MCQs on Electrical Machines

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01․ If input of a transformer is square wave the output will be
pulsed wave.
square wave.
triangular wave.
sine wave.

In a square wave the changes in amplitude, takes place only at the side edge of wave form hence transformation action takes place at these edges only. Because of that, the output voltage will be in shape of pulsed wave.

02․ Two transformers with identical rating, are designed with flux densities of 1.2 and 1.4 Wb / m2 respectively. The weight of first transformer per kVA is
less than that of second transformer.
equal to that of second transformer.
more than that of second transformer.
none of these.

The voltage equation of a transformer is V = 4.44.φ fT Volt. Hence for the identical voltage rating flux φ of both transformers must be same if we consider f and T of both transformer are same. Again φ = BmA. Where Bm is maximum flux density and A is the area of cross-section of the transformer core. As φ is same for both transformers, the area of cross section of transformer core of the first transformer is more than that of the second transformer means the weight of the first transformer is more.

03․ The flux involved in the emf equation of a transformer has
rms value.
maximum value.
average value.
total value.

Derivation of emf equation of a transformer involves derivative of a function of alternating flux wave in the core. The expression of flux function is the product of maximum flux and a cosine function that is φ cosθt. Obviously, an emf equation in its expression involves the maximum value of flux.

04․ The no load current in a transformer lags the applied voltage by
about 75°.
about 115°.

Ideally a transformer draws the magnetizing current, lags primary applied voltage by 90°. But the transformer also has core loss current component which will be in phase with applied voltage. And no load current is nothing but the vector summation of these two currents. Hence, the no load current will not lag behind applied voltage by exactly 90° but it lags somewhat less than 90°. It is in practice generally about 75°.

05․ The leakage flux in a transformer depends upon
the applied voltage.
the load current.
the frequency.
the mutual flux.

Whenever the transformer is loaded, load current starts to flow in secondary winding which produces secondary flux in the core of transformer. For neutralizing this secondary flux primary winding draws current from source to produce counter flux in the core. But in this phenomenon some of the flux produced in the secondary will not linked with primary winding and similarly some of the flux produced in primary will not link with secondary winding, as all the flux will not pass through the core. Those fluxes pass outside the core and do not link with other winding, are called leakage flux. As this phenomenon of leakage flux comes into picture only when load is connected to the transformer and obviously varies with load.

06․ For satisfactory parallel operation of two transformers which of the following conditions is most essential,
polarities of all transformers must be same.
percentage impedance of all transformers must be same.
voltage ratio of all transformers must be same.
MVA rating of all transformer inversely proportional to impedance.

Percentage impedance of all transformers operated in parallel should be same but if there is a difference, parallel operation is still possible but load sharing by the transformers may not be as desired. If voltage ratio of all transformers operated in parallel is not same, still parallel operation of transformers is possible but there will be local circulating current in-between transformers. We can always put different MVA rated transformers in parallel but load sharing would be according to there MVA ratings if other conditions are properly fulfilled. But if polarities two transformers are not properly connected there will not be any output at all since total current will locally circulated in between transformers no current will come to the load. We can compare this phenomenon with two oppositely connected battery.

07․ If a transformer has turns ratio K, the primary and secondary current are I1 and I2 respectively and magnetizing current and core loss component of no load current Iμ and Iw respectively, then
I1 = KI2 + Iw.
I1 = KI2 + Iw + Iμ.ok
I1 = KI2 + Iμ.
I1 = KI2.

Total primary current drawn from source by an electrically loaded transformer has to supply the load at secondary, has to magnetize the core and also has to compensate the core loss of transformer. So primary current of electrically loaded transformer has three components, primary equivalent of secondary load current i.e., product of turns ratio and secondary current, second one is magnetizing component and third one is obviously core loss component.

08․ Which of the following transformer insulating oil generally used in India?
Paraffin based insulating oil.
Naphtha based insulating oil.
Aromatics based insulating oil.
Olefin based insulating oil.

Transformer insulating oil consists of four major classes of organic compounds, namely, paraffin, naphtha, aromatics and olefins. Generally paraffin based and naphtha based insulating oil are used in transformer. Paraffin based insulating oil has faster deterioration rate than naphtha oil and the sludge of paraffin oil is not soluble, hence can obstruct the convectional flow of oil during cooling, but still in India this oil is hugely being used due to its easy availability and insolubility of sludge is somewhat overcome due to warm weather condition of India.

09․ Winding conductors of an electrical power transformer are mainly insulated by
cotton insulation.
rubber insulation.
paper insulation.
wooden insulation.

Normally crepe kraft papers are used for insulation purpose inside oil immersed transformer as because the deterioration rate of paper inside oil is very slow compared to other insulating materials.

10․ The overall power factor of an on load transformer
depends on the power factor of the load.
is always lagging.
is always unity.
is always leading.

Whenever we calculate over all power factor of a transformer, we consider the impedance of the electrically loaded transformer which consists of impedance of that transformer and impedance of its load. As power factor of load depends upon its nature of impedance, obviously overall power factor of an on load transformers depends on the power factor of the load.