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A soft starter for an induction motor works by

  • A.varying the frequency applied to the machine from zero upwards
  • B.ramping up the stator voltage with thyristors
  • C.inserting and removing resistance in the stator circuit in graded steps
  • D.changing the number of poles for which the stator is wound

Answer: B

Phase-angle control of back-to-back thyristors raises the voltage gradually, limiting the current at the cost of torque. It cannot give more torque per ampere, which is where a full converter with frequency control differs fundamentally.

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