DOL Diagram
This is a free printable dol diagram: download the diagram as SVG or open it and print to paper or PDF.
A direct on-line (DOL) starter connects a three-phase induction motor directly to full supply voltage, using a main contactor and overload relay to control and protect the motor from a single push-button station.
Direct on-line (DOL) starting is the simplest method of starting a three-phase induction motor. The motor is connected directly across the full supply voltage — typically 400 V three-phase — via a main contactor. The result is the maximum available starting torque, but also the maximum starting current: typically 5–8 times the motor's full-load current (FLC). For small motors, this inrush is acceptable. Above approximately 4–5 kW (the limit varies by electricity supplier and jurisdiction), the voltage dip caused by the starting inrush may disturb other equipment on the same supply, making reduced-voltage starting methods (star-delta, soft starter, VFD) necessary.
A DOL starter circuit has two interconnected sections:
1. Power circuit (main circuit): The three-phase supply (L1, L2, L3) enters through the main isolator, then passes through a set of three normally-open main contactor contacts (one per phase), and continues to the thermal overload relay's current-sensing elements before reaching the motor terminals (U, V, W). All three phases must be connected and correctly phased to ensure the motor rotates in the intended direction. Swapping any two phases at the motor terminals reverses the direction of rotation.
2. Control circuit (auxiliary circuit): Operates at reduced voltage in most industrial installations (110 V AC or 24 V AC/DC) for personnel safety. The control transformer (if used) reduces the main supply voltage for the control circuit. The start-stop station consists of a normally-closed (NC) stop push-button in series with a normally-open (NO) start push-button. A normally-open auxiliary contact on the main contactor is wired in parallel with the start push-button — this is the latching (sealing-in or hold-in) contact, which maintains the contactor energised after the start button is released. The thermal overload relay's NC contact is in series with the control circuit: when the overload trips, it opens this contact, de-energising the contactor and stopping the motor.
Phase failure protection is a critical consideration. A standard DOL starter with a thermal overload does not reliably protect against single phasing (loss of one supply phase) — a motor running on two phases overheats rapidly. Electronic motor protection relays or phase-failure relays provide this protection.
Local and remote control can be added by placing additional NO start contacts in parallel with the main start button, and additional NC stop contacts in series with the stop button, at remote locations.
How to wire dol diagram
- Calculate and select the correctly rated components Identify the motor's full-load current (FLC) from its nameplate. Select a main contactor with an AC3 utilisation category current rating equal to or greater than the motor's FLC. Select a thermal overload relay with an adjustment range that includes the motor's FLC — set the dial to the motor's nameplate current after installation. Select the main isolator rated above the motor's starting current (approximately 8× FLC).
- Wire the power circuit (main circuit) Connect the three-phase supply (L1, L2, L3) through the main isolator to the main contactor's input terminals. Connect the contactor's output terminals through the overload relay's current-sensing elements to the motor terminals U, V, W. Connect the motor's earth terminal to the PE earth bar. Torque all power circuit terminals to the manufacturer's specified torque — under-torqued connections arc and overheat under starting current.
- Wire the control transformer and control supply (if applicable) If a separate control voltage is used (110 V AC or 24 V), wire the control transformer primary between one phase and neutral (or between two phases) of the main supply, via a separate MCB. Connect the secondary to the control circuit live and neutral rails. Add a fuse on the secondary circuit for protection.
- Wire the stop button (NC) in series on the control circuit live rail The stop push-button uses its normally closed contact in series with the control circuit live conductor — pressing stop interrupts the circuit, de-energising the contactor. Multiple stop buttons (remote stop stations) are wired in series with the main stop button.
- Wire the start button (NO) in series after the stop button The start push-button's normally open contact is wired in series in the control circuit after the stop button. Wire the auxiliary NO contact of the main contactor in parallel (bridging) across the start button terminals — this is the latching contact.
- Wire the overload relay NC contact in series with the contactor coil The overload relay's normally closed auxiliary contact is placed in series in the control circuit between the start/stop arrangement and the contactor coil (A1 terminal). When the relay trips, this contact opens, breaking the coil circuit and dropping the contactor out, stopping the motor.
- Test the control circuit before energising the power circuit Apply control voltage only (main isolator open). Press the start button — the contactor should energise and latch. Press stop — the contactor should de-energise. Manually trip the overload relay — the contactor should drop out. Reset the overload and verify normal control before closing the main isolator and starting the motor.
Specifications
| Typical starting current (DOL) | 5–8× motor full-load current (FLC) |
|---|---|
| Contactor utilisation category (motor starting) | AC3 (IEC 60947-4-1) |
| Overload relay trip class | Class 10 (trips within 10 s at 7.2× FLC) — standard for most induction motors |
| Maximum recommended motor size for DOL (three-phase) | Typically up to 7.5–15 kW (consult local utility supply rules) |
| Thermal overload relay set point | Motor nameplate FLC (typically set at 100–110% of FLC) |
| Control circuit voltage (European practice) | 110 V AC (reduced-voltage control); or 24 V DC in modern panels |
| Applicable standards | IEC 60947-4-1 (low-voltage motor starters), IEC 60364 (electrical installation), BS 7671, NEC/NFPA 70 |
Safety warnings
- All motor control panel work must be carried out by a competent and licensed electrician in compliance with IEC 60364, BS 7671 (UK), NEC/NFPA 70 (USA), or AS/NZS 3000 (Australia/NZ). Three-phase industrial supplies carry lethal voltages and must be treated with the highest level of caution.
- Always lock out and tag out (LOTO) the main isolator before working on the starter circuit or motor. Verify dead using a CAT III/IV rated voltage tester on all three phases, between each pair of phases and between each phase and earth, before touching any conductors.
- Capacitors fitted to some motor control circuits can retain dangerous charge after the supply is isolated. Verify that capacitors are discharged (using a discharge resistor or appropriate tool) before working on circuits containing them.
- The motor's mechanical load must be considered when selecting the contactor — pumps, fans, and compressors present different starting profiles. A contactor inadequately rated for motor starting duty wears prematurely and may fail closed (welded contacts), causing the motor to run continuously.
- Ensure the thermal overload relay is correctly set to the motor's nameplate full-load current. Under-setting causes nuisance trips; over-setting allows the motor to overheat without protection — either condition shortens motor life.
Tools needed
- Digital multi-meter (CAT III rated, AC/DC voltage, continuity, and resistance modes)
- Insulated screwdrivers and nut drivers (1 000 V rated)
- Torque screwdriver (for terminal connections to manufacturer torque specifications)
- Non-contact voltage tester
- Lock-out/tag-out device (suitable for the isolator type)
- Cable strippers for power and control cable sizes in use
- Phase rotation meter (to confirm three-phase sequence before starting motor)
Common mistakes
- Setting the overload relay to a current well above the motor's FLC — this provides inadequate protection and allows the motor to overheat without tripping.
- Wiring the latching (hold-in) auxiliary contact in series rather than in parallel with the start button — the contactor energises only while the start button is held, and de-energises the moment it is released.
- Omitting the normally-closed overload relay contact from the control circuit — the motor has no automatic protection against overload and runs until manually stopped or until a winding failure occurs.
- Connecting the motor without checking the phase rotation — the motor runs backwards, potentially damaging the driven load (pump, conveyor). Always verify phase sequence with a rotation meter before first start.
- Using a contactor with an AC1 (resistive) duty rating instead of AC3 (motor) — AC3-rated contactors are designed for the high inrush current and breaking duty of induction motors; AC1 contacts wear prematurely.
Troubleshooting
- Motor does not start when the start button is pressed
- Cause: Possible causes: overload relay has tripped (contact open); stop button contacts have failed open; control circuit supply is absent; contactor coil is open-circuit. Fix: Check and reset the overload relay. Verify control supply voltage at the panel supply terminals. Test continuity through the stop button, start button, and overload NC contact with a multi-meter. Check coil voltage at terminals A1 and A2 of the contactor while pressing start — if voltage is present but the contactor does not pull in, the coil is faulty.
- Motor starts but the overload relay trips within a few seconds
- Cause: Overload relay set point is too low; motor load is too heavy for the motor rating; the motor is single-phasing (one supply phase lost); the motor windings have deteriorated. Fix: Verify the set point matches the motor nameplate current. Check all three supply phases for voltage with the motor running. Measure running current on all three phases with a clamp meter — if one phase reads zero, there is a phase break. If currents are balanced but all above FLC, the mechanical load is excessive.
- Contactor hums, vibrates, or chatters continuously
- Cause: Supply voltage to the contactor coil is below the minimum specified; the shading ring on the contactor's iron core is broken or missing (AC contactors only); or the contactor is not pulling in fully due to a mechanical obstruction. Fix: Measure the coil supply voltage — it must be within the coil's rated range (typically ±10% of nominal). Inspect the contactor face for debris, worn guides, or a missing/broken shading ring. A broken shading ring always requires contactor replacement — this is not repairable in the field.
Frequently asked questions
What is the purpose of the auxiliary contact wired in parallel with the start button in a DOL circuit?
This is the latching (or hold-in) contact. When the start button is pressed, the contactor energises and its auxiliary NO contact closes. This contact maintains (latches) the control circuit closed after the start button is released and springs back open. Without this contact, the motor would stop the moment the operator releases the start button.
What does the thermal overload relay protect against?
The thermal overload relay protects the motor from sustained overcurrent — caused by mechanical overload, single-phase operation, blocked ventilation, or a starting problem. Its bimetallic elements heat up proportionally to the current they carry; at the trip threshold (set to approximately 100–115% of motor FLC), the relay trips and opens the control circuit, stopping the motor.
How do I reverse the direction of rotation of a three-phase motor started by a DOL starter?
Swap any two of the three phase conductors at the motor terminals (U, V, W) or at the main contactor output. Never swap phases at the incoming supply side — this affects all equipment on the same panel. A reversing starter uses two contactors with an interlocking scheme to achieve forward and reverse without manual intervention.
What is the maximum motor size suitable for DOL starting?
There is no universal limit — it depends on the supply capacity and the electricity utility's rules. Many suppliers limit DOL starting to motors up to 4 kW on single-phase or up to 7.5–15 kW on three-phase without prior approval. Larger motors may require reduced-voltage starting (star-delta, soft starter, or VFD) to limit supply disturbance. Always check the local network operator's connection requirements.
Can a DOL starter be used on a single-phase motor?
DOL starting principles apply to single-phase motors, but the circuit is simpler: one contactor switches the single live conductor. Single-phase motors have a starting capacitor (or start winding) for initial torque — the starter must not interrupt the capacitor circuit during normal running. Single-phase DOL circuits are common for small pumps, compressors, and fans.
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