In a Star-Delta Starter used for induction motors, the starting torque is reduced to approximately:

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  1. 33% of full-load torque
  2. 66% of full-load torque
  3. 50% of full-load torque
  4. 75% of full-load torque

Answer (Detailed Solution Below)

Option 1 : 33% of full-load torque
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Star-Delta Starter

In star delta starting an induction motor is connected in through a star connection throughout the starting period. Then once the motor reaches the required speed, the motor is connected through a delta connection.

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As the winding is star-connected, the voltage across each phase of the winding is \({V_L\over \sqrt{3}}\).

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In a delta connection, \(V_L=V_P\)

The starting torque is proportional to the square of the phase voltage.

\({T_Y\over T_{\Delta}}=({V_L\over \sqrt{3}\times V_L})^2\)

\({T_Y\over T_{\Delta}}={1\over 3}\)

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