Electrical machines miscellaneous

Electrical machines miscellaneous

Easy Questions
Moderate Questions

Electrical Machines

Electrical machines miscellaneous
  1. A single phase, 50 kVA, 1000V/100 V two winding transformer is connected as an autotransformer as shown in the figure.

    The kVA rating of the autotransformer is ______.








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    KVA rating of Auto transfer = 1000 V × 550 A = 550 KVA
    or, = 1100 V × 500 A = 550 KVA.

    Correct Option: B


    KVA rating of Auto transfer = 1000 V × 550 A = 550 KVA
    or, = 1100 V × 500 A = 550 KVA.

  1. A 15kW, 230 V dc shunt motor has armature ci r cuit r esi st ance of 0.4 Ω and field cir cuit resistance of 230 Ω. At no load and rated voltage, the motor runs at 1400 rpm and the line current drawn by the motor is 5 A. At full load, the motor draws a line current of 70A. Neglect armature reaction. The full load speed of the motor in rpm is ________.








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    Given 15kW, 230V dc shunt motor armature resistance Ra = 0.4 Ω
    Field resistance, Rsh = 230 Ω

    ∴ Eb1 = 230 – 4 × 0.4 = 228.4 V

    ∴ Eb2 = 230 – 69 × 0.4 = 202.4 V

    ∴
    N2
    		 =
    Eb2
    		 ×
    φ1
    N1Eb1φ2

    =
    Eb2
    		 (∵ Flux is constant)
    Eb1

    N2 = N1 ×
    Eb2
    Eb1

    = 1400 ×
    202.4
    		 = 1240 rpm
    228.4

    Correct Option: C

    Given 15kW, 230V dc shunt motor armature resistance Ra = 0.4 Ω
    Field resistance, Rsh = 230 Ω

    ∴ Eb1 = 230 – 4 × 0.4 = 228.4 V

    ∴ Eb2 = 230 – 69 × 0.4 = 202.4 V

    ∴
    N2
    		 =
    Eb2
    		 ×
    φ1
    N1Eb1φ2

    =
    Eb2
    		 (∵ Flux is constant)
    Eb1

    N2 = N1 ×
    Eb2
    Eb1

    = 1400 ×
    202.4
    		 = 1240 rpm
    228.4

  1. A star connected 400V, 50Hz, 4 pole synchronous machine gave the following open circuit and short circuit test results :
    Open circuit test : Voc = 400 V (rms, line-to-line) at field current, If = 2.3 A
    Short circuit test : Isc = 10 A (rms, phase) at field current, If = 1.5 A
    The value of per phase synchronous impedance in Ω at rated voltage is _____________








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    For open circuit test :
    VOC = 400 V (L – L)1 If = 2.3 A
    For short circuit test :
    ISC = 10 A (phase), If = 1.5 A
    We know,

    ∴ ISC =
    2.3
    		 × 10 = 15.33 A
    1.5

    ∴ Zs =
    VOC
    ISC

    =
    400 / √3
    		 = 15.06
    15.33

    = 15.06 Ω / ph

    Correct Option: B

    For open circuit test :
    VOC = 400 V (L – L)1 If = 2.3 A
    For short circuit test :
    ISC = 10 A (phase), If = 1.5 A
    We know,

    ∴ ISC =
    2.3
    		 × 10 = 15.33 A
    1.5

    ∴ Zs =
    VOC
    ISC

    =
    400 / √3
    		 = 15.06
    15.33

    = 15.06 Ω / ph

  1. An 8-pole, 3-phase, 50 Hz induction motor is operating at a speed of 700 rpm. The frequency of the rotor current of the motor in Hz is _______








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    Given, P = 9, f = 50 Hz, N = 700 rpm
    4 Frequency of Rotor current = s.f.

    S =
    Ns - N
    		 =
    750 - 700
    		 = 0.067
    Ns750

    ∴ fr = (0.067) × 50 = 3.33 Hz

    Correct Option: C

    Given, P = 9, f = 50 Hz, N = 700 rpm
    4 Frequency of Rotor current = s.f.

    S =
    Ns - N
    		 =
    750 - 700
    		 = 0.067
    Ns750

    ∴ fr = (0.067) × 50 = 3.33 Hz

  1. With an armature voltage of 100V and rated field winding voltage, the speed of a separately excited DC motor driving a fan is 1000 rpm, and its armature current is 10A. The armature resistance is 1Ω. The load torque of the fan load i s proportional to the square of the rotor speed. Neglecting rotational losses, the value of the armature voltage (in Volt) which will reduce the rotor speed to 500 rpm is ___________.








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    NA

    Correct Option: A

    NA