously without an interlock: without both mechanical and electrical interlocking, a simultaneous forward and reverse command creates a bolted three-phase fault. Never commission an RDOL starter without verifying both interlock systems are functional before applying power.
Troubleshooting
- Motor runs in the same direction regardless of forward or reverse selection
- Cause: Reverse contactor output phases not transposed, or reverse contactor mechanically jammed by a misaligned interlock Fix: De-energise and lock out. Verify KM2 output terminal wiring: T1 should connect to motor W, T2 to V, T3 to U (assuming L1/L3 swap). Check the mechanical interlock moves freely and does not prevent KM2 from fully closing. Apply power, jog in reverse, and check phase sequence at motor terminals with a phase rotation meter.
- Overload relay trips immediately when reverse is selected while motor is running
- Cause: Plugging current: reversing a running motor causes a 6–10× FLA current surge that exceeds the overload relay trip threshold Fix: This is normal motor behaviour during direct reversal. Add an on-delay timer relay (anti-plugging delay of 2 to 10 seconds after stop) to prevent reverse contactor energisation until the motor has stopped. Alternatively, install an anti-plugging relay that monitors back-EMF to confirm zero speed before permitting reversal.
- Contactor KM1 or KM2 will not energise despite button press
- Cause: Electrical interlock NC contact (KM2-b or KM1-b) open-circuit or wired incorrectly, or overload relay NC contact open Fix: With power off, measure continuity across the overload relay NC contact (95–96) — it should be closed in the reset state. Measure continuity across the electrical interlock NC contact (e.g. KM2-b) with KM2 de-energised — it should be closed. Trace the control circuit wiring against the schematic to identify any open contact or incorrect wiring connection.
Frequently asked questions
Why must both mechanical and electrical interlocking be used?
Electrical interlocking alone can fail if both contactors are commanded simultaneously by a control circuit fault — the NC contacts may not open fast enough if the coils energise together. Mechanical interlocking is a failsafe physical barrier. IEC 60947-4-1 and good engineering practice require both: electrical interlocking as primary logic and mechanical interlocking as a physical failsafe against catastrophic phase-to-phase short circuit.
Which two phases must be swapped to reverse motor direction?
Any two of the three phases can be swapped — swapping L1 and L3 is the most common convention. Swapping L1 and L2 or L2 and L3 produces the same reversal effect. What matters is that exactly two phases are interchanged in the reverse contactor wiring — swapping all three has no effect on direction.
Can an RDOL starter reverse the motor while it is still running?
Electrically yes — the starter can command reversal while the motor runs. However, this subjects the motor to plugging: the motor acts as a generator against the reversing supply, briefly drawing 6 to 10 times FLA. This causes extreme mechanical shock. For applications requiring frequent reversals, a soft-starter or VSD with ramp control should be used instead of hard reversal.
What size cable is needed between the contactors and the motor for an RDOL starter?
Cable sizing follows the same rules as standard DOL starters: the cable must carry 100% of motor FLA continuously with appropriate derating for installation method (conduit, trunking, direct burial). Both contactors carry the same current at all times — they are not in parallel, only one is closed at a time. Consult cable manufacturer derating tables and applicable codes.
Is an RDOL starter appropriate for all reversing applications?
No. RDOL starters are suitable for infrequent reversals with loads that can tolerate the mechanical shock of direct-on-line reversal — conveyors, hoists with dynamic braking, and positioning applications. High-inertia loads (fans, centrifuges) should not be reversed directly as the motor may overheat and couplings can fail. A VSD with controlled ramp reversal is more appropriate for such applications.
Related diagrams
- automatic star delta starter control circuit diagram
- control circuit of star delta starter
- control wiring of star delta starter
- dol starter control circuit
- dol starter control diagram
- dol starter control diagram with indicator