DOL Starter Diagram: Power & Control Circuit Explained
This is a free printable dol starter diagram: download the diagram as SVG or open it and print to paper or PDF.
A DOL (Direct-On-Line) starter connects a three-phase induction motor directly to full supply voltage through a contactor and thermal overload relay. The DOL starter diagram shows both the high-current power circuit and the low-voltage control circuit that start and stop the motor safely.
Direct-On-Line starting is the most straightforward method for running three-phase induction motors rated up to approximately 7.5 kW (10 HP) on standard industrial supply systems. The DOL starter diagram consists of two clearly separated but interdependent circuits: the power (main) circuit and the control (auxiliary) circuit. The power circuit carries three-phase full-voltage supply through the main contactor's power contacts to the motor terminals. A thermal overload relay in series with the power conductors monitors current and trips if the motor draws excessive current for an extended period. The control circuit operates at a lower voltage—often 110 V AC from a step-down transformer, or 24 V AC/DC—and contains the start push button (normally open), stop push button (normally closed), the thermal overload relay's normally-closed contact, and the main contactor coil. When the operator presses Start, current flows through the control circuit to energize the contactor coil. The coil pulls in the armature, closing the power contacts (which connect the motor to supply) and simultaneously closing an auxiliary normally-open contact wired in parallel with the Start button. This auxiliary contact holds the coil energized after the Start button is released—the self-latching or "hold-in" function. Pressing Stop opens the control circuit, de-energizing the coil, releasing the power contacts, and stopping the motor. A thermal overload trip also opens the control circuit through the overload relay's NC contact, stopping the motor and requiring manual reset at the overload relay body once the motor has cooled. IEC 60947-4-1 governs the design and testing requirements for DOL motor starters in industrial applications worldwide. The starter enclosure must be rated for the installation environment (IP54 for outdoor, IP20 for clean indoor panels).
A Direct-On-Line (DOL) starter is the simplest method of starting a three-phase induction motor, applying full mains voltage to the motor terminals immediately. The control circuit — typically drawn as a ladder diagram — sequences the start push-button, hold-on contact, stop button, overload relay trip contact, and contactor coil in a logical rung-by-rung layout. Ladder diagrams are especially useful for DOL starters because each rung maps to a discrete control action, making fault-finding straightforward. Draw your DOL ladder diagram in the free online editor to document the power and control circuits side by side.
How to wire dol starter diagram
- Mount and isolate the starter Install the DOL starter enclosure with the main isolator above it. Connect three-phase supply L1, L2, L3 to the isolator input. Do not connect the output until all internal wiring is complete.
- Wire the power circuit Run supply cables from isolator output to main contactor line terminals (L1, L2, L3). From the contactor load terminals (T1, T2, T3), run cables through the thermal overload relay to motor terminals U, V, W. Connect the motor PE terminal to the earth bar.
- Set the overload relay Adjust the overload relay current dial to the motor nameplate full-load amps. The pointer must align precisely with the FLA value—do not estimate.
- Wire the control circuit Wire: control supply → Stop NC button → Start NO button → Overload NC contact → contactor coil A1. Wire A2 to control neutral. Wire the contactor NO auxiliary contact in parallel with the Start button for self-latching.
- Test before motor connection Close the isolator with the motor disconnected. Energize control circuit only (motor cables disconnected at motor terminal box). Press Start—contactor should click and hold. Press Stop—contactor releases. Trip overload manually—contactor releases and will not restart until reset.
Specifications
| Typical supply voltage | 380-415 VAC, 3-phase 50 Hz |
|---|---|
| DOL starting inrush | 5-7 x motor FLA |
| Overload relay setting | 100% of motor nameplate FLA |
| Max motor size (DOL) | 7.5 kW / 10 HP (typical) |
Safety warnings
- Both the power and control circuits inside the starter enclosure are live when the main isolator is closed—always lock out the isolator before opening the enclosure.
- The motor frame must be bonded to the protective earth (PE) conductor—a winding insulation failure without earth bonding energizes the motor frame at full supply voltage.
- Set the thermal overload relay current dial to exactly the motor nameplate FLA—never increase the setting to prevent nuisance tripping; overloaded motors burn out windings.
Tools needed
- Clamp-on ammeter (true RMS, AC, three-phase capable)
- Multimeter (AC/DC voltage and continuity)
- Insulated screwdrivers and terminal torque wrench
- Phase sequence meter
Common mistakes
- Wiring the hold-in auxiliary contact in series instead of parallel with the Start button—the motor stops immediately when the operator releases Start.
- Connecting all three power phases to the same terminal on a three-pole contactor—only one phase reaches the motor, which draws excessive current and burns the winding.
- Setting the overload relay higher than motor FLA to stop nuisance trips—this removes protection and allows the winding to overheat before tripping.
Troubleshooting
- Motor hums but does not turn after Start
- Cause: One phase missing—open fuse, failed contactor contact, or disconnected cable on one phase Fix: Clamp-meter all three phases at the motor terminals while energized. The missing phase will read zero amps; trace that phase back to the fault.
- Contactor chatters rapidly on and off
- Cause: Low control voltage causing the coil to pull in then drop out cyclically, or a loose connection in the hold-in auxiliary contact circuit Fix: Measure control voltage at A1-A2 during operation. Must be within 85-110% of coil rated voltage. Inspect the hold-in auxiliary contact for dirt or damage.
- Motor runs but overload trips after several minutes
- Cause: Motor mechanically overloaded, or overload relay setting is slightly low for this motor Fix: Clamp-meter motor current on all three phases under load. If current exceeds FLA, reduce driven load. If current is at or below FLA, the overload relay class or setting needs adjustment.
Frequently asked questions
What is the maximum motor size for a DOL starter?
The practical limit for DOL starting depends on supply system impedance. Most utilities and local regulations permit DOL for motors up to 7.5 kW (10 HP) on standard industrial supplies. Some jurisdictions allow up to 15 kW if the voltage dip during starting does not exceed 3-5% of nominal. Above these limits, reduced-voltage starters (star-delta, autotransformer, or soft-starter) are required to limit inrush current and voltage dip effects on other connected equipment.
How does the thermal overload relay protect the motor?
The thermal overload relay (TOR) contains bimetallic heating elements in series with each motor phase. Overload current heats these elements, causing them to bend and trip a mechanism that opens the NC contact in the control circuit, de-energizing the contactor coil and stopping the motor. The trip time is inversely proportional to the degree of overload—a small overload takes minutes to trip; a severe overload trips in seconds. After tripping, the motor must cool before the TOR can be manually reset.
Why does my DOL starter trip immediately when I press Start?
Immediate tripping on Start usually indicates: (1) the thermal overload relay is set below the motor FLA and the starting inrush trips it instantly; (2) a mechanical jam on the driven load—if the motor cannot accelerate, it draws locked-rotor current indefinitely; (3) a phase-to-phase or phase-to-ground fault in the motor winding or power cables; (4) a failed main contactor contact causing unbalanced phase voltage. Check the overload setting first, then disconnect the motor and test the motor insulation resistance.
What is the difference between a DOL starter and a soft starter?
A DOL starter applies full voltage instantaneously, causing 5-7x FLA inrush current and mechanical shock to the driven machine. A soft starter uses thyristors (SCRs) to gradually ramp up the voltage applied to the motor over a programmable time period (typically 2-30 seconds), limiting inrush to 2-3x FLA and reducing mechanical stress. Soft starters improve power quality, reduce belt/coupling wear, and are required above the DOL size limit. They add cost and complexity compared to the simple contactor-based DOL starter.
Can a DOL starter run a motor in reverse?
A standard single-contactor DOL starter runs the motor in one direction only. To enable reversing, a reversing starter uses two contactors with mechanical and electrical interlocks: the forward contactor connects L1-L2-L3 to motor terminals T1-T2-T3; the reverse contactor swaps two phases (L1-L3-L2), reversing the phase sequence and motor direction. Both contactors cannot energize simultaneously—the interlock prevents direct reversal under load, which would cause very high current and mechanical shock.
What does a ladder diagram for a DOL starter look like?
A DOL starter ladder diagram has two vertical power rails (L1 supply and neutral or L2) with rungs between them. Rung 1 contains the normally-closed stop button, the normally-closed overload relay contact, and the normally-open start button in series with the contactor coil (KM1). Rung 2 places a normally-open auxiliary contact from KM1 in parallel with the start button to provide latching (hold-on). When the start button is pressed, KM1 energises and seals in via its auxiliary contact; pressing stop or an overload trip de-energises KM1 and disconnects the motor.
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