DOL Starter Power Diagram: Main Contactor, Overload Relay, and Motor Wiring Reference

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A DOL starter power diagram shows the mains contactor, overload relay, and motor terminal connections that form the power circuit of a direct-on-line motor starter.

A Direct-On-Line (DOL) starter is the simplest and most common method of starting a three-phase induction motor. It connects the motor directly to the full supply voltage via a main contactor, with an overload relay (OLR) in series with the motor supply conductors to provide thermal overload protection. The DOL starter consists of two distinct circuits: the power circuit (covered here) and the control circuit, which governs coil energisation.

In the power circuit, three-phase supply enters through a main isolating switch and a set of fuses or a moulded-case circuit breaker (MCCB). The MCCB provides short-circuit protection for the wiring and the starter components. The output of the MCCB connects to the line-side terminals of the main contactor, conventionally labelled L1, L2, and L3 on the contactor body.

The main contactor's load-side terminals (T1, T2, T3) connect to the input terminals of the overload relay. Crucially, the overload relay must be wired in the motor leads — between the contactor output and the motor terminals — not in the incoming supply. This positioning means the overload relay measures the actual current drawn by the motor during all operating conditions, including starting inrush and sustained overload.

The overload relay output terminals connect directly to the motor terminals: U1, V1, W1 on the motor's terminal block. The motor terminal block also has U2, V2, W2 terminals used when reconfiguring the winding connection. For a motor running in star configuration permanently, U2, V2, W2 are linked together internally (star point). For delta connection, U1-W2, V1-U2, W1-V2 are bridged.

Motor direction of rotation (phase sequence) depends on the order in which L1, L2, L3 are connected to U1, V1, W1. Swapping any two of the three supply phase conductors at the contactor input or at the motor terminal block reverses rotation. Rotation must be verified by observing the motor shaft — or the driven load — before coupling to the mechanical system.

The overload relay's trip current is set to the motor's full-load current (FLC) rating, which appears on the motor nameplate. Setting the relay above FLC allows persistent overload without tripping; setting it significantly below FLC causes nuisance tripping during normal starting. The overload relay trips the control circuit via its normally closed (NC) auxiliary contact, which is wired in series with the start push-button circuit — a design principle that fails safe: any trip opens the contactor coil circuit, de-energising the contactor and disconnecting the motor.

How to wire dol starter power diagram

Size and install the main isolating switch and MCCB Select an MCCB or fused switch-disconnector rated for the motor's starting current (typically use a Type D or motor-rated MCCB with a breaking capacity exceeding the prospective short-circuit current at the installation point). Mount on a DIN rail or back plate in a suitably rated enclosure. Connect the three-phase supply to the MCCB's line (top) terminals.
Connect MCCB load-side terminals to contactor line-side Run three phase conductors from the MCCB load-side terminals to the main contactor's line-side input terminals, labelled L1, L2, and L3 on the contactor. Use cable sized for the motor's full-load current with appropriate derating for the installation method. Apply phase-colour identification at both ends of each conductor.
Connect contactor load-side to overload relay input Connect the contactor's T1, T2, and T3 load-side output terminals to the overload relay's input terminals (also typically labelled L1, L2, L3 on the OLR, or 1/L1, 3/L3, 5/L5 depending on manufacturer). Maintain phase sequence — do not transpose conductors between contactor and OLR.
Connect overload relay output to motor terminal block Connect the OLR's output terminals (2/T1, 4/T2, 6/T3 or T1, T2, T3) to the motor's U1, V1, and W1 terminals respectively. Use cable rated for the motor FLC and route in conduit or trunking to the motor junction box. Fit appropriate cable glands at the motor junction box.
Configure motor terminal block for star or delta Open the motor terminal box and configure the U2, V2, W2 terminals for the supply voltage. For a 400 V supply with a 400/690 V motor: connect in delta (U1-W2, V1-U2, W1-V2 bridged). For a 230 V supply with a 230/400 V motor: connect in delta. For a 400 V supply with a 230/400 V motor: connect in star (U2, V2, W2 linked together). Verify from the motor nameplate voltage and connection diagram.
Set the overload relay trip current Read the motor's full-load current (FLC) from the nameplate. Set the OLR trip current dial to the nameplate FLC. Confirm the OLR's manual/auto reset mode — for most machinery, manual reset is preferred so a tripped motor does not restart automatically without an operator investigation.
Test before connecting the load Ensure the driven mechanical load (pump, fan, conveyor) is uncoupled or safe to run freely. Restore MCCB power and initiate a test start from the control circuit. Observe motor rotation direction before coupling. Measure running current on all three phases with a clamp meter and verify against nameplate FLC — balanced three-phase current within ±5% indicates correct wiring and no mechanical overload.

Specifications

Starting Current (DOL)	Typically 6–8 times motor full-load current (FLC) at full voltage
Contactor Utilisation Category	AC-3 (squirrel cage motors, switching at full load) per IEC 60947-4-1
OLR Trip Current Setting	Set to motor nameplate FLC; trip at 105–125% FLC over the thermal time constant
Overload Relay Class	Class 10 (typical for standard motors); Class 20 for high-inertia loads with extended start times
Protective Earth Conductor Size	Per IEC 60364-5-54 Table 54.2, based on phase conductor cross-section
MCCB Short-Circuit Breaking Capacity	Must exceed prospective short-circuit current (Isc) at the installation point
Motor Terminal Voltage (standard EU/IEC)	400 V AC line-to-line, 50 Hz (three-phase)
Applicable Standards	IEC 60947-4-1 (contactors and motor starters), NEC Article 430, BS 7671 Regulation 552

Safety warnings

All DOL starter wiring at mains voltage must be performed by a licensed electrician and must comply with applicable wiring regulations and motor installation standards (NEC/NFPA 70, BS 7671, IEC 60364, AS/NZS 3000). Isolate the supply at the MCCB and apply lockout/tagout before any work on power circuit connections. Verify dead with a calibrated voltage tester at every point of work, including at the motor terminal box — capacitive motor starters can retain charge briefly after isolation.
The main contactor and overload relay must be rated for AC-3 motor duty (squirrel cage induction motor loads) not AC-1 (resistive loads). Using an AC-1 rated contactor for motor starting will cause rapid contact erosion due to the inductive breaking duty involved.
Never bypass or short-circuit the overload relay — even temporarily during fault-finding. The OLR is the primary protection against motor winding damage from sustained overloads. Operating a motor without functional overload protection risks winding failure, bearing damage, and fire.
Ensure the driven mechanical system is safe to start in either direction before the first test run. Reverse rotation of pumps can cause impeller damage; reverse rotation of conveyors or machines with ratchet mechanisms can cause serious mechanical damage or injury. Verify rotation direction with the load uncoupled first.
All motor enclosures and metal conduit must be bonded to the protective earth (PE) system. Verify earth continuity from the motor frame to the main earthing terminal with a low-resistance ohmmeter before energising.

Tools needed

Calibrated voltage tester, CAT III (for verifying dead before work)
Clamp meter / current clamp (for measuring three-phase running current after commissioning)
Digital multimeter (resistance, continuity, voltage)
Insulated VDE-rated screwdrivers
Hydraulic cable lug crimping tool (for motor cable lugs)
Wire strippers and cable knife
Lockout/tagout (LOTO) padlocks and hasps
Low-resistance ohmmeter (for verifying earth continuity)

Common mistakes

Wiring the overload relay between the MCCB and the contactor (on the supply side of the contactor) instead of between the contactor and the motor — in DOL starters the result is the same current measured, but this approach fails dangerously in star-delta starters where the OLR would only measure a fraction of winding current.
Setting the OLR trip current to a value higher than the motor nameplate FLC — 'to avoid nuisance tripping' — which removes thermal overload protection and allows the motor to overheat without tripping.
Using an AC-1 (resistive duty) contactor instead of AC-3 (motor duty) — the AC-3 rating accounts for the inductive breaking duty and high inrush; an AC-1 contactor will suffer accelerated contact wear and early failure.
Failing to verify motor terminal block configuration (star vs delta) for the supply voltage — connecting a 230/400 V motor in star on a 400 V supply applies 230 V across each winding (correct), but connecting it in delta on 400 V applies 400 V across each 230 V winding, causing immediate insulation failure.
Omitting the motor earth connection or using an undersized earth conductor — the motor frame must be earthed with a protective earth conductor sized to carry fault current for long enough to operate the upstream protective device.

Troubleshooting

Contactor closes but motor does not start or hums without turning: Cause: Single-phasing: one phase conductor open-circuit at MCCB, contactor, OLR, or motor terminal; or motor capacitor fault (single-phase motor) Fix: With contactor held closed (by control circuit), measure voltage at the motor terminal box across all three phase pairs (U1-V1, V1-W1, U1-W1). A missing phase shows as approximately zero volts on two of the three measurements. Trace back from the motor through OLR to contactor to MCCB to identify the open conductor or contact.
OLR trips within seconds of starting: Cause: OLR set below motor FLC, motor mechanically jammed or heavily overloaded, or motor winding fault (low insulation resistance between phases or to earth) Fix: Confirm OLR setting matches nameplate FLC. Uncouple the driven load and try starting — if the motor starts freely the driven load is jammed or excessively loaded. If the motor still trips, test winding insulation resistance with a 500 V megohmmeter between each phase pair and between each phase and earth — should read >1 MΩ.
Motor runs hot and draws above nameplate FLC on all three phases: Cause: Mechanical overload, incorrect motor terminal configuration (star vs delta), or supply voltage low Fix: Measure supply voltage at the MCCB — must be within ±10% of motor nameplate voltage. Verify motor terminal block configuration (star or delta) against nameplate. Check mechanical system for overload — bearing friction, pump back-pressure, conveyor loading. Compare measured three-phase current balance — significantly unbalanced currents indicate a supply or winding fault.

Frequently asked questions

Why must the overload relay be in the motor leads, not on the supply side of the contactor?

The overload relay must measure the actual current flowing through the motor windings. If placed on the supply side of the contactor, it would only measure inrush current and running current when the contactor is closed, which would be correct — however, in a star-delta starter, placing the overload relay in the wrong position would measure only a fraction of the motor current. For a DOL starter, either position measures the same current, but placing the OLR in the motor leads is the standard practice and the only safe practice for all starter types.

How do I set the overload relay trip current on a DOL starter?

Set the OLR's adjustable dial to the motor's full-load current (FLC) rating as printed on the motor nameplate. Most thermal overload relays have a setting range (e.g., 4–6 A) and a dial that selects within that range. If the nameplate FLC is 5.5 A, set the relay dial to 5.5 A. Do not set higher than FLC to avoid losing overload protection, and do not set below FLC to avoid nuisance tripping during normal operation.

What is the difference between a DOL starter and a star-delta starter?

A DOL starter connects the motor directly to full voltage on all three phases simultaneously, causing a starting current of typically 6–8 times the full-load current. A star-delta starter first connects the motor windings in star (reducing the voltage across each winding to 1/√3 of line voltage), then switches to delta after the motor approaches full speed. This limits starting current to approximately 1/3 of the DOL inrush but also produces only 1/3 of the starting torque.

What does the overload relay do when a motor is overloaded?

When motor current exceeds the OLR's set point for longer than the relay's thermal time constant, the relay trips. In the power circuit, the OLR does not directly interrupt the motor current — it opens a normally closed (NC) auxiliary contact in the control circuit. This de-energises the contactor coil, causing the contactor's main contacts to open and disconnect the motor from the supply. The OLR must be reset manually (or is set to auto-reset) before the motor can be restarted.

How do I reverse a three-phase motor wired through a DOL starter?

Swap any two of the three phase supply conductors feeding the contactor input terminals (e.g., swap the conductors on L1 and L2). This reverses the phase rotation seen by the motor, reversing its direction of rotation. Never attempt to change motor rotation while the motor is running — always stop the motor, isolate the supply at the MCCB or isolator, confirm dead, then swap the conductors.

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