Electrical machines miscellaneous

Electrical machines miscellaneous

Easy Questions
Moderate Questions

Electrical Machines

Electrical machines miscellaneous
  1. In an induction motor, if air gap is increased, then








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    NA

    Correct Option: C

    NA

  1. If a three phase slip ring induction motor is fed from the rotor side with startor winding short circuited, then frequency of the current flowing in the short circuited startor is








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    Induction motor is similar to a transformer with secondary short-circuited if supply is supplied across primary or primary short-circuited if supply is applied across secondary. If supply is applied across rotor, then flux which is revolving in nature is developed. It sweeps past the startor surface and cuts conductor of startor and so emf is induced in the startor which is revolving at natural but with lesser speed than the rotor. So due to this relative speed, the rotor starts to rotate. This explanation is vice-versa to the explanation, when the startor is energised with supply. So frequency of startor at the above specific condition is slip frequency.

    Correct Option: A

    Induction motor is similar to a transformer with secondary short-circuited if supply is supplied across primary or primary short-circuited if supply is applied across secondary. If supply is applied across rotor, then flux which is revolving in nature is developed. It sweeps past the startor surface and cuts conductor of startor and so emf is induced in the startor which is revolving at natural but with lesser speed than the rotor. So due to this relative speed, the rotor starts to rotate. This explanation is vice-versa to the explanation, when the startor is energised with supply. So frequency of startor at the above specific condition is slip frequency.

  1. The power input to a 415 V, 50 Hz, 6 pole, 3-phase induction motor running at 975 rpm is 40 kW. If startor losses are 1 kW and friction and windage losses total 2 kW, then efficiency of the motor is








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    NA

    Correct Option: A

    NA

  1. A three phase, wound rotor induction motor is to be operated with slip energy recovery in the constant torque mode, when it delivers an output power Po at slip s. Theoretically, maximum power available for recovery at the rotor terminals, is








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    Tfl =
    3
    		 .
    V²
    ωsr2/sfl

    and   T =
    3
    		 .
    V'²
    ωsr2/s

    Given: Tfl = T and V' = (1. 1) V
    ∴  s =
    V
    		2 ⋅ sfl = 0.03 ×
    1
    		0.6 = 2.48%
    V'1.1

    Correct Option: B

    Tfl =
    3
    		 .
    V²
    ωsr2/sfl

    and   T =
    3
    		 .
    V'²
    ωsr2/s

    Given: Tfl = T and V' = (1. 1) V
    ∴  s =
    V
    		2 ⋅ sfl = 0.03 ×
    1
    		0.6 = 2.48%
    V'1.1

  1. A 3-phase induction motor has a full-load slip of 3 percent at normal voltage. Which of the following will be the value or slip of the motor if it develops same torque theoretically while operating at 110 percent of its normal voltage?








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    NA

    Correct Option: A

    NA