University Of Pune Question Paper
S.E. (Electrical) (Semester – I) Examination, 2011
ELECTRICAL MACHINES – I
(2003 Course)
Time : 3 Hours Max. Marks : 100
N.B. : i) Answer 3 questions from Section I and 3 questions from II.
ii) Answers to the two Sections should be written in
separate books.
iii) Neat diagrams must be drawn wherever necessary.
iv) Black figures to the right indicate full marks.
v) Use of logarithmic tables, pocket calculator and steam tables
is allowed.
vi) Assume suitable data, if necessary.
SECTION – I
1. a) Prove the condition for maximum efficiency of single phase transformer. 4
b) Draw the Phasor diagram of 1-phase transformer on load condition at :
i) Unity power factor
ii) Leading power factor. 6
c) The required no load voltage ratio in a 150 KVA, 50 Hz, single phase transformer
is 5000/250 V. Find efficiency at half rated KVA, Unity power factor and also
efficiency at full load 0.8 p.f. lagging if the full load copper losses are 1800 W;
care losses are 1500 W. 6
OR
2. a) Explain the various features of on ideal transformer. 4
b) How approximate equivalent circuit is different from accurate equivalent circuit of
transformer. 6
c) A 250 KVA single phase transformer has iron loss of 1.8 KW. The full load
copper loss is 2000 Watts, calculate :
i) Efficiency at full load, 0.8 lagging p.f.
ii) KVA supplied at maximum efficiency
iii) Maximum efficiency at 0.8 lagging p.f. 6
3. a) Explain load sharing of transformers connected in parallel with equal voltage
ratios and unequal voltage ratios. 8
b) Two 2200/110 V Transformers are operated in parallel to share a load of
120 KVA at 0.8 p.f. lagging. Transformers are rated below.
A : 100 KVA : 0.8% resistance and 10% reactance
B : 60 KVA : 1% resistance and 5% reactance
Find the load carried by each transformer. 8
OR
4. a) How equivalent circuit parameters are obtained from open and short circuit
tests on transformer ? 8
b) A 5 KVA, 500/250 V, 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
5. a) With proper connection and phasor diagrams describe the different ways of
connecting three phase transformers. 8
b) With the help of neat connection diagram, explain the Sumpner’s test on
1-phase transformer. Also state its limitations. 10
OR
6. a) What are the advantages of single three phase transformer unit over a bank of
single phase transformer ? 6
b) State the conditions which must be fulfilled for successful parallel operation
of 3 phase transformers. 6
c) Write the short notes on Testing of transformer as per B.I.S. (2026). 6
SECTION – II
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 d.c. compound generator drives 20 lamps, all are connected in
parallel. Terminal voltage is 550 V with each lamp resistance as 500 Ω. If
Rsh = 25 Ω , Ra
= 0.06 Ω and Rse = 0.04 Ω , calculate the armature current and
the generated e.m.f. 8
OR
8. a) Distinguish between lap and wave type of windings in DC machines. 4
b) Explain the following terms in brief :
i) Pole pitch
ii) Back pitch
iii) Resultant pitch
iv) Commutator pitch. 4
c) A d.c. series generator has armature resistance of 0.5 Ω and series field
resistance of 0.03 Ω . It drives a load of 50 A. If it has 6 turns/coil and total
540 coils on the armature and is driven at 1500 r.p.m. calculate the terminal
voltage at the load. Assume 4 poles, lap type winding, flux per pole as 2 mwb
and total brush drop as 2V. 8
9. a) Draw the circuit diagrams for separately excited generators and self excited
series generator indicating all the currents and voltages. 8
b) A d.c. shunt motor runs at a speed of 1000 r.p.m. on no load taking a current
of 6A, from the supply when connected to 220 V d.c. supply. Its full load
current is 50 A. Calculate its speed on full load. Assume Ra
= 0.3 Ω and
Rsh = 110 Ω. 8
OR
10. a) Draw the performance characteristics of different types of d.c. generators
and explain them. 8
b) Obtain the torque equation of d.c. motor. 4
c) Explain the various losses in d.c. machine. 4
11. a) Describe Hopkinson’s test in detail with its advantages and disadvantages. 10
b) What is commutator ? What is meant by good commutation ? How is it
achieved ? 8
OR
12. a) Explain Swinburne’s test for finding the efficiency of a d.c. machine. Can this
method applicable to d.c. series motors. 8
b) A retardation test is carried out on a 1000 r.p.m. d.c. machine. The time taken
for the speed to fall from 1030 rpm to 970 rpm is
i) 36 seconds with no excitation
ii) 15 seconds with no excitation
iii) 9 seconds with full excitation and the armature supporting on extra load of
10 A. at 219 V. Calculate
1) Moment of inertia of the armature in Kg.m2
2) Iron loss
3) Mechanical loss at the mean speed of 1000 r.p.m. 10
——————
S.E. (Electrical) (Semester – I) Examination, 2011
ELECTRICAL MACHINES – I
(2003 Course)
Time : 3 Hours Max. Marks : 100
N.B. : i) Answer 3 questions from Section I and 3 questions from II.
ii) Answers to the two Sections should be written in
separate books.
iii) Neat diagrams must be drawn wherever necessary.
iv) Black figures to the right indicate full marks.
v) Use of logarithmic tables, pocket calculator and steam tables
is allowed.
vi) Assume suitable data, if necessary.
SECTION – I
1. a) Prove the condition for maximum efficiency of single phase transformer. 4
b) Draw the Phasor diagram of 1-phase transformer on load condition at :
i) Unity power factor
ii) Leading power factor. 6
c) The required no load voltage ratio in a 150 KVA, 50 Hz, single phase transformer
is 5000/250 V. Find efficiency at half rated KVA, Unity power factor and also
efficiency at full load 0.8 p.f. lagging if the full load copper losses are 1800 W;
care losses are 1500 W. 6
OR
2. a) Explain the various features of on ideal transformer. 4
b) How approximate equivalent circuit is different from accurate equivalent circuit of
transformer. 6
c) A 250 KVA single phase transformer has iron loss of 1.8 KW. The full load
copper loss is 2000 Watts, calculate :
i) Efficiency at full load, 0.8 lagging p.f.
ii) KVA supplied at maximum efficiency
iii) Maximum efficiency at 0.8 lagging p.f. 6
3. a) Explain load sharing of transformers connected in parallel with equal voltage
ratios and unequal voltage ratios. 8
b) Two 2200/110 V Transformers are operated in parallel to share a load of
120 KVA at 0.8 p.f. lagging. Transformers are rated below.
A : 100 KVA : 0.8% resistance and 10% reactance
B : 60 KVA : 1% resistance and 5% reactance
Find the load carried by each transformer. 8
OR
4. a) How equivalent circuit parameters are obtained from open and short circuit
tests on transformer ? 8
b) A 5 KVA, 500/250 V, 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
5. a) With proper connection and phasor diagrams describe the different ways of
connecting three phase transformers. 8
b) With the help of neat connection diagram, explain the Sumpner’s test on
1-phase transformer. Also state its limitations. 10
OR
6. a) What are the advantages of single three phase transformer unit over a bank of
single phase transformer ? 6
b) State the conditions which must be fulfilled for successful parallel operation
of 3 phase transformers. 6
c) Write the short notes on Testing of transformer as per B.I.S. (2026). 6
SECTION – II
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 d.c. compound generator drives 20 lamps, all are connected in
parallel. Terminal voltage is 550 V with each lamp resistance as 500 Ω. If
Rsh = 25 Ω , Ra
= 0.06 Ω and Rse = 0.04 Ω , calculate the armature current and
the generated e.m.f. 8
OR
8. a) Distinguish between lap and wave type of windings in DC machines. 4
b) Explain the following terms in brief :
i) Pole pitch
ii) Back pitch
iii) Resultant pitch
iv) Commutator pitch. 4
c) A d.c. series generator has armature resistance of 0.5 Ω and series field
resistance of 0.03 Ω . It drives a load of 50 A. If it has 6 turns/coil and total
540 coils on the armature and is driven at 1500 r.p.m. calculate the terminal
voltage at the load. Assume 4 poles, lap type winding, flux per pole as 2 mwb
and total brush drop as 2V. 8
9. a) Draw the circuit diagrams for separately excited generators and self excited
series generator indicating all the currents and voltages. 8
b) A d.c. shunt motor runs at a speed of 1000 r.p.m. on no load taking a current
of 6A, from the supply when connected to 220 V d.c. supply. Its full load
current is 50 A. Calculate its speed on full load. Assume Ra
= 0.3 Ω and
Rsh = 110 Ω. 8
OR
10. a) Draw the performance characteristics of different types of d.c. generators
and explain them. 8
b) Obtain the torque equation of d.c. motor. 4
c) Explain the various losses in d.c. machine. 4
11. a) Describe Hopkinson’s test in detail with its advantages and disadvantages. 10
b) What is commutator ? What is meant by good commutation ? How is it
achieved ? 8
OR
12. a) Explain Swinburne’s test for finding the efficiency of a d.c. machine. Can this
method applicable to d.c. series motors. 8
b) A retardation test is carried out on a 1000 r.p.m. d.c. machine. The time taken
for the speed to fall from 1030 rpm to 970 rpm is
i) 36 seconds with no excitation
ii) 15 seconds with no excitation
iii) 9 seconds with full excitation and the armature supporting on extra load of
10 A. at 219 V. Calculate
1) Moment of inertia of the armature in Kg.m2
2) Iron loss
3) Mechanical loss at the mean speed of 1000 r.p.m. 10
——————
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