DOL Circuit Diagram

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A DOL (Direct Online) starter circuit diagram shows how a three-phase induction motor is started by connecting it directly to the full supply voltage through a contactor and overload relay, without any current-limiting device.

A Direct Online (DOL) starter is the simplest and most common method of starting an AC induction motor. In a DOL circuit, the motor is connected directly to the full supply voltage the moment the start button is pressed. There is no reduced-voltage starting, no resistor insertion, and no auto-transformer — the motor sees full voltage from the first instant of starting.

This simplicity comes with a characteristic consequence: DOL starting produces a high inrush current, typically five to eight times the motor's full-load current (FLC), lasting from one to several seconds depending on the load. This current surge can cause voltage dips on the supply network and mechanical stress on the motor windings and driven equipment. For this reason, DOL starting is generally acceptable only for motors up to approximately 5.5 kW to 11 kW in most network configurations, although local utility regulations vary. Larger motors typically require soft starters, star-delta starters, or variable frequency drives (VFDs) to limit starting current.

A DOL starter circuit has two electrical circuits: the power circuit and the control circuit. The power circuit carries the full motor current through the main contacts of the contactor (sometimes called the main contactor) and the overload relay thermal elements. The control circuit operates at a lower voltage (often 110 V AC or 24 V AC/DC) and controls the contactor coil via start and stop push buttons and the overload relay's normally closed (NC) auxiliary contact.

The overload relay is a thermal or electronic device that monitors the current through each motor phase. If the motor draws excessive current for a sustained period (e.g. due to mechanical overload, single-phasing, or undervoltage), the overload relay trips its NC contact, de-energising the contactor coil and disconnecting the motor. The three-wire control circuit includes a 'hold-on' or 'latching' auxiliary contact on the contactor that maintains the circuit after the start button is released, providing electrical interlocking and the ability to stop from any remote stop button. Standards governing DOL starters include IEC 60947-4-1 (low-voltage motor starters) and NEMA ICS 2.

How to wire dol circuit diagram

Determine motor parameters and select components Identify the motor's supply voltage (e.g. 400 V, 3-phase, 50 Hz), full-load current (FLC) from the nameplate, starting current (typically 5–8× FLC), and power factor. Select a contactor rated at minimum 100% of FLC and an overload relay adjustable around the nameplate FLC.
Select upstream overcurrent protection (fuses or MCCB) Size the main fuses or motor circuit breaker for the motor starting current. For fuses, use type gM or gL motor fuses rated at approximately 2–3× FLC for starting time allowance. For a motor circuit breaker (MPCB), set the instantaneous trip at 8–12× FLC per IEC 60947-4-1.
Wire the power circuit Connect the three-phase supply (L1, L2, L3) to the main circuit breaker or fuse isolator, then to the main contactor input terminals (L1, L2, L3 on the contactor), then through the overload relay (T1, T2, T3 on the contactor to 1T, 2T, 3T on the overload relay), then to the motor terminals (U, V, W).
Wire the control circuit Take a control supply (typically via a control transformer to 110 V AC, or 24 V AC/DC). Wire the following series string: control supply phase → fuse → E-stop (NC) → Stop button (NC) → Overload relay NC contact → Start button (NO) → Contactor coil → control supply neutral. Wire the contactor's NO auxiliary contact in parallel with the start button to provide latching (hold-on).
Set the overload relay Adjust the overload relay current setting to the motor nameplate full-load current. Most thermal overload relays have a graduated dial; set it to match the FLC exactly. Verify the trip class (Class 10 is standard for most industrial motors; Class 20 for high-inertia loads like fans and centrifuges).
Test the control circuit before applying power to the motor Apply only the control circuit supply (if isolated from the power circuit via a control transformer). Press the start button and verify the contactor closes and latches. Press the stop button and verify the contactor drops out. Simulate an overload trip by operating the overload relay test button and verify the contactor drops out.
Commission with motor connected Apply full power. Start the motor and measure current on all three phases with a clamp meter during starting and at full load. Verify currents are balanced and within rated values. Confirm motor rotation direction and check that the overload relay thermal elements are not heating excessively during normal running.

Specifications

Supply voltage (typical, IEC regions)	400 V AC, 50 Hz, 3-phase (or as specified by local utility)
Starting inrush current (typical)	5–8 × motor full-load current (FLC)
Contactor utilisation category	AC-3 (squirrel cage induction motor: making at full locked rotor current, breaking at full-load current)
Overload relay trip class (IEC 60947-4-1)	Class 10 (standard); Class 20 (high-inertia loads)
Control circuit voltage (typical)	110 V AC (IEC standard); 24 V AC or DC (modern controls); 230 V AC (some older installations)
Maximum recommended motor size for DOL starting	Approximately 5.5–11 kW (subject to local network operator requirements)
Governing standard	IEC 60947-4-1 (low-voltage motor starters); NEMA ICS 2 (North America)
Motor nameplate current setting	Overload relay set to 100% of motor nameplate full-load current (FLC)

Safety warnings

DOL starter installation, wiring, and commissioning must be performed by a licensed electrician or qualified electrical engineer in compliance with applicable standards including IEC 60947-4-1, IEC 60364, NEC/NFPA 70, BS 7671, or AS/NZS 3000. This diagram is for educational and reference purposes only.
Always isolate and lock out / tag out (LOTO) the supply to the starter before working on any power circuit wiring. Verify all three phases and the neutral (where present) are at zero voltage with a calibrated CAT III/IV voltage tester before touching any terminal.
The emergency stop button must be wired to produce a Category 0 (power removal) or Category 1 stop in compliance with IEC 60947-5-5 and the machinery safety standard ISO 13849. Never wire the E-stop into a circuit that allows the motor to restart automatically when the E-stop is released.
The overload relay protects the motor windings, not the supply wiring. The upstream fuses or motor circuit breaker protect the wiring. Both must be correctly sized and coordinated. Never remove or bypass the overload relay to keep a faulty motor running — this will cause winding failure and risk of fire.
DOL starting of large motors (above approximately 11 kW) without utility approval can cause voltage dips that affect neighbouring customers and may violate network connection agreements. Check with the local distribution network operator before installing large DOL starters.

Tools needed

Calibrated CAT III/IV voltage tester and multimeter
Clamp meter (for measuring motor phase currents during commissioning)
Insulated screwdrivers rated to IEC 60900
Torque screwdriver (terminal tightening to manufacturer specifications)
Wire ferrule crimping tool (for control circuit wiring)
Phase rotation meter (to verify motor rotation direction before starting a connected load)
Continuity tester (for checking control circuit wiring before energising)

Common mistakes

Wiring the overload relay NC contact in the power circuit rather than the control circuit, so overload trips disconnect only one phase rather than the whole motor.
Omitting the contactor auxiliary NO contact in parallel with the start button — the circuit fails to latch, and the motor stops the moment the start button is released.
Setting the overload relay to a value higher than the motor nameplate FLC, removing overload protection and risking motor winding failure.
Connecting the motor terminals (U, V, W) to the supply and the supply (L1, L2, L3) to the motor — this results in the motor running in reverse, potentially damaging the driven equipment.
Using a contactor rated for AC-1 (resistive load) duty rather than AC-3 (motor squirrel cage) duty — AC-1 contactors are not designed for the high making and breaking currents of motor starting and will arc-weld their contacts closed.

Troubleshooting

Contactor energises but drops out immediately: Cause: The auxiliary latching contact is not wired or is faulty; the overload relay is already in a tripped state; the stop button is stuck closed, preventing latching Fix: Check continuity through the overload relay NC contact — reset the relay if tripped. Verify the auxiliary NO contact is wired in parallel with the start button. Check the stop button contact opens and closes correctly with a multimeter.
Motor runs but overload relay trips shortly after starting: Cause: Overload relay set too low; motor running with a mechanical overload; phase loss causing the remaining two phases to carry excess current Fix: Check all three phase currents with a clamp meter during running. Verify the relay setting matches the nameplate FLC. Check for single-phasing (one phase voltage missing or very low) at the motor terminals.
Motor runs in the wrong direction: Cause: Phase sequence of the supply to the motor is L1, L3, L2 instead of L1, L2, L3 (or equivalent transposition) Fix: Isolate and lock out the supply. Swap any two of the three phase conductors at the contactor output or the motor terminals (not the supply side). Reapply power and check direction.
Contactor coil overheats: Cause: Control supply voltage too high for coil rating, or contactor is not fully closing due to a mechanical fault and is humming in the linear region Fix: Measure control circuit voltage with a multimeter and compare to the contactor coil voltage rating. Listen for the characteristic hum of a partially energised AC contactor. Check the contactor contacts for wear, contamination, or corrosion causing the main contacts not to close fully.

Frequently asked questions

What does DOL stand for and what does a DOL starter do?

DOL stands for Direct On Line. A DOL starter connects a three-phase AC induction motor directly to the supply voltage through a main contactor. It provides start, stop, and overload protection. The motor starts at full voltage, which produces a high starting inrush current — typically five to eight times the full-load current.

What is the maximum motor size suitable for DOL starting?

The maximum motor size for DOL starting depends on the supply network's capacity to tolerate the inrush current without unacceptable voltage dips. As a general guideline, DOL is commonly accepted for motors up to 5.5–11 kW on a three-phase supply, but local utility regulations and the available fault level determine the exact limit for any specific installation.

What is the difference between the power circuit and the control circuit in a DOL starter?

The power circuit carries full motor current: from the supply through the main circuit breaker or fuses, through the main contactor contacts, through the thermal overload relay, to the motor terminals. The control circuit carries only the small current needed to energise the contactor coil, controlled by stop/start push buttons and the overload relay's NC contact.

Why is the stop button wired as normally closed and the start button as normally open?

The stop button is normally closed (NC) so that if the button or its wiring fails open-circuit, the motor stops — a fail-safe condition. The start button is normally open (NO) so the motor cannot start from a wiring or contact fault. This is a fundamental safety principle in motor control: loss of control signal must result in the safe state (motor stopped).

What does an overload relay do in a DOL starter?

The thermal overload relay monitors current through the motor's phase conductors. If the motor draws excessive current for a sustained period — due to mechanical overload, single-phasing, or voltage imbalance — the relay trips its normally closed control contact. This de-energises the contactor coil, opening the main contacts and disconnecting the motor, preventing overheating damage.

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