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Code No: V3111
R07
Set No: 1
III B.Tech. I Semester Supplementary Examinations, December - 2013
ELECTRICAL MACHINES -III
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) What is a distribution factor? What is its effect? Derive an expression for distribution
factor of a winding having Q slots per pole per phase and a slot angle of β.
b) For a 3-phase winding with coil span of 1600, determine the distribution and winding
factors in case the winding has a phase spread of 1200. Take the three phase winding
(i) to be uniformly distributed and
(ii) to have 9 slots per pole.
2.
a) What are the causes of harmonics in the emf waveforms of synchronous generators and
what means are adopted to minimize them.
b) A 3-phase alternator has 2 slots per pole per phase and coil span of 5 slot pitches. The
flux density wave of alternator consists of a fundamental and a 25% third harmonic.
Calculate the percentage increase in the phase voltage due to harmonic.
3.
Explain the merits and demerits of EMF and MMF methods. Explain what are the
assumptions made in each case.
4.
a) In what respect is the operation of an alternator on infinite bus bar different from parallel
operation of two alternators? What is the effect of change in excitation on the operation of
an alternator on infinite bus?
b) A 2MVA, 3-phase, star connected, 8 pole, 750 rpm alternator is operating on 6000V bus
bars, Xs is 6 ohm per phase. Find synchronizing power and torque per mechanical degree of
displacement for full load, 0.8 power factor lagging.
5.
a) What are the advantages and disadvantages of the synchronous motor?
b) A Synchronous motor takes 25KW from 400V supply mains. The synchronous reactance
of the motor is 4Ω. Find the power factor at which the motor would operate when the
exciting current is so adjusted that the generated emf is 500V.
6.
a) Mention the different methods of starting a synchronous motor. Explain any one of them
in detail.
b) Show that the current locus of a synchronous motor developing constant power circle.
Determine its center and radius.
7.
Write short notes on following:
a) Double revolving field theory.
b) Capacitor Start single phase induction motor.
8.
a) Explain the construction and operation of A.C series motor with neat sketch?
b) What are the changes must be made in a d.c motor that is to operate on a.c supply?
**
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Code No: V3111
R07
Set No: 2
III B.Tech. I Semester Supplementary Examinations, December - 2013
ELECTRICAL MACHINES -III
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) Discuss the effect of pitch and distribution factors in an alternator.
b) For a 3-phase winding with 3 slots per pole per phase and coil span of 8 slots, compute
the breadth and pitch factors.
2.
a) With neat circuit diagrams, explain the various tests conducted on an alternator to
determine its synchronous reactance.
b) A 16 pole, 3 phase, star connected alternator has 144 slots. The coils are short pitched by
one slot. The flux per pole is Φ= 100sinθ + 30 sin3θ + 20 sin 5θ. Find harmonics as
percentage of phase voltage and line voltage.
3.
a) Discuss the ZPF method of calculating voltage regulation.
b) Explain how the above method is superior to synchronous impendence and mmf
methods.
4.
a) What is the effect of synchronizing power and Torque on parallel operation of
Alternators?
b) A 750 kVA, 11 kV, 4 pole 3 phase star connected alternator has percentage resistance
and reactance of 1 and 15 ohm respectively. Calculate synchronizing power per mechanical
degree of displacement at
(i) no-load (ii) at full- load at 0.8 pf lag. The terminal voltage in each case is 11kV.
5.
Write short notes on the following
a) V and Λ curves of synchronous motor.
b) Synchronous condenser for Power factor improvement.
6.
a) What are the uses of damper windings in a synchronous motor?
b) Why it is necessary to increase the excitation to obtain minimum current with the
application of load.
7.
a) Explain what is meant by the split-phase method of motor starting
b) Compare various types of single phase induction motors in terms of construction and
performance
8.
a) Write the characteristics and applications of A.C series motor?
b) Describe the construction and operation of reluctance motor with a neat sketch?
**
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Code No: V3111
R07
Set No: 3
III B.Tech. I Semester Supplementary Examinations, December - 2013
ELECTRICAL MACHINES -III
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) Explain the main constructional features of cylindrical rotor and salient pole alternators.
b) For a 3-phase winding having 3-slots per pole per phase, calculate the breadth factor in
case phase spread is (i) 600
and (ii) 1200.
2.
a) What are slot harmonics? How can they be reduced?
b) Derive the phasor diagram of a cylindrical rotor alternator. What is the effect of
armature reaction and how is it included in phasor diagram? Draw phasor diagrams for
leading, unity and lagging power factors.
3.
a) Explain the MMF method of determing the voltage regulation of alternator
b) A 3 phase star connected alternator is rated at 1600kVA, 13.5l kV. The effective
armature resistance and synchronous reactance are 1.5 ohms and 30 ohms per phase
respectively. Find regulation for a load of 1280kW, rated voltage and power factor of
(i) unity (ii) 0.8 lagging.
4.
a) Define the significance of transient and sub-transient reactances in an alternator.
b) Two 15KVA, 400V, 3-phase alternators in parallel supply a total load of 25 KVA at 0.8
p.f. lagging. If one alternator shares half the power at unity power factor, determine the
power factor and KVA shared by the other alternator.
5.
a) Explain the construction and principle of operation of synchronous motor.
b) Explain the characteristics features of a synchronous motor.
6.
a) Explain the phenomenon of hunting and how it can be suppressed.
b) A 1000 kVA, 11 kV, 3 phase star connected synchronous motor has an armature
resistance and reactance per phase of 3.5 ohms and 40 ohms respectively. Determine the
angular retardation of motor induced emf when fully loaded at unity PF.
7.
a) Explain the cross field theory of a single phase induction motor.
b) A 1/4 h.p. 110 V, 60 Hz, 4 pole capacitor start motor has the following constants and
losses
Auxiliary winding ra= 2 xa= 2.79
Main winding rm=4 xm= 2.0.
Magnetizing reactance is 66.8.
Core loss=24W rotational loss of 13W.
Determine the stator current, power factor and efficiency when the motor is running as a
single-phase motor at the rated voltage and frequency with its starting winding open.
8.
Describe the principle of operation of permanent magnet and reluctance motors.
**
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Code No: V3111
R07
Set No: 4
III B.Tech. I Semester Supplementary Examinations, December - 2013
ELECTRICAL MACHINES -III
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) Derive an expression for an induced e.m.f in a synchronous generator. Also explain how
the e.m.f is having sinusoidal wave form.
b) An alternator has 3 slots per pole per phase. For a coil span of 8 slots, find the winding
factor.
2.
a) Sketch and explain the open circuit and short circuit characteristics of a synchronous
machine. How voltage regulation can be calculated by the using these characteristics.
b) A 3-Phase star - connected alternator is rated at 1600KVA, 13,500V.The armature
effective resistance and synchronous reactance are 1.5 and 30 respectively per phase.
Calculate the percentage regulation for a load of 1280KW at power factors of
(i) 0.8 leading, and
(ii)
0.8 lagging.
3.
a) Describe the slip test method for the measurement of Xd to Xq of synchronous machines.
b) A 3.5 MVA, slow-speed, 3-phase synchronous generator rated at 6.6KV has 32 poles its
direct and quadrature axis synchronous reactances as measured by the slip test are 9.6 Ω
and 6 Ω respectively. Neglecting armature, determine the regulation and the excitation emf
needed to maintain 6.6KV at the terminals when supplying a load of 2.5MW at 0.8pf
lagging. What maximum power can the generator supply at the rated terminal voltage, if the
field becomes open-circuited?
4.
a) Explain the necessity of parallel operation of alternators.
b) Two 50MVA, 3-phase alternators operate in parallel. The settings of the governors are
such that the rise in speed from full-load to no-load in 2% in one machine and 3% in the
other, the characteristics being straight lines in both cases. If each machine is fully loaded
when the total load is 100 MW, what will be the load on each machine when the total load
is reduced 60 MW?
5.
a) Draw and explain the phasor diagram of a synchronous motor operating at
(i) lagging power factor
(ii) leading power factor.
b) A 2300V, 3 phase star connected synchronous motor has a synchronous reactance of 10
ohm per phase. When the motor delivers 255 hp the efficiency is 90% (exclusive of field
loss). The power angle is 20˚. Calculate
(i) E per phase (ii) current, I and
(iii) power factor. Neglect resistance.
6.
a) Explain briefly the effect of varying excitation up on the armature current and pf of a
synchronous motor when input power to the motor is maintained constant.
b) Explain the operation of a synchronous induction motor.
**
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Code No: V3111
R07
Set No: 4
7. a) Prove that a single phase motor winding when excited by a single phase supply produces
two equal and opposite revolving fields.
b) “The centrifugal switch of a single phase motor failed to open”. Explain the after effects in
the performance.
8. a) List out the characteristics of reluctance motor?
b) Why are small fractional horse power ac series motors are called as universal motors?
**
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