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Which of the following methods of starting is required for induction motors of high rating?
1. Auto-transformer starters
2. Rotor resistance starters
3. Start delta starters
4. Direct on line starters

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Correct Answer - Option 1 : Auto-transformer starters

Methods of Starting 3-Phase Induction Motors:

The method to be employed in starting a given induction motor depends upon the size of the motor and the type of the motor.

The common methods used to start induction motors are:

  1. Direct-on-line starting
  2. Stator resistance starting
  3. Autotransformer starting
  4. Star-delta starting
  5. Rotor resistance starting

Methods (1) to (4) are applicable to both squirrel-cage and slip ring motors.

However, method (5) is applicable only to slip-ring motors.

If any one of the first four methods is used for starting squirrel cage motors, depending upon, the size of the motor.

Note: Slip ring motors are invariably started by rotor resistance starting.

Direct-on-line Starting:

  • In this method, the motor is started by connecting it directly to a 3-phase supply.
  • The impedance of the motor at standstill is relatively low and when it is directly connected to the supply system, the starting current will be high (4 to 10 times the full-load current) and at a low power factor.
  • This method of starting is suitable for relatively small (up to 10 HP) machines.

Star-delta (Y-Δ) Starting:

  • The stator winding of the motor is designed for delta operation and is connected in star during the starting period.
  • When the machine is up to speed, the connections are changed to the delta.
  • With star-connection during starting, stator phase voltage is (1/√3) times the line voltage.
  • Consequently, starting torque is (1/√3)or 1/3 times the value it would have with Δ-connection.
  • This is rather a large reduction in starting torque. The reduction in voltage is fixed.
  • This method of starting is used for medium-size machines (up to about 25 HP).

Autotransformer Starting:

  • This method also aims at connecting the induction motor to a reduced supply at starting and then connecting it to the full voltage as the motor picks up sufficient speed.
  • The tapping on the autotransformer is so set that when it is in the circuit, 65% to 80% of line voltage is applied to the motor.
  • Autotransformer starting has several advantages viz low power loss, low starting current, and less radiated heat.
  • For large machines (over 25 HP), this method of starting is often used. 

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