MCQ & answers on electrical engineering

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353) The overall power factor of a on load transformer

A. depends on the power factor of the load.

B. is always lagging.

C. is always unity.

D. is always leading.

 A  B  C  D

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.

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

A. 7

B. 12

C. 4

D. 3

 A  B  C  D

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 K1.K2.

355) A short circuit test on a transformer gives

A. Copper loss at full load

B. Copper loss at any load

C. Copper loss at half load

D. Copper loss at over load

 A  B  C  D

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.

356) A step up transformer increases

A. Power

B. Voltage

C. Current

D. Frequency

 A  B  C  D

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.

357) In a step down transformers which one statement of following is true?

A. number of secondary turns is less than primary turns

B. secondary power is less than primary power

C. secondary current is less than primary current.

D. number of secondary turns is more than that of primary

 A  B  C  D

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.

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

A. zero

B. leading

C. unity

D. lagging

 A  B  C  D

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

359) When a transformer is loaded, the secondary terminal voltage gains. That means power factor is

A. lagging

B. zero

C. unity

D. leading

 A  B  C  D

Approximate relation of no load and full load voltages in a transformer is E2 = V2 + (R.cosΘ + X.sinΘ). Where E2, V2, R, X &.cosΘ are no load voltage,full load voltage, secondary resistance of transformer, secondary leakage reactance of transformer & power factor respectively. If power factor is leading, Θ is negative which means {R.cos( - Θ) + X.sin( - Θ)} or, {R.cosΘ - X.sinΘ} may be less than zero if R.cosθ < X.sinΘ. In that case full load secondary voltage V2 may be greater than no load secondary voltage E2.

360) If Kh is hysteresis constant, f is supply frequency, Bm maximum flux density in core of transformer, the expression of hysteresis loss in transformer, is given by

A. Kh.f.Bm1.6

B. Kh.f.Bm3

C. Kh.f2.Bm1.6

D. Kh.f.Bm.

 A  B  C  D

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

361) If Ke is eddy current constant, Kf is form factor, f is supply frequency, Bm maximum flux density in core of transformer, the expression of eddy current loss in transformer, is given by

A. Ke.f2. KfBm2

B. Ke.f. Kf2.Bm2

C. Ke.f2.Kf2.Bm2

D. Ke.f2. Kf2.Bm

 A  B  C  D

If Ke is eddy current constant, Kf is form factor, f is supply frequency, Bm maximum flux density in core of transformer, the expression of eddy current loss in transformer, is given by
Ke.f2. Kf2.Bm2

362) Stray losses in transformer occurs in

A. Steel core

B. Winding conductors and steel structures of transformer

C. Transformer insulating oil

D. Paper insulation in transformers

 A  B  C  D

Stray loss occurring in the mechanical structure and winding conductor due to the stray fluxes.

363) If iron loss and full load copper loss of any transformer are denoted by Pi and Pc, then the load corresponding to maximum efficiency is given by

A. (Pi / Pc)0.5 X Full Load

B. (Pi / Pc)2 X Full Load

C. (Pi / Pc) X Full Load

D. (Pi / Pc)1.6 X Full Load

 A  B  C  D

At maximum efficiency of transformer, iron-loss = copper-loss e.i. Pi = I22. R2’. Where I2 is load current at maximum efficiency and R2’ resistance of transformer referred to secondary.Therefore,
I2 = (Pi / R2’)0.5 = (Pi / IL2.R2’)0.5 X IL
= (Pi / Pc)0.5 X IL Where IL is full load current of transformer.

364) Secondary winding of an auto transformer is also called

A. compensating winding

B. common winding

C. tertiary winding

D. damping winding

 A  B  C  D

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.

365) Saving of copper in auto transformer is increased as compared to two winding transformer, when transformation ratio K approaches to

A. unity

B. infinity

C. two

D. zero

 A  B  C  D

Saving of copper weight in auto transformer = Transformation ratio (K) X Weight of copper in two winding transformer. Thus it can be visualized that saving will increase as K approaches unity.

366) Tertiary winding in a transformer mostly

A. star connected

B. delta connected

C. zig-zag star connected

D. zig-zag delta connected

 A  B  C  D

When faults occur on the primary or secondary sides, considerable unbalanced of phase voltage, may be produced which is compensated by large circulating current through this closed delta connected tertiary winding.

367) The main purpose of providing tertiary winding in a three phase transformer

A. to suppress harmonic voltages and to limit voltage unbalance when the main load is asymmetrical

B. to inter connect three supply systems operated at different voltages

C. to carry adding extra load to the transformer which for some reason must be kept isolated

D. none of the above

 A  B  C  D

The purposes of providing tertiary winding in a three phase transformer to carry adding extra load to the transformer which for some reason must be kept isolated, to inter connect three supply systems operated at different voltages but most important purpose is to suppress harmonic voltages and to limit voltage unbalance when the main load is asymmetrical.

368) When operating two or more transformers in parallel, % impedance of these transformers should be

A. inversely proportional to MAV rating of transformers.

B. proportionate to MAV rating of transformers.

C. same.

D. such that actual impedance of all transformers would be same

 A  B  C  D

Load sharing by transformers operated in parallel are as per their MVA rating if % impedances of the transformers are same.


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