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A soft start achieved by ramping the frequency from zero differs from one achieved by ramping the voltage in that it

  • A.gives full torque per ampere throughout
  • B.gives less torque per ampere than voltage ramping does
  • C.requires no converter to be fitted at all
  • D.cannot be used with an induction machine

Answer: A

Frequency ramping keeps the flux and therefore the slip small, so the machine develops rated torque at rated current from standstill. Voltage ramping reduces the flux and obtains low speed only through large slip, which gives poor torque and heats the rotor.

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