Download 2013

Code No: R31025
R10
Set No: 1
III B.Tech. I Semester Regular and Supplementary Examinations, December -2013
ELECTRICAL MACHINES-III
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. Explain the constructional details and working of single phase induction motors. Also
mention their applications.
2. a) What is the difference between integral slot and fractional slot windings for
synchronous machine? Explain
b) Calculate the line voltage of an alternator if p=10, 3 phase, f=50 Hz, no. of slots =
60, conductors per slot = 4, coil span = 150 0, Ф = 0.12 Wb, winding is sinusoidally
distributed and star connected.
3. What is the reason for harmonics in generated e.m.f. of a synchronous machine? What
are the effects of harmonics? Explain the ways to reduce the harmonics.
4. What is voltage regulation? What is its importance? Explain the synchronous
impedance method for the determination of voltage regulation of an alternator.
5. a) What are the conditions required for parallel operation of alternators? Also explain
the advantages of parallel operation of alternators.
b) A 3-ph, 3000kVA, 6 pole, 50 Hz alternator running in parallel with 3300V bus-bar.
Zs=25%. Calculate synchronous power and torque for one mechanical degree of
displacement.
6. a) Explain the variation of current and power factor of a synchronous motor with
excitation.
b) A 3-ph, 400V, 50 Hz, 27.5 kW synchronous motor has 88% efficiency at full load.
Za=(0.2+j1.6) ohms. If the excitation is adjusted to give a leading p.f. of 0.9. Calculate
i) the excitation emf ii) total mechanical power developed.
7. Why the synchronous motor is not a self starting? Explain the starting methods of
synchronous motors. Also list the main applications of synchronous motors.
8. a) Explain the principle of operation of A. C. series motor. Also list their applications.
b) Explain the operation of Universal motor.
**
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Code No: R31025
R10
Set No: 2
III B.Tech. I Semester Regular and Supplementary Examinations, December -2013
ELECTRICAL MACHINES-III
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) Explain in detail the double field revolving theory.
b) Explain the working of a capacitor start and capacitor run single-phase induction
motor.
2. a) What is the difference between distributed and concentrated windings for
synchronous machines?
b) From fundamentals, derive the expression for e.m.f. of an alternator.
3. a) What is short pitching? Explain the reduction of harmonics with short pitching in
synchronous machines.
b) A 16-pole, 3-phase star connected alternator has 144 slots. The coils are short
pitched by one slot. The pole flux Ф= 100sinθ+30sinθ+20sinθ. Find harmonics as
percentage of phase voltage.
4. a) Explain the ampere-turn method for the determination of voltage regulation of an
alternator.
b) A 3-ph, 100 kVA, 3000V, star-connected alternator has an effective R a of 0.2
ohms. A field current of 40 A produces a short circuit current of 200A and open
circuit emf of 1040 V line. Calculate the full-load voltage regulation at i) unity p.f. ii)
0.8 p.f. lagging and iii) 0.8 p.f. leading.
5. a) What is the need for parallel operation of alternators? Explain the division of load
between two parallel alternators.
b) Two 3-ph alternators are working in parallel with the following particulars:
Alternator 1: Z1 = (0.2+j2) ohms/ph; E1 = (2000+j0) V/ph
Alternator 2: Z2 = (0.2+j2) ohms/ph; E2 = (2200+j100) V/ph
Load: . ZL = (3+j4) ohms/ph.
Determine the KW output and p.f. of each alternator.
6. A star connected 6.6 kV, 3-ph, synchronous motor working at constant voltage and
excitation with Zs=(0+j20) ohms. It is working at 0.8 p.f. lead when input is 1000 kW.
Find input current and p.f. when the machine is loaded until the input power is 1500
kW.
7. a) What is meant by hunting? Explain the suppression of hunting.
b) .Why the synchronous motor is not self starting? Describe the method of starting a
synchronous motor.
8. a) Explain the construction and principle of operation of permanent magnet
synchronous motors. Also list their advantages and disadvantages.
b) Explain the operation of Universal motor.
**
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Code No: R31025
R10
Set No: 3
III B.Tech. I Semester Regular and Supplementary Examinations, December -2013
ELECTRICAL MACHINES-III
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) List and explain different types of single-phase induction motors.
b) Derive the torque equation of split phase single-phase induction motor.
2. a) Define pitch factor, distribution factor and winding factor.
b) Calculate the pitch factor and distribution factors for a 3-ph winding with 4 slots
per pole per phase and with the coil span of 10 slots.
3. a) What is armature reaction? Explain the effect of armature reaction in alternators.
b) What is synchronous reactance? Explain the experimental determination of
synchronous reactance.
4. a) Explain the Z.P.F. method for the determination of voltage regulation of an
alternator.
b) In a single-phase alternator, a voltage of 50V is generated in O.C. test and a current
of 200A is generated in S.C. test. Armature resistance is 0.1 ohms. Calculate
synchronous impedance, synchronous reactance, generated voltage and voltage
regulation when it is supplying a current of 100A at 200V, 0.8 p.f. lagging.
5. Two alternators A and B operate in parallel and supply a load of 10 MW at 0.8 p.f.
lagging.
a) By adjusting steam sypply of A, its power output is adjusted to 6000 kW and by
changing its excitation, its p.f. is adjusted to 0.92 lag. Find the p.f. of alternator B.
b) If steam sypply of both machines is left unchanged, but excitation of B is reduced so
that its p.f. becomes 0.92 lead, find new p.f. of A.
6. a) Why does synchronous motor runs at synchronous speed?
b) What is synchronous condenser? Explain.
c) A 3-phase, 3300V, star connected synchronous motor has Z=(0.5+j5) ohms and
taking an input power of 1500 kW when back emf is 4000V. Calculate i) Line current ii)
power factor for the above conditions.
7. Write short notes on the following:
a) Hunting of synchronous motors
b) Synchronous induction motor
8. What is a reluctance motor? Explain its principle of operations. Also explain the
torque production mechanism in reluctance motors.
**
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R10
Code No: R31025
Set No: 4
III B.Tech. I Semester Regular and Supplementary Examinations, December -2013
ELECTRICAL MACHINES-III
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) Explain the operation of a shaded pole single-phase induction motor.
b) Explain the operation of a capacitor start single-phase induction motor.
2. With neat diagrams, explain the constructional features of salient pole and non-salient
pole synchronous machines. Also derive the expression for e.m.f. of alternator.
3. a) What is leakage reactance? What are its effects in synchronous machines?
b) Define synchronous impedance and synchronous reactance. Also explain the
experimental determination of synchronous reactance.
4. The following data were obtained for the OCC of a 10 MVA, 13 kV, 3-phase, 50 Hz,
star-connected synchronous generator:
If (A)
50
75
100
125
150
162.5
200
250
300
Voc (line) (kV)
6.2
8.7
10.5
11.6
12.8
13.7
14.2
15.2
15.9
An excitation of 100 A causes the full-load current to flow during the short-circuit
test. The excitation required to give the rated current at zero p.f. and rated voltage is
290A.
(i) Calculate the adjusted synchronous reactance of the machine
(ii) Calculate the leakage reactance of the machine assuming the resistance to be
negligible
(iii) Determine the excitation required when the machine supplies full-load at 0.8 p.f.
lagging by using the leakage reactance and drawing the mmf phasor diagram. What is
the voltage regulation of the machine?
5. What are the conditions required for parallel operation of alternators? Discuss any one
method of synchronizing of alternators. Also explain the advantages of parallel
operation of alternators.
6. a) Draw the power flow diagram of a synchronous motor and explain different losses.
b) A three-phase, 11,000V, star-connected synchronous motor is taking 60A current at
0.8 p.f. lead. Its impedance Z=(1+j30) ohms. Calculate i) power input ii) back emf
and iii) mechanical power developed.
7. a) Briefly describe the phenomenon of ‘hunting’ in a synchronous motor. How is it
rectified?
b) Explain different starting methods of synchronous motor.
8. a) Explain the principle of operation and applications of A.C. series motors.
b) Discuss the principle of operation of a reluctance motor. Also mention its
advantages and disadvantages.
**
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