Direct-On-Line (DOL) Motor Starter Circuit Diagram
This is a free printable dol wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
Direct-on-line (DOL) motor starters provide simple on-off control for three-phase motors through electromagnetic contactors.
Direct-on-line (DOL) motor starting applies full three-phase voltage directly to motor terminals, enabling instant torque development. The DOL circuit uses three main components: a magnetic contactor (relay) switching high-current connections, an overload relay providing motor protection, and a control circuit energizing the contactor. A pushbutton control station contains a Start button energizing the contactor coil. Energizing the contactor pulls an armature that closes three heavy-duty power contacts, simultaneously supplying all three phases to the motor windings. The contactor includes a held-in circuit maintaining coil energization after the Start button is released. A Stop button de-energizes the contactor. The overload relay monitors motor current continuously—if current exceeds the setpoint (typically 125% of motor full-load current), the relay trips, de-energizing the contactor. This protection prevents motor overheating from jamming, low voltage, or phase loss. The control circuit operates at reduced voltage (120V) independent of the three-phase power level.
How to wire dol wiring diagram
- Identify the motor full-load current from the motor nameplate
- Select a contactor rated for at least this current with 25% safety margin
- Mount the contactor and overload relay in a control enclosure with proper spacing
- Wire the three-phase input to the contactor main contacts
- Connect the motor terminals to the contactor output
Frequently asked questions
What is the difference between DOL starting and soft-starting?
DOL starting applies full voltage instantly, producing high inrush current (6-10x normal). Soft-starting gradually ramps voltage, reducing inrush to 2-3x normal.
How does the overload relay protect the motor?
The overload relay monitors motor current and compares it to a setpoint. If current exceeds the setpoint, the relay trips after a few seconds.
What voltage should the DOL control circuit use?
Most DOL control circuits operate at 120V AC, derived from a control transformer stepping down the main three-phase voltage.
Can I manually reset an overload relay after it trips?
Yes. Once the motor overload condition is removed, press the manual reset button to restore normal operation.
What size contactor should I select for a three-phase motor?
Select a contactor rated for at least the motor full-load current at the required voltage, with 25% safety margin.
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