Electrical Machines-Transformers

IES previous year question papers

1. If the applied voltage of a certain transformer is increased by 50% and the frequency is reduced to 50% (assuming) that the magnetic circuit remains unsaturated), the maximum core /flux density will
[IES-1997]
a. Change to three, times the original value
b.  Change to 1.5, times the original value
c. Change to 0.5, times the original value
d. Remains as same value

2. In a 3-phase transformer shown in the figure, the phase displacement of secondary line voltages with corresponding primary line voltages will be [IES-1998]
a. 0 degrees          b. 30 degrees lag
b. 30 degrees lead    d. 180 degrees

3. If an ideal centre-tapped 1: 4 transformer is loaded as shown in the figure, the impedance measured across the terminals AA would be (figure contains primary side AA terminal and secondary side centertapped with loads 2z & z ) [IES-1999]
a. 3z/64   b. 3z/32   c. 2z/3   d. z/6

4. Consider the following statements:
1. Interpole windings are connected in series with armature winding
2. Polarity of interpole must be the same as that of the main pole in advance
3. Distortion of the main field under the pole shoes is not affected by the use of interpoles Which of these statements regarding interpole used in dc motors is/are correct? [IES-2000]
a. 1&2. b. 2&3  c. 1&3  d. 1 alone

5. In case of auto transformers ,which of the following statements are correct?
1. An auto transformers requires less copper as compared to a conventional, 2-winding transformers of the same capacity. 2. An auto transformer provides isolation between the primary and secondary windings.
3. An auto transformer has less leakage reactance as compared to the conventional, 2-winding transformer of the same capacity.

Select the correct answer using the codes given below: [IES-2001]
a. 1,2&3  b. 1&2  c. 1&3   d. 2&3

6. match List-I(Transformer) with List-II(Voltage ratio)and select the correct answer [IES-2002]
List-I                               List -II
A. Power transformer 1. 230V/230V B. Auto Transformer   2.220V/240V
C. Welding T/F.           3.400V/100V
D. Isolation T/F.         4 .132KV/11KV

a. A-4, B-2, C-3, D-1
b. A-4, B-2, C-1, D-3
c. A-2, B-4, C-1, D-3
d.  A-2, B-4, C-3, D-1

6. The function of oil in a transformer is to provide [IES-2003]
a. Cooling & Insulation
b. Protection against lightening
c. Protection against short circuit
d. Lubrication

7. Assertion (A): Delta-connected tertiary windings tend to act as an automatic feed back control system. Reason(R) : In the event of unbalance secondary load currents, both secondary and primary load voltages are restored to their normal phase magnitude and angle. [IES-2004]
a. Both A and R are true and R is the correct explanation of A
b. Both A and R are true but R is not the correct explanation of A
c.  A is true but R is false
d.  A is false but R is true

8. Match List-1 (Type of coil) with List-II (Use of coil) and select the correct answer using the codes given below the lists: [IES-2005]
List-I                              List-II
A. Sandwitch coils     1. Low voltage coils for currents above 100A
B. Disc coils                2. High voltage windings of small transformers
C. Cross -over coils     3. Cooling oils is contact with each turn of the winding
D.Spiral type               4. Shell-type transformer core
a. A-2, B-3, C-4, D-1
b. A-4, B-1, C-2, D-3
c. A-2, B-1, C-4, D-3
d A-4, B-3, C-2, D-1

9. What is the efficiency of an auto transformer in comparison to that of a two-winding transformer of the same rating? [IES-2006]
a. Slightly less than that of a two-winding transformer
b. Same as that of a two winding transformer
c. More than that of a two winding transformer
d. As low as 1/5 th of the efficiency of a two winding transformer

10. A 500 KVA transformer has constant losses of 500 W and copper losses at full load are 2000 W. at what load, is the efficiency maximum? [IES-2007]
a. 250    b.500   c. 1000    d. 125

11. Core of large power transformers are made from which one of the following? [IES-2008]
a. Hot rolled steel
b. Cold rolled steel
c. Cold-rolled grain oriented steel
d. Ferrite

12. What is the power transferred conductively from primary to secondary of an auto-transformer having trans-formation ratio of 0.8 supplying a load of 3 KW? [IES-2009]
a. 0.6  b. 2.4  c. 1.5  d. 2.7

13. A delta/delta connected transformer is connected to Y/Y connected transformer. The ratio of VA rating of Y/Y connected transformer and delta/delta connected transformer [IES-2010]
a. 57.7% b.100% c. 50% d. 75%

14. The full load copper and iron loss of a transformer are 6400 W and 500 W, respectively. The above copper loss and iron loss at Half load will be  [IES-2011]
a. 3200 W & 500 W
b. 3200 W & 250 W
c. 1600 W & 125 W
d. 1600 W & 500 W

15. Match List-I with List-II and select the correct answer using the code given below the lists: [IES-2012]
List-I                            List-II
A. Magnetic Flux       1. Resistance
B. Magneto motive force 2. Electric current
C. Reluctance         3. Conductivity
D. Permeability      4. Electromotive force
a. A-2, B-1, C-4, D-3
b. A-3, B-1, C-4, D-2
c.  A-2, B-4, C-1, D-3
d. A-3, B-4, C-1, D-2

16. For power transformers of larger ratings, the tapings are located in the middle portion of the winding to  [IES-2013]
a. Increase the break-down strength of the winding
b. Enable better cooling of windings
c. Enable better distribution inter turn voltage
d. Reduce the mechanical forces affecting the windings during short circuit