Question

With control circuit diagram, explain the working of direct on-line motor starter.

Hint:

The DOL starter provides built-in under-voltage/no-voltage protection. If power fails, the contactor coil drops out and opens the contacts. When power returns, the motor will not start unexpectedly until the operator presses the START button again.

Answer 1

Step 1: What is a Direct On-Line (DOL) Starter?
A DOL starter connects a three-phase induction motor directly across the full line voltage. It consists of a contactor with a magnetic coil, an auxiliary contact (for latching), and a thermal bimetallic Overload Relay (OLR) for safety.

Step 2: Step-by-Step Working of the Control Circuit:
The operation of the control circuit is highly structured:

• Starting Sequence: When the operator presses the green START button (Normally Open - NO), the circuit closes.
  Current flows from Phase ($L_1$) through the closed Overload Relay (OLR) contact, the closed red STOP button (Normally Closed - NC), and the pressed START button to energize the Contactor Coil ($C$) to Neutral ($N$).
  The energized coil creates a strong magnetic field that pulls the main contactor plunger down, closing the three main power contacts to connect the motor directly to the $3$-phase supply lines.
• Latching Holding Circuit: Simultaneously, the contactor closes its auxiliary NO Contact connected in parallel with the START button.
  When the operator releases the START button, it springs back to its open position. However, current continues to flow to the coil through this closed auxiliary contact. This keeps the contactor energized, allowing the motor to run continuously.
• Stopping Sequence: When the red STOP button (NC) is pressed, it breaks the control circuit.
  The contactor coil is de-energized, releasing the main and auxiliary contacts. The motor is disconnected from the supply and comes to a halt.
• Overload Protection: If the motor draws excessive current, the bimetallic OLR heats up and opens its NC contact, breaking the control circuit. This de-energizes the contactor and safely disconnects the motor.