TESLA INSTITUTE: Electrical Engineering

Friday, 17 July 2020

How works a 1-Phase Motor Direct Online Starter (DOL) ?

July 17, 20201-Phase Motor, automation, circuit, Direct Online Starter, DOL, Electrical Engineering, STEM Education No comments:

In this post I want to explain you how Direct Online Starter (DOL) circuit for 1-Phase Motor works.

For smaller motors a starting current is not too large, so DOL stater can be use safely.

It is very popular and often use circuit for starting a small motors.

Look what we have on this circuit

Terminal block X1 for connect power supply to the circbuit
MCB F1 is used as main power On / Off switch and also as a motor protection.
A electromagnetic contactor K1 is used to turn on and off a power to the motor M.
1-Phase motor M
This is a power part of the circuit.
Then we have an elements of control part of the circuit:
Exactly we use a main contacts of contactor K1 for it .
The contactor K1 has also one auxiliary normal open contact (terminals 13 and 14)
Here we have a coil of contactor K1.
In a circuit we have two push buttons S1 and S2.
Push button S1 is use for stop the motor, and it is normal close NC.
Push button S2 is use for start a motor, and it is normal open NO.

1-Phase Motor - Direct Online Starter (DOL)

Now, how this all works ?

When we turn on a circuit breaker F1 we put a voltage on main contacts of contactor K1 and on a push button S1. Because button S1 is normal close type, we have a voltage on on terminals 13 of button S2 and auxiliary contact K1.

In this state DOL starter is ready to switch a motor.

So now if we push button S2, we give a voltage on contactor's coil K1. Main contacts K1 get close, and a motor M get start. In a same time auxiliary contact K1 will also get close, so when we release start bush button S2, coil K1 will be energized by this contact, and motor will stay in run state.

When we press Start button S2 current path for contactor's coil K1 is closed from L terminal of terminal block X1 through circuit breaker F1, button S1, button S2, coil K1 and terminal N.

Then when we release a Start button S2 all is same, but now current is flow through closed auxiliary contact K1 instead of through S2 button. In that way auxiliary contact K1 keeps a coil K1 energized. This contact is known as holding contact as it keeps contactor hold on.

Now when we press Stop button S1, we will break this path, coil K1 will be stop energized, main contacts, and auxiliary contact K1 get back to open position, and the motor stops.

Same effect will be when it will be make by open circuit breaker F1. It can happen in case of motor overload or if some short-circuit occur in a circuit.

We can use the circuit breaker F1 also for cut off a power from the circuit for some maintenance work.

When circuit breaker F1 is Off we also can not turn off a motor.

When circuit breaker F1 is ON our Direct Motor Starter is ready to work, and we can run the motor by press Start button S2.