Saturday, December 26, 2015

S.E. (Electrical) (Semester – I),ELECTRICAL MACHINES – I,University Of Pune Question Paper,2010 Question Paper,

University Of Pune Question Paper
S.E. (Electrical) (Semester – I) Examination, 2010
(2003 Course)
Time : 3 Hours Max. Marks : 100
N.B. : i) Answers to the two Sections should be written in separate
answer books.
ii) From Section I solve Q. No. 1 or Q. No. 2, Q. No. 3 or
Q. No. 4, Q. No. 5 or Q. No. 6. Similarly from Section II
solve Q. No. 7 or Q. No. 8, Q. No. 9 or Q. No. 10, Q. No. 11
or Q. No. 12.
iii) Figures to the right indicate full marks.
iv) Use of electronic calculator is allowed.
v) Assume suitable data, if necessary.
1. a) Prove the condition for maximum efficiency of a transformer. 4
b) Draw the phasor diagram of a transformer on : 6
i) load with unity power factor
ii) load with leading power factor.
c) A 600 KVA, single phase transformer has an efficiency of 92% both at full
load and half-load at unity power factor. Determine its efficiency at 60% of
full load at 0.8 power factor lag. 6
2. a) Explain the various features of an ideal transformer. 4
b) Compare the core type and shell type transformers. 4
c) A 4-KVA, 200/400 V, 1-phase transformer has equivalent resistance and
reactance referred to low voltage side equal to 0.5Ω and 1.5Ω respectively.
Find the terminal voltage on the high voltage side when it supplies 3/4
th full load
at power factor of 0.8, the supply voltage being 200 V. Hence, find the output
of the transformer and its efficiency if the core losses are 100 W. 8
3. a) How equivalent circuit parameters are obtained from open and short circuit
tests on transformer. 8
b) A 5 KVA, 500/250V, 50 Hz, single phase transformer gave the following readings
O.C. Test : 500 V, 1 A, 50 W (L.V. side open)
S.C. Test : 25 V, 10 A, 60 W (L.V. side shorted)
Determine :
i) The efficiency on full load, 0.8 lagging p.f.
ii) The voltage regulation on full load, 0.8 leading p.f.
iii) The efficiency on 60% of full load, 0.8 leading p.f. 8
4. a) Explain load sharing of transformers connected in parallel with equal voltage
ratios and unequal voltage ratios. 8
b) Two 2,200/110 V transformers are operated in parallel to share a load of 125
KVA at 0.8 p.f. lagging. Transformers are rated as below :
A : 100 KVA; 0.9% resistance and 10% reactance
B : 50 KVA; 1.0% resistance and 5% reactance.
Find the load carried by each transformer. 8
5. a) Explain the conditions which must be fulfilled for successful parallel operation
of 3 phase transformer. 6
b) With proper connection and phasor diagrams describe the different ways of
connecting three phase transformers. 8
c) Explain scott connection. 4
6. a) What are the advantages of a single three phase transformer unit over a bank
of single phase transformers. 6
b) Explain the Sumpner’s test on single phase transformer with the help of neat
connection diagram. 6
c) Explain the testing of transformer as per B.I.S. (2026). 6
7. a) Draw a neat sketch of a D.C. machine. Label it. List the various parts and material
used for them. Also state the function of each parts. 8
b) A long-shunt compound generator delivers a load current of 50 A at 500 V
and has armature, series field and shunt field resistances of 0.05Ω , 0.03Ω
and 250Ω respectively. Calculate the generated voltage and the armature current.
Allow IV per brush for contact drop. 8
8. a) Distinguish between lap and wave type of windings in D.C. machines. 4
b) Explain the significance of the Back e.m.f. 4
c) A shunt generator has F.L. current of 196 A at 220 V. The stray losses are 720 W
and shunt field coil resistance is 55Ω . If it has a F.L. efficiency of 88%, find
the armature resistance. Also, find the load current corresponding to maximum
efficiency. 8
9. a) Explain speed control methods for a d.c. series motor in details. 8
b) A series motor, with an unsaturated field and negligible resistance when running
at a certain speed on a given load takes 50 A at 400 V. If the load torque varies
at the cube of speed, calculate the resistance required to reduce the speed by
25%. 8
10. a) Draw the performance characteristics of different types of d.c. generators
and explain them. 8
b) Explain the various losses in d.c. machine. 4
c) A 250-V shunt motor with armature resistance of 0.5Ω runs at 600 r.p.m. on
full load and takes an armature current of 20 A. If resistance of 1.0Ω is placed
 in armature circuit find the speed at full-load torque. 4
11. a) Describe Hopkinson’s test in details with its advantages and disadvantages. 8
b) What is commutator ? What is meant by good commutation ? How is it
achieved ? 10
12. a) A 4 pole, 50 KW, 250 V wave-wound shunt generator has 400 armature
conductors. Brushes are given a bead of 4 commutator segments. Calculate
the demagnetisation amp-turns/pole if shunt field resistance is 50Ω . Also,
calculate extra shunt field turns/pole to neutralize the demagnetisation. 10
b) Explain Swinburne’s test for finding the efficiency of a d.c. machine. Can this
method be applicable to d.c. series motors. 8
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