Induction motors not kept running in Star Connections

Why Induction Motors Are Not Kept Running in Star Connection

Induction motors are sometimes started in Star connection (using a Star-Delta starter) but are always run in Delta for normal operation. The reasons are:

1. Voltage Reduction in Star

• In Star connection,

  $$V_{phase} = \frac{V_{line}}{\sqrt{3}}$$

• This means each winding receives only 57.7% of the line voltage.

2. Current Reduction

• Since $I_{phase} = I_{line}$, the line current drawn is lower.

• Current is reduced in proportion to the reduced voltage.

3. Torque Reduction

• Motor torque is proportional to the square of the applied voltage:

  $$T \propto V^2$$

• So, in Star:

  $$T_{star} = \left(\frac{1}{\sqrt{3}}\right)^2 \times T_{delta} = \frac{1}{3} T_{delta}$$

• This means the motor can produce only one-third of the rated torque.

4. Power Reduction

• Motor input power is proportional to voltage × current.

• Because both are reduced in Star, the motor delivers only about one-third of its rated power compared to Delta.

5. Inefficiency & Underutilization

• Running in Star keeps the motor underloaded, unable to handle its designed mechanical load.

• Efficiency and performance drop significantly.

• That’s why Star is used only for starting, to limit inrush current, and then switched to Delta for normal running.

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✅ Conclusion:
Induction motors are not kept running in Star connection because they deliver only one-third torque and power, making them inefficient and unsuitable for normal operation. Hence, Star is used for starting only, while Delta is used for continuous running.

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