DOL Starter (Direct On Line Starter) Diagram And Working Principle

What is DOL Starter?

A DOL starter (also known as a direct on line starter or across the line starter) starts a 3 phase induction motor by connecting it straight to the supply. In a DOL starter an induction motor is connected across its 3-phase supply, so the starter applies full line voltage to the motor terminals.

Even with that direct connection, the motor still has protection. A DOL motor starter includes protective devices and, in some models, condition monitoring. Below is a wiring diagram of a DOL starter:

Since the DOL starter connects the motor directly to the main supply line, the motor draws a high inrush current compared with full-load current. Locked-rotor current is often about 5 to 8 times full-load current, depending on the motor design. That current falls as the motor approaches rated speed.

A direct on line starter is used where that inrush does not cause an excessive voltage drop on the supply, or where the drop is accepted. If the drop must be limited, a reduced-voltage method such as a star delta starter is one option. Direct on line starters are common on small motors, especially 3 phase squirrel cage induction motors. The permitted kW depends on the supply, not on a single universal rating.

The image above is the DC armature form I = (V – E)/R, where back emf E rises with speed N. A 3-phase induction motor is different: start current is set by locked-rotor impedance, then falls as slip falls.

At standstill there is no generated back emf in a DC machine, so that formula gives a large current. Small low-inertia motors often accelerate quickly, so the high current lasts only a short time.

As speed rises, that DC armature current falls. A small 3-phase motor on a stiff supply can also DOL-start smoothly for the same reason: the high-current interval is short.

A large motor has more inertia and draws a long high-current pulse, so DOL may dip the supply or stress the driven machine. Geometry (short core, large diameter) is only one factor. Some large motors still DOL-start on a stiff bus, including with oil-immersed switchgear.

DOL Starter Wiring Diagram

The wiring diagram for a DOL starter is shown below. A direct online starter has two buttons: a GREEN button to start and a RED button to stop. The DOL starter comprises an MCCB or circuit breaker, a contactor and an overload relay for protection. Those start and stop buttons control the coil circuit.

To start the motor, pressing the green button closes the contact and applies full line voltage to the motor. Contactors may have 3 or 4 poles; the diagram below shows a 4-pole contactor.

It contains three NO (normally open) contacts that connect the motor to supply lines, and the fourth contact is “hold on contact” (auxiliary contact) which energizes the contactor coil after the start button is released.

If a fault or an overload opens the coil circuit, the contactor drops out and the starter disconnects the motor from the supply.

3 Phase Motor Starter with Overload Protection

When a motor draws excessive current to meet the load requirements such that this load requirement goes beyond the rated limit, this is known as overload.

Thermal overload protection opens the coil circuit when sustained current overheats the motor. Overload relays provide that delayed trip.

That relay is sized for overload, not for a bolted short circuit. A fuse or MCB clears high fault current. Overload protection opens at currents a little above motor rating, after a delay.

A long overload can damage the winding. A short high-current pulse such as DOL starting should not trip a correctly set overload.

Overload protection is often an overload relay. Relays may be electronic with an adjustable trip, thermal (heater or sensor) or magnetic (instant excess current).

A common US setting is 125% of full-load amperes for many motors. That figure is a code-style maximum, not a worldwide rule. IEC thermal relays are chosen by trip class.

DOL Starter Working Principle

The working principle of a DOL starter begins with the connection to the 3-phase main with the motor. The control circuit is connected to any two phases and energized from them only.

When we press the start button, current flows through the contactor coil (magnetizing coil) and the rest of the control circuit.

The current energises the contactor coil and leads to close the contacts, and hence 3-phase supply becomes available to the motor. The control circuit for a DOL Starter is shown below.

Pressing the stop button opens the coil circuit, so the contactor drops out and the motor coasts to rest. An overload trip does the same.

Even after releasing the start button, the contactor coil (Magnetizing Coil) continues to receive power from the primary contacts, as shown in the DOL Starter diagram.

Advantages of DOL Starter

The advantages of a DOL starter include:

1. Simple and most economical starter.
2. More comfortable to design, operate and control.
3. Provides nearly full starting torque at starting.
4. Easy to understand and troubleshoot.
5. A DOL starter applies line voltage to the motor terminals. Many LV motors are delta-connected at that voltage, but that is a nameplate connection.

Disadvantages of DOL Starter

The disadvantages of a DOL starter include:

1. High starting current (5-8 times of full load current).
2. DOL starting can dip the supply voltage, so it is often limited to smaller motors or a stiff supply.
3. DOL Starter repeated starts add thermal and mechanical stress.
4. The start is mechanically abrupt.
5. Unnecessary high starting torque

DOL Starter Applications

The applications of DOL starters are primarily motors where a high inrush current does not cause excessive voltage drop in the supply circuit (or where this high voltage drop is acceptable).

Direct on line starters are commonly used to start small water pumps, conveyor belts, fans and compressors. In the case of an asynchronous motor (such as the 3-phase squirrel-cage motor) the motor will draw a high starting current until it has run up to full speed.