Electrical machines-Transformers

Everytime I tried while googling of gate questions topic wise,never found useful data.I am trying blog about gate and IES previous questions (topic wise).I am starting with machines subject and in machines opening with transformers.

1. Transformer Losses and Efficiency

GATE-1. A single phase transformer has a maximum efficiency of 90% at 111 load and unity power factor. Efficiency at half load at the same power factor is: [GATE-2003]
(a) 86.7%      (c) 88.9%  
(b) 88.26%    (d) 87.8%

GATE-2. A 500 kVA, 3-phase transformer has iron losses of 300 W and full load copper losses of 600 W. The percentage load at which the transformer is expected to have maximum efficiency is:
[ GATE-2003]
(a) 50.0%      (b) 70.7% 
(c) 141.4%    (d) 200.0%

Data for Q.3 and Q.4 are given below. Solve the problems and choose the correct answers.

A 300 kVA transformer has 95% efficiency at full load 0.8 pf lagging and 96% efficiency at half load, unity pf.

GATE-3. The iron loss (Pi) and copper loss (Pc) in kW, under full load operation are: [GATE-2006]
(a) Pc= 4.12, Pi= 8.51    (b) Pc= 6.59, Pi = 9.21   (c) Pc= 8.51, Pi = 4.12    (d) Pc= 12.72, Pi = 3.07

GATE-4. What is the maximum efficiency (in %) at unity pf load? (a) 95.1   (b) 96.2  (c)96.4 (d)98.1
 

And now some IES based questions

IES-1. A single-phase transformer when supplied from 220V, 50Hz has eddy current loss of 50W. If the transformer is connected to a voltage of 330V, 50Hz, the eddy current loss will be: [IES-2001]
(a) 168.75W          (b) 112.5W
(c) 75W                  (d) 50W

IES-2.  If cP and scP represent core and full-load ohmic losses respectively, the maximum kVA delivered to load corresponding to maximum efficiency is equal to rated kVA multiplied by: [IES-2001]  (a) Pc /Psc 
(b) square root of Pc/Psc   
(c) Square of Pc/Psc    
(d) Square of Pc/Psc * Square of Psc/Pc

IES-3. The full-load copper-loss and iron-loss of a transformer are 6400W and 5000W respectively. The copper-loss and iron-loss at half load will be, respectively [IES-2002] 
(a) 3200 W and 2500 W  
(b) 3200 W and 5200 W  
(c) 1600 W and 1250 W  
(d) 1600 W and 5000 W 

DIRECTION:
The following three items consist of two statements, one labelled as ‘Assertion A’ and the other labelled as ‘Reason R’. You are to examine these two statements carefully and decide if the Assertion A and the Reason R are individually true and if so, whether the Reason is a correct explanation of the Assertion. Select your answers to these items using the codes given below: Codes: (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

IES-4. Assertion (A): The distribution transformers are designed for minimum core losses. [IES-2002] 
Reason (R): Primary windings of distribution transformers are energized throughout the day. 

IES-5. If P1 and P2 be the iron and copper losses of a transformer at full load, and the maximum efficiency of the transformer is at 75% of the full load, then what is the ratio of P1 and P2 ? [IES-2006]  (a) 9/16  (b) 10/16 (c) 3/4 (d) 3/16 

IES-6. If the iron core of a transformer is replaced by an air core, then the hysteresis losses in the transformer will: [IES-2006] 
(a) Increase     (b) Decrease 
(c) Remain unchanged   
(d) Become zero 

IES-7. The equivalent circuit of a transformer has the leakage reactances X1, X2′ and the magnetising reactance Xm. What is the relationship between their magnitudes? [IES-2006]
(a) X1>>X′2>>Xm 
(b) X1<<X′2<< Xm 
(c) X1≈X′2>>Xm  
(d) X1≈X′2<< Xm

IES-8. If the voltage applied to a transformer primary is increased by keeping the V/f ratio fixed, then the magnetizing current and the core loss will, respectively, [IES-2006] 
(a) Decrease and remain the same  (b) Increase and decrease
(c) Remain the same and remain the same 
(d) Remain the same and increase 

IES-9. A 500 kVA transformer has constant losses of 500 Wand copper losses at full load are 2000 W. Then at what load, is the efficiency maximum? [IES-2007]
(a) 250 kVA        (b) 500 kVA
(c) 1000 kVA.     (d) 125 kVA

IES-10. On which of the following factors does hysteresis loss depend? [IES-2007]
1. Flux density   
2. Frequency  
3. Thickness of lamination 
4. Time 
Select the correct answer using the code given below:
(a) 2 and 3.           (b) 1 and 2
(c) 3 and 4              (d) 1 and 4 

IES-11. A single-phase transformer rated for 220/440 V, 50 Hz operates at no load at 220 V, 40 Hz. This frequency operation at rated voltage results in which one of the following? [IES-2008]
(a) Increase of both eddy-current and hysteresis losses 
(b) Reduction of both eddy-current and hysteresis losses 
(c) Reduction of hysteresis loss and increase in eddy-current loss 
(d) Increase of hysteresis loss and no change in the eddy-current loss 

IES-12. What is the load at which maximum efficiency occurs in case of a 100 kVA transformer with iron loss of 1kW and full-load copper loss of 2 kW? [IES-2008] 
(a) 100 kVA            (b) 70.7 kVA
(c) 50.5 kVA            (d) 25.2 kVA

IES-13. The full-load copper loss and iron loss of a transformer are 6400 Wand 5000 W, respectively. What are the above copper loss and iron loss, respectively at half-load? [IES-2008] 
(a) 3200 W, 2500 W   
(b) 3200 W, 5000 W 
(c) 1600 W, 1250 W   
(d) 1600 W, 5000 W
                                           
IES-14. When are eddy-current losses in a transformer reduced? [IES-2008]
(a) If laminations are thick
(b) If the number of turns in primary winding is reduced
(c) If the number of turns in secondary winding is reduced
(d) If laminations are thin

Some other questions

1. If cu loss of a transformer at 7/8 th full load is 4900 watts. then,its full-load cu loss would be
(a) 5600 (b) 6400 (c) 375 (d) 429

2. The iron loss of a transformer at 400 Hz is 10 watts.
Assuming the eddy current and hysteresis losses vary as the square of flux density,the iron loss of the transformer at rated voltage.but,at 50 Hz would be ........ Watts
(a) 80 (b) 640 (c) 1.25 (d) 100

3. A 200kva transformer has an iron loss of 1 KW and full-load CU loss of 2KW.its load kva corresponding to maximum efficiency is
(a) 100 (b) 141.4 (c) 50 (d) 200

4. The ordinary transformer has maximum efficiency when
(a) it runs at half-load
(b) it runs at full-load
(c) its cu-loss equal to iron-loss
(d) its runs slightly over load

5. Hysteresis loss depends on which factors ?
(a) flux density
(b) frequency
(c) quality of amount of iron in core
(d) All of the above

6. For distribution transformer, the increase in voltage effect the efficiency
(a) at any load increases [Fractional/Full-load/Over-load]
(b) at any load decreases
(c) at fractional load decreases & at full-load/over-load increases
(d) None of the above

7. For constant kva output is assumed,how will effect the cu-loss & regulation of a transformer by change in voltage
(a) varies as square of voltage.but, decreases with increase in voltage. (b) varies as voltage.but, decreases with increase in voltage.
(c) varies as square of voltage.but, increases with decrease in voltage. (d) varies as square of voltage.but, increases with decrease in voltage.

8. Iron loss of a transsformer changes with change in voltage as
(a) varies as square of voltage.
(b) varies as voltage.
(c) varies as 1/square of 'V'
(d) varies as 1/voltage

9. For constant kva output ,the increase in voltage will effect the heating of transformer as
(a) Iron temperature increases.
(b) copper temperature decreases.
(c) iron temperature increases & copper temperature decreases.
(d) iron temperature decreases & copper temperature increases.
(e) None of the above.

10. The increase in frequency of transformer will effect the iron losses
(a) Increases (b) decreases
(c) no change

11. The increase in frequency of transformer will effect the cu losses
(a) Increases (b) decreases
(c) no change

12. The decrease in frequency of transformer will effect the heating of transformer (temperature)
(a) Increases    (b) decreases
(c) no change    (d) independent

13. Efficiency of transformer at lower frequencies .............. compared with higher frequencies.
(a) Less (b) More (c) equal

14. On which of the following factors does eddy current loss depend?
1. Flux density   
2. Frequency  
3. Thickness of lamination 
4. Time 
Select the correct answer using the code given below:
(a) 2 and 3.           (b) 1,2 and 3
(c) 1 and 3            (d) 1 and 2

Answers for above questions:

GATE:
1. d  2. b  3. c  4. b

IES :
1.b  2.b  3.b  4.a  5.a  6.d  7.d  8.d 9.a  10.b   11.d.  12.b   13.d   14.d

Extra Questions :
1.b 2.b 3.b 4.c 5.c 6.c 7.a 8.a 9.c 10.b 11.a 12.a 13.a 14.b

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