DOL Starter Control Circuit Diagram – Direct On-Line Motor Starting
This is a free printable dol control diagram: download the diagram as SVG or open it and print to paper or PDF.
Understand and wire a direct on-line (DOL) motor starter control circuit, including contactor, overload relay, start/stop buttons, and self-holding contact.
A direct on-line (DOL) starter connects a three-phase induction motor directly to the full supply voltage at the moment of starting, producing the simplest and lowest-cost motor starting method. Because the motor receives full voltage immediately, the starting current is typically five to eight times the full-load current — a characteristic that limits DOL use to smaller motors (commonly up to 7.5 kW on a stiff supply, though local utility limits vary) or applications where the inrush is acceptable.
The DOL control circuit consists of two distinct parts: the power circuit and the control circuit. The power circuit carries the full motor current and includes the main contactor contacts (K1), the thermal overload relay (OL), and the motor terminals. The control circuit operates at a lower voltage (often 230 V or 110 V AC, or 24 V DC on modern installations) and includes the START pushbutton (normally open), STOP pushbutton (normally closed), overload relay auxiliary contact (normally closed), and the contactor coil (K1).
The self-holding (or sealing) contact is a key feature: an auxiliary normally-open contact of K1 is wired in parallel with the START button. When the START button is pressed, K1 energises; its auxiliary contact closes, bypassing the START button so the contactor stays energised when the button is released. Pressing STOP — or a trip of the overload relay — breaks the control circuit and de-energises K1, disconnecting the motor.
The thermal overload relay provides protection against sustained overcurrent by bending a bimetallic strip that opens the auxiliary contact in the control circuit. Its trip current is set to the motor's full-load current rating.
This diagram is a generic educational reference. All motor control installations must comply with IEC 60947 (low-voltage switchgear), NEC/NFPA 70, or the applicable national standard, and must be designed and commissioned by a competent electrical engineer or licensed electrician.
How to wire dol control diagram
- Isolate and verify the supply dead Switch off and lock out the upstream isolator or circuit breaker feeding the control panel. Verify all three phase conductors and the neutral (if present) are dead using a calibrated voltage tester before working inside the panel.
- Install and wire the main contactor Mount the contactor (K1) on the DIN rail. Connect the three supply phases to the main contactor's input terminals (L1, L2, L3). These are the power circuit supply terminals.
- Wire the overload relay in series with the motor Connect the main contactor output terminals (T1, T2, T3) to the overload relay input terminals. Connect the overload relay output terminals to the motor terminal box (U, V, W). Set the overload relay trip current to the motor nameplate FLC.
- Wire the control circuit From the control supply (L1 or a step-down transformer secondary): connect one terminal to the STOP pushbutton (NC). Wire the STOP output to the overload relay NC auxiliary contact. Wire the auxiliary contact output to the START pushbutton (NO). Wire the START pushbutton output to the K1 contactor coil. Return the other coil terminal to the neutral or control supply return. Connect the K1 auxiliary NO contact in parallel with the START pushbutton for self-holding.
- Check wiring before energising Inspect all connections for tightness. Verify the overload relay setting. Confirm that auxiliary contacts are correctly identified as NC or NO. Confirm no exposed conductors can contact the enclosure or adjacent terminals.
- Restore power and test the control sequence Restore supply. Press START — the contactor should close and the motor should run. Release the START button — the motor should continue running (self-hold confirmed). Press STOP — the contactor should open and the motor should stop. Trip the overload relay manually if it has a test button, and confirm the motor stops.
Specifications
| Starting method | Direct on-line (full voltage) |
|---|---|
| Typical application motor size | Up to approximately 7.5 kW – 15 kW (supply authority dependent) |
| Starting current (typical) | 5× to 8× motor full-load current |
| Control circuit voltage (common) | 230 V AC, 110 V AC, or 24 V DC |
| Applicable standard | IEC 60947-4-1, NEC/NFPA 70, BS EN 60947 |
| Overload relay type | Thermal bimetallic, adjustable to motor FLC |
| Contactor contact life | Per manufacturer's mechanical and electrical endurance rating |
| Number of power poles | 3 (three-phase) or 2 (single-phase) |
Safety warnings
- Isolate and lock out all supply conductors before working inside the panel. Three-phase 400 V AC is lethal — test all phases with a calibrated voltage tester after isolation.
- The thermal overload relay protects against sustained overcurrent but does not provide short-circuit protection. A separate fuse or circuit breaker upstream of the starter is mandatory.
- Fail-safe wiring requires all protective contacts (STOP, overload) to be normally closed in the control circuit. If these are wired normally open, loss of control continuity will not stop the motor — a dangerous configuration.
- All motor control equipment must be selected and installed in accordance with IEC 60947, NEC/NFPA 70, BS EN 60947, or the applicable national standard. Engage a licensed electrician or competent electrical engineer.
- Never reset a tripped overload relay immediately and repeatedly. Investigate the cause of the overload before restarting to prevent motor winding damage.
Tools needed
- Calibrated voltage tester or multimeter
- Lockout/tagout kit
- Insulated screwdrivers (flathead and Phillips)
- Wire strippers and crimping tool
- Ferrule crimper and insulated ferrules
- DIN rail cutters
- Continuity tester or multimeter
Common mistakes
- Wiring the self-holding auxiliary contact in series with the START button instead of in parallel — the motor will only run while the START button is held.
- Setting the overload relay to a higher current than the motor FLC to avoid nuisance trips — this removes overload protection and risks burning out the motor windings.
- Using a normally-open overload relay auxiliary contact in the control circuit instead of normally-closed — the contactor will not energise at all.
- Omitting a fuse or MCB on the control circuit — a fault in control wiring can cause a persistent short that damages the contactor coil.
- Connecting the control circuit phase the same as L1 of the motor without checking the transformer or control supply arrangement — this can create phase-to-phase faults if the motor neutral is not correctly referenced.
- Failing to test the STOP button and overload trip function before leaving the installation — commissioning must include verifying all protective functions actually stop the motor.
Troubleshooting
- Pressing START does not energise the contactor
- Cause: Control circuit supply absent, STOP button open-circuit, overload relay not reset, or contactor coil open Fix: Check control supply voltage. Verify STOP button is not stuck open. Check if overload relay has tripped and needs manual reset. Measure voltage across contactor coil with START pressed; if voltage present but contactor does not pull in, the coil is faulty.
- Motor starts but stops when START button is released
- Cause: Self-holding auxiliary contact is not wired in parallel with the START button, or auxiliary contact is faulty/open Fix: Verify that the K1 auxiliary NO contact is connected across the two terminals of the START pushbutton. Check that the auxiliary contact closes when the contactor energises (measure continuity). Replace auxiliary contact block if faulty.
- Overload relay trips repeatedly on starting
- Cause: Trip current set too low, motor or driven machine is mechanically overloaded, or motor windings have a fault Fix: Confirm the trip dial is set to the motor nameplate FLC, not a lower value. Measure actual motor current with a clamp meter during starting. Check driven machine for mechanical binding. If current exceeds rated FLC during running, investigate the mechanical load.
Frequently asked questions
What is the difference between the power circuit and the control circuit in a DOL starter?
The power circuit carries the full motor operating current through the main contactor contacts and overload relay to the motor. The control circuit carries only the small coil current and uses pushbuttons, auxiliary contacts, and the overload relay's trip contact to energise or de-energise the main contactor coil. Keeping them separate makes troubleshooting straightforward.
Why is a self-holding (sealing) contact used instead of a maintained-contact switch?
A momentary START pushbutton with a self-holding auxiliary contact allows the motor to run continuously after a brief press, and ensures the motor stops safely if the control supply fails (fail-safe behaviour). A maintained switch would require the operator to manually switch it off, and the motor would restart automatically on power restoration — a significant safety hazard.
How do I set the overload relay trip current?
Set the overload relay's adjustable dial to the motor's nameplate full-load current (FLC) in amperes. Do not set it to the starting current or a higher value to prevent nuisance tripping — overload relays are designed to tolerate short-duration starting inrush and will trip only on sustained overcurrent that indicates a real overload condition.
What happens when the overload relay trips?
The bimetallic trip mechanism opens the normally-closed auxiliary contact wired in series with the contactor coil in the control circuit. The contactor de-energises, opening the main contacts and disconnecting the motor from the supply. The overload relay must be manually reset (after allowing the bimetal to cool) before the motor can be restarted.
Can a DOL starter be used for large motors?
DOL starting is generally limited to smaller motors — typically under 7.5 kW to 15 kW depending on the supply authority's regulations — because the starting current surge can cause voltage dips that affect other equipment on the same supply. Larger motors use star-delta, autotransformer, or soft-starter methods to reduce starting current.
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