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Code No: R05310204
Set No. 1
III B.Tech I Semester Regular Examinations, November 2007
ELECTRICAL MACHINES-III
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks

1. (a) What is short pitch winding & distributed winding? Why the armature wind-
ing is distributed & short pitch type?
(b) Calculate the EMF of a 4 pole, 3-F, star connected alternator running at 1500
r.p.m from the following data: Flux per pole = 0.1 wb, Total number of slots
= 48, Conductors per slot (in two layers) = 4, coil span = 150o. [8+8]

2. (a) Explain the load characteristics of an alternator.
(b) The phase EMF of a 3-F alternator consist of fundamental, 20 % 3r d harmonic
& 10 % fth harmonic. The amplitude of fundamental is 1000 V. Calculate
the RMS value of line & phase voltage, when the alternator is connected in
i. Star
ii. Delta. [8+8]

3. (a) Explain the voltage regulation metho d of an alternator by which the armature
reaction & leakage reactance can be separated.
(b) A 3-F, star connected salient pole synchronous generator is driven at a speed
near synchronous with the eld circuit open and the stator is supplied from
a balanced 3-F supply. Voltmeter connected across the line gave minimum
and maximum readings of 1196 V & 1217 Volts. The line current uctuated
between 120 & 225 Amp. Find the direct and quadrature axis reactances per
phase. Neglect armature resistances. [8+8]

4. (a) Explain the ‘two bright one dark’ & ‘all dark’ method of synchronization of
alternators.
(b) The EMFs of two alternators are 3000 200 & 2900 00 V. Their synchronous
impedances are 2 + j20 /ph & 2.5 + j30 /ph. The load impedance is 10 +
j4 /ph. Find the circulating current. [8+8]

5. (a) Explain the construction & working principle of Synchronous motor.
(b) A 3- F, 400 V star connected SM has e ective per phase armature resistance
& synchronous reactance of 0.2 & 2 respectively. It takes 20 A to deliver
a certain load. Calculate the excitation EMF induced in the motor if it works
with
i. 0.8 pf lagging
ii. 0.8 pf leading


Code No: R05310204
Set No. 1
iii. UPF. [8+8]

6. (a) Explain the construction of ‘ ’ for a synchronous motor.
(b) Calculate the synchronous coe cient (in kW & Nm per mechanical degree) at
full load for a 1 MVA, 0.8 pf lagging, 6.6 kV, 8 pole, star connected generator
of negligible resistance & synchronous reactance of 0.8 pu. [8+8]

7. (a) Explain the operation of ‘ ’ with
vector diagram.
(b) Explain how the direction of a single phase Induction motor cab be reversed.
[8+8]

8. (a) Compare AC series motor & Universal motor.
(b) Compare variable reluctance stepper motor & permanent magnet stepper mo-
tor. [8+8]


Code No: R05310204
Set No. 2

1. (a) Explain the various winding factors? Explain the e ects of each of them.
(b) Determine the slot distribution and the pole phase group sequence for a 45
slot, 6 pole, 3-F winding. [6+10]

2. (a) Explain the e ect of armature reaction on the EMF induced. Is it possible to
obtain load voltage more than EMF induced? if yes, how?
(b) A 200 kVA, 480 V, 50 Hz, star connected synchronous generator with a rated
eld current of 5A was tested and the following data were obtained:
OC test: 540 V between lines on open circuit.
SC test: 300 A.
When a DC voltage of 10 V was applied to two of its terminals, a current of
25 A was measured, nd the value of synchronous impedance, synchronous
reactance voltage regulation of 0.6 p.f leading. [8+8]

3. (a) How the MMF method is di erent from EMF metho d in nding voltage reg-
ulation of an alternator? Explain the drawbacks of each method.
(b) A 1 MVA, 11 kV, 3-F, star connected synchronous machine has following OCC
test data: [6+10]
f 50 110 140 180
OL-kV 7 12.5 13.75 15
(where EOL is the line voltage at no load)
The short circuit test yielded full load current at a eld current of 60 A. The
ZPF yielded a full load current at terminal voltage for a eld current of 150
A. The armature resistance is negligible. Calculate the voltage regulation at
full load 0.866 pf lagging by Potier triangle method.

alternators.

5. A 20 pole, 30 kW, 660 V, 50 Hz, star connected synchronous motor is operating
with it’s per phase generated voltage exactly equal to the phase voltage applied
to armature. At loaded condition the motor is retarded by 5o mechanical from its
synchronous position. Per phase synchronous reactance & the e ective armature
resistance are 10 & 1 respectively. Calculate:


Code No: R05310204
Set No. 2
(a) Armature current
(b) The total power drawn by the motor from bus.

(c) The developed power. [16]

6. (a) Explain the construction of damper winding. Clearly show the location of
damper winding.
(b) With neat diagram & explanation, show how damper winding prevents oscil-
lations. [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) Give the applications of various types of permanent magnet motors.
(b) Explain the working principle of permanent magnet motors. [8+8]


Code No: R05310204
Set No. 3

1. (a) Obtain the expression for the RMS value of EMF induced in an alternator.
(b) An alternator is operating at no load has an induced EMF of 346.4 V/ph and
a frequency of 60 Hz. If the pole ux is deceased by 15 % & the speed is
increased by 6.8 %; determine [8+8]
i. The induced EMF
ii. Frequency.

2. (a) A 16 pole, 3-F star connected alternator has 144 slots. The coils are short
pitched by one slot. The ux per pole is F = 100 sin + 30 sin 3 + 20 sin
5 . Find the harmonics as percentage of phase voltage & line voltage.
(b) De ne
i. synchronous reactance
ii. Synchronous impedance and
iii. leakage reactance in an alternator. [10+6]

3. (a) With proper explanation & diagram, Justify the statement ‘

’.
(b) The no load excitation of an alternator required to give rated voltage is 160 A.
In a short circuit test with full current owing in the armature, the eld exci-
tation was 135 A. Determine the approximate excitation that will be required
to give full load current at 0.8 PF lagging at the rated terminal voltage. [8+8]

alternators.

5. (a) Explain, why synchronous motor is not self starting?
(b) A 3-F, 600 V, star connected SM has e ective per phase armature resistance
& synchronous reactance of 0.4 & 3.6 respectively. Calculate the induced
EMF per phase if the motor works on full load delivering 326 kW. The full
load e ciency is 87 % having power factor of 0.8 leading. Also calculate the
load angle. [6+10]


Code No: R05310204
Set No. 3
(b) Explain the characteristics of synchronous induction motor. [8+8]

7. (a) Explain,
3 - F , why?
(b) The name plate of single phase IM, 4 pole Induction motor gives the following
data:
Output = 410 W, Supply voltage = 230 V, Frequency = 50 Hz,
Input current = 3.2 A, Power factor = 0.7, speed = 1410 RPM.
Calculate:
i. The e ciency of the motor
ii. The slip of the motor when delivering rated output. [8+8]

8. (a) Compare AC series motor & Universal motor.
(b) Compare variable reluctance stepper motor & permanent magnet stepper mo-
tor. [8+8]


Code No: R05310204
Set No. 4

1. (a) Explain the various winding factors? Explain the e ects of each of them.
(b) Determine the slot distribution and the pole phase group sequence for a 45
slot, 6 pole, 3-F winding. [6+10]

2. (a) Justify the statement ‘The terminal voltage of an alternator is not only de-
pends on the load current, but also on the nature of load’.
(b) A 3-F, star connected alternator is rated at 1600 kVA, 13500 V. The arma-
ture e ective resistance & reactance is 1.5 /ph. & 30 /ph. respectively.
Calculate percentage regulation for a load of 1280 kW at power factors of
i. 0.8 lagging
ii. 0.8 leading. [6+10]

3. A 3-F, 200 kVA, 1.1 kV, 50 Hz star connected alternator having an e ective per
phase resistance of 0.62 gave the following results: [16]

Field Current (A) 20 35 50 80 100 120
OC Voltage L 692.82 1120 1450 1750 1953 2180
SC Current (A) 0 22 44 66 88 110
Using MMF method, nd the voltage regulation at 100 A

(a) 0.8 pf lagging
(b) 0.8 pf leading.

4. (a) Explain all the necessary conditions for successful parallel operation of alter-
nators.
(b) A 2 MVA, 3-F, star connected, 4 pole, 750 RPM alternator is operating on
6000 V bus bars, ×s is 6 /ph. Find synchronizing power and torque for full
load 0.8 power factor lagging. [8+8]

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.
(b) Derive the expression for the maximum power developed by a synchronous
motor. [8+8]


Code No: R05310204
Set No. 4
(b) Explain the characteristics of synchronous induction motor. [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 static characteristics of stepper motor.
(b) Explain the role of compound winding in the operation of AC series motor.
[8+8]

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