MCQs on Electrical Machines

01․ If in a sub – station there is one 132 / 33 kV transformer whose secondary is connected with primary of one 33 / 11 kV transformer, the total transformation ratio of the sub – station will be,

If two transformers are connected in such a way that secondary of first one is connected with primary of second, and if transformation ratios of these two transformers are K₁ and K₂, total or overall transformation ratio of this system will be K₁K₂.

02․ A short circuit test on a transformer gives

In a short circuit test we apply variable voltage at HV terminals of a transformer keeping LV side short circuited. The applied voltage is varied until the input current reaches its rated value or full load value. As the current drawn from source is full load current the test normally measures the copper losses of transformer at full load. But since, by adjusting variable supply voltage we can control the supply current at any value, a short circuit test can give copper loss at any load.

03․ A step up transformer increases

A step up electrical transformer transforms a power from one circuit to another without changing its frequency to higher voltage level and proportionate lower current level.

04․ In a step down transformer which one of the following statements is true?

In a transformer voltage per turn is same in both primary and secondary winding hence, induced voltage across the winding is proportional to number of turns in the winding. As in step down transformer secondary voltage is less than primary, the number of turns in secondary is obviously less than that of primary.

05․ When a transformer is loaded, the secondary terminal voltage drops. That means power factor is

Approximate relation of no load and full load voltages in a transformer is, Where E₂, V₂, R, X and cosθ  are no load voltage, full load voltage, secondary resistance of transformer, secondary leakage reactance of transformer and power factor respectively. If power factor is lagging, θ  is positive which means ( Rcosθ  + Xsinθ ) > 0. Thus E₂ > V₂.

06․ If K_h is hysteresis constant, f is supply frequency, B_m maximum flux density in core of transformer, the expression of hysteresis loss in transformer, is given by

If K_h is hysteresis constant, f is supply frequency, B_m maximum flux density in core of transformer, the expression of hysteresis loss in transformer is given by K_hfB_m^x watts. Where x ranges from 1.5 to 2.5.

07․ If K_e is eddy current constant, K_f is form factor, f is supply frequency, B_m maximum flux density in core of transformer, the expression of eddy current loss in transformer, is given by

If K_e is eddy current constant, K_f is form factor, f is supply frequency, B_m maximum flux density in core of transformer, the expression of eddy current loss in transformer is given by K_ef²K_f²B_m².

08․ Stray losses in transformer occurs in

Stray loss occurs in the mechanical structures and winding conductors due to stray fluxes.

09․ If iron loss and full load copper loss of any transformer are denoted by P_i and P_c, then the load corresponding to maximum efficiency is given by

At maximum efficiency of transformer, iron-loss = copper-loss e.i. P_i = I₂²R₂. Where I₂ is load current at maximum efficiency and R₂ resistance of transformer referred to secondary. Where I_L is full load current of transformer and R₂ is the equivalent resistance of the transformer referred to load side.

10․ Secondary winding of an auto transformer is also called

The auto transformer has a single continuous winding which is used for both input and output voltages. Secondary winding of auto transformer carries both the supply current and load current. But these two currents are practically in phase opposition. Hence secondary winding of auto transformer is also called common winding.