Motor starter
Working principle, Types and Connection diagram.

  1. Motor starter.
  2. Working principle of motor starter.
  3. Types of motor starter.
  4. Why it is used?

1- Motor Starter:

Motor starter: types and working principle.

Motor starter is an electrical device. It is used for motor start and stop safely. It is also used for motor protectionas overload, short circuit and phase prevention.

It can be categorized in basically two types controlling like as electrical and digital. Other type we can categories as capacity, phase, operating type etc.

If we talk about electrically controlled starter is used as two types as one for below 5hp motor as direct on-line starter and second for above 5hp motor as star delta starter.

If we talk about digitally controlled starter, it is used as two types as one for frequency controlling as variable frequency drive (VFD) and second for controlling starting current as soft starter.

2- Working principle:

Here we will know that how to work motor starter. The Motor starter, it means motor start and stop with protection. Now what is the protection, Starter is manufactured with protection like as overload, short circuit, single phasing etc.

Direct On Line Starter

When press the start button it will provide the supply to contactor coil after that contactor will hold and motor will start. The overload relay is connected with main contactor below side for measuring the current. If the load current is more as per motor capacity then motor will be tripped.
If we want to stop the motor, the push button will be pressed then motor will be stopped.

Star Delta Starter

When on push button is pressed, then star and main contact will hold and motor will start in star connection. It will move as per time set current after crossing time limit star contactor will stop and delta contactor will start. Now motor will move with full rpm.


variable frequency drive is used for motor start stop and speed control. The frequency is controlled in this process.

Soft Starter:

: Soft starter is used for only motor start and stop purpose. The speed can note be controlled in this process. So it is called as soft starter. As same star delta the starting voltage of motor is decreased with the help of soft starter.

3 - Types of Motor Starter?

Motor starter is used commonly in the world. It is available in different types. Types of starters is below mentioned.

1- Electrical Starter:-
  • Single Phase
  • Three phase
2- Digital Starter:-
  • VFD- Variable frequency drive.
  • Soft starter.
  • GSM mobile network starter.
3- According starting mode :-
  • Manual start starter.
  • Auto start starter.
4- According winding connection :-
  • Direct on-line starter.
  • Star delta starter.
  • Auto transformer starter.
5- According Resistance control :-

4- Why motor starter is used:

Motor starter basically is used for switching the motor in safe condition. Starter protects as safe guard of motor like as overload, overheating, switching time jerking load, single phasing. Short circuit etc.

Small motors (below 5hp):

Single phase or below 5 hp three phase motor is operated with direct on-line starter (DOL). These motors are connected with supply without decreasing voltage as normal because these type motors take heavy current just second.
So, contactors are used for safely switching. Relays are also used for motor protection from overload, short circuit etc.

Big motors (above 5hp):

During starting above 5 hp motors draw a heavy current from line so supply line and other parts may be damage. For minimizing heavy current, the motor is operated in first as star connection after set time the motor is operated in delta connection.
So, this type starter is called star delta starter. It provides other protection for motor as overload, short circuit, single phasing etc.

So in this way we can say the starter is more important for motor starting and protection.

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