Different Starting methods are used for starting motors depending upon the rating of motor.
Direct online starter is simplest starter used for starting of Induction motors and in short it is also known as DOL starter.
Direct online starters are used only for motors upto 7.5 KW as they draw heavy inrush current during starting of motor. In these starters full voltage is comes across motor and draws 6-8 times rated current at starting of motor. Due to heavy current drawn during starting of motor these starters are used only for motors upto 7.5 KW rating only.
Main Parts of Direct online starter:-
Direct online Starter consists of following parts:-
1. Electromagnetic Contactor
2. Overload relay
3. Main MCB
4. ON/OFF switches
All Starters has two circuit diagrams :-
1. Power circuit Diagram
2. Control circuit diagram
Let’s discuss one be one:-
1. Power circuit diagram:-
Power circuit is usually also called three phase diagram. In That diagram Overload relay comes at end of circuit and is below:-
|DOL Power Circuit|
Above diagram is known as power circuit diagram as ON/OFF switches are not shown in this diagram, instead only 3-Phase wiring is shown. In this diagram you can easily see that Overload relay is connected nearest to motor end so that if any abnormality occurs then Overload get tripped immediately and protects both motor and circuit.
2. Control Circuit diagram
In control circuit O/L Relay comes at starting of circuit and is shown as below:-
|Direct Online Starter Control Circuit|
In this circuit as we see O/L Relay NC is used and Stop Push Button NC is used.
Let’s discuss how this DOL Control circuit works as power circuit will not work until Control circuit gets right.
Whenever start PB is pressed Phase 1st goes through Overload relay NC and then goes through Stop PB and Start PB and thereafter holds power contactor now as soon as Power contactor get holds it’s NO becomes NC and thereafter power supply continue to flow to power contactor through 13-14 point and Start PB goes to initial position.
In above control circuit 220V circuit is shown but it may be 440V circuit depends upon Power contactor coil voltage supply.
This is how a Simple DOL Starter works.
For other Direct online Starter applications visit at link below:-
Advantages of DOL Starter:-
1. These are most simple type of starters.
2. Most economical and cheapest starter.
3. Maintenance cost is very low.
Disadvantages of DOL Starter:-
1. Draws very high inrush current at starting i.e. 6-8 times rated current of motor which restricts its use to motors upto 7.5 KW only.
2. It will put high Thermal Stress on motors that will reduce motor life.
3. High Starting current will leads to dips in voltage.To know Circuit of Star-Delta Starter click below:-