In previous we gone through losses and efficiency.
Now,
2. Auto - Transformers
GATE-1. A 50 kVA, 33001230 V single-phase transformer is connected as an autotransformer shown in figure. The nominal rating of the autotransformer will be: [GATE-2004]
(a) 50.0 kVA. (c) 717.4 kVA
(b) 53.3 kVA. (d) 767.4 kVA
GATE-2. A single-phase 50 kVA, 250 V/500V two winding transformer has an efficiency of 95% at full load, unity power factor. If it is reconfigured as a 500V/750V autotransformer, its efficiency at its new rated load at unity power factor will be: [GATE-2007]
(a) 95.752% (b) 97.851%
(c) 98.276 % (d) 99.241%
Statement for Linked Answer Questions Q 3 and Q 4 :
The figure above shows coils 1 and 2, with dot markings as shown, having 4000 and 6000 turns respectively. Both the coils have a rated current of 25 A. Coil 1 is excited with single phase, 400 V, 50 Hz supply.
GATE-3. The coils are to be connected to obtain a single-phase, 400, 1000V autotransformer to drive a load of 10 kVA.Which of the options given should be exercised to realize the required auto-transformer? [GATE-2009]
(a) Connect A and D; Common B (b) Connect B and D; Common C
(c) Connect A and C; Common B
(d) Connect A and C; Common D
GATE-4. In the autotransformer obtained in Question 3, the current in each coil is: [GATE-2009]
(a) Coil-1 is 25 A and Coil-2 is 10 A (b) Coil-1 is 10 A and Coil-2 is 25 A (c) Coil-1 is 10 A and Coil-2 is 15 A (d) Coil-1 is 15 A and Coil-2 is 10 A.
IES-1. In case of auto-transformers, which of the following statements are correct? [IES-2001]
1. An auto-transformer requires less copper as compared to a conventional, 2-winding transformer 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:
(a) 1, 2 and 3. (b) 1 and 2 (c) 1 and 3. (d) 2 and 3
IES-2. A two-winding transformer is used as an autotransformer. The kVA rating of the autotransformer compared to the two-winding transformer will be: [IES-2002]
(a) 3 times (b) 2 times
(c) 1.5 times (d) Same
IES-3. A two-winding transformer is converted into an auto-transformer. If we apply additive polarity and subtractive polarity for the connections, then the secondary voltage is 2640 V and 2160 V, respectively. What is the ratio of primary to secondary voltage of the original transformer? [IES-2006] (a) 66 : 54 (b) 54 : 66
(c) 10 : 1 (d) 1 : 10
IES-4. A single-phase, 10kVA, 2000/ 200V, 50Hz transformer is connected to form an auto transformer as shown in the figure given below. What are the values of V1 and I2, respectively?[ figure contains V2 is 2000V & I1 is 50A]
(a) 2200V, 55A (b) 2200V, 45A (c) 2000V, 45A (d) 1800V, 45A
IES-5. If the wave form of the voltage impressed on the primary of a Y−Δ bank contains 5th harmonics, what are the wave forms of the resultant voltages of the primary and the secondary? [IES-2005]
Primary Secondary
(a) Peaked Peaked
(b) Peaked Flat-topped
(c) Flat-topped Peaked
(d) Flat-topped Flat-topped
IES-6. 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 1/5th of the efficiency of a two-winding transformer
IES-7. What is the power transmitted inductively in an auto-transformer which supplies a load at 161 volts with an applied primary voltage of 230 volts? [IES-2009]
(a) 35% of the input
(b) 70% of the input
(c) 15% of the input
(d) 30% of the input
IES-8. What is the power transferred conductively from primary to secondary of an auto-transformer having transformation ratio of 0.8 supplying a load of 3 kW? [IES-2009]
(a) 0.6 kW (b) 2.4 kW (c) 1.5 kW (d) 0.27 kW
Extra Questions:
DIRECTION:
1.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.
Assertion (A) : An auto-transformer is more efficient in transferring energy from primary to secondary circuit.
Reason (R) : Because it does both inductively and conductively.
Statement for Linked Answer Questions Q 2 and Q 3 :
The weight of transformer is W1 (copper weight).it is converted into auto-transdormer.
2. How much copper is going to save by conversion ?
a.K*W1 b. (1/K)*W1
c.(1-K)*W1 d. (1+K)*W1
3. How much copper is going to save by conversion ?
a.K*W1 b. (1/K)*W1
c.(1-K)*W1 d. (1+K)*W1
Statement for Linked Answer Questions Q 4and Q 5 :
An auto-transdormer suppiies a load of 3Kw at 115 volts at a unity power factor. If the applied primary voltage is 230 volts.
4. calculate the power transferred to the load inductively ?
a. 1.5Kw b. 2Kw c.1.8Kw d.2.5Kw
5. calculate the power transferred to the load conductively ?
a. 1Kw b. 1.5Kw c.1.8Kw d.2.5Kw
Statement for Linked Answer Questions Q6,Q7 and Q8 :
A 500kva, 50 Hz, single-phase, 6600/5000-V auto-transdormer, assuming following approximate values : e.m.f per turn 8V, maximum flux density 1.3 Wb/m^2
6. Determine the area of core ?
a. 255cm^2 b. 266cm^2
c. 277cm^2 d. 288cm^2
7. Number of turns on h.v side &
l.v side are :
a. 625 & 825 b. 825 & 625
c. 660 & 500 d. 500 & 600
8. The position of tapping point of transformer ?
a. 200 turns from low voltage end or 625 turns from common end.
b. 200 turns from high voltage end or 650 turns from common end.
c. Only 650 turns from common end
d. Only 200 turns from low/high voltage end
Answers :
GATE :
1.d 2.c 3.a 4.d
IES :
1.c 2.b 3.c 4.d 5.b 6.c 7.d 8.b
Extra Questions :
1.a 2.a 3.c 4.a 5.b 6.c 7.b 8.b
In this I have to attach some figures. I will complete it soon.....
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