Question

Star delta motor starters are used for ________ motors.

• A. less than 1 kW
• B. 2 kW
• C. greater than 4 kW
• D. 3 kW

Hint:

To remember:

• DOL Starters = For small motors $\leq 4\text{ kW}$ (or $5\text{ HP}$).
• Star-Delta Starters = For medium-to-large motors $> 4\text{ kW}$ (or $5\text{ HP}$).

Answer 1

The Correct Option is C

Step 1: Purpose of Motor Starters:
At starting, a three-phase induction motor is stationary (slip $s = 1$), and its back emf is zero. As a result, the motor acts like a short-circuited transformer and draws a massive starting current (typically $5\text{ to }7$ times its full-load running current).

Step 2: Categorizing Starter Applications:

• Direct-On-Line (DOL) Starter: Connects the motor directly across the full supply voltage. This starter is typically limited to small motors up to 4 kW (or 5 HP) because their lower starting currents do not cause significant voltage drops in the local supply network.
• Star-Delta Starter: Connects the stator windings in a Star ($\text{Y}$) configuration during startup, which reduces the voltage across each phase to $1/\sqrt{3}$ ($\approx 58\%$) of the line voltage. This reduces the starting current and torque to $1/3$ of their full-voltage delta values. Once the motor accelerates to speed, the starter switches the windings to a Delta ($\Delta$) configuration for normal run operation. This method is typically used for larger motors greater than 4 kW (5 HP).

Step 3: Conclusion:
Therefore, Star-Delta starters are used for motors greater than 4 kW, which corresponds to option (C).