A DOL (direct-on-line) starter connects a three-phase induction motor directly to the full supply voltage in one step. It is the simplest and cheapest motor starter: a contactor switches the motor on and off, an overload relay protects it from overheating, and two push buttons (start and stop) control it. Because the motor gets full voltage at once, it draws a starting current of about 6 to 8 times its full-load current, so DOL starters are used mainly for small motors, typically up to about 5 kW (7.5 HP).
Main parts of a DOL starter
| Part | Function |
|---|---|
| Contactor | An electrically operated switch with three main contacts for the motor lines and an auxiliary (hold-on) contact. |
| Contactor coil | When energised, it pulls the contacts closed; when de-energised, a spring opens them. |
| Thermal overload relay (OLR) | Bimetal strips heat up with excess current and open the control circuit, stopping the motor. |
| Start push button (green, NO) | Normally open; pressing it energises the coil. |
| Stop push button (red, NC) | Normally closed; pressing it breaks the coil circuit. |
| MCB / fuses | Short-circuit protection for the motor and control circuit. |
How a DOL starter works
- Start: pressing the green start button completes the control circuit, so the contactor coil is energised.
- Motor runs: the coil closes the three main contacts and full line voltage reaches the motor terminals. The motor accelerates to full speed, usually within a few seconds.
- Holding (latching): an auxiliary contact of the contactor, wired in parallel with the start button, also closes. This “hold-on” contact keeps the coil energised after you release the button.
- Stop: pressing the red stop button breaks the coil circuit, the contacts open and the motor stops.
- Overload trip: if the motor draws too much current for too long (a jammed load, for example), the overload relay opens its contact in the coil circuit and the motor stops.
- No-volt protection: if the supply fails, the coil drops out. When power returns, the motor will not restart on its own until someone presses start again, which is a safety feature.
Starting current and torque
At the instant of starting, an induction motor has no back EMF, so it behaves almost like a short circuit. With DOL starting:
- Starting current is about 6 to 8 times the full-load current.
- Starting torque is high, roughly 1.5 to 2.5 times full-load torque, depending on the motor design.
Worked example. A 5.5 kW, 415 V motor has a full-load current of about 11 A. Starting DOL, it may draw 6 × 11 = 66 A to 8 × 11 = 88 A for a few seconds. On a weak supply this surge causes a voltage dip that can flicker lights or trip other equipment, which is why larger motors use reduced-voltage starters.
DOL vs star-delta vs soft starter
| Starter | Starting current | Starting torque | Cost | Typical use |
|---|---|---|---|---|
| DOL | 6 to 8 × FLC | High | Lowest | Small motors, pumps, fans, compressors |
| Star-delta | About 1/3 of DOL | About 1/3 of DOL | Moderate | Medium motors that start on light load |
| Autotransformer | Adjustable (tap-based) | Adjustable | Higher | Large motors needing more torque than star-delta |
| Soft starter / VFD | Controlled, smooth | Controlled | Highest | Large motors, conveyors, precise control |
Advantages and disadvantages of a DOL starter
Advantages
- Simple design, easy to wire, understand and maintain.
- Low cost and compact.
- Gives the maximum starting torque, useful for loads that are hard to start.
Disadvantages
- High inrush current causes voltage dips on the supply.
- Sudden full torque gives mechanical shock to belts, couplings and gearboxes.
- Not suitable for large motors; supply authorities often restrict DOL starting above a certain rating.
Where DOL starters are used
- Small water pumps and borewell motors.
- Fans, blowers and small air compressors.
- Small conveyors and workshop machines such as lathes and drill presses.
Frequently asked questions
What is the full form of DOL starter?
DOL stands for Direct On Line. The starter connects the motor directly to the full supply voltage.
Up to what rating can a DOL starter be used?
DOL starters are normally used for small motors up to about 5 kW (7.5 HP). Larger motors usually use star-delta starters, soft starters or drives to limit starting current.
Why does a DOL starter draw high current?
At standstill the motor has no back EMF, so full voltage across its low impedance draws about 6 to 8 times the full-load current until it speeds up.
What is the purpose of the hold-on contact in a DOL starter?
The hold-on (auxiliary) contact keeps the contactor coil energised after the start button is released, so the motor keeps running.
References
- B. L. Theraja and A. K. Theraja, A Textbook of Electrical Technology, Vol. II, starting of induction motors.
- NPTEL – Electrical Machines courses