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Code No: W0221/R07

II B.Tech I Semester (R07) Regular Examinations, Nov- 2010

ELECTRICAL MACHINES - I

(Electrical and Electronics Engineering)Time: 3 hours Max Marks: 80

Answer any FIVE QuestionsAll Questions carry equal marks

1. a) State the principle of conversion of energy and apply it to an electric motor aselectromechanical conversion device? (8M)

b) Derive the expression for the energy stored in the magnetic field for singly excited

electromechanical system? (8M)

2. a) Discuss the function of commutator along with its constructional details giving suitablediagrams. (8M)

b) A 4 pole lap connected dc generator having 50 slots on its armature with 6 conductors per slot,

the flux per pole 30 mWb and generates an open circuit voltage of 180 V. (i) Find the speed at

which the motor will run for the above condition, (ii) keeping the speed constant, suggest a

change in the armature of the generator such that the generator is capable to generate at no load a

voltage of 90 V, with the same rated flux. (8M)

3. a) What is meant by armature reaction? Briefly describe that the effect of armature mmf on themain field is entirely cross magnetizing? (8M)

b) A 50 kW, 400V, 6-pole dc generator has 720 lap wound conductors. The brushes lead of 2.5angular degrees (mech.) from the geometric neutral. Calculate the cross and demagnetizing turns

per pole. Neglect the shunt field current. (

4. a) Explain with suitable diagrams (8M)i) Critical resistance of the field circuit of the shunt generator

ii) Critical speed of the generator.

b) A separately excited dc generator has armature circuit resistance of 0.1 and the total drop at

the brushes is 2V. When running at 1000 rpm, it delivers a current of 100A at 250 V to a load of

constant resistance. If the generator speed drops to 700 rpm, with field current unaltered, find the

terminal voltage and current delivered to load? (8M)

5. a) Draw the load characteristics of shunt, series and compound generators? Describe thesecharacteristics nature and applications? (8M)

b) Two Shunt-wound generators running parallel each have an armature resistance of 0.03 and

field resistance of 60. The combined external load current is 4500 A. The fields of the

generators are so excited that the electromotive force induced in the one machine is 500 V and in

the other machine is 510 V. Calculate the bus bar voltage and output of the each machine?

(8M)1 of 2

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Code No: W0221/R07

6. a) Derive the expression for torque developed by the dc motor using BIlconcept? (8M)b) A 220 V dc shunt motor takes no load current of 10 A and runs at 1500 rpm. At full load the

armature current is 100 A and the motor runs at 1470 rpm. Resistance of the armature is 0.1

calculate the following (i) back emf at no load and full load and (ii) percentage of reduction in

flux due to armature reaction (8M)

7. a) Describe and compare the various methods of speed control of dc motors? (8M)b) A 440 V dc series motor a regulating resistance of R in series. The motor resistance is 0.3.

When R = 0 and the current I is 20 A, the motor runs at 1200 rpm. Find the speed when R=3

and I =15A. Also find the ratio of total mechanical output in two cases. Given that the flux with I

= 15A is 80 % of that with I = 20A. (8M)

8. a) Draw and discuss analytically, the shape of the efficiency curve of a shunt motor. Determinethe condition for maximum efficiency? (8M)

b) Describe with suitable diagram of Fields Test to find the efficiency of dc machine as a motor

and as a generator? (8M)

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Code No: W0221/R07



(Electrical and Electronics Engineering)

Time: 3 hours Max Marks: 80Answer any FIVE Questions

All Questions carry equal marks

1. a) Derive an expression for the field energy stored in doubly excited electromechanical system.(8M)

b) Show that the torque developed in doubly excited magnetic system is equal to the rate of

increase of field energy with respect to displacement at constant currents (8M)

2. a) Describe the constructional details of the armature of a dc machine giving suitable diagrams?(8M)

b) A 4 pole wave connected dc generator having 60 slots on its armature with 6 conductors per

slot, runs at 750 rpm and generates an open circuit voltage of 230 V. i) Find the useful flux per

pole needed for the above condition, ii) keeping the flux constant, suggest a change in the

armature of the generator such that the generator is capable to generate at no load a voltage of

115 V, when running at same speed. (8M)

3. a) What is meant by ideal commutation? Describe the difference between actual commutationand ideal commutation? (8M)

b) A 4-pole dc generator supplies a current of 143A. it has 492 armature conductors (i) wave

connected (ii) lap connected. When delivering full load, the brushes are given an actual lead of

100

. Calculate the demagnetizing armature ampere turns per pole. The field winding is shuntconnected and takes 10A. Find the number of extra shunt field turns necessary to neutralize the

demagnetization? (8M)

4. a) A shunt generator, when driven at its normal speed, fails to self excite. Discuss the reason forthis, and state various conditions for the buildup of voltage? (8M)

b) The open circuit characteristics of a dc shunt generator driven at 1000 rpm is as follows

Field current (A): 0 1 2 3 4 6 8 10

Emf (V) : 4 115 230 315 360 405 430 450

Based on the above, Calculate

i) The emf to which the machine will excite with shunt field resistance of 50.

ii) The additional resistance in the field circuit to reduce the emf to 392 V.

iii) Critical resistance of shunt field circuit at 600 rpm. (8M)

5. a) Explain how the external and internal characteristics of the dc shunt generator can be obtained practically.

b) Two shunt generators are running parallel to supply a load of 5000A. Each machine has a field

resistance of 50 and armature resistance of 0.02 . The emf generated is 600 V and 610 V

respectively. Find the bus bar voltage and kilowatt output of the each machine. (8M)1 of 2

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Code No: W0221/R07

6. a) Why the emf induced in dc motor is called as back emf and then explains principle ofoperation of dc motor? (8M)

b) A 4-pole dc series motor has 944 wave connected armature conductors. At a certain load the

flux per pole is 34.6 mWb and the total mechanical power developed is 4 kW. Calculate the linecurrent taken by the motor and the speed at which it will run with an applied voltage of 500 V.

The total motor resistance is 3 . (8M)

7. a) Draw a labeled diagram of a three point starter, showing also the protective devices? Discussthe function of the above starter? (8M)

b) A 240V, 50A, 800 rpm dc shunt motor has armature circuit resistance of 0.2 . If load torque

is reduced to 60% of its full-load value and a resistance of 2 is inserted in series with armature

circuit, find the motor speed. Armature reaction weakens the field flux by 4% at full load and by

2% at 60% of full load? (8M)

8. a) Discuss various losses occurring in dc shunt motor and generator? State how each loss willvary when the load on the machine is varied? (8M)

b) Describe with suitable diagram of Hopkinsons Test to find the efficiency of dc machine as a

motor and as a generator? (8M)

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Code No: W0221/R07






1. a) Derive an expression for the mechanical force developed for singly excited magnetic fieldsystem? (8M)

b) A doubly excited magnetic field system has the following inductances in terms of angle

between the stator and rotor coils.

Stator Self inductance, Lss= 0.16 H

Rotor self inductance, Lrr= 0.04+0.02cos 2 H

Mutual inductance, Msr= 0.07cos H

If the current flowing the stator and rotor are 10 A and 8 A respectively Calculate the mechanical

torque developed? (8M)

2. a) What is the fundamental difference between a simple lap winding and a simplex wavewinding? Draw simple diagrams to show the above windings? (8M)

b) The armature of a 4-pole lap wound shunt generator has 120 slots with 4 conductors per slot.

The useful flux per pole is 0.05Wb. The armature resistance is 0.05 and the shunt field

resistance is 50 . Find the speed of the machine when applying 450A at a terminal voltage of

250 V. (8M)

3. a) What is meant by commutation? Briefly describe the difference between good commutationand bad commutation (8M)b) A 4-pole dc generator supplies a current of 143 A. it has 492 armature conductors i) wave

connected ii) lap connected. When delivering full load, the brushes are given an actual lead of

100. Calculate the demagnetizing armature ampere turns per pole. The field winding is shunt

connected and takes 10A. Find the number of extra shunt field necessary to neutralize the

demagnetization? (8M)

4. a) A shunt generator, when driven at its normal speed, fails to self excite. Discuss the reason forthis, and state various conditions for the buildup of voltage? (8M)

b) The magnetizing characteristics of a separately excited dc generator driven at 400 rpm is as

follows

Excitation current (A): 2 3 4 5 6 7 8 9Induced Emf (V) : 110 155 186 212 230 246 260 271

The machine is connected as shunt generator and runs at 400 rpm. Calculate

i) The voltage generated by the generator, when the field circuit resistance of 50

ii) It is desired to reduce the emf to 220 V, find the value of additional resistance required

in the field circuit

iii)The critical value of the shunt field resistance. (8M)1 of 2

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Code No: W0221/R07

5. a) Draw the external characteristics of various types of dc generators and compare them? Giveits applications (8M)

b) A 110kW, short shunt compound generator has no load voltage of 500V and is overcompounded 10% producing a linear external characteristic. If the armature resistance be 0.09 ,

series field resistance 0.04 , and shunt resistance 100 , Obtain numerical values of the internal

characteristics and then plot the internal characteristic. (8M)

6. a) Discuss how the torque is produced in dc motor? Derive the expression for it? (8M) b) A 100V long shunt compound motor takes 45A when running at 750 rpm. Its armature

resistance is 0.22 while the series and shunt field resistances are 0.13 and 100 respectively.

Iron and friction losses amounts to 750W. Find i) shaft power, ii) Total torque and iii) Shaft

torque (8M)

7. a) Draw and explain the speed versus torque characteristics and current versus torquecharacteristics of dc shunt motor? (8M)

b) A 250 V shunt motor has an armature resistance of 0.5 is excited to give the constant main

field. At full load the motor runs at 500 rpm and takes an armature current of 30A. If a resistance

of 1 is placed in the armature circuit find the speed at (i) full load torque and (ii) double full

load torque. (8m)

8. a) Briefly describe, how to separate the stray losses of a dc motor? Explain the test method withsuitable diagrams? (8M)

b) In a brake-test on a small shunt motor the speed was 1500 rpm the load on one side of the

brake band was 28.9 N and on the other 1.67 N. The diameter of the brake pulley was 15.2 cm. If

the input current was 2 A at 250 V, calculate the torque, efficiency and the brake horse power.

(8M)

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Code No: W0221/R07






1. a) Draw and explain with block diagram, the various energies involved in an electromechanicalenergy conversion device? (8M)

b) Distinguish between the field energy and co-energy? (8M

2. a) Define front pitch, back pitch, resultant pitch for the lap and wave windings with the help ofsuitable diagrams (8M)

b) A 4-pole lap connected dc generator has no load generated emf of 500 V when driven at 1200

rpm. Calculate the flux per pole if armature has 120 slots with 6 conductors per slot. If each

conductor has a resistance of 0.01 , find the armature resistance for lap and wave wound cases

the armature windings. (8M)

3. a) Explain the methods of improving commutation in dc machine? (8M) b) How are demagnetizing and cross magnetizing ampere-turns/pole in a D.C Machines are

calculated? (8M)

4.

a) What are the different causes of shunt generator fails to self excite? Also discuss theirremedies? (8M)

b) A long shunt compound generator supplies a load at 250V. The load consists of five motors

each drawing 60A and lighting load of 250 lamps of 100W each. Armature, series field and shunt

field resistance are 0.02, 0.05 and 50, respectively. Find (i) armature current and (iii) emf

generated. How will be the ampere turns of the series winding be changed if a diverter of 0.1 is

connected across the series field. (8M)

5. a) Draw the external and internal characteristics of compound generators? What is meant by levelcompounding and Explain how it is done? (8M)

b) A d.c series generator having internal characteristics which is a straight line through zero to50V at 200A is connected as a booster between a station bus bar and a feeder of 0.3 resistance.

Calculate the voltage difference between the station bus bar and the far end of the feeder at a

current of

i) 1200A and

ii) 50A. (8M)1 of 2

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Code No: W0221/R07

6. a) Discuss how the torque is produced in dc motor? Derive the expression for it? (8M)b) A 600 V dc motor drives a 60 kW loads at 700 rpm. The shunt field resistance is 100 and

the armature resistance is 0.16 . If the motor efficiency is 85 %, determinei) The input current

ii) Armature torque developed?

iii) The rotational losses. (8M)

7. a) Describe the ward-Leonard system of speed control of dc motor? (8M)b) A 250 V, dc shunt motor has an armature current of 20A when running at1000 rpm against

some load torque. The armature resistance is 0.5 and brush contact drop is one volt per brush.

By how much main flux to be reduced to raise the speed by 50% with the developed torque is

maintained constant? Ignore the effects of armature reaction and magnetic saturation. (8M)

8. a) Briefly describe the Fields test to find the efficiency of dc series machine as a motor and as agenerator? (8M)

b) Describe a method of determining the efficiency of dc shunt motor at various loads, without

actually putting the load on the motor? State the assumptions made in the method described.

(8M)

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