Star–Delta starting is a common method used to start a three-phase induction motor while reducing the high starting current. It is mainly used for medium and large motors.

1. Basic Idea

When an induction motor starts directly from the line (DOL), it draws a very large current (about 5–7 times the rated current). This can cause:

• Voltage drop in the supply line

• Overheating of windings

• Stress on electrical equipment

To avoid this, the Star–Delta starter first connects the motor windings in star (Y) during starting and later switches to delta (Δ) for normal operation.

2. Working Principle

Step 1: Starting in Star Connection

• The three stator windings are connected in star (Y).

• Each phase receives $V_L / \sqrt{3}$voltage instead of full line voltage.

• Because the voltage is lower:

  • Starting current reduces to about one-third

  • Starting torque reduces to about one-third

Step 2: Running in Delta Connection

• After the motor reaches about 80–90% of rated speed, the starter switches the winding connection from star to delta.

• In delta connection, each winding receives full line voltage.

• The motor then operates at rated torque and power.

3. Components of a Star–Delta Starter

A typical starter consists of:

• Main Contactor (KM1) – connects motor to supply

• Star Contactor (KM2) – forms star connection during starting

• Delta Contactor (KM3) – forms delta connection during running

• Timer – controls transition time from star to delta

• Overload Relay – protects motor from excessive current

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