DOL Starter Control Circuit Diagram
This is a free printable dol starter control diagram: download the diagram as SVG or open it and print to paper or PDF.
A DOL starter control circuit connects a Start push button, Stop push button, contactor hold-in auxiliary contact, and overload relay NC contact in a series control loop — pressing Start latches the contactor coil in, and pressing Stop or a fault trips it out.
Direct-on-line (DOL) starting is the simplest method of starting a three-phase or single-phase electric motor. The motor is connected directly to the full supply voltage with no series impedance, autotransformer, or reduced-voltage device. The full line voltage appears at the motor terminals the instant the main contactor closes, producing maximum starting torque and maximum starting current (typically 5–8× full-load current).
The DOL starter consists of two separate circuits:
1. The power circuit carries the full motor current from the supply through a set of fuses or an MCCB, through the main contactor's three power poles, through a thermal overload relay (OLR), and then to the motor terminals. This is typically high-current wiring sized for the motor's full-load amps.
2. The control circuit operates at a lower voltage (often the same as the power supply, e.g. 230 V, but sometimes stepped down through a control transformer to 110 V or 24 V for safety). The control loop consists of: - Stop push button: normally closed (NC) — momentary; in series with the coil - Start push button: normally open (NO) — momentary; in parallel with the hold-in contact - Contactor auxiliary hold-in contact (NO): closes when the contactor energises, creating a latch that keeps the coil energised after the Start button is released - Overload relay NC contact: in series with the coil; opens on overload, de-energising the coil and dropping out the contactor
When the Start button is pressed, current flows through the coil (stop NC → overload NC → start NO closed → coil). As the coil energises, the main contacts close and the auxiliary hold-in contact closes simultaneously. The operator releases Start, but the auxiliary contact now maintains the coil circuit. Pressing Stop opens the NC contact, breaking the coil circuit, dropping out the contactor and opening the power circuit.
DOL starting is generally used for motors up to 7.5 kW on systems where the supply authority permits the starting current surge. Above this size, reduced-voltage starters (star-delta, autotransformer, or soft starter) are typically required.
How to wire dol starter control diagram
- Isolate the supply and apply lockout-tagout Switch off and lock out the main isolation at the distribution board or MCC before opening the starter enclosure. Verify all phases are dead using a calibrated voltage tester at the starter busbars.
- Wire the power circuit Connect three-phase supply (L1/L2/L3) to the input of the fuses or MCCB, then from the fuse outputs to the top (line side) of the main contactor power poles. Connect the contactor bottom (load side) to the top of the overload relay. Connect the overload relay load side to the motor terminal block (U/V/W or T1/T2/T3).
- Identify the control circuit supply Decide whether the control circuit will run at line voltage (e.g. 230 V from L1 and N, or between two phases) or at a stepped-down voltage via a control transformer. If using a control transformer, install it between the main fuses and the control circuit, fuse the secondary, and use the secondary voltage for all control wiring.
- Wire the Stop push button (NC) in series Connect one end of the control circuit supply to one terminal of the Stop push button NC contact. The Stop button is the first series element — any open here immediately drops out the coil. Use a momentary NC push button (not a latching switch).
- Wire the Start push button (NO) in parallel with the hold-in contact From the Stop button output, connect to one terminal of the Start push button NO contact. Also connect to one terminal of the contactor auxiliary NO hold-in contact. Connect the other terminals of both the Start button and the hold-in contact together — they are now in parallel with each other.
- Wire the overload relay NC auxiliary contact in series From the parallel Start/hold-in junction, connect to the overload relay NC auxiliary contact, then from the OLR NC contact to the contactor coil terminal. Complete the coil circuit by connecting the other coil terminal back to the neutral or second phase of the control supply.
- Label, test, and commission Label all conductors per your panel schedule. With motor disconnected, energise the control circuit only and verify: Start latches (hold-in works), Stop drops out the contactor, tripping the OLR test button drops out the contactor. Then connect the motor, set the overload relay to motor FLA, and carry out a loaded test run monitoring current on all three phases.
Specifications
| Power circuit voltage (typical IEC) | 380–415 V AC three-phase, 50 Hz; or 220–240 V AC single-phase |
|---|---|
| Power circuit voltage (typical NEMA) | 460 V three-phase, 60 Hz; or 120/208 V single-phase |
| Control circuit voltage (common options) | 230 V AC (from phase to neutral) or 110 V AC (via control transformer) or 24 V AC/DC |
| Starting current (DOL) | 5–8× motor full-load current (FLA) — varies by motor design |
| Typical application size (IEC guidance) | Motors up to approximately 7.5 kW — above this, check supply authority requirements |
| Overload relay setting | Set to motor nameplate full-load current (FLA) — do not set higher |
| Hold-in contact type | Normally open (NO) auxiliary contact on main contactor, wired in parallel with Start button |
| Applicable standards | IEC 60947-4-1 (low-voltage switchgear and controlgear), local wiring regulations per jurisdiction |
Safety warnings
- Isolate and lock out all three phases of the main supply before working inside the starter enclosure. Verify that all phases are dead at the starter input terminals using a calibrated three-phase voltage tester — the power circuit carries lethal voltages and the control circuit may be derived from a different source that requires separate isolation.
- The thermal overload relay NC contact must be wired in series in the control coil circuit. If the overload contact is omitted or bypassed, the starter has no automatic motor protection — the motor will run until it burns out on an overload condition.
- The hold-in auxiliary contact creates a latched (self-sealing) circuit. Understand that pressing Stop breaks this latch — if the Stop button wiring fails open-circuit, the motor can only be stopped by tripping the overload or isolating the main supply. For critical applications, add a second emergency stop.
- All DOL starter installations must be designed, installed, and commissioned by a qualified electrician in accordance with the applicable standard for your jurisdiction (IEC 60947, NEC/NFPA 70, BS 7671, SANS 10142-1). This diagram is for reference and educational purposes only.
- Set the overload relay to the motor nameplate full-load current before commissioning. An overload set too high will not protect the motor from thermal damage; set too low it will trip unnecessarily on normal starting current.
Tools needed
- Calibrated three-phase voltage tester
- Lockout-tagout kit (three-phase rated)
- Digital multimeter with AC voltage and continuity functions
- Clamp meter (for verifying motor FLA during commissioning)
- Insulated screwdrivers (flathead and Phillips)
- Crimping tool and ferrule kit for control wiring
- Wire strippers
- Cable ferrule and label printer (or marker)
Common mistakes
- Wiring the Stop push button as normally open instead of normally closed — the circuit will never energise and the button cannot stop the motor safely.
- Omitting the hold-in auxiliary contact in parallel with the Start button — the contactor drops out immediately when the Start button is released, so the motor only runs while the button is held.
- Not including the overload relay NC contact in the control circuit series loop — the overload relay will detect the fault but cannot disconnect the motor.
- Setting the overload relay above the motor nameplate FLA to prevent nuisance trips — this defeats overload protection and risks motor winding damage.
- Failing to fuse the control circuit separately when it is derived from the power circuit — a fault in the control wiring can then affect the main protection.
- Using a latching (maintained) push button for the Start function instead of a momentary NO button — the Start button must auto-release, with the hold-in contact providing the latch.
Troubleshooting
- Contactor does not pick up when Start is pressed
- Cause: No control circuit voltage, open circuit in control loop (Stop button open, OLR contact open, wiring break), or failed contactor coil Fix: Check control circuit supply voltage at the first terminal. With a multimeter, probe through the series control loop: Stop NC contact closed? OLR NC contact closed (reset the OLR if tripped)? Confirm voltage across the coil terminals when Start is pressed. If voltage is present but coil does not pull in, measure coil resistance — an open coil requires contactor replacement.
- Contactor picks up but drops out as soon as Start button is released
- Cause: Hold-in auxiliary NO contact not wired in parallel with Start button, or auxiliary contact has failed open Fix: Verify the NO auxiliary contact wiring — it must connect the same two terminals as the Start button (in parallel). With the contactor energised by holding Start, check for continuity across the auxiliary contact. If no continuity while energised, the auxiliary contact is faulty — replace the auxiliary contact block or contactor.
- Overload relay trips shortly after motor starts
- Cause: Overload set too low, motor actually overloaded (driven load too heavy or mechanical fault), single phasing (one phase missing) Fix: Check the overload relay current setting against the motor nameplate FLA. Use a clamp meter to measure current on all three phases during starting and running. If one phase reads zero or very low, check the corresponding fuse and contactor pole. If all three phases are correct and current is above FLA, investigate the driven load.
- Motor runs but cannot be stopped with the Stop button
- Cause: Stop button NC contact has welded closed (internal fault), Stop button wired as NO, or Stop button bypassed in the control circuit Fix: Isolate the supply immediately using the main isolator or by tripping the MCCB. Inspect Stop button wiring — confirm it is wired as NC and in series with the coil. Test the button contact with a multimeter — it should be closed at rest and open when pressed. Replace a faulty button.
- Contactor chatters (buzzes repeatedly) during operation
- Cause: Low control circuit voltage, shorted shading ring on AC contactor, or mechanical obstruction preventing full contact closure Fix: Measure control voltage at the coil terminals — must be within ±10% of coil rating. Inspect the contactor face for foreign matter or worn contact tips preventing full closure. A broken shading ring on an AC contactor causes persistent chattering and requires contactor replacement.
Frequently asked questions
Why does the DOL starter not hold in after releasing the Start button?
The auxiliary hold-in contact is either not wired in parallel with the Start button, has failed open, or is not rated for the control circuit voltage. Without the hold-in contact latching, the coil circuit opens as soon as the Start button is released. Check the auxiliary contact wiring and verify the contact closes when the contactor energises.
What is the purpose of the overload relay NC contact in the control circuit?
The overload relay monitors motor current. If current exceeds the setpoint for long enough to indicate an overload condition, its NC auxiliary contact opens, breaking the control circuit and dropping out the contactor. This removes power from the motor before thermal damage occurs. Without this contact in the control loop, the overload relay cannot stop the motor.
Can a DOL starter be used with a single-phase motor?
Yes. A single-phase DOL starter uses a single-pole or double-pole contactor and a single-phase overload relay. The control circuit principle — Stop NC, Start NO with hold-in auxiliary — is the same. The power circuit routes single-phase supply through the contactor to the motor.
What happens if I wire the Stop button as normally open instead of normally closed?
A normally open Stop button means the control circuit is open by default — the contactor will never energise, regardless of pressing Start. Additionally, a normally open Stop means the motor cannot be stopped by pushing the button (it would need to be closed to stop, which is the opposite of a safety stop). Always wire the Stop button as NC.
How do I add a second start/stop station to a DOL starter?
Wire the additional Stop button's NC contact in series with the existing Stop button in the control loop — so either Stop can de-energise the coil. Wire the additional Start button's NO contact in parallel with the existing Start button (and the hold-in contact) — so either Start can energise the coil. Multiple Stop buttons are always in series; multiple Start buttons are always in parallel.
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