ELECTRICAL MACHINES-III

Code No: R05310204 Set No. 1
III B.Tech I Semester Regular Examinations, November 2008
ELECTRICAL MACHINES-III
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
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1. (a) Explain the classification of alternators based on rotor used with the help of
neat diagrams.
(b) A 16 pole, 3- , alternator is coupled to a engine running at 375 rpm. It
supplies an IM that has a full load speed of 1450 rpm. Find the slip & number
of poles of the motor. [8+8]
2. (a) Explain the sources of harmonics. What are the various effects of harmonics
on generated emf in an alternator?
(b) Find the RMS value of fundamental & third harmonic EMF per phase for an
alternator having following data: 50 Hz, 3- , 20 poles, 4 slots/pole/phase,
double layer winding with 6 conductors/slot, coil span of 150o, flux per pole:
fundamental is 0.1 wb, third harmonic 17 % of fundamental. All coils of a
phase are connected in series. [8+8]
3. (a) What is the synchronous impedance method? Why the method is called so?
What are the limitations of this theory?
(b) The following table gives the OCC & SCC of a 2 pole, 11kV, 50 Hz, 3- star
connected alternator. The stator resistance between two terminals is 0.2
.
Calculate the regulation at full load current of 125 A at 0.8 pf lagging (by
synchronous impedance method) [6+10]
If 16 20 25 32 45
EOL-kV 4.4 5.5 6.6 7.7 8.8
(where EoL is line voltage at no load)
4. (a) Explain the various methods of synchronization of alternators.
(b) Two similar 4 MVA alternators operate in parallel. The governor of first
machine is such that frequency drops from 50 Hz at no load to 47.5 Hz at full
load. The corresponding drop for second machine is 50 Hz to 48 Hz. [8+8]
i. How will they share a load of 6 MW?
ii. What is the maximum load they can share at UPF without over loading
any generator?
5. (a) A sub-station operating at full load of 1200 kVA supplies a load at 0.7 power
factor lagging. Calculate the permissible additional load at this power factor
and the rating of synchronous condenser to raise the substation power to 0.9
lagging.
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Code No: R05310204 Set No. 1
(b) Derive the expression for the maximum power developed by a synchronous
motor. [8+8]
6. (a) Explain the procedure to plot ‘V curves’ & ‘inverted V’ curves for a given
synchronous machine with help of its circles diagrams.
(b) A 2 pole, 50 Hz, 3- turbo alternator is excited to generate a bus-bar voltage of
11 kV on no load. The machine is star connected and the short circuit current
for this excitation is 1000 A. Calculate the synchronizing power per degree
of mechanical displacement of the rotor and the corresponding synchronizing
torque. [8+8]
7. (a) Why single phase motors are not self starting?
(b) Explain the necessary arrangements made to make single phase Induction
motor self starting & with neat diagram explain the operations of same. [8+8]
8. (a) Explain the working principle of reluctance motor.
(b) Compare working of AC series motor & Universal motor. [8+8]
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Code No: R05310204 Set No. 2
III B.Tech I Semester Regular Examinations, November 2008
ELECTRICAL MACHINES-III
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
1. (a) Explain the differences between stationary armature and rotating armature.
What are the advantages of rotating armature over stationary armature?
(b) A 4 pole alternator has an armature with 25 slots and 8 conductors per slot
and rotates at 1500 rpm and the flux per pole is 0.05 wb. Calculate the EMF
generated, if winding factor is 0.96 and all the conductors in a phase are in
series. [8+8]
2. (a) Explain the effects of harmonics on electrical power system & utility.
(b) Calculate the RMS value of EMF induced per phase of a 10 pole, 3- , 50 Hz,
alternator with 2 slots per pole per phase and 4 conductors per slot in two
layers. The coil span is 150o electrical. The flux per pole has a fundamental
component of 0.12 wb & a 20% of third harmonic component. [8+8]
3. (a) Explain the AT method of finding voltage regulation.
(b) A 1 MVA, 6.6 kV, 3- star connected synchronous generator has a synchronous
reactance of 25
 per phase. It supplies full load current at 0.8 lagging pf and a
rated terminal voltage. Compute the terminal voltage for the same excitation
when the generator supplies full load current at 0.8 leading pf. [8+8]
4. (a) Explain the operational differences in parallel operation of two alternators &
synchroning an alternator to infinite bus bars.
(b) Two star connected alternators supply a load of 3 MW at 0.8 pf lagging and
share the load equally. The excitation of second machine is adjusted so that
it is supplying 150 A at a lagging pf. The synchronous impedances are 0.4 +
j12
/ph& 0.5 + j10
/ph. Find current, power factor, induced EMF and
load angle of each machine. Terminal voltage is 6.6 kV. [8+8]
5. (a) An industrial plant has a load of 800 kW at power factor of 0.8 lagging. It is
desired to purchase a synchronous motor of sufficient capacity to deliver a load
of 200 kW and also serve to correct the over all plant power factor to 0.92.
Assuming that the synchronous motor has an efficiency of 92%, determine its
kVA input rating and power factor at which it will operate.
(b) Explain the load angle characteristics of a synchronous motor. [8+8]
6. A 1000 HP, 6.0 kV, 3- , star connected synchronous motor has a synchronous
impedance of 1.5 +j16
/ph. It is excited to develop an open circuit EMF of 5
kV. Draw the locus diagram of current for loads up to 1250 HP, with constant
excitation. Determine the maximum value of power factor. [16]
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Code No: R05310204 Set No. 2
7. (a) Explain the various types of single phase Induction motor.
(b) What is splitting of phases? Why splitting of phase is necessary in single
phase Induction motor. [8+8]
8. Write short notes on following:
(a) AC series motor
(b) Universal motor
(c) Variable reluctance stepper motor
(d) Permanent magnet stepper motor. [4×4=16]
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Code No: R05310204 Set No. 3
III B.Tech I Semester Regular Examinations, November 2008
ELECTRICAL MACHINES-III
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
1. (a) Derive the expression for the EMF induced in a 3- alternator?
(b) Calculate the no load terminal voltage of a 3- , 8 pole, star connected alter-
nator running at 750 rpm having the following data:
Sinusoidaly distributed flux per pole = 55 mwb, total number of slots on the
armature = 72, conductors per slot = 10, distribution factor = 0.96. Assume
full pitch coils. [8+8]
2. (a) Explain the causes of harmonics? Explain the concept of fictitious poles.
(b) A 10 pole, 3- , 50 Hz, alternator has 8 slots per pole & 6 conductors per slot.
The winding is 7/8 pitch. There are 0.03 wb entering the armature from each
north pole & this flux is sinusoidaly distributed along the air gap. The star
armature coils are connected in series. Determine the open circuit EMF of the
alternator. Find the breadth factor for 3rd & 5th harmonics. [6+10]
3. (a) Explain the various tests conducted on an alternator to find the voltage reg-
ulation of an alternator by Potier triangle method.
(b) The no load excitation of an alternator required to give rated voltage is 1
pu. In a short circuit test with full current flowing in the armature, the field
excitation was 0.85 pu. Determine the approximate excitation that will be
required to give full load current at 0.78 PF leading at the rated terminal
voltage. [8+8]
4. (a) Explain the term synchronization, and hence explain, synchronizing power.
(b) Two identical 3 MVA alternators are running in parallel. The frequency drops
from no load to full load for the two alternators are 50 Hz to 47 Hz and 50 Hz
to 48 Hz respectively. [8+8]
i. How they will share a load of 4000 kW?
ii. What is maximum unity factor load which they can supply jointly supply
without any one of them over loaded?
5. (a) A 3- , synchronous motor observing 60 kW is connected in parallel with a
factory load of 240 kW having lagging pf of 0.8. If the combined load has a
pf of 0.9 lagging, what is the value of leading kVAR supplied by the motor &
at what power factor it is working?
(b) Explain the ‘power factor v/s field current’ & ‘armature current v/s field
current’ characteristics of synchronous motor. [8+8]
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Code No: R05310204 Set No. 3
6. (a) Explain the procedure to plot ‘V curves’ with the help of ‘power circle’ &
‘excitation circle’.
(b) Explain the effect of damper winding on the performance of a synchronous
machine. [8+8]
7. (a) Explain the ‘doubly revolving field theory’ related to single phase Induction
motor.
(b) Explain the construction & give the applications of single phase Induction
motor. [8+8]
8. Give at least four applications of each of following:
(a) AC series motor
(b) Universal motor
(c) Variable reluctance stepper motor
(d) Permanent magnet stepper motor. [4×4=16]
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Code No: R05310204 Set No. 4
III B.Tech I Semester Regular Examinations, November 2008
ELECTRICAL MACHINES-III
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
1. (a) Obtain the expression for the short pitch factor & distributed winding factor?
(b) A 3- , 4 pole, star connected alternator has 60 slots with 2 conductors per
slot. The pitch of the coil is 3 slots less than pole pitch. The flux per pole is
0.125 wb. Calculate the no load terminal voltage if the speed of alternator is
1500 rpm. [8+8]
2. (a) What is armature reaction? Explain armature reaction for different power
factors of load?
(b) Data from tests performed to determine the parameters of a 200 kVA, 480 V,
60 Hz, 3- , stat connected alternator are Voc=480 V, Isc=209.9A for constant
If & for DC test VDC = 91.9 V, IDC=72.8 A (stator). Determine synchronous
impedance & the SCR of the alternator. [8+8]
3. (a) Explain the ‘Zero power factor’ method of finding voltage regulation of an
alternator.
(b) The no load excitation of an alternator required to give rated voltage is 1
pu. In a short circuit test with full current flowing in the armature, the field
excitation was 0.75 pu. Determine the approximate excitation that will be
required to give full load current at 0.866 PF lagging at the rated terminal
voltage. [8+8]
4. (a) Explain the effect of change in excitation on the parallel operation of two
alternators.
(b) Two similar stat connected alternators 3- alternators share a load of 7500
kW equally at 6000 V and 0.8 pf lagging. The synchronous impedance of 2.5
+ j50
/ph. The excitation of second machine is changed, so that it delivers
40 A at a lagging pf. Find: [8+8]
i. Armature current of first machine
ii. EMF of each machine
iii. Power factor of each machine.
5. (a) An industrial load of 4 MW is supplied at 11 kV, the power factor being
0.8 lagging. A synchronous motor is required to meet an additional load of
1103.25 kW and at the same time to raise the resultant power factor to 0.95
lagging. Determine the kVA capacity of the motor and the power factor at
which it must operate. The efficiency of motor is 80 %.
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Code No: R05310204 Set No. 4
(b) Explain the various power stages of synchronous motor. What are the various
losses taking place in synchronous motor. [8+8]
6. The effective resistance & synchronous reactance per phase of a 50 Hz, 6.6 kV, 1
MVA, 3- , star connected synchronous motor are 0.5
 & 12
 respectively. Plot
the V curves for this machine when the input is maintained constant at 250 kW.
No load saturation curve: [16]
If 50 100 150 175 200
V 3400 5700 7200 7900 8400
7. (a) Explain the construction & operation of ‘capacitor start and run’ single phase
Induction motor.
(b) Explain ‘double revolving field theory’ of single phase Induction motor. [8+8]
8. With neat diagram explain the construction & working of variable reluctance step-
per motor. Also explain its static & dynamic characteristics. [16]
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