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R10
Code No: R22024
SET - 1
II B. Tech II Semester Supplementary Examinations Jan/Feb - 2015
ELECTRICAL MACHINES - II
(Electronics and Electronics Engineering)
Time: 3 hours
Max. Marks: 75
or
ld
Answer any FIVE Questions
All Questions carry Equal Marks
~~~~~~~~~~~~~~~~~~~~~~~~
a) Explain the classification of transformers.
b) Draw and explain the phasor diagram of transformer for lagging and leading loads?
(5M+10M)
2.
a) Explain how variation in frequency affects the operation of a given transformer.
b) The parameters of a 2300/230 V, 50-Hz transformer are given below:
R1 = 0.286 Ω R2' = 0.319 Ω R0 = 250 Ω X1 = 0.73 Ω X2' = 0.73 Ω, X0 = 1250 Ω. The
secondary load impedance ZL = 0.387 + j 0.29. Solve the exact equivalent circuit with
normal voltage across the primary.
(5M+10M)
3.
a) With a neat circuit diagram explain about the Sumpner’s test conducted on transformer.
b) Explain the working of an auto transformer
(8M+7M)
4.
A ∆-∆ bank consisting of three 20-kVA, 2300/230-V transformers supplies a load of 40 kVA.
If one transformer is removed, find for the resulting V- V connection
i) kVA load carried by each transformer
ii) per cent of rated load carried by each transformer
iii) total kVA rating of the V-V bank
iv) ratio of the V-V bank to ∆ - ∆ bank transformer ratings.
v) per cent increase in load on each transformer when bank is converted into V-V bank. (15M)
TU
W
1.
a) Derive the condition for max torque under running conditions in 3 phase induction motors
induction motors.
b) A 3-phase, slip-ring, induction motor with star-connected rotor has an induced e.m.f. of 120
volts between slip-rings at standstill with normal voltage applied to the stator. The rotor
winding has a resistance per phase of 0.3 ohm and standstill leakage reactance per phase of
1.5 ohm. Calculate: i) rotor current/phase when running short-circuited with 4 percent slip
and ii) the slip and rotor current per phase when the rotor is developing maximum torque.
(8M+7M)
JN
5.
6.
a) Explain about cogging and crawling in 3 phase induction motors.
b) Draw and explain phasor diagram of 3 phase induction motors. Obtain its equivalent circuit.
(8M+7M)
2
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R10
Code No: R22024
SET - 1
A 3-ph, 400-V induction motor gave the following test readings;
No-load : 400 V, 1250 W, 9 A, Short-circuit : 150 V, 4 kW, 38 A
Draw the circle diagram. If the normal rating is 14.9 kW, find from the circle diagram, the fullload value of current, p.f. and slip.
(15M)
8.
a) A 4-pole induction motor and a 6-pole induction motor are connected in cumulative cascade.
The frequency in the secondary circuit of the 6-pole motor is observed to be 1.0 Hz.
Determine the slip in each machine and the combined speed of the set. Take supply
frequency as 50 Hz.
b) Explain the speed control of 3-phase induction motor using stator voltage control. (8M+7M)
JN
TU
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7.
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R10
Code No: R22024
SET - 2
II B. Tech II Semester Supplementary Examinations Jan/Feb - 2015
ELECTRICAL MACHINES - II
(Electronics and Electronics Engineering)
Time: 3 hours
Max. Marks: 75
or
ld
Answer any FIVE Questions
All Questions carry Equal Marks
~~~~~~~~~~~~~~~~~~~~~~~~
a) A single phase transformer takes 10A on no load at p.f. of 0.2 lagging. The turns ratio is 4 : 1
(step down). If the load on the secondary is 200 A at a p.f. of 0.85 lagging, find the primary
current and power factor. Neglect the voltage-drop in the winding.
b) Derive the e.m.f equation of a single phase transformer.
(8M+7M)
2.
a) Find the all-day efficiency of 500-kVA distribution transformer whose copper loss and iron
loss at full load are 4.5 kW and 3.5 kW respectively. During a day of 24 hours, it is loaded
as under :
No. of hours
Loading in kW
Power factor
6
400
0.8
10
300
0.75
4
100
0.8
4
0
–
b) Derive the condition for max efficiency in a transformer.
(8M+7M)
3.
a) The equivalent circuit for a 200/400-V step-up transformer has the following parameters
referred to the low-voltage side.
Equivalent resistance = 0.15 Ω; Equivalent reactance = 0.37 Ω
Core-loss component resistance = 600 Ω; Magnetizing reactance = 300 Ω. When the
transformer is supplying a load at 10 A at a power factor of 0.8 lag, Calculate: i) the primary
current ii) secondary terminal voltage.
b) An 11500/2300 V transformer is rated at 100 kVA as a 2 winding transformer. If the two
windings are connected in series to form an auto-transformer, what will be the possible
voltage ratios?
(8M+7M)
JN
TU
W
1.
4.
a) Two T-connected transformers are used to supply a 440-V, 33-kVA balanced load from a
balanced 3-phase supply of 3300 V. Calculate: i) voltage and current rating of each coil
ii) kVA rating of the main and teaser transformer.
b) A balanced 3-phase load of 150 kW at 1000 V, 0.866 lagging power factor is supplied from
2000 V, 3-phase mains through single-phase transformers (assumed to be ideal) connected
in i) delta-delta ii) V-V. Find the current in the windings of each transformer and the
power factor at which they operate in each case. Explain your calculations with circuit and
vector diagrams.
(8M+7M)
2
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R10
Code No: R22024
SET - 2
Derive the inter relationship Rotor power input, rotor copper loss and mechanical power
developed of an induction motor?
(15M)
6.
a) Explain about the power stages in 3 phase induction motors.
b) Explain the relation between torque and rotor power factor in 3 phase induction motors.
(8M+7M)
7.
Draw the circle diagram for a 3.73 kW, 200V, 50-Hz, 4-pole, 3-phase star-connected induction
motor from the following test data :
No-load : Line voltage 200 V, line current 5 A; total input 350 W
Blocked rotor : Line voltage 100 V, line current 26 A; total input 1700 W Estimate from the
diagram for full-load condition, the line current, power factor and also the maximum torque in
terms of the full-load torque. The rotor Cu loss at standstill is half the total Cu loss.
(15M)
8.
a) Two 50-Hz, 3-Ph induction motors having six and four poles respectively are cumulatively
cascaded, the 6-pole motor being connected to the main supply. Determine the frequencies
of the rotor currents and the slips referred to each stator field if the set has a slip of 2
percent.
b) Explain the speed control of 3-phase induction motor using frequency control.
(8M+7M)
JN
TU
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R10
Code No: R22024
SET - 3
II B. Tech II Semester Supplementary Examinations Jan/Feb - 2015
ELECTRICAL MACHINES - II
(Electronics and Electronics Engineering)
Time: 3 hours
Max. Marks: 75
or
ld
Answer any FIVE Questions
All Questions carry Equal Marks
~~~~~~~~~~~~~~~~~~~~~~~~
a) Explain the working principle of an ideal transformer by using phasor diagram?
b) A 25-kVA transformer has 500 turns on the primary and 50 turns on the secondary winding.
The primary is connected to 3000-V, 50-Hz supply. Find the full-load primary and
secondary currents, the secondary e.m.f. and the maximum flux in the core. Neglect leakage
drops and no-load primary current.
(8M+7M)
2.
A 5-kVA, 2,300/230-V, 50-Hz transformer was tested for the iron losses with normal excitation
and Cu losses at full-load and these were found to be 40 W and 112 W respectively. Calculate
the efficiencies of the transformer at 0.8 power factor for the following kVA outputs :
1.25
2.5
3.75
5.0
6.25
7.5
Plot efficiency vs kVA output curve.
(15M)
3.
a) A 5 kVA 200/1000 V, 50 Hz, single-phase transformer gave the following test results :
O.C. Test (L.V. Side) : 2000 V, 1.2 A, 90 W
S.C. Test (H.V. Side) : 50 V, 5A, 110 W
i) Calculate the parameters of the equivalent circuit referred to the L.V. side.
ii) Calculate the output secondary voltage when delivering 3 kW at 0.8 p.f. lagging, the
input primary voltage being 200 V.
iii) Find the percentage regulation.
b) A 5 kVA, 110/110 V, single-phase, 50 Hz transformer has full-load efficiency of 95% and
an iron loss of 50 W. The transformer is now connected as an auto-transformer to a 220 V
supply. If it delivers a 5 kW load at unity power factor to a 110 V circuit, calculate the
efficiency of the operation and the current drawn by the high-voltage side.
(8M+7M)
JN
TU
W
1.
4.
A load of 1000 kVA at 0.866 p.f. lagging is supplied by two 3 phase transformers of 800 kVA
capacity operating in parallel. Ratio of transformation is same: 6600/400 V, delta/star. If the
equivalent impedances referred to secondary are (0.005 + j.015) ohm and (0.012 + j0.030) ohm
per phase respectively. Calculate load and power factor of each transformer.
(15M)
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R10
Code No: R22024
SET - 3
a) Explain the working principle of 3 phase induction motors.
b) A 4-pole, 3-phase induction motor operates from a supply whose frequency is 50Hz.
Calculate:
i) the speed at which the magnetic field of the stator is rotating.
ii) the speed of the rotor when the slip is 0.04.
iii) the frequency of the rotor currents when the slip is 0.03.
iv) the frequency of the rotor currents at standstill.
(10M+5M)
6.
a) Explain the torque slip characteristics of 3 phase induction machines
b) The power input to the rotor of 440 V, 50 Hz, 6-pole, 3-phase, induction motor is 80kW.
The rotor electromotive force is observed to make 100 complete alterations per minute.
Calculate: i) the slip, ii) the rotor speed,
iii) rotor copper losses per phase.
(8M+7M)
7.
Draw the circle diagram from no-load and short-circuit test of a 3-phase 14.92 kW, 400V, 6pole induction motor from the following test results (line values).
No-load : 400V, 11 A, p.f. = 0.2
Short-circuit : 100V, 25 A, p.f. = 0.4
Rotor Cu loss at standstill is half the total Cu loss. From the diagram, find: a) line current, slip,
efficiency and p.f. at full-load b) the maximum torque.
(15M)
8.
a) Explain the working principle of Induction generators.
b) Explain about the cascaded operation of 3-phase induction motors.
JN
TU
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(8M+7M)
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R10
Code No: R22024
SET - 4
II B. Tech II Semester Supplementary Examinations Jan/Feb - 2015
ELECTRICAL MACHINES - II
(Electronics and Electronics Engineering)
Time: 3 hours
Max. Marks: 75
1.
or
ld
Answer any FIVE Questions
All Questions carry Equal Marks
~~~~~~~~~~~~~~~~~~~~~~~~
a) Explain about the construction of Transformer.
b) Explain about a single phase transformer on load by using phasor diagram.
(8M+7M)
a) In a 25-kVA, 2000/200 V, single-phase transformer, the iron and full-load copper losses are
350 W and 400 W respectively. Calculate the efficiency at unity power factor on i) full load
ii) half full-load.
b) Explain about various losses in a transformer.
(8M+7M)
3.
The low voltage winding of a 300kVA, 11,000/2500V, 50-Hz transformer has 190 turns and a
resistance of 0.06. The high-voltage winding has 910 turns and a resistance of 1.6Ω. When the
l.v. winding is short-circuited, the full-load current is obtained with 550-V applied to the h.v.
winding. Calculate: i) the equivalent resistance and leakage reactance as referred to h.v. side
and ii) the leakage reactance of each winding.
(15M)
4.
Explain about three phase to two phase transformation using Scott connection .
5.
a) Explain the production of rotating magnetic field in 3 phase induction motors.
b) List out the advantages and disadvantages of 3 phase induction motors
(15M)
(10M+5M)
TU
W
2.
a) A 440V, 3-phase , 50-Hz, 4-pole, Y-connected induction motor has a full-load speed of 1425
rpm. The rotor has an impedance of (0.4 + j4) ohm and rotor/stator turn ratio of 0.8.
Calculate: i) full-load torque ii) rotor current and full-load rotor Cu loss iii) power output
if windage and friction losses amount to 500 W. iv) maximum torque and the speed at
which it occurs v) starting current and vi) starting torque.
b) The maximum torque of a 3-phase induction motor occurs at a slip of 12%. The motor has
an equivalent secondary resistance of 0.08Ω /phase. Calculate the equivalent load resistance
RL, the equivalent load voltage VL and the current at this slip if the gross power output is
9,000 watts.
(8M+7M)
JN
6.
7.
8.
A 415V, 29.84 kW, 50-Hz, delta-connected motor gave the following test data :
No-load test : 415 V, 21 A, 1,250 W
Locked rotor test : 100 V, 45 A, 2,730 W
Construct the circle diagram and determine:
a) the line current and power factor for rated output
b) the maximum torque. Assume stator and rotor Cu losses equal at standstill.
(15M)
a) Explain the speed control of 3-phase induction motor using pole changing method
b) A certain 3-phase, 6-pole, 50-Hz induction motor when fully-loaded, runs with a slip of 3%.
Find the value of the resistance necessary in series per phase of the rotor to reduce the speed
by 10%. Assume that the resistance of the rotor per phase is 0.2 ohm.
(8M+7M)
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