DOL(Direct on line) starters for Three phase motors

Direct on line starting is a starting technique of three phase induction motors.During starting period induction motors will draw more starting current,which is six times greater then normal rated current of that motor.so it is important to reduce the starting current,for these purpose starters are widely used.There are different types of starters they are

1. Direct online starters
2. Star delta Starters
3. Auto transformer starters

Wiring of DOL Starter

1. Main Contact

• Contactor is connecting among Supply Voltage, Relay Coil and Thermal Overload Relay.
• L1 of Contactor Connect (NO) to R Phase through MCCB
• L2 of Contactor Connect (NO) to Y Phase through MCCB
• L3 of Contactor Connect (NO) to B Phase through MCCB.

NO Contact (-||-):

• (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)
• Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is connected to Common wire of Start/Stop Button.

NC Contact (-|/|-):

• (95-96) is a normally closed NC contact (opens when the thermal overloads trip if associated with the overload block)

2. Relay Coil Connection

• A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermal over Load Relay’s NC Connection (95).

3. Thermal Overload Relay Connection:

• T1,T2,T3 are connect to Thermal Overload Relay
• Overload Relay is Connecting between Main Contactor and Motor
• NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and Common Connection of Start/Stop Button.

Working principle of DOL Starter

The main heart of DOL starter is Relay Coil. Normally it gets one phase constant from incoming supply Voltage (A1).when Coil gets second Phase relay coil energizes and Magnet of Contactor produce electromagnetic field and due to this Plunger of Contactor will move and Main Contactor of starter will closed and Auxiliary will change its position NO become NC and NC become (shown Red Line in Diagram)  .

Pushing Start Button

When We Push the start Button Relay Coil will get second phase from Supply Phase-Main contactor(5)-Auxiliary Contact(53)-Start button-Stop button-96-95-To Relay Coil (A2).Now Coil energizes and Magnetic field produce by Magnet and Plunger of Contactor move. Main Contactor closes and Motor gets supply at the same time Auxiliary contact become (53-54) from NO to NC .

Release Start Button

Relay coil gets supply even though we release Start button. When We release Start Push Button Relay Coil gets Supply phase from Main contactor (5)-Auxiliary contactor (53) – Auxiliary contactor (54)-Stop Button-96-95-Relay coil (shown Red / Blue Lines in Diagram).

In Overload Condition of Motor will be stopped by intermission of Control circuit at Point 96-95.

Pushing Stop Button

When we push Stop Button Control circuit of Starter will be break at stop button and Supply of Relay coil is broken, Plunger moves and close contact of Main Contactor becomes Open, Supply of Motor is disconnected.