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Transformers and Generators Question Paper - Jun 18 - Electrical And Electronics (Semester 3) - Visveswaraya Technological University (VTU)
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Transformers and Generators - Jun 18
Electrical And Electronics (Semester 3)
Total marks: 80
Total time: 3 Hours
INSTRUCTIONS
(1) Question 1 is compulsory.
(2) Attempt Q1 or Q2, Q3 or Q4, Q5 or Q6, Q7 or Q8, Q9 or Q10.
(3) Draw neat diagrams wherever necessary.
1.a.
Draw and explain the full load phasor diagrams of single phase transformer for lagging and
leading process factor loads.
(6 marks)
00
1.b.
Find the All day efficiency of single phase transformer having maximum efficiency of 98$\%$ at 15 $\mathrm{KVA}$ at UPF and loaded as follows:
(6 marks)
00
12 hours $-2 \mathrm{KW}$ at 0.5 power factor lagging
6 hours $-2 \mathrm{KW}$ at 0.8 power factor lagging
6 hours- no load.
1.c.
Draw the approximate Equivalent circuit of a transformer referred to primary side.
(4 marks)
00
OR
2.a.
State the advantages of single three phase transformers over bank of single phase
transformer.
(5 marks)
00
2.b.
Explain with the help of connection and phasor diagrams, how scott connections are used to
obtain two base supply from three phase supply mains.
(6 marks)
00
2.c.
The following results were obtained on a
(5 marks)
00
50 KYA. 2400$/ 120 \mathrm{Y}$ , transformer.
O.C test: $396 \mathrm{W}, 9.65 \mathrm{A}, 120 \mathrm{V}$
S.C test $: 810 \mathrm{W}, 20.8 \mathrm{A}, 92 \mathrm{V}$
Determine :
i) The circuit constants
ii) The efficiency at foll-load, 0.8 p.f. lagging
iii) The approximate vtg regulation.
Module-2
3.a.
Discuss the necessary conditions for the parallel operation of 2 transformers.
(5 marks)
00
3.b.
Drive an expression for the currents shared by between 2 transformers connected in parallel
supplying a common load when no load voltages of these transformers are unequal.
(6 marks)
00
3.c.
How stabilization is achieved due to tertiary winding.
(5 marks)
00
OR
4.a.
With the help of neat sketches, explain the working of ON load tap changer and OFF load tap changer.
(10 marks)
00
4.b.
Define auto transformer? Derive an expression for the saving of copper in an Auto transformer.
(6 marks)
00
Module-3
5.a.
Discuss the causes of noise in transformers? How to reduce the noise in transformers.
(5 marks)
00
5.b.
Explain current Inrush phenomenon in transformers.
(5 marks)
00
5.c.
With a circuit diagram, explain in detail Sumpner's test for determining the efficiency and voltage regulation of transformer.
(6 marks)
00
OR
6.a.
With a neat circuit diagram, explain armature reaction in DC machines.
(6 marks)
00
6.b.
Draw and explain the characteristics of DC shunt generator.
(5 marks)
00
6.c.
Derive EMF Equation of synchronous generator.
(5 marks)
00
Module-4
7.a.
With phasor diagram, explain the concept of two reaction theory in a salient pole
synchronous machine
(8 marks)
00
7.b.
Define voltage regulation of an alternators.
(3 marks)
00
7.c.
What is synchronization of alternators? Need for parallel operation of alrernators.
(5 marks)
00
OR
8.a.
With a neat circuit diagram explain the slip test on salient pole synchronous machine to
determine $X_{d}$ and $X_{9}$ from slip test.
(8 marks)
00
8.b.
Write a note on V-curves of synchronous generator.
(4 marks)
00
8.c.
Define electrical load diagram of a synchronous generator.
(4 marks)
00
Module-5
9.a.
What are the various methods of determining the yoltage regulation for 3$\phi$ alternator and
explain any one method in detail.
(8 marks)
00
9.b.
The open and short circuit test reading for a $3 \phi-$ star connected $1000 \mathrm{KVA} .200 \mathrm{V}, 50 \mathrm{Hz}$ synchronous generator are,
(8 marks)
00
| Field maps | 10 | 20 | 25 | 30 | 40 | 50 |
|---|---|---|---|---|---|---|
| OC terminal vtg | 800 | 1500 | 1760 | 2000 | 2350 | 2600 |
| SC armature current in amp | - | 200 | 250 | 300 | - | - |
The armature effective resistance is 0.2 ohm per phase. Draw the characteristic curves and estimate the full load percentage regulation i) 0.8 p.f lagging ii) 0.8 p.f leading.
OR
10.a.
Write a short note on capability curves of synchronous generator.
(6 marks)
00
10.b.
Discuss about hunting in synchronous machines. Also explain the role of damper winding.
(6 marks)
00
10.c.
Discuss about short circuit ratio and its significance.
(4 marks)
00
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