Direct-On-Line (DOL) Starter Wiring Diagram

Direct-On-Line (DOL) Starter Wiring Diagram — interactive diagram. Open it in the editor to customise components and wiring.

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A direct-on-line (DOL) starter diagram shows how a contactor, overload relay, start/stop pushbuttons, and associated control wiring connect to start and protect a three-phase induction motor by connecting it directly to the full supply voltage.

A direct-on-line (DOL) starter is the simplest and most widely used method for starting a three-phase squirrel-cage induction motor. It connects the motor stator windings directly across the full line voltage in one step — there is no voltage reduction, auto-transformer, or soft-start ramp. The benefit is simplicity and low cost; the drawbacks are a high starting current (typically 5–8× full-load current) and significant starting torque jerk, which makes DOL unsuitable for motors above the utility-permitted limit for direct starting (commonly 5–7.5 kW on most distribution networks, though the limit varies by grid operator).

The power circuit consists of: a main isolator or fuse switch, three-pole main contactor (KM1), and a thermal or electronic overload relay (FR/OL) connected in series between the incoming supply and the motor terminals. The overload relay monitors the motor current on all three phases and trips the control circuit if any phase exceeds the set threshold for a defined time period.

The control circuit operates at line voltage (common) or via a control transformer at 24 V or 110 V (safer for personnel). It comprises: a normally-closed (NC) stop pushbutton (SB1), normally-open (NO) start pushbutton (SB2), the contactor's own NC auxiliary contact for overload trip indication, and a holding (latch) contact in parallel with SB2 so the contactor remains energised after the start button is released.

Operation sequence: pressing SB2 energises the coil of KM1 through the stop button and overload contact. KM1 closes its main power contacts (connecting the motor to the supply) and simultaneously closes its auxiliary NO holding contact, which bypasses SB2 and keeps KM1 energised. Pressing SB1 (stop) or an overload trip breaks the coil circuit, KM1 drops out, disconnecting the motor.

Indicator lamps (green = running, red = stopped/fault) and remote control contacts can be added to the control circuit without modifying the power circuit.

How to wire direct on line starter diagram

Isolate and verify the supply is dead Open and lock out the upstream isolator or circuit breaker before any wiring work. Verify absence of voltage on all three phases and the neutral (if applicable) with a calibrated voltage tester.
Wire the power circuit Connect the three-phase supply to the input terminals (L1, L2, L3) of the contactor KM1. Connect the output terminals (T1, T2, T3) through the overload relay to the motor terminals (U, V, W). Observe phase sequence and motor rotation direction.
Set the overload relay current Adjust the overload relay dial to the motor's rated full-load current (FLC) as stated on the motor nameplate. Most thermal overload relays have a setting range; select the range that covers the FLC and set the dial to match.
Wire the control circuit From one line (or the control transformer secondary), wire in series: NC stop pushbutton (SB1) → NC overload relay auxiliary contact → contactor coil (A1–A2) → neutral/return. Wire the NO start pushbutton (SB2) in parallel with the NO holding contact of KM1, bridging between the junction after SB1 and the coil.
Connect indicator lamps and auxiliary outputs Wire a green running lamp across the KM1 NO auxiliary contact (or across the coil terminals). Wire a red lamp to indicate stop/fault state. Connect any remote monitoring contacts to the appropriate auxiliary terminals.
Perform insulation resistance check Before energising, use a 500 V insulation resistance tester (megohmmeter) to verify insulation resistance between each conductor and earth is >1 MΩ. This confirms no wiring faults before first start.
Commission and test Restore the supply, press Start, and verify the motor runs in the correct direction. Check running current on all three phases with a clamp meter — it should be at or below the nameplate FLC under normal load. Test Stop and overload trip functions.

Specifications

Applicable motor type	Three-phase squirrel-cage induction motor
Contactor utilisation category	AC-3 (IEC 60947-4-1)
Typical starting current multiple	5–8 × full-load current (FLC)
Starting torque (DOL)	Full motor locked-rotor torque (approximately 1.5–2.5 × rated torque)
Overload relay setting	Motor nameplate FLC (±5%)
Control circuit voltage (common options)	Line voltage (230 V / 400 V) or 110 V / 24 V via control transformer
Maximum motor size for DOL (typical UK/EU grid limit)	≤5.5 kW (varies by distribution network operator)
Applicable standards	IEC 60947-4-1, IEC 60364, BS 7671, AS/NZS 3000, SANS 10142-1

Safety warnings

Always isolate and lock out (lock-out/tag-out, LOTO) the incoming supply before wiring or modifying the starter. Contactors and overload relays may store energy in coil inductance; treat all terminals as live until proven dead.
The overload relay protects against overload only. It does not provide short-circuit protection. Always install correctly rated fuses or a motor circuit breaker upstream for short-circuit fault protection.
Never wire the stop pushbutton as normally-open. A normally-open stop button in series with the coil circuit means that a loose connection or broken wire prevents stopping the motor — a serious safety hazard.
Verify the contactor AC utilisation category is AC-3 (squirrel-cage motor starting). Using an AC-1 (resistive load) contactor for motor starting will cause premature contact wear and welding due to the high inrush current.
All installation, wiring, and commissioning must comply with the applicable national wiring standard (e.g., IEC 60364, BS 7671, AS/NZS 3000, SANS 10142) and must be performed or supervised by a qualified and authorised electrician.

Tools needed

Calibrated voltage tester or multimeter (CAT III minimum)
Insulation resistance tester (megohmmeter, 500 V DC)
Clamp meter (for current measurement during commissioning)
Screwdrivers (flat and Pozidriv/Phillips, multiple sizes)
Wire strippers and ferrule crimping tool
Lock-out/tag-out (LOTO) padlock and hasp
Cable ties and cable markers

Common mistakes

Omitting the holding contact in parallel with the start button, causing the motor to run only while the start button is held.
Setting the overload relay to the supply voltage rating rather than the motor nameplate current, leaving the motor with no effective overload protection.
Wiring the stop button as normally-open instead of normally-closed, which is both a safety fault and means a wire break cannot stop the machine.
Connecting the motor terminals in reverse phase sequence and not checking rotation direction before coupling to driven machinery, which can damage pumps, fans, or conveyors.
Using motor cable that is sized only for running current without accounting for the voltage-drop impact of the high starting current, causing prolonged low-voltage start conditions.

Troubleshooting

Motor does not start when Start button is pressed: Cause: Control circuit open: blown control fuse, tripped overload relay, loose terminal, or defective pushbutton Fix: With supply on, use a multimeter to trace voltage through the control circuit in sequence: verify voltage at the coil input terminal. If 0 V at the coil but supply present before SB1, check each series element (SB1, overload auxiliary, SB2) in turn. Reset the overload relay if tripped.
Motor starts but stops immediately when Start button is released: Cause: Holding contact not wired, or holding contact defective/not closing Fix: Verify the NO auxiliary contact of KM1 is wired in parallel with SB2. With the contactor pulled in, measure continuity across the holding contact terminals — it should be closed. Replace the auxiliary contact block if defective.
Overload relay trips repeatedly during normal operation: Cause: Overload relay set below motor FLC, single-phasing (one phase open), or motor genuinely overloaded Fix: Measure current on all three phases with a clamp meter while running at normal load. Compare with nameplate FLC. If all three phases are balanced and near FLC, adjust relay setting upward within its rated range. Investigate the driven load for mechanical binding.
Contactor chatters or hums excessively: Cause: Low control voltage at coil, worn shading ring on AC coil, or loose coil connection Fix: Measure voltage at the coil terminals (A1–A2) while contactor is pulled in. It should be within ±10% of rated coil voltage. Inspect the shading ring on the magnet face — a broken shading ring causes 100/120 Hz chatter on AC coils. Tighten all coil wiring terminals.
Motor runs in wrong direction: Cause: Phase sequence at motor terminals is reversed relative to driven machinery requirement Fix: Isolate the supply completely. Swap any two of the three motor supply cables (T1/T2, T2/T3, or T1/T3) at the contactor output or at the motor terminal box. Recommission and verify rotation before coupling to load.

Frequently asked questions

Why does a DOL starter use a holding (seal-in) contact?

The holding contact is a normally-open auxiliary contact on the contactor wired in parallel with the start pushbutton. Once the contactor pulls in, this contact closes and maintains the coil circuit without the operator holding the start button. Removing it would require the start button to be held continuously throughout operation.

What is the purpose of the overload relay in a DOL starter?

The overload relay monitors motor current and trips the control circuit if the motor is overloaded for a sustained period, protecting the motor windings from overheating. It is not a short-circuit protection device — short circuits are interrupted by the upstream fuses or circuit breaker.

What starting current does a DOL starter draw?

At the moment of connection, a squirrel-cage induction motor draws its locked-rotor current, typically 5–8 times its full-load current. A 5.5 kW motor with a full-load current of 11 A may draw 55–88 A at startup. This inrush lasts 2–10 seconds depending on load inertia.

When should a star-delta or soft starter be used instead of DOL?

When the motor is above the utility's permitted direct-start rating, when mechanical loads cannot tolerate high torque jerk (conveyors, pumps with rigid couplings), or when the starting current causes unacceptable voltage dips on the supply. Star-delta reduces starting current to roughly one-third of DOL values.

Does the stop button in a DOL starter need to be normally-closed?

Yes, the stop button must be wired as normally-closed (NC) in series with the control circuit. This is a fail-safe design: if the stop button cable is cut or a connection breaks, the circuit opens and the motor stops. Using a normally-open stop button is a serious safety error — a broken wire would prevent stopping.