Electrical Machines- Transformers

Hey,below questions are gate previous years.........

1. A 220/440 V ,SO Hz, 5 kVA single phase transformer operates on 220 V,40 Hz supply with secondary winding. Then [GATE-1993]
a. the eddy cu rrent loss and hysteresis loss of the transformer decreases
b.  the eddy cu rrent loss and hysteresis loss of the transformer increases
c. the hysteresis loss of the transformer increases while eddy current loss remains the same
d. The hysteresis loss remains the same whereas eddy current loss decreases.

2. Two transformers of identical voltages but of different capacities are operating in parallel. For satisfactory load sharing [GATE-1994]
a.  impedences must be equal
b. pu impedences must be equal
c. pu impdences and X/R ratio must be equal
d. impdences and X/R ratio must be equal

3. Supply to one terminal of a connected three-phase core type transformer which is on no-load, fails. Assuming magnetic circuit symmetry, voltages on the secondary side will be [GATE-1995]
a. 230,230,115
b. 230,115,115
c. 345,115,115
d.345,0,345

3. Auto-transformer is used in transmission and distribution [GATE-1996]
a. when operator is not available
b. When iron losses are to be reduced
c. when efficiency considerations can be ignored
d. When transformation ratio is small

4. The low voltage winding ofa 400/230V, 1-phase, 50 Hz transformer is to be connected to a 25 Hz supply at 25 Hz, the supply voltage should be [GATE-1997]
a. 230    b. 460    c. 115    d. 65

5. The magnetizing current in a transformer is rich in [GATE-1998]
a. 3rd harmonic.    b. 5th harmonic
c. 7th harmonic.    d. 13thHarmonic

6. A 10 kVA, 400 V/200V single-phase transformers with 10% impedance draws a steady short circuit line current of [GATE-1999]
a. 50A.  b.150A. c. 250A  d. 350A

7. 1) A 3-phase delta/star transformer is supplied at 6000 V on the delta-connected side. The terminal voltage on the secondary side when supplying full load as 0.8 lagging power-factor is 415 V. The equivalent resistance and reactance drops for the transformer are 1% and 5% respectively. The turn’s ratio of the transformer is: [GATE-2000]
a.14.     b. 24.    c.42.     d.20

8. The core flux of a practical transformer with a resistive load [GATE-2001]
a. is strictly constant with load changes.
b. increases linearly with load.
c.  increases as the square root of the load.
d.  decreases with increase

9. For a linear electromagnetic circuit ,the following statement is true [GATE-2004]
a.  Field energy is equal to the co-energy
b. Field energy is greater than the co-energy
c.  Field energy is less than the co-energy
d. Coenergy is zero

10. Which three –phase connection can be used in a transformer to introduce a phasedifference of between its output and corresponding input line voltages [GATE-2005]
a. star-star         b.delta-star
c. delta-delta.    d. zig-zag

9. Two transformers are to be operated in parallel such that they share load in proportion to their kVA ratings . The rating of the first transformer is 500 kvA and its pu leakage impedance is 0.05 pu. If the rating of second transformer is 250 kVA, its pu leakage impedance is [GATE-2006]
a. 0.20   b. 0.10   c. 0.005. d. 0.025

10. It is desired to measure parameters of 230V/115 V, 2 kVA, single-phase transformer . The following wattmeters are available in a laboratory :
W1 : 250 V, 10 A, low power factor
W2 : 250 V, 05 A, low power factor
W3 : 150 V, 10 A, high power factor
W4 : 150 V, 05 A, high power factor
The wattmeters used in open circuit and short circuit test of the transformer will be respectively be [GATE-2008]
a. W1 & W2.     b. W2 & W4
c.  W1 & W4.     c. W2 & W3

11. A single phase 10 kVA 50 Hz transformer with 1kV primary winding draws 0.5 A and 55 W at rated voltage and frequency on no load. A second transformer has a core with all its linear dimensions times the corresponding dimensions of the first transformer. The core material and lamination thickness are the same in both transformers. The primary windings of both the transformers have the same number of turns. If a rated voltage of 2 kV at 50 Hz is applied to the primary of the second transformer then the no load current and power rgespectively are [GATE-2012]
a. 0.7 A,77.8 w
b. 0.7 A, 155.6 w
c. 1A, 110 w
d. 1A, 220 w

12. A single-phase transformer has no-load loss of 64 W, as obtained from an open- circuit test. When a short-circuit test is performed on it with 90% of the rated currents flowing in its both LV and HV windings, he measured loss is 81 W. The transformer has maximum efficiency when operated at [GATE-2013]
a. 50.0% Rated current
b. 64.0%Rated current
c. 80.0% Rated current
d. 88.8%Rated current