DOL is an acronyms for direct online starter which is used in the industry to start induction motors and its uses main contactor.
Note: in DOL, they are basically two circuit used for the connection. such as, power and control circuit.
COMPONENTS USED FOR DIRECT ONLINE STARTER
MCB
- POWER OR MAGNETIC CONTACTOR
- THERMAL OVERLOAD RELAY
- THREE PHASE INDUCTION MOTOR
- STOP PUSH BUTTON
- START PUSH BUTTON
- R Y AND B WHICH IS THE SOURCE.
Meaning and working principal of the components
MAGNETIC CONTACTOR: contactors are advance relay that is mainly used where higher current regulations is required.
Contactors have different points label as follows: T1, T2, T3, T4, T5, AND T6
NO Which Normally Open
NC which Normally Close.
A1 and A2 which are the coil of the contactor.
Working principal of contactor
When power is supplied to the coil from the control circuit, a magnetic field is produced, thereby magnetizing the electromagnetic.
The magentic field attracts the armature to the magnet, which in turn closes the contacts.
With contact closed, current flows through the power circuit from the line to the load.
THERMAL OVERLOAD RELAY: Thermal overload relay (ORL) is a device that works like a relay, but it’s majorly work is to trip off when ever excessive current that can damage the motor wants to pass through the circuit.
Working principal of ORL
The thermal overload relay is an electromechanical protection device. In the main circuit, this device protect the motor in the event of phase overload or failure.
Thermal overload relay does not directly shutdown the load.it trigger one or several auxiliary contacts which then disconnect the motor.
MCB: MCB is a circuit breaker that helps to trip off when their is a short circuit current coming inside the circuit. It has several parts that helps to Carry out the job perfectly. They are as follows:
- Supply terminal
- Frame
- Operating knob
- Bimetallic strip
- Load terminal
- Fixed contact
- Arc quench knob.
HOW TO CONNECT THE MOTOR WITH DOL
For power circuit connection
Firstly, you connect your power supply to your MCB as labelled R, Y, B, and N and this wire will be connected to the point labelled 1, 3 and 5.
Secondly, you connect another wire from the point labelled 2, 4 and 6 of the MCB to the point labelled 1, 3 and 5 of your contactor.
Thirdly, you then connect another wire from the point labelled 2, 4 and 6 of your contactor to the point labelled 1, 3 and 5 of your overload relay.
Lastly, you connect another wire from the point labelled 2, 4 and 6 of your overload relay to your motor as shown on the motor connection U1, V1 and W1. Which will be in Delta connection.
FOR CONTROL CIRCUIT
Connect wire to your MCB and connect it to 95 of your ORL and 96 to the stop push button.
Then the other end of your stop push button, connect another two wire, one to your start push button and the other one to your contactor labelled 54 and then connect two wire to the second leg of your start push button and one to the contactor labelled A1 and one to 55.
Then connect your A2 to the neutral and start your motor.
Conclusion
DOL is a simple circuit that helps to control our motor with the help of contactors, ORL and MCB.