D.O.L ( Direct Online Starter ) - Definition And Working

D.O.L ( Direct Online Starter ) - Definition And Working

Definition Of D.O.L (Direct Online Starter) Starter

A starter which connects a motor directly across the line is called a Direct-On-Line starter, i.e. in this method of motor starting, the motor is direct connected by means of a starter along with the full supply voltage.

Wiring Diagram Of D.O.L ( Direct Online Starter )

Wiring or Circuit Diagram Of DOL Starter

• L1, L2, L3     Lines
• M                  Main contacts
• MA               Auxiliary or maintaining contact
• S1                 START push button
• S2                 OFF push button
• OLC              Overload relay coil
• OL                 Overload relay contact
• C                   Magnetic or operating coil

Upper Figure shows a wiring diagram of a push button type Direct-On-Line starter, which is commonly used. It is very simple, inexpensive, easy to install and maintain. 

It consists of a 'START' and 'OFF' push buttons. Electromagnetic contactor and overload relay. Switching operation by this starter is directly from line without any provision to control the starting current i.e. here there is no any device to reduce the starting current in this starter.

Operation Or Working Of DOL Starter

Single Line Diagram Of Control Circuit Of DOL Starter

Figure shows a line diagram of a control circuit. So when the START push button 'S1' is pressed, the circuit is completed from L1, through S2, S1, C (operating coil) and normally closed contacts (O.L.) of the overload relay to L2.

• i.e. L₁ S₂ S₁ - C - O.L. - L₂

So at that time, the coil C is energised, and it closes contacts M and connects the motor across the line.

An Auxiliary or Maintaining contact M, is used to keep the holding or operating coil energised after the finger is removed from the 'START' push button S₁.

When the 'OFF' push button 'S₂' is pressed, the supply through the contactor coil C is disconnected. So when the coil C is de-energised, the main contacts M of the starter are opened. Thus supply to the motor is disconnected and the motor stops.

In this type of starter, there will be no current limiting device to limit the starting high current, only protection against under voltage and overload is provided in this type of starter.

Under Voltage Protection

When the supply voltage is below a certain value or in the event of failure of power supply, the coil C is de-energised, therefore motor will be disconnected from the main supply.

Overload Protection

The motor is protected against overload by a thermal overload relay which open circuit the control circuit when overload occurs. 

In case of an overload on the motor, overload Relay coils (O.L.C.) are energised. The normally closed contacts O.L. is opened and the contactor cell C is de-energised to disconnect the motor from the supply.

In this method of starting of induction motor, the rate of the rise temperature is high and motor may also get damaged if the starting period is large, which may due to high load or more voltage drop in the supply lines. 

So small size squirrel cage induction motors upto 5 kW may be started by this method