SET - 1
R10
Code No: R21025
II B.Tech I Semester, Regular Examinations, Nov - 2011
ELECTRICAL MACHINES -I
(Electrical and Electronics Engineering)
Time: 3 hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
1. a) Explain the principle of energy conversion of electromechanical system.
(7M+8M)
b) What are the significances of energy and co-energy of energy conversion system?
2.
a) Explain about different excitation methods of DC Generators.
(9M+6M)
b) An 8 pole wave connected DC generator has 900 armature conductors and flux/pole of 0.04
Wb. At what speed it must be driven to generate 500V.
3.
a) Explain with help of neat sketches the phenomenon of commutation in DC machines. State
and discuss the methods adopted for minimizing the sparking at the brushes.
b) Calculate the reactance voltage for a 4 pole lap wound generator if speed is 300 rpm,
diameter of commutator is 1.12 m, number of commutator segments are 450, brush width is 2.25
cm, length of each conductor is 1 m, effective length of core is 0.3 m, turns per commutator
segment is 2, and full load armature current is 900 A.
4.
(8M+7M)
a) Draw OCC of a dc shunt generator and define critical speed and critical resistance.
b) A dc shunt generator has the following open circuit magnetization curve running at 800 rpm
Field current (A) : 0
0.5
1.0
2.0
3.0
4.0
5.0
EMF (V)
50
100 175
220
245
262
: 10
Find graphically the critical resistance of shunt field current. If the field resistance is changed to
75 ohms, what will be the critical speed for the machine to build up.
5.
(9M+6M)
a) Explain with neat diagrams how two DC compound generators are operated in parallel.
b) Two DC shunt generators with emfs of 120 V and 115 V, armature resistance of 0.05 ohms
and 0.04 ohms and field resistances of 20ohms and 25ohms respectively are in parallel
supplying a load of 25 kW. How do they share load?
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(8M+7M)
Code No: R21025
6.
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SET - 1
a) Derive the expression for the speed of a dc motor interms of back emf and flux per pole
b) A 250V shunt motor runs at 1000 rpm at no load and takes 8A. The total armature and shunt
field resistances are 0.2 and 250 ohms respectively. Calculate the speed when loaded and taking
50A. Assume the flux to be constant.
7.
(8M+7M)
a) Explain the various method of speed control of DC shunt motor. Discuss their merits and
demerits.
b) A 220V, DC shunt motor runs at 760rpm and takes armature current of 48A. Find the
resistance to be added to the field circuit to increase the speed to 950 rpm at an armature current
of 78A. Assume flux is proportional to field current. Armature resistance and field resistance is
0.15Ω and 240Ω respectively.
8.
(8M+7M)
a) Explain the regenerative method of testing of DC motors. On which type of machines this test
is conducted? Explain.
(8M+7M)
b) The iron losses in a dc machine, when driven at normal speed and at normal excitation are
8kW. When driven at half speed and same excitation the losses are reduced to 3 kW. Determine
the values of eddy current and hysteresis losses at i) half speed and ii) full speed
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R10
Code No: R21025
SET - 2
II B.Tech I Semester, Regular Examinations, Nov - 2011
ELECTRICAL MACHINES -I
(Electrical and Electronics Engineering)
Time: 3 hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
1.
a) Derive an expression for the energy stored in a magnetic field.
b) A 10 kW, 1440 rpm DC shunt generator has a time constant Lf/Rf of 0.2 sec for its field
winding. Under normal operating conditions, the If2rf loss in the field winding is 400 watts.
Compute the energy stored in the magnetic field produced by the field winding, under normal
operating conditions
(7M+8M)
2.
a) Give the materials and functions of the following parts of a DC machine:
i) Field poles
ii) Yoke
iii) Commutator
iv) Commutating poles
v) Armature
b) An 8-pole lap wound armature rotated at 350 rpm is required to generate 260 V. The useful
flux per pole is about 0.05 Wb. If the armature has 120 slots, calculate a suitable number of
conductors per slot and hence determine the actual value of flux required to generate the same
voltage.
(10M+5M)
3.
a) Explain about demagnetizing AT/pole and cross magnetizing AT/pole in DC generators.
b) A lap wound, 4 pole DC generator with 480 armature conductors supplies 72 A. The brushes
are given an actual lead of 120 (mechanical). Calculate the cross magnetizing AT/ pole.
(9M+6M)
4.
a) What are the factors which will affect the voltage build up in self excited DC generators?
b) A 500V, 250kw, long shunt compound generator induces an emf of 480V when running at
1000 rpm on no-load. On full load the speed of the machine drops to 975 rpm, the flux increased
by 15% and the terminal voltage rises to 500V. If the series and shunt field resistances are
0.02Ω and 100Ω respectively, calculate the armature resistance. Assume a voltage drop of 1V
per brush.
(7M+8M)
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Code No: R21025
5.
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SET - 2
a) “If two compounded inter pole DC generators are connected in parallel, one equalizing bar
has to be used”. Explain the function of equalizing bar and justify the statement.
b) Two shunt generators running at parallel have the following data.
EMF’s: 120V and 115V;
Common load: 25kW;
Armature resistances: 0.05 Ω and 0.04Ω.
How do they share load?
6.
(7M+8M)
a) From the fundamentals, derive the expression for torque produced in DC motor.
b) The no-load armature current of a 230V, DC shunt motor is 2A at a speed of 1200rpm. If the
full load armature current is 40A, find the full load speed and torque developed. Assume that the
armature resistance is 0.25Ω and the field flux remains unaltered.
7.
(7M+8M)
a) Explain with neat sketch how speed control of a DC motor is done by Ward Leonard control
system.
b) A 440V shunt motor while running at 1500rpm takes an armature current of 30A and delivers
output of 15h.p, the load torque varies as the square of the speed. Calculate the value of
resistance to be connected in series with the armature for reducing motor speed to 1300rpm.
(7M+8M)
8.
a) Explain how the field’s test is conducted on DC series machines?
b) Two identical dc machines are tested by Hopkinson’s method with an input of 40A at 220V.
The motor armature current was 200A and its field current was 6A, while the generator field
was 7A. Also each machine had an armature resistance of 0.05ohms. Calculate the efficiency of
the machine.
(7M+8M)
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Code No: R21025
R10
SET - 3
II B.Tech I Semester, Regular Examinations, Nov - 2011
ELECTRICAL MACHINES -I
(Electrical and Electronics Engineering)
Time: 3 hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
1.
a) Show how mechanical output can be determined in the multiple excited nonlinear systems.
b) What are the advantages of analyzing energy conversion devices by field energy concept?
(7M+8M)
2.
a) Mention the differences between lap and wave windings.
b) A dynamo has a rated armature current at 250 A. What is the current per path of the armature
if the armature winding is simplex wave wound or simplex lap wound? The machine has 12
poles.
3.
(7M+8M)
a) Explain the process of commutation in DC generator? Mention its types.
b) A 4-pole, wave wound, 750rpm, shunt generator has armature and field resistances of 0.4Ω
and 200Ω respectively. The armature has 720 conductors and the flux per pole is 29×10-3 Wb. If
the load resistance is 10Ω, determine the terminal voltage of the machine.
4.
(7M+8M)
a) What are the essential conditions for voltage build up of a self excited DC generator?
b) A long shunt generator supplies a load current of 180A at a terminal voltage of 400V. The
series field, shunt field and armature resistances are 0.03 Ω, 200 Ω and 0.04 Ω respectively.
Contact drop per brush is 1V. Armature reaction may be ignored. Determine the EMF
generated.
(7M+8M)
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Code No: R21025
5.
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SET - 3
a) What are the conditions necessary for parallel operation of DC shunt generators?
b) Explain the conditions under which two shunt generators operating in parallel do not share
the total load in exact proportion to the machine ratings.
c) Two DC generators operating in parallel have linear characteristics. One machine has
terminal voltage of 270 V on no load and 220 V at a load of 30 A. The other machine has a
voltage of 280V on no load and 220V at 40A. Calculate the output current of each machine and
the bus voltage when i) The total load current is 50 A and ii) Load resistance is 10 ohms.
(3×5 =15M)
6.
a) Draw and explain the Electrical characteristics of DC series and shunt motors.
b) A dc shunt motor takes 1.5A on no-load when connected to 250V mains with an armature
resistance of 1.0Ω. When the field current is 0.5A, determine the load current corresponding to
maximum efficiency.
7.
(7M+8M)
a) What are the applications of DC motors?
b) A 230V shunt motor is taking a current of 50A. Resistance of shunt field is 46Ω and the
resistance of armature is 0.02Ω. There is a resistance of 0.6Ω in series with the armature and
speed is 800rpm. What alteration must be made in the armature circuit to raise the speed to
850rpm, the torque remaining the same?
8.
(7M+8M)
a) Describe the brake test on DC motor to determine the efficiency characteristics of DC motor.
b) Hopkinson’s test on two similar DC shunt machines gave the following data:
Line voltage 230V, line current excluding both the field currents is 40A, motor armature current
350A, field currents 5A and 4.2A.
Calculate the efficiency of each machine. Armature resistance of each machine is 0.02Ω.
(7M+8M)
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R10
Code No: R21025
SET - 4
II B.Tech I Semester, Regular Examinations, Nov - 2011
ELECTRICAL MACHINES -I
(Electrical and Electronics Engineering)
Time: 3 hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
1.
a) All energy conversion devices use magnetic field as a coupling medium rather than electrical
field. Explain why?
b) Derive the expression for the magnetic force developed in linear magnetic system. (8M+7M)
2.
a) Explain the principle of operation of D.C. generator and derive EMF equation for the same.
b) A 6-pole lap wound DC generator has an armature with 90 slots and 6 conductors per slot and
rotates at 1200 rpm. The flux per pole is 50 m Wb. Calculate EMF generated.
3.
(9M+6M)
a) What is the purpose of compensating winding? How the number of turns per pole for
compensating winding determined?
b) A 4-pole dc generator has a simple wave wound armature with 100 slots and 4 conductors per
slot and, delivers a current of 50A on full load. If a brush lead is 9 mechanical degrees, calculate
4.
the per pole demagnetizing and cross magnetizing AT/pole.
(7M+8M)
a) What is critical speed? How do you calculate the critical speed in laboratory?
(9M+6M)
b) Find the resistance of the load which takes a power of 6kW from a dc generator whose
external characteristic is given by the equation V= 300-0.5IL; where IL is load current.
5.
a) What are the necessary conditions for parallel operation of DC generators?
(7M+8M)
b) Two shunt generators running in parallel share a load of 100kW equally at a terminal voltage
of 230V. On no-load, their voltage rises to 24V and 245V respectively. Assuming that their
volt-ampere characteristics are rectilinear, find how they would share the load when the total
load current is reduced to half of its original value. Also fine the new terminal voltage.
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Code No: R21025
6.
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SET - 4
a) Explain the principle of operation of DC motors. What is back EMF? What is its effect?
b) A 4-pole, 500V, DC shunt motor has 700 wave connected conductors on its armature. The
full load armature current is 60A and flux per pole is 30 mWb. Calculate the full load speed if
the motor armature resistance is 0.2Ω and the brush drop is 1V per brush.
7.
(7M+8M)
a) Explain the necessity of starter in DC motors. Draw and explain the operation of a 4-point
starter.
b) A 120V, DC shunt motor has an armature resistance of 0.2Ω; a brush voltage drop of 2V, the
rated full load current is 75A. Calculate i) the current at the instant of starting as a percentage of
full load current and ii) the value of resistance to limit the motor current at the instant of starting
to 150 percent of the rated load current.
8.
(7M+8M)
a) Explain Swinburne’s test. How efficiency can be predetermined for DC generator and DC
motor using this test?
b) A 500V, shunt motor takes 4A on no-load. The resistance of the armature including the
brushes is 0.2Ω and field current is 1A. Estimate the output and the efficiency when the input
current is 100A.
(7M+8M)
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