When a Star Delta Starter is used with 3 – phase squirrel cage motors, in which sequence is the connection made?

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ALP CBT 2 Electronic Mechanic Previous Paper: Held on 21 Jan 2019 Shift 2
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  1. First Delta, then Star modes
  2. First delta, then Star and again Delta modes
  3. First Star, then Delta modes
  4. First Star, then Delta and again Star mode 

Answer (Detailed Solution Below)

Option 3 : First Star, then Delta modes
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Detailed Solution

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  • In the star-delta starting method of induction motor, a two-way switch is used to connect the stator winding in star while starting and in delta while running at normal speed.
  • In the star-delta starting method of induction motor, the starting current is reduced approximately by 1/3 times as compared to the original.
  • The starting torque will 1/3 times that it will be for delta connected winding.
  • During starting the stator winding is star connected, so that voltage over each phase in the motor will be reduced by a factor of 1/√3 of that would be for delta connected winding.
  • Hence a star-delta starter is equivalent to an auto-transformer of ratio 1/√3 or 57.73 % reduced voltage.
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