SET - 1
R10
Code No: R22024
II B.Tech II Semester, Regular Examinations, April – 2012
ELECTRICAL MACHINES -II
(Electrical and Electronics Engineering)
Time: 3 hours
Max. Marks: 75
Answer any FIVE Questions
All Questions carry Equal Marks
~~~~~~~~~~~~~~~~~~~~~~
1.
a) Define a transformer. How is the energy transferred from one circuit to another?
Distinguish between primary and secondary windings.
b) A 400/100V, ideal 1-phase transformer is connected to a load of 8+j6Ω and its primary is
fed from 400V, 50Hz source. Calculate the current, power factor and power on both primary
and secondary sides. Neglect magnetising current.
2.
a) Draw the exact equivalent circuit of a transformer and describe briefly the various
parameters involved in it.
b) A 200V, 60 Hz single phase transformer has hysteresis and eddy current losses of 250 watts
and 90 watts respectively. If the transformer is now energised from 230 V, 50 Hz supply,
calculate its core losses. Assume Steinmentz’s constant equal to 1.6.
3.
a) In open-circuit test, the ohmic losses are negligible in comparison with normal core loss.
Explain.
b) In short-circuit test, the core loss is negligible in comparison with full load ohmic losses.
Explain.
c) A 5 KVA, 1000/200V, 50Hz single phase transformer gave the following test results:
Open-circuit test: (l.v side): 200V, 1.2 A, 90 W
Short-circuit test (h.v Side): 50 V, 5 A, 110 W
Compute the parameters of the approximate equivalent circuit referred to l.v side. Also draw
the exact equivalent circuit referred to l.v side.
4.
a) Discuss various possible poly-phase connections with three winding transformers.
b) Explain in detail the working of an On-load tap-changer for a transformer.
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Code No: R22024
5.
SET - 1
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a) Describe the constructional features of both squirrel-cage induction motor and slip-ring
induction motor. Discuss the merits of one over the other.
b) A 3-phase slip-ring, induction motor with star-connected rotor has an induced e.m.f. of 120V
between slip-rings at standstill with normal voltage applied to the stator. The rotor winding has
a resistance per phase of 0.3 Ω and standstill leakage reactance per phase of 1.2 Ω. Calculate
i) Rotor current/phase when running short-circuited with 4% slip and ii) the slip and rotor
current per phase when the rotor is developing maximum toque.
6.
a) Sketch and explain the torque-slip characteristics of a 3-phase induction motor.
b) For a 3-phase induction motor, the rotor ohmic loss at maximum torque is 16 times that at
full load torque. The slip at full load torque is 0.03. If stator resistance and rotational losses are
neglected, then calculate the starting torque in terms of full load torque.
7.
A 3-phase star connected, 440V, 4-pole, 50Hz, slip ring induction motor gave the following
test results,
No Load Test : 440V,9A, pf=0.2
Blocked Rotor Test ; 100V, 22A, pf= 0.3
All the above are line values. The ratio of primary to secondary turns=3.5, stator and rotor
copper losses are equally divided in blocked rotor test. The full load current is 20A. Plot the
circle diagram and for full load find
i) The line current, power factor, slip
ii) Staring torque
iii) Resistance to be inserted in series with rotor circuit for giving starting torque 200% of
full load torque. Also find the current and power factor under this condition.
8.
a) Explain the principle of operation of induction generator and mention its advantages and
disadvantages.
b) Two motors A and B with 10 poles and 12 poles respectively are cascaded. The motor A is
connected to a 50Hz supply. Find i) Speed of set
ii) the electrical power transferred to the
motor B when the input to the motor A is 60kW. Neglect losses.
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R10
Code No: R22024
SET - 2
II B.Tech II Semester, Regular Examinations, April – 2012
ELECTRICAL MACHINES -II
(Electrical and Electronics Engineering)
Time: 3 hours
Max. Marks: 75
Answer any FIVE Questions
All Questions carry Equal Marks
~~~~~~~~~~~~~~~~~~~~~~
1.
a) Explain the principle of transformer action.
b) Derive an expression for the e.m.f induced in a transformer winding. Show that e.m.f per
turn in the primary is equal to e.m.f per turn in the secondary
2.
a) Suggest a suitable test to pre determine the regulation of a transformer and explain it.
b) In a test for the determination of losses of a 440 V, 50 Hz transformer, the total iron losses
were found to be 2500 W at normal voltage and frequency. When the applied voltage and
frequency were 220 V and 25 Hz, the iron losses found to be 850 W. Calculate the eddy current
and hysteresis losses at normal voltage and frequency.
3.
a) Explain why open circuit and short circuit tests should be performed on l.v and h.v sides
respectively.
b) A 10 KVA, 500/250 V, single phase transformer gave the following results:
S.C test (h.v side): 60V, 20 A, 150 W.
The maximum efficiency occurs at unity pf and at 1.2 full-load current. Determine full load
efficiency at 0.8 pf. Also calculate the maximum efficiency.
4.
a) Explain the principle of three-phase to two-phase conversion using scott-connected
transformers.
b) A 200 kVA, 3-phase, 50 Hz, 6600/400 V transformer is star connected on the primary side
and delta connected on the secondary side. The resistance of the primary winding is 2.4
Ω/phase and that of the secondary winding is 0.028 Ω/phase. Calculate the iron losses of the
transformer at the normal voltage and frequency if its full load efficiency is 93 % at a power
factor of 0.80 lagging.
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Code No: R22024
5.
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SET - 2
a) Describe the principle of operation of a 3-phase induction motor. Explain why the rotor is
forced to rotate in the direction of rotating magnetic field?
b) The power input to a 500V, 50Hz, 6-pole, 3-phase induction motor running at 975 rpm is 40
kW. The stator losses are 1 kW and the friction and windage losses total 2 kW. Calculate:
i) the slip ii) the rotor copper loss iii) shaft power and iv) the efficiency.
6.
a) Draw and explain the phasor diagram of a 3-phase induction motor.
b) Discuss the phenomenon of crawling and cogging in an induction motor.
7.
a) With neat diagram explain the construction and operation of auto transformer starter.
b) A Cage induction motor when started by means of a star-delta starter takes 180% of full load
current and develops 35% of full load torque at starting. Calculate the
starting current and
torque in terms of full load torque when started by means of an auto transformer with 75%
tapping.
8.
a) Compare the speed control of 3-phase induction motor by rotor resistance control and
variable frequency control.
b) A 4-pole, 50Hz, wound rotor has a rotor resistance of 0.56 Ω/phase and runs at 1430 r.p.m at
full load. Calculate the additional resistance per phase to be inserted in the rotor circuit to lower
the speed to 1200 r.p.m, if the torque remains constant.
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R10
Code No: R22024
SET - 3
II B.Tech II Semester, Regular Examinations, April – 2012
ELECTRICAL MACHINES -II
(Electrical and Electronics Engineering)
Time: 3 hours
Max. Marks: 75
Answer any FIVE Questions
All Questions carry Equal Marks
~~~~~~~~~~~~~~~~~~~~~~
1.
Give the concept of single-phase ideal transformer. Describe its performance with the help of
e.m.f equations, its phasor diagrams at no-load and on load.
2.
Describe briefly the various losses in a transformer and explain how each loss varies with the
load current.
3.
a) State the essential and desirable conditions which should be satisfied before two single phase
transformers may be operated in parallel.
b) Two similar 40-kVA, 1-phase transformers gave the following results when tested by the
back to back method:
W1 in the supply line: 800watts
W2 in the secondary’s series circuit at rated current: 1000watts.
Calculate the efficiency of each transformer at full load and unity p.f.
4.
a) Why are tap-changing transformers required? Explain the operation of no-load tap changing
transformer.
b) Two single-phase furnaces working at 100V are connected to 3300-V, 3-phase mains
through Scott-connected transformers. Calculate the current in each line of the 3-phase mains
when the power taken by each furnace is 400-KW at a power factor of 0.8 lagging. Neglect
losses in the transformer.
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SET - 3
a) Show that the voltage generated in the rotor circuit of a 3-phase induction motor at any slip s
is equal to s times the voltage generated at stand-still.
b) A 4-pole induction motor is energized from a 50Hz supply. If the machine runs on full load
at 2% slip, determine the
i) Rotor speed with respect to resultant rotor field.
ii) Stator resultant field speed with respect to rotor structure, and
iii) Frequency of rotor currents.
6.
a) Derive the torque equation of induction motor. From this derive the condition for maximum
torque.
b) An 8-pole, 50Hz, 3-phase slip ring induction motor has effective resistance of 0.08 Ω/phase.
The speed corresponding to maximum torque is 650 r.p.m. What is the value of resistance to be
inserted in rotor circuit to obtain maximum torque at starting?
7.
a) Explain the procedure of drawing the circle diagram of an induction motor. What
information can be obtained from the circle diagram?
b) A 4-pole, 3-phase, 50Hz induction motor has a starting current which is 5 times its full load
value when switched on directly. What will be the % reduction in starting torque if the motor is
started with,
i) Star-delta starter
ii) Auto transformer starter with 65% tapping?
8.
a) Explain the following schemes of speed control with neat sketches
i) Cascade method
ii) Pole changing method.
b) State the applications of induction generator.
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R10
Code No: R22024
SET - 4
II B.Tech II Semester, Regular Examinations, April – 2012
ELECTRICAL MACHINES -II
(Electrical and Electronics Engineering)
Time: 3 hours
Max. Marks: 75
Answer any FIVE Questions
All Questions carry Equal Marks
~~~~~~~~~~~~~~~~~~~~~~
1.
a) Explain how the primary current increase as the current on the secondary side of the
transformer is increased.
b) A 100KVA 2400/240 V, 50Hz single phase transformer has an exciting current of 0.64A and
a core loss of 700watts, when its high-voltage side is energised at rated voltage and frequency.
Calculate the two components of the exciting current.
2.
a) Define voltage regulation of a transformer. What causes a change in secondary terminal
voltage of a transformer, as it is loaded?
b) A 100 KVA, 50 Hz, 440/11000 V, 1-phase transformer has an efficiency of 98.5% when
supplying full-load current at 0.8pf and an efficiency of 99% when supplying half-full load
current at unity pf. Find the iron losses and ohmic losses corresponding to full load current. At
what value of load current will the maximum efficiency be attained?
3.
a) Show that in an Auto-Transformer the conductor weight= (1-k) weight of conductor in two
winding transformer.
b) Two transformers A and B are connected in parallel to a load of (2+j1.5) Ω. Their
impedances in secondary terms are ZA = (0.15+j0.5) Ω and ZB = (0.1+j0.6) Ω. Their no load
terminal voltages are EA=207 ∟00 volt and EB=205 ∟00 volt. Find the power output and
power factor of each transformer.
4.
a) Explain the role of on load tap changers in transformers for achieving voltage regulation.
b) Two 2200/110-V transformers operated in parallel to share a load of 125 KVA at 0.8 power
factor 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.
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5.
SET - 4
R10
Code No: R22024
a) Derive the expression for (rotor side) starting current, starting power factor, standstill
frequency and standstill E.M.F of slip ring induction motor.
b) A 100-KW (output), 3300-V, 50Hz, 3-phase, star connected induction motor has a
synchronous speed of 500 r.p.m. The full-load slip is 1.8% and full load power factor is 0.85.
Stator copper loss = 2440 W. Iron loss = 3500 W. Rotational losses = 1200 W. Calculate
i) the rotor copper loss
6.
ii) the line current and
iii) the full-load efficiency.
a) With the help of rotor equivalent circuit of an induction motor, show that the power
transferred magnetically from a stator to rotor is given by I 22
r2
per phase.
s
b) Explain the necessity of using double cage and deep bar rotors in induction motor.
7.
a) Explain Auto-transformer starter with neat sketch and obtain the expression for starting
torque in terms of full load torque.
b) Explain the procedure to conduct blocked rotor test and no-load test on induction motor.
8.
a) Explain the speed control of 3-phase induction motor using ‘rotor e.m.f injection method.
b) A 4-pole 3-phase, 50Hz, slip ring induction motor rotates at 1440 r.p.m with slip ring
terminals short-circuited. The per phase rotor resistance and stand still reactance are 0.1 Ω and
0.6 Ω respectively. If an extra resistance of 0.1 Ω /phase is added to the rotor circuit, what will
be the new full-load speed?
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