electrical machines-iii 5

8/10/2019 Electrical Machines-III 5

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Code: 9A02505

B.Tech III Year I Semester (R09) Regular & Supplementary Examinations December/January 2013/14

ELECTRICAL MACHINES - III

(Electrical & Electronics Engineering)

Time: 3 hours Max. Marks: 70

Answer any FIVE questionsAll questions carry equal marks

*****

1 (a) Briefly describe the phase spread, phase band. Calculate their values for integral slot

winding with suitable example.

(b) A 3- phase star connected alternator has rating of 120 KVA at 2.5 KV with 12 poles. The

rated peed of the alternator is 800 rpm. There are three slots per pole per phase and the

number of conductors per slot is 5. Determine the flux per pole for full pitched winding to

give the rated terminal voltage at no load.

2 The effective resistance of a 2200 V, 50 Hz, 440 KVA single phase alternator is 0.5 ohm.

On short circuit, a field current of 4 amp, gives the full load current. The emf on open

circuit for the same field current is 1160 V. Find synchronous impedance, synchronous

reactance and %regulation of 0.6 p.f lagging.

3 Explain the zero power factor method of finding voltage regulation of an alternator.

4 A 4 KVA, 3-phase, 110 V, 50 Hz, star connected alternator has Xd = 3 & Xq = 2. The

machine is delivering full load current at 0.8 pf lagging at rated voltage. Find the induced

emf, load angle, maximum power delivered.

5 (a) What is a synchronous condenser? What is the use of synchronous condensers?

(b) A 220 V, 3-phase, star connected synchronous motor has a resistance of 0.22 ohm per

phase and a synchronous reactance of 2.4 ohm per phase. The motor is operating at 0.6

pf leading with a line current of 180 A. Determine the value of generated e.m.f.

6 (a) Explain the effect of damper winding on the performance of a synchronous machine.

(b) Describe the induction-motor starting of synchronous motor. Why it is necessary to short

circuit the field winding of the motor at the time of the starting?

7 (a) Explain how the direction of a single-phase induction motor can be reversed.

(b) Explain the necessary arrangements made to make single phase induction motor self

starting & with neat diagram explain the operations of same.

8 Give at least four applications of each of following:

(a) A.C. series motor.

(b) Universal motors.

(c) Variable reluctance stepper motor.

(d) Permanent magnet stepper motor.

*****

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Code: 9A02505






*****

1 (a) Explain the effect of short chording and spreading the winding in a 3-phase alternator on

the emf induced in the winding. Derive the expressions for these effects in terms of slot

pitch, number of slots per pole per phase and short chording.

(b) Describe the merits of fractional slot winding. Why its armature slots should be divisible

by 3 in case of 3-phase machine?

2 (a) Explain the effects of harmonics on electrical power system and utility.

(b) Calculate the RMS value of emf induced per phase of a 10 pole, 3-phase, 50 Hz,

alternator with 2 slots per pole per phase and 4 conductors per slot in two layers. The

coil span is 150oelectrical. The flux per pole has a fundamental component of 0.12 Wb &

a 20% of third harmonic component.

3 A 3-, 440 V, 50 Hz, delta connected alternator has a direct axis and quadrature axis

reactance of 0.12 and 0.09 respectively. If the alternator supplies 900 A at 0.8 p.f

lagging, calculate the following:

(a) The excitation e.m.f neglecting saliency (Xd = Xq)

(b) The excitation e.m.f taking into account the saliency.

Neglect armature resistance.

4 Explain the two bright one dark and all dark method of synchronization of alternators.

5 (a) Explain the construction and working principle of synchronous motor.

(b) A 400 V, 3-phase synchronous motor takes 52.5 A at a power factor 0.8 leading.

Calculate the power supplied and induced e.m.f. The impedance per phase is (0.25+j3.2)

ohm.

6 (a) Explain the construction of damper winding. Clearly show the location of damper

winding.

(b) With neat diagram and explanation, show how damper winding prevents oscillations.

7 (a) Explain the operation of split phase single phase Induction motor with vector diagram.

(b) Explain how the direction of a single phase Induction motor can be reversed.

8 Write the short notes on the following,

(a) A.C series motor.

(b) Universal motor.

(c) Variable reluctance stepper motor.

(d) Permanent magnet stepper motor.

*****

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1 (a) Explain the differences between stationary armature and rotating armature. What are the

advantages of rotating armature over stationary armature?

(b) Describe the merits of fractional slot winding. Why its armature slots should be divisible by 3 in

case of 3-phase machine?

2 (a) The phase e.m.f of a 3-phase alternator consists of fundamental, 20% third harmonic & 10% fifth

harmonic. The amplitude of fundamental is 1000 V. Calculate the r.m.s value of line & phase

voltage, when the alternator is connected in (i) Star. (ii) Delta.(b) Explain the load characteristics of an alternator.

3 A 1 MVA, 11 KV, 3-phase star-connected synchronous machine has following OCC test data:

If 50 110 140 180

Eol - KV 7 12.5 13.75 15

( Where Eolis the line voltage at no load ) the short circuit test yielded full load current at a field

ZPF yielded a full load current at terminal voltage for a field current of 150 A. the armature

resistance is negligible. Calculate the voltage regulation at full load of 0.866 p.f lagging by Potier

triangle method.

4 Explain the various methods of synchronization of alternators.

5 (a) Write short notes on synchronous condenser for power factor improvement.

(b) A 3-phase, 400 V star connected synchronous motor has effective per phase armature resistance

and synchronous reactance of 0.2 ohm and 2 ohm 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 p.f lagging

(ii) 0.8 p.f leading

(iii) UPF.

6 (a) Explain the importance of power circles in determining the characteristics of a synchronous

machine.

(b) Explain the characteristics of synchronous induction motor.

7 (a) Why a capacitor start motor is better than capacitor start and run? Explain.

(b) With neat diagrams explain the construction and working of shaded pole motor?

8 Compare the performance characteristics of A.C series motor when it is connected across:

(a) A.C supply and

(b) D.C supply.

*****

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Answer any FIVE questions

All questions carry equal marks*****

1 (a) Explain the construction and working principle of alternators with a neat diagram.

(b) A 4-pole, 3-phase, 50 Hz, star connected alternator has 60 slots with 4 conductors per slot.

Coils are short pitched by 3 slots. If the phase spread is 60 o, find the line voltage induced for

a flux per pole of 0.0943 Wb distributed sinusoidal in space. All the turns in phase are in

series.

2 (a) A 16-pole, 3-phase star connected alternator has 144 slots. The coils are short pitched by

one slot. The flux per pole is phase = 100 sin + 30 sin3 + 20 sin5. Find the harmonics as

percentage of phase voltage & line voltage.

(b) Define:

(i) Synchronous reactance.

(ii) Synchronous impedance and

(iii) Leakage reactance in an alternator.

3 A 3- , 200 KVA, 1.1 KV, 50 Hz star connected alternator having an effective per phase

resistance of 0.62 gave the following results:

Field current (A) 20 35 50 80 100 120

O.C. Voltage V1 692.82 1120 1450 1750 1953 2180

S.C. Current (A) 0 22 44 66 88 110

Using MMF method, find the voltage regulation at 100 A.(a) 0.8 p.f. lagging.

(b) 0.8 p.f leading.

4 Explain the operational differences in parallel operation of two alternators and synchronizing

an alternator to infinite bus bars.

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) Explain why synchronous motor is not self starting.

6 (a) Explain the importance of power circles in determining the characteristics of a synchronous

machine.

(b) Explain the phenomenon of hunting in synchronous machine.

7 (a) Why a capacitor start motor is better than split-phase motor? Explain.

(b) Describe the construction and working of shaded pole motor.

8 Explain the operating characteristics of an A.C series motors.

*****

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