Fifth Examination, June/July D.C. Machines and · PDF fileD.C. Machines and Synchronous...

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Time: 3 hrs.

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Max. Marks:100

(05 Marks)

(06 Marks)(06 Marks)

(08 Marks)

Fifth Semester B.E. Degree Examination, June/July 2014

D.C. Machines and Synchronous Machines

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Explain the process of commutation in a D.C. machines. Briefly explain the methods toimprove the commutation. (10 Marks)

Note: Answer FIVE fuII questions, selectingst least TWO questions from each part.

PART _ ADerive an emf equation for D.C. generator.

Explain the characteristics of D.C. series motor.Derive an expression for the armature torque of a D.C. motor.

armature reaction.

Derive the e.m.f. equation of an alternator.

With the help of a neat diagram, explain the Word-Leonard method of speed control. Whatare the advantages and disadvantages of this method? (10 Marks)What is the necessity of starters? With a neat figure, explain the working of three pointstarters. (t0 }Iarks)

With a neat figure, explain Swinburne's test and also find the efficiency when running as a

c. A 4-pole wave-wound motor armature has 880 conductors and delivers 120 A. The brusheshave been displa-ced through 3 angular degrees from the geometrical axis. Calculate:i) Demagnetising atno-,rrrns/pole.ii) Cross-magnetising amp-turns/pole. (05 Marks)

2a.b.c.

3a.

b.

A D.C. shunt motor runs at 1000 rpm on 200 V supply its armature resistance is 0.8fJ andthe armature current drawn is 40 amps. What resistance must be connected in series with thearmature to reduce the speed to 600 rpm, the armature current remaining same? Neglect the

4a.

b.

c.

5a.

b.

c.

generator and motor. (08 Marks)What are the losses in D.C. machines and prove that iron losses : copper losses. (06 Marks)A:500 Volts D.C. shunt motor when running on no load takes 5 amp. armature resistance is0.5 O and shunt field resistance is 250 O. Find the output in KW and efficiency of the motorwhen running full load and taking a current of 50 amps. (06 Marks)

PART _ BExplain the construction of the rotor of the salient pole and cylindrical rotor of alternator.


A 3-phase, 16-pole alternator has a star-connected winding with 144 slots and 10 conductorsper slot. The flux per pole is 0.03 Wb. Sinusoidally distributed and the speed is 375 rpm.Find the frequency and the phase and line emf. Assume full-pitched coil. (08 Marks)

6 a. Describe the synchronous impedance method to determine regulation of an alterrrator forlagging and leading power factor. (10 Marks)

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b. A 3-phase 6000 V altemator has the folloUI wrng o.c.c. at nolrnaField current t4 18 23 30 43Terminal volts 4000 5000 6000 7000 8000

With armature short-circuited and full load cument flowing the field current is 17 A andwhen the machines is supplying full-load of 2000 kVA at zero power factor, the field currenf'is 42.5 A and the terminal voltage is 6000 V. Determine the filed current required when themachine is supplying the fuIl-load at 0.8 p.f. lagging. (10 Marks)

,,,,,7 a. DiSbuss the factors on which the division of load between two alternators takes place when

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thelidne working in parallel.b. ExplairQpgrating characteristics of alternators.

a.

b.

c.

Explain Huntih$ in s,ynchronous machines. '.',i. "Explain V-curve and inverted V curves of synchronous rnachines.A 2300 V, 3 - phaSg star-connected synchronous motod has a resistance of 0.2 ohm perphase and a synchronous reactance of 2.2 ohmperphase. The motoroperating at 0.5 powerfactor leading with a line"current of 200 A. Determine the value of the generated emf perphase. (10 Marks)

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Fifth Examination, June/July D.C. Machines and · PDF fileD.C. Machines and Synchronous...

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