Code No: R05220204 Set No. 1
II B.Tech II Semester Regular Examinations, Apr/May 2008
ELECTRICAL MACHINES-II
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
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1. (a) Derive the EMF equation of transformer? Hence derive the voltage ratio.
(b) A 15kVA 2400-240-V, 60 Hz transformer has a magnetic core of 50-cm2 cross
section and a mean length of 66.7 cm. The application of 2400 V causes
magnetic field intensity of 450 AT/m (RMS) and a maximum flux density of
1.5 T . Determine
i. The turn’s ratio
ii. The numbers of turns in each winding
iii. The magnetizing current [8+8]
2. A 5 kVA, 25 Hz transformer has 1.3 % for copper losses, 0.7 % hysterisis losses &
0.3 % eddy current losses. What will be the % losses when transformer is used on
10 kV, 50 Hz system? Compare full load efficiency in both the cases. [16]
3. (a) What are the advantages of Sumpner’s test? Give the related calculation to
find the efficiency of a transformer.
(b) In Sumpner’s test on two identical transformer rated 500 kVA, 11/0.4 kV,
50 Hz, the wattmeter reading on HV side is 6 kW on rated voltage and on
LV side is 15 kW when circulated full load current. Find the efficiency of
each transformer on 3/4th load & 0.8 pf lagging. What will be the maximum
efficiency of each transformer? [8+8]
4. (a) With neat phasor diagram, explain the voltage regulation of three-phase trans-
former.
(b) An ideal 3- step down transformer connected in delta/star delivers power
to a balanced 3- load of 120 kVA at 0.8 pf. The input line voltage is 11
kV and the turn’s ratio of transformer (phase to phase) is 10. Determine the
line voltage line currents, phase voltages, phase currents on both primary &
secondary sides. [8+8]
5. (a) Explain clearly the principle of operation of Induction motor.
(b) The frequency of stator EMF is 50 Hz for an 8-pole induction motor. If the
rotor frequency is 2.5 Hz, calculate the slip and the actual speed of rotor.
[8+8]
6. (a) Explain term Maximum torque, Full load torque, Starting torque & No-load
torque.
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Code No: R05220204 Set No. 1
(b) An 8-pole, 50 Hz, 3- slip ring IM has effective resistance of 0.08 /phase.
The speed correspond to maximum torque is 650 rpm. What is the value of
resistance to be inserted in rotor circuit to obtain maximum torque at starting?
[8+8]
7. (a) Calculate reduction in starting current & starting torque when the supply
voltage is reduced by 25 %.
(b) A 3-, cage IM has a short circuit current equal to 5 times the full load current.
Find the starting torque as the % of full load torque, if the motor is started
by
i. DOL starter
ii. Star- starter
iii. an auto transformer starter with x % tapping
iv. Rotor resistance starter.
Starting current in (iii) & (iv) is to be limited to 2.5 times the full load current.
Full load slip is 4 %. [4+12]
8. (a) With neat diagram explain the operation of 3- IM as induction generator.
(b) Two motors A & B with 10 poles & 12 poles respectively are cascaded. The
motor A is connected to a 50 Hz supply. Find
i. Speed of the set
ii. The electrical power transferred to the motor B when the input to the
motor A is 60 kW. Neglect losses. [8+8]
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Code No: R05220204 Set No. 2
II B.Tech II Semester Regular Examinations, Apr/May 2008
ELECTRICAL MACHINES-II
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
1. (a) Derive the EMF equation of transformer? Hence derive the voltage ratio.
(b) A 15kVA 2400-240-V, 60 Hz transformer has a magnetic core of 50-cm
2 cross
section and a mean length of 66.7 cm. The application of 2400 V causes
magnetic field intensity of 450 AT/m (RMS) and a maximum flux density of
1.5 T . Determine
i. The turn’s ratio
ii. The numbers of turns in each winding
iii. The magnetizing current [8+8]
2. (a) What is the efficiency of transformer? How the efficiency of transformer can
be calculated?
(b) The turn’s ratio of a single phase transformer is 4. The resistance & leakage
reactance of HV windings are 1.4 & 1.6 respectively and that of LV
windings are 0.06 & 0.08 respectively. If 200 V is applied to HV winding
& LV winding is short circuited, find the current supplied by the source.
(Neglect magnetising current) [6+10]
3. (a) Draw the vector diagrams of transformer at load with lagging & leading power
factors.
(b) Calculate the voltage regulation for a 200/400 V, 4 kVA transformer at full
load & pf. 0.8 lagging with following test data:
OC test: 200 V, 0.8 A, 70 W (LV side)
SC test: 20 V, 10 A, 60 W (HV side) [8+8]
4. (a) What are the disadvantages of current & voltage harmonics in transformers.
Explain how these harmonics can be eliminated.
(b) Two single phase furnaces are supplied at 250 V from a 6.6 kV, 3-f system
through a pair of Scott connected transformer, if the load on the main trans-
former is 85 kW at 0.9 pf lagging and that on the teaser transformer is 69
kW at o.8 pf lagging. Find the values of line currents on the three phase side.
Neglect the losses. [8+8]
5. (a) Explain clearly the principle of operation of Induction motor.
(b) The frequency of stator EMF is 50 Hz for an 8-pole induction motor. If the
rotor frequency is 2.5 Hz, calculate the slip and the actual speed of rotor.
[8+8]
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Code No: R05220204 Set No. 2
6. (a) With neat diagram explain the equivalent circuit of 3- Double Cage IM.
(b) A 400 V, 4 pole, 7.5 kW, 50 Hz, 3-, IM develops its full load torque at a
slip of 4 %. The per phase circuit parameters of the machine are: r1 = 1.08
, x1 = 1.41 , r
′
2 =? x
′
2 = 1.41 . Mechanical, core & stray losses may be
neglected. Find: rotor resistance/ph referred to secondary, maximum torque
and corresponding rotor speed. [8+8]
7. With neat diagram explain the various tests to be conducted on 3- IM to plot the
circle diagram. [16]
8. (a) Explain the speed control of IM by rotor resistance control method. How this
method of speed control is different from stator side speed control methods.
(b) A 4 pole, 50 Hz, wound rotor IM has a rotor resistance of 0.56 ph and runs
at 1430 rpm at full load. Calculate the additional resistance per phase to
be inserted in the rotor circuit to lower the speed to 1200 rpm, if the torque
remains constant. [8+8]
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Code No: R05220204 Set No. 3
II B.Tech II Semester Regular Examinations, Apr/May 2008
ELECTRICAL MACHINES-II
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
1. (a) With neat phasor diagram explain the operation of transformer with capacitive
load
(b) The voltage ratio of single phase 50 Hz transformer is 5000/500 V at no-load.
Calculate the number of turns in each winding, if the value of the flux in the
core is 7.82 mWb. [8+8]
2. (a) Derive the condition for maximum efficiency of a transformer.
(b) A single phase 150 kVA transformer has efficiency of 96 % at full load, 0.8 pf
and at half load, 0.8 pf lagging. Find maximum efficiency of transformer and
corresponding load. [8+8]
3. With neat diagram, explain the various tests conducted on transformer to obtain
its equivalent circuit. Derive all related equations. [16]
4. (a) Explain tests to determine the equivalent circuit parameters of a three-phase
transformer.
(b) A 3-. 100 kVA, 5000/400 V Star/Star, 50 Hz transformer has an iron loss
of 1400 W. The maximum efficiency of transformer occurs at 80% of load.
Calculate:
i. The efficiency of transformer at full load and 0.85 pf lagging
ii. The maximum efficiency at UPF. [8+8]
5. (a) Explain the construction of Induction Motor.
(b) An 8 pole, 3- alternator is coupled to a prime mover running at 750 rpm. It
supplies an induction motor which has a full load speed of 960 rpm. Find the
number of poles of IM and slip. [8+8]
6. (a) A 4 pole, 400 V, 3- IM has a standstill rotor EMF of 100 V per phase.
The rotor has resistance of 50 m/ph and standstill reactance of 0.5 /ph.
Calculate the maximum torque & slip at which it occurs. Neglect stator
impedance.
(b) Explain the various losses taking place in IM. Explain the effect of slip on the
performance of IM. [8+8]
7. (a) A cage IM when started by means of a star- starter takes 180 % of full load
current & develops 35 % of full load torque at starting. Calculate the starting
current & torque in terms of full load torque when started by means of an
auto transformer with 75 % tapping.
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Code No: R05220204 Set No. 3
(b) Compare DOL starter, Star starter, Auto transformer starter & Rotor re-
sistance starter with relate to the following:
i. Starting current
ii. Starting torque
iii. Flexibility
iv. Cost & efficiency [8+8]
8. Explain all the modes of operation of Induction machine. Plot the neat character-
istics. [16]
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Code No: R05220204 Set No. 4
II B.Tech II Semester Regular Examinations, Apr/May 2008
ELECTRICAL MACHINES-II
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
1. (a) With neat phasor diagram explain the operation of transformer with inductive
load
(b) The exciting current for a 50 kVA, 480/240V 50 Hz transformer is 2.5% of
rated current at a phase angle of 79.80. Find the components of magenetising
current & loss component. Also find the magnetising reactance & core loss
resistance. [8+8]
2. (a) Derive the condition for maximum efficiency of a transformer.
(b) A single phase 150 kVA transformer has efficiency of 96 % at full load, 0.8 pf
and at half load, 0.8 pf lagging. Find maximum efficiency of transformer and
corresponding load. [8+8]
3. (a) Explain the various simple tests conducted on a single transformer to find the
approximate equivalent circuit of transformer.
(b) OC test is preferred to conduct on LV side & SC test is preferred to conduct
on HV side. Explain the reasons. [10+6]
4. (a) With neat phasor diagram, explain the voltage regulation of three-phase trans-
former.
(b) An ideal 3- step down transformer connected in delta/star delivers power
to a balanced 3- load of 120 kVA at 0.8 pf. The input line voltage is 11
kV and the turn’s ratio of transformer (phase to phase) is 10. Determine the
line voltage line currents, phase voltages, phase currents on both primary &
secondary sides. [8+8]
5. (a) Explain clearly the principle of operation of Induction motor.
(b) The frequency of stator EMF is 50 Hz for an 8-pole induction motor. If the
rotor frequency is 2.5 Hz, calculate the slip and the actual speed of rotor.
[8+8]
6. (a) In approximate equivalent circuit of 3- IM, explain step by step the devel-
opment of equivalent load resistance.
(b) A 440 V, 19 kW, 50 Hz, 8 pole, IM has its stator & rotor connected in star.
The effective stator to rotor turn is 2.5:1. The parameters of its circuit model
are: r1 = 0.4 , x1 = 1.03 , r2 = 0.07 , x2 = 0.18 , rm = 25.9 , ri = 127.4
(including rotational losses) Neglect any change in mechanical losses due to
change in speed, calculate the maximum added rotor resistance required for
the motor to run up to the speed for a constant load torque of 300 Nm. [6+10]
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Code No: R05220204 Set No. 4
7. A 3-, star connected, 440 V, 4-pole, 50 Hz slip ring IM gave the following test
results:
No load Test: 440 V, 9 A, PF = 0.2
Blocked rotor test: 100 V, 22 A, PF = 0.3
All above are the line values. The ratio of primary to secondary turns = 3.5, stator
& rotor copper losses are equally divided in blocked rotor test. The full load current
is 20 A. Plot the circle diagram and for full load find:
(a) The line current, The power factor, Slip
(b) Starting torque
(c) Resistance to be inserted in series with rotor circuit for giving starting torque
200 % of full load torque. Also find current & power factor under this condi-
tion. [16]
8. (a) Compare the speed control of 3- IM by rotor resistance control & variable
frequency control.
(b) Two slip ring IMs having 10 & 6 poles respectively are mechanically coupled.
i. Calculate the possible speed when first motor is supplied from a 50 Hz
supply line.
ii. Calculate the ratio of power shared by the two motors.
iii. If the smallest possible speed is to be attained independently by each
machine,
calculate the frequency of the voltage to be injected in the rotor circuit.
[8+8]
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