DIRECT ON LINE STARTER

DIRECT ON LINE STARTER

As we saw on the last topic about jogging circuits; when the JOG button was pressed the motor starts running when it is released it immediately stopped running.
However there are some industrial applications which need the motor to run by itself after the button was pressed only once. The DOL starter is the right device for this job.

In DOL the contactor is the one responsible to connect the motor to the supply at the power circuit, its coil is controlled at the control circuit by the ‘start’ and ‘stop’ button. A Normally Open auxiliary contact on the contactor is used to retain the supply to the contactor coil even after the start button is released, this is referred as ‘self holding’, the circuit will be on afterwards until its deactivated by the stop button.

Below there is a DOL circuit;

NOTE THAT WE ASSUME THAT YOU ALREADY KNOW OPERATIONS OF MOTOR CONTROL COMPONENTS IF NOT THEN WAIT FOR OUR SECTION ON MOTOR COMPONENTS DESCRIPTION

 

COMPONENTS

F1-Thermal Overload Relay,       

KM1-Contactor

3phase Induction Motor

S1-Stop push button (NC)

S2-Start push button (NO)

CB1-3phase circuit breaker

CB2-Single phase circuit breaker

OPERATION

When the start button S2 is pressed the contactor coil KM1 will energize, the Normally Open auxiliary contact of KM1 will close and after even after S2 is released the contact will remain closed ,the contactor contacts at the power circuit will also close when S2 is pressed and current will flow continuously until the stop button is pressed. When the stop button S1 is pressed the coil of KM1 will de-energize thus opening both the N.O auxiliary contact and the contactor power circuit contacts, the motor will stop running until S2 is pressed again.

NOTE: motor power rating should be considered when the selecting the current rating of the contactor and cables

The next topic will be on combination of jogging circuit and the DOL.