Starter Diagram: DOL Motor Starter Power and Control Circuit Connections
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A starter diagram shows the complete power and control circuit for a direct-on-line motor starter, including main contactor, thermal overload relay, start-stop push-buttons, and seal-in auxiliary contact for sustained motor operation.
A direct-on-line (DOL) motor starter is the simplest and most common motor starting method for motors up to approximately 7.5 kW on 400 V systems where the grid impedance can withstand the full locked-rotor starting current. The DOL starter consists of a main contactor that connects full line voltage directly to the motor terminals on command. The starting current during DOL starting can reach 6–8 times motor full-load amps, causing a voltage dip on the supply bus. For larger motors where this voltage dip is unacceptable, reduced-voltage starters such as star-delta, autotransformer, or electronic soft starters are used instead. The DOL starter circuit divides into two distinct sections: the power circuit and the control circuit. The power circuit carries full motor current: three-phase supply enters the main isolator or circuit breaker, passes through the main contactor main contacts, and continues through the thermal overload relay heater elements to the motor terminals. The power circuit conductors are sized for motor full-load amps with a 25 percent margin. The control circuit operates at reduced voltage — typically 24 V AC or DC, or 230 V AC depending on the installation — and carries only the small currents required to energise contactors, relays and indicators. It consists of a normally-closed stop push-button in series with a normally-open start push-button, which connects the control supply through a normally-closed overload relay contact to the contactor coil A1. Simultaneously, an auxiliary normally-open contact (13-14) of the contactor is wired in parallel with the start push-button, forming the hold-in circuit. Once the operator presses and releases start, the hold-in contact maintains coil energisation indefinitely until the stop button is pressed or the overload relay trips. The motor runs continuously until a stop command is given or a fault condition opens the overload relay contact.
How to wire starter diagram
- Mount components Mount the main MCB or isolator, contactor, and overload relay in the enclosure. DIN rail mount provides a neat, secure installation. Maintain 50 mm clearance above and below the contactor for heat dissipation.
- Wire main power circuit Connect three-phase supply to MCB. Wire MCB output to contactor main terminals 1, 3, 5. Wire contactor output terminals 2, 4, 6 to overload relay input. Connect overload relay output to motor cable terminals.
- Connect control supply Bring control supply (24 V or 230 V) from the main supply through a control fuse or MCB to the stop button input terminal.
- Wire stop-start buttons and coil Stop button (NC) to start button (NO) to overload relay NC contact (95-96) to contactor coil A1. Connect coil A2 to control supply return (neutral or 0 V). Wire auxiliary contact 13 to junction after stop button; 14 to junction before start button (parallel with start button).
- Set overload and test Set overload relay FLA dial to motor nameplate current. Restore power, press start, confirm motor runs and contactor holds in. Test stop button interrupts operation. Check running current on all three phases.
Specifications
| Starting method | Direct-on-line (DOL) — full voltage applied immediately |
|---|---|
| Starting current | 6–8 × motor FLA at locked rotor |
| Overload relay class | Class 10 (trips within 10 s at 7.2 × Iset) |
| Control circuit voltage | 24 V AC/DC or 230 V AC (application dependent) |
Safety warnings
- Thermal overload relay must be set to motor nameplate full-load current before first start — incorrect setting either fails to protect the motor or causes nuisance trips.
- The stop push-button must be a normally-closed (NC) type — a normally-open stop button cannot interrupt the coil circuit and fails to stop the motor.
- Ensure main isolator is locked open before working on any starter component — the main contactor main terminals remain live even when the contactor is de-energised.
Tools needed
- Clamp-on ammeter to measure and record starting and running current
- Digital multimeter for control circuit voltage and contact resistance checks
- Torque wrench with electrician's bit set for main terminal torquing
- Insulation resistance tester for motor winding check before first start
Common mistakes
- Connecting the hold-in auxiliary contact (13-14) to the wrong circuit node, causing the motor to start only while the start button is held.
- Wiring a normally-open push-button as the stop button — the motor cannot be stopped without the correct NC-type stop push-button.
- Not bonding the motor frame to the protective earth bus with a conductor sized per the fault clearing requirements of the protection device.
Troubleshooting
- Motor starts only while start button held
- Cause: Hold-in auxiliary contact not wired correctly — 13-14 not in parallel with start button Fix: De-energise, check auxiliary contact terminal 13 connects to the junction between stop button and start button, and terminal 14 connects to the junction between start button and coil input.
- Motor does not start — contactor does not click
- Cause: Control supply absent, stop button open, or overload relay tripped Fix: Measure control supply voltage. Press overload relay reset button if it has tripped. Probe stop button terminals to confirm NC contact is closed. Trace voltage through each series element to find the open point.
- Contactor clicks but motor does not run
- Cause: Open main contact on one phase or motor winding fault Fix: Measure voltage at motor terminals with contactor energised — all three phases should show line voltage. Any missing phase indicates an open main contact or blown main fuse. Check motor winding continuity.
Frequently asked questions
What is the difference between DOL and star-delta starting?
DOL (direct-on-line) connects full supply voltage to the motor immediately, producing maximum starting torque but also maximum starting current (6–8 times FLA). Star-delta starting first connects in star, reducing starting current and torque to one third, then transitions to delta for full operation. DOL is simpler, cheaper, and faster to start for small motors. Star-delta is chosen for larger motors where starting current must be limited to protect grid voltage quality.
How does the seal-in auxiliary contact work?
The seal-in or hold-in contact is the normally-open auxiliary contact (13-14) of the contactor, wired in parallel with the start push-button. When the contactor first closes (triggered by pressing start), this auxiliary contact also closes. If the start button is then released, the auxiliary contact provides an alternative current path that keeps the coil energised. Only the stop button (or overload relay) can break this hold-in path and de-energise the coil.
What size overload relay should I select?
Select a thermal overload relay with an adjustment range that includes the motor nameplate full-load amps (FLA). Set the dial to the exact FLA value. The overload relay provides class 10 protection — it trips within 10 seconds at 7.2 times the set current (locked-rotor current), and provides inverse-time-current protection for overloads between 100% and 700% of set current.
Can I run the motor continuously without pressing start?
Yes — the DOL starter with a hold-in contact is designed for maintained (continuous) motor operation without requiring an operator present at the push-button. The motor runs continuously until the stop button is pressed, the overload relay trips, or supply power fails. Power failure automatically de-energises the contactor; when power is restored, the motor does not restart automatically unless an auto-restart circuit is specially added.
What causes the overload relay to trip repeatedly?
Repeated overload tripping indicates the motor is consistently drawing more current than its nameplate FLA. Root causes include: mechanical overload on the driven machine (excessive friction or head pressure), supply voltage below motor rated voltage causing higher current for the same power, a single phase open causing remaining phases to carry 173 percent of normal current, or incorrect overload relay setting. Measure current on all three phases and compare to FLA.
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