BRAINWAVE
GATE-2012
Electrical Machines
Max. Marks: 45(Q.1 – Q.45 1 Mark) Max. Time: 90 Min.s
1.For understanding the behaviour of a transformer, the following laws may be called for
1. Lenz’s law 2. Newton’s second law 3. Faraday’s laws of electromagnetic induction
4. Ohm’s law 5. Fleming’s right hand rule 6. Right hand grip rule
From these the correct answer is
(a) 1,3,4 (b) 2,3,4,5 (c) 1,3,4,5,6 (d) 1,3,4,6
2.In an ideal transformer, if K is some constant, then supply voltage V, in terms of its magnetizing current Im can be expressed as
(a) jKfIm (b) jKf/Im (c) –jKfIm (d) –jKf/Im
3.A transformer has sometimes two or more ratings depending upon the use of
(a) the cooling techniques (b) the type of windings
(c) the type of core (d) the type of insulation
4.A transformer at no load excited at rated voltage. Now a cut is made in the transformer yoke thus creating a small air gap. With this, the transformer core flux
(a) will decrease and magnetizing current Im will increase
(b) will remain constant and Im will increase
(c) as well as Im both will increase
(d) as well as Im both will decrease
5.If the secondary winding of an ideal transformer shown in the circuit has 40 turns, the number of turns in the primary winding for maximum power transfer to the 2 resistor will be
(a) 20 (b) 40 (c) 80 (d) 160
6.In a single phase transformer, polarities of terminals a & b at any instant are shown in the fig. At the same instant,
(a) c is +ve, d is +ve and flux is clockwise (cw) (b) c is -ve, d is +ve and flux is counter clockwise (ccw)
(c) c is -ve, d is +ve and flux is clockwise (cw) (d) c is +ve, d is -ve and flux is counter clockwise (ccw)
7.CRGO laminations in a transformer are used to minimise
(a) eddy current loss (b) hysteresis loss (c) both eddy current and hysteresis losses (d) ohmic loss
8.If a transformer primary is energised from a square wave voltage source, then its output voltage will be
(a) zero (b) a sine wave (c) a triangular wave (d) a pulsed wave
9. The no load current in a transformer lags the applied voltage by
(a) 900 (b) about 750 (c) 00 (d) about 1100
10. The leakage flux in a transformer depends upon
(a) the applied voltage (b) the frequency (c) the load current (d) the mutual flux
11. The useful flux of a transformer is 1 Wb. When it is loaded at 0.8 p.f lag, then its mutual flux
(a) may decrease to 0.8Wb (b) may increase to 1.01 Wb
(c) remains constant (d) may decrease to 0.99 Wb
12.Two transformers of the same type, using the same grade of iron and conductor materials are designed to work at the same flux and current densities; but the linear dimensions of one are two times those of the other in all respects. Then the ratio of kVA ratings of the two transformers closely equals
(a) 16 (b) 8 (c) 4 (d) 2
13.A 220/440 V, 50Hz, 5 kVA, single phase transformer operates on 220 V, 40 Hz supply with secondary open circuited. Then
(a) both eddy current & hysteresis looses decrease (b) both eddy current & hysteresis looses increase
(c) eddy current loss remains the same but hysteresis loss increase
(d) eddy current loss increases but hysteresis loss remains the same
14.The hysteresis & eddy current losses of a single phase transformer working on 200 V, 50 Hz supply are Ph & Pe respectively. The percentage decrease in these, when operated on a 160 V, 40 Hz supply are
(a) 32, 36 (b) 20, 36 (c) 25, 50 (d) 40, 80
15.The maximum efficiency for a transformer occurs at 80% of full load. Its core loss is Pc and ohmic loss is Poh. For this transformer, the ratio Pc/Poh is
(a) 0.8 (b) 1.25 (c) 0.64 (d) 0.8944
16. Frequency of the supply voltage to a transformer at no load is increased but the supply voltage is held constant. With this
(1) eddy current loss remains constant but hysteresis loss increases
(2) eddy current loss remains constant but hysteresis loss decreases
(3) magnetizing current increases but core loss current decreases
(4) both magnetizing and core loss currents decrease
From these, the correct answer is
(a) 2,3 (b) 2,4 (c) 1,3 (d) 1,4
17.The applied voltage to a transformer primary is increased keeping V/f constant. With this, the core loss will
(a) decrease & magnetizing current Im will increase (b) increase & Im will also increase
(c) remain constant & Im will also remain constant (d) increase & Im will remain constant
18. As the load on a transformer is increased, the core losses
(a) decrease slightly (b) increase slightly
(c) remain constant (d) may increase or decrease slightly depending upon the nature of load
19. Can a 50 Hz transformer be used for 25 Hz, if the input voltage is maintained constant at the rated value corresponding to 50 Hz?
(a) Yes. Since the voltage is constant, current levels will not change
(b) No. Flux will be doubled which will drive the core to excessive saturation
(c) No. Owing to decreased reactance of transformer, input current will be doubled at load
(d) Yes. At constant voltage, insulation will not be overstressed
20. In a transformer, if primary leakage impedance is neglected then
1. magnetizing current lags the applied voltage V1 by 900
2. Core loss current lags V1 by 900
3. Exciting current lags V1 by 900
4.core loss current is in phase with V1
5.Exciting current lags V1 by about 800
6. Magnetizing current lags V1 by about 800
From these, the correct statements are
(a) 1,4,5 (b) 3,4,6 (c) 1,4 (d) 1,2,6
21. A transformer secondary is connected to pure resistive load. The power factor on the primary side will be
(a) Near about .95 lead (b) near about .095 lag (c) zero (d) unity
22. A 50 Hz transformer having equal hysteresis and eddy current losses at rated excitation is operated at 45 Hz at 90% of rated voltage compared to rated operating point, the core loss under this condition
(a) reduces by 10% (b) reduces by 19% (c) reduces by 14.5% (d)Remains unchanged
23. In a 1-phase transformer, the magnitude of leakage reactance is twice that of resistance of both primary and secondary. With secondary short circuited, the input p.f is
(a) 1/√2 (b) 1/√5 (c) 2/√5 (d) 1/√3
24. A multimeter, for measuring resistance is connected to one terminal of primary and the other terminal of secondary. The multimeter reading would be
(a) zero (b) infinity (c) zero or infinity (d) equal to the resistance of the windings
25. For given base voltage and base volt amperes, the per unit leakage impedance of a transformer is x. What will be the per unit leakage impedance of this transformer when the voltage and volt-ampere bases are both doubled?
(a) 0.5x (b) 2x (c) 4x (d) x
26. At 50 Hz operation, a single phase transformer has hysteresis loss of 200 W and eddy current loss of 100 W. Its core loss at 60 Hz operation will be
(a) 432 W (b) 408 W (c) 384 W (d) 360 W
27. In a transformer, eddy current loss is 100 W which is half of the total core loss. If both the thickness of laminations and frequency are increased by 10%, the new core loss would be
(a) 256.41 W (b) 231 W (c) 267.41 W (d) 242 W
28. A 220 V, 50 Hz transformer with 0.35 mm thick laminations has eddy current loss of 120 W which is two third of the total core loss at no load. If this transformer is built with 0.7 mm thick laminations and is worked from 110 V, 25 Hz, then total no load loss would be
(a) 150 W (b) 510 W (c) 200 W (d) 45 W
29. A transformer fed from an alternator at 230 V, 50 Hz has eddy current loss of 50 W and hysteresis loss of 100 W. If the speed of the prime mover driving the alternator drops to 80% of its previous speed, then eddy current and hysteresis losses in the transformer would be
(a) 40 W, 80 W (b) 32 W, 80 W (c) 32 W, 64 W (d) 40 W, 64 W
30. Two windings of a transformer are indicated by terminals AB and CD as shown in fig. When a voltage of 110 V is applied across AB with CD short circuited, a voltage of 200 V appears across AC. The turns ratio from CD to AB is
(a) 3 (b) 1 (c) 3 or 1 (d) 2 or 1
31. A 400V/200/200V, 50Hz three winding transformer is connected as shown fig. The reading of the voltmeter V will be
(a) 0V (b) 400V (c) 600V (d) 800V
32. A transformer has leakage impedance of Ze= re+jxe. Its maximum voltage regulation occurs at a power factor of
(a) re/xe leading (b)re/ze lagging (c) xe/ze leading (d)re/ze leading
33.A transformer has leakage impedance of Ze= re+jxe. Zero voltage regulation for this transformer occurs at a power factor of
(a) re/xe leading (b)re/ze lagging (c) xe/ze leading (d)re/ze leading
34.A 1-phase transformer has p.u. leakage impedance of 0.02+j0.04. Its regulation at power factor 0.8 lagging and 0.8 leading are respectively
(a) 4%, 0.8% (b) 4%, -0.8% (c) 2.4%,-0.8% (d) 4%,-1%
35.The voltage regulation of transformer is mainly influenced by
(a) no-load current and load power factor (b) winding resistance and load power factor
(c) leakage fluxes and load power factor (d) winding resistance and core loss
36.A 1-phase transformer has a maximum efficiency of 90% at full load unity p.f. Efficiency at half load at the same p.f is
(a) 86.7% (b) 88.26% (c) 88.9% (d) 87.8%
37. full-load voltage regulation of a power transformer is zero when power factor of the load is near
(a)unity and leading (b) zero and leading
(c) Zero and lagging (d) unity and lagging
38. The voltage regulation of transformer at full load 0.8 pf lagging is 4%.Its voltage regulation at full-load 0.8pf leading
(a) will be positive (b) will be negative (c) may be positive (d)may be negative
39. The voltage regulation of transformer depend on its
1. Equivalent reactance 2.equivalent resistance 3. Load power factor
4. transformer size 5.load current
From these, the correct answer is
(a) 1 , 2 ,3 ,5 (b)1, 2, 3, 4, 5 (c) 1, 2, 4, 5 (d) 1, 2, 3, 4
40. The voltage regulation of a transformer at full load and 0.8 pf lagging is 2.5%.The voltage regulation at full load 0.8 leading will be
(a) -2.5% (b) zero (c) -0.9% (d) 2.5%
41.The efficiency of a transformer at full load and 0.8 pf lag is 90%.its efficiency at full load 0.8 pf leading will be
(a) Somewhat less than 90% (b) somewhat more than 90%
(c) 90% (d) 91%
42.Transformer maximum efficiency ,for a constant load current, occurs at
(a) at unity (b)zero pf leading (c) zero pf lagging (d) unity pf
43. Transformer at no –load behaves like
(a) a resistor, pf=0 (b) an inductive reactor, pf=0.2lagging
(c) a capacitive reactor, pf=0.2 leading (d) an inductive reactor, pf=0.8 lagging
44.which of the following statements are incorrect
1. Maximum voltage regulation of a transformer occurs at leading power factor
2. voltage regulation of a transformer is the maximum when load power factor (lagging) angle has the same value as the angle of equivalent impedance
3. voltage regulation of a transformer at zero power factor is always zero
4.voltage regulation of a transformer can be negative at leading power factor
Select the correct answer using the code given bellow:
(a) 1 and 3 (b) 2 and 3 (c) 2and 4 (d) 1 and 4
45.A transformer designed for operation of 60Hz supply is worked on 50Hz supply system without change its voltage and current ratings. When compared with full load efficiency at 60Hz, the transformer efficiency on full load at 50Hz will
(a)increases marginally (b) increases by a factor of 1.2
(c) remain unaltered (d) decreases marginally
