Multiple Choice Questions on Transformer | Page – 11

  1. Major insulation of transformer includes

    Two major insulation of transformer -
    1. Insulation between windings and earthed i. e. LV and core, HV and yoke
    2. Insulation between LV winding and HV winding.

  2. What is the limitation imposed on machine design?

    Temperature rise : If temperature rises under higher output the insulation is deteriorated and it ultimately affects the life of machine.
    Insulation : Breakdown may occur due to high voltage gradient which may lead to temperature rise and hence mechanical damage will take place.
    Efficiency : If efficiency of machine increases then cost of the machine will also increase.

  3. Types of air cooling of transformer

    Types of air cooling of transformer – AN ( air natural ), ANON ( air natural oil natural ), ABAF ( air blast / air force).

  4. Specific magnetic loading is the ratio of

    Specific magnetic loading is the ratio of total flux in air gap to area of flux path in air gap.

  5. Area of yoke in transformers is what % of?

    By keeping the yoke area 15 - 20 % more, the yoke flux density is reduced thereby reducing the iron losses in the yoke position.

  6. How is the window space factor related to the voltage rating of transformer?

    As voltage rating increases more and more space is needed for insulation and clearances thereby reducing window space factor.

  7. What is the effect of frequency variation on eddy current loss in transformer core if voltage remains constant?

    Eddy current loss is unchanged in transformer core due to frequency variation even if voltage remains same.

  8. Which of these is necessary for making connection of external circuit with windings?

    Bushing is necessary to make connection of external circuit i.e. overhead line with windings of transformer.

  9. With 30 % tapping on its auto transformer starter, the starting torque of a 3 phase induction motor is 100 N-m if 60 % tapping is utilized the new starting torque would be

    T ∝ V2, when 60 % of tapping is used voltage applied to motor is doubled when compared to 30 % of tapping hence torque becomes 4 times.

  10. A 500 kVA 2300 / 230 V, 60 Hz single phase transformer has a high voltage winding resistance of 0.65 ohm and a low voltage winding resistance of 0.0035 ohm the equivalent resistance of the machine referred to the secondary is

    V1 = 2300 V; V2 = 230 V; R1 = 0.65 ohm; R2 = 0.0035 ohm;
    K = V2 / V1 = 230 / 2300 = 1/10
    Equivalent resistance referred to secondary R02 = R2 + K2 X R1
    or R02 = 0.0035 + (1/10)2 X 0.65 = 0.01 ohm.

  11. A 400/200 V transformer has a pu impedance of 0.05. The HV side voltage required to circulate the full load current during short circuit test is

    In general about 20% of voltage is required to get full load current.

  12. Leakage reactance of transformer vary with number of turns as

    Leakage reactance of transformer is directly proportional to the square of number of turns.

  13. Leakage reactance of transformer depends upon

    Leakage reactance of transformer depends upon- no. of turns, winding configuration, supply frequency.

  14. Mineral insulating oil are the most widely used for liquid insulation for

    Mineral insulating oil are used widely in liquid insulation due to its easy availability, low price and good electrical as well as heat transfer properties.

  15. Transformer with current rating of 2000 A has what type of winding?

    HELICAL WINDING is mostly used for L.V coils of medium and high capacity transformer with currents as high as say 2000 A.

  16. Minor insulations include

    Minor insulation includes conductor insulation, insulation between turns and in layers and coils.

  17. What is the flash point of transformer oil in degree centigrade?

    Flash point of transformer oil is the ignition temperature of oil vapour ie lowest temperature at which the oil gives off sufficient inflammable vapour to produce a momentary flash on application of a small flame. It is about 140 degree centigrade.

  18. Important property of core of a transformer is

    Core of a transformer has high permeability, as it causes the magnetic field lines to be concentrated in the core material.