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Code No: W0221/R07                                             Set No. 1
II B.Tech I Semester Regular Examinations, November 2009
ELECTRICAL MACHINES-I
(Electrical & Electronic Engineering)
Time: 3 hours                                           Max Marks: 80

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All Questions carry equal marks

1. With the help of neat diagram obtain the expression for the energy stored in a
magnetic system for a simple attracted armature type relay. Explain the operation
of system.                                                                   [16]

2. (a) Explain the role of brushes in DC machines. What is the material used for
brushes? What is importance of brush pressure on performance of machine?
(b) A armature is wound with lap winding and 8 poles. There are 72 slots on the
armature surface and 8 conductors per slot. If the resistance of one conductor
is 2 mΩ, ﬁnd the resistance of armature. Neglect the resistance of overhang.
Find the value of EMF induced if ﬂux per pole is 60mwb and generator is
rotated at 300 rpm.                                                     [6+10]

3. (a) With a neat sketch explain the function of commutator in a d.c. machine.
(b) A 2-pole, 25KW, 220V wave-wound shunt generator has 400 armature con-
ductors. Brushes are given a lead of 4 commutator segments. Calculate the
demagnetization amp-turns/pole if shunt ﬁeld resistance is 110Ω. Also calcu-

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late extra shunt ﬁeld turns/pole to neutralize the demagnetization.    [8+8]

4. What is critical speed? How do you calculate the critical speed in laboratory. [16]

5. (a) What are the reasons for the parallel operation of DC generators?
(b) What are the conditions necessary for parallel operation of DC shunt genera-
tors?
(c) Explain the procedure of parallel operation of DC shunt generators. [4+4+8]

6. (a) Diﬀerentiate between generator action and motor action of a DC machine.
(b) Explain the applications of DC shunt and series motors with the help of their
characteristics and equations.                                         [8+8]

7. (a) Explain the functions of NVC and OLC in 4 Point starter. What are the
advantages and limitations of this starter on 3 point starter?
(b) The EMF developed in the armature of a shunt generator at 1155 RPM is
240 Volts for a ﬁeld current of 4.5 Amps and 255 V for a ﬁeld current of 5
Amps. The generator is now used as a motor on a 260 V supply and takes an
armature current of 75 Amps. Find the motor speed, when the ﬁeld current
is adjusted to 4.8 Amps, total armature resistance is 0.12 Ω.      [6+10]

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Code No: W0221/R07                                              Set No. 1
8. (a) A 110 V DC series motor has an armature resistance of 0.25 Ω and series ﬁeld
resistance of 0.2 Ω. When the shaft torque is 28 Nm, the speed is 600 RPM. At
this speed core and mechanical losses together are equal to 350 W. Calculate
i.   Torque required to overcome the losses.

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ii.   Armature current
iii.   Total copper losses
iv.    over all eﬃciency.
(b) Explain the various losses taking place in DC compound motor. How they

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Code No: W0221/R07                                             Set No. 2
II B.Tech I Semester Regular Examinations, November 2009
ELECTRICAL MACHINES-I
(Electrical & Electronic Engineering)
Time: 3 hours                                           Max Marks: 80

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All Questions carry equal marks

1. A electromechanical system, which is U shaped, is excited from a voltage source
ν= Vm sin ωt. The core of the system is made of homogenous material of uniform
cross sectional area. Find the expression for average force experienced on the
armature, when the armature is held at a distance of x from the core.      [16]

2. (a) What is an equalizer connection? What is necessity of equalizer connection?
(b) An 8 pole DC generator has per pole ﬂux of 40mWb and winding is connected
in lap with 960 conductors. Calculate the generated EMF on open circuit
when it runs at 400 rpm. If the armature is wave wound at what speed must
the machine be driven to generate the same voltage.                 [8+8]

3. (a) Explain about demagnetizing Ampere turn per pole and Cross magnetizing
Ampere Turn per pole.
(b) What is the purpose of compensating winding? Explain in detail.           [8+8]

4. What is critical speed? How do you draw magnetization characteristics in labora-
tory.                                                                       [16]

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5. (a) Explain clearly why an equilizer connection makes it possible for two com-
pound generators to operate in parallel in stable equilibrium.
(b) Discuss the necessity for parallel operation of generators. Explain the parallel
operation of DC Series generators.                                        [8+8]

6. (a) Deﬁne torque. Derive the expression for torque developed by a D.C. motor
from fundamentals.
(b) Determine the torque developed when a current of 30A passes through the
armature of a motor with the following particulars: lap winding, 310 conduc-
tors, 4-pole, pole-shoes 16.2 cm long subtending an angle of 600 at the centre,
bore radius 16.2 cm, ﬂux density in air gap 0.7 tesla.                   [8+8]

7. (a) A 220 V DC shunt motor, with an armature resistance of 0.1 Ω is running at
1000 RPM and takes an armature current of 50 A. If the ﬁeld ﬂux is suddenly
reduced by 10%, obtain
i. the maximum value of current at this instant and the corresponding
torque.
ii. Ultimate speed and armature current after the transients are over. As-
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Code No: W0221/R07                                            Set No. 2
(b) Explain the use of diverters in speed control of DC series motors.     [10+6]

8. (a) Compare Swinburne’s test and Hopkinson’s test conducted on DC machines.
List the advantages and limitations of both the methods.

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(b) The Hopkinson’s test on two DC machines gave the following results for full
load: line voltage 250 V, line current = 45 A excluding ﬁeld currents, motor
armature current = 385 A, ﬁeld currents 5 A and 4 A. Calculate the eﬃciency
of each machine. Armature resistance of each machine is 0.015 Ω.      [6+10]

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Code No: W0221/R07                                             Set No. 3
II B.Tech I Semester Regular Examinations, November 2009
ELECTRICAL MACHINES-I
(Electrical & Electronic Engineering)
Time: 3 hours                                           Max Marks: 80

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All Questions carry equal marks

1. In a rotary device the inductances of coils with rotor poison at 45o are approximated
as L11 = L22 = 2+ π 1− 45o andL12 = π 1− 90o . Calculate the torque of ﬁeld origin
2
θ
2
θ
o
if rotor is ﬁxed at 45 and i1 = 5 Amp and i2 = 0. Also ﬁnd the average torque, if
current in coil 1 is 5sin 314 t and coil 2 is short circuited.                   [16]

2. (a) What is an equalizer connection? What is necessity of equalizer connection?
(b) An 8 pole DC generator has per pole ﬂux of 40mWb and winding is connected
in lap with 960 conductors. Calculate the generated EMF on open circuit
when it runs at 400 rpm. If the armature is wave wound at what speed must
the machine be driven to generate the same voltage.                 [8+8]

3. (a) Explain about demagnetizing Ampere turn per pole and Cross magnetizing
Ampere Turn per pole.
(b) What is the purpose of compensating winding? Explain in detail.           [8+8]

4. What is critical ﬁeld resistance? How do you calculate the critical ﬁeld resistance
from magnetic characteristic in laboratory.                                    [16]

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5. (a) Is the magnetization characteristic similar for all types of DC generators. Jus-
tify
(b) Two compound generators G1 and G2 ﬁtted with an equilizing bar has gener-
ated e.m.f.’s of 250 V and 254 V, series ﬁeld resistance of 0.008 Ω and 0.012 Ω
and armature resistance of 0.02 Ω and 0.04 Ω respectively. They are operating
in parallel supply a load of 475A. Determine
i. current in each armature
ii. current in each series windingand
iii. Bus bar voltage.                                                     [8+8]

6. (a) Discuss Armature reaction and commutation in DC motors. Explain their
eﬀects on the performance of the motor and give remedies to their eﬀects.
(b) A 6-pole DC motor has a wave connected armature with 87 slots, each slot
containing 6 conductors. The ﬂux per pole is 20 m.wb and the armature has
a resistance of 0.13 ohm when the motor is connected to 240V supply and the
armature draws a current of 80A driving a load of 15KW. Calculate
i. Speed
ii. Armature Torque and
iii. shaft Torque.                                                       [10+6]
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Code No: W0221/R07                                            Set No. 3
7. (a) What are the diﬀerent speed control methods of DC Shunt motor? Explain
(b) A DC series motor is driving a fan load, whose torque varies a cube of speed.
The total armature and series ﬁeld resistance is 1 Ω. It takes 10 Amp from

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200 V mains and runs at 1000 RPM. Find the resistance to be connected in
series with the motor to make it run at 800 RPM.                       [6+10]

8. (a) The following readings are obtained when performing a brake test on DC shunt
motor. Spring Balances, 08 kgs and 30 kgs. Diameter of drum = 42 cm. Speed
of the motor = 1000 rpm. Applied voltage = 220 V. Line current = 50 A.
Calculate out put power and eﬃciency.
(b) A 500 V DC shunt motor takes a current of 5 A on no-load. The resistances
of the armature and ﬁeld circuit are 0.2 Ω and 300 Ω respectively. Find,
the eﬃciency when loaded and taking a current of 125 A and the percentage
change of speed.                                                    [8+8]

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Code No: W0221/R07                                             Set No. 4
II B.Tech I Semester Regular Examinations, November 2009
ELECTRICAL MACHINES-I
(Electrical & Electronic Engineering)
Time: 3 hours                                           Max Marks: 80

www.andhracolleges.com
All Questions carry equal marks

1. A electromechanical system, which is U shaped, is excited from a voltage source
ν= Vm sin ωt. The core of the system is made of homogenous material of uniform
cross sectional area. Find the expression for average force experienced on the
armature, when the armature is held at a distance of x from the core.      [16]

2. (a) What is an equalizer connection? What is necessity of equalizer connection?
(b) An 8 pole DC generator has per pole ﬂux of 40mWb and winding is connected
in lap with 960 conductors. Calculate the generated EMF on open circuit
when it runs at 400 rpm. If the armature is wave wound at what speed must
the machine be driven to generate the same voltage.                 [8+8]

3. (a) Explain about demagnetizing Ampere turn per pole and Cross magnetizing
Ampere Turn per pole.
(b) What is the purpose of compensating winding? Explain in detail.           [8+8]

4. (a) What is meant by voltage build-up of generator. Explain the various causes
for the failure of generator to build-up.

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(b) What is classiﬁcation of compound generators? Give their application.[10+6]

5. (a) Explain the exact procedure for connecting a shunt generator in parallel with
(b) Six DC Shunt Generators are running in parallel. Each generator supplies a
load current o f 400A at 210V. The shunt ﬁeld current of each generator is 4A
and the armature resistance of each generator is 0.04 ohm. If one generator is
suddenly switched oﬀ, determine the % change in terminal voltage, the total
load current being kept unchanged.                                      [8+8]

6. (a) Distinguish between motor and generator action. Derive the equation for the
back e.m.f induced in a DC motor.
(b) A 6-pole DC motor has a wave connected armature with 87 slots, each slot
containing 6 conductors. The ﬂux per pole is 20 m.wb and the armature has
a resistance of 0.13 ohm when the motor is connected to 240V supply and the
armature draws a current of 80A driving a load of 16KW. Calculate
i. Speed
ii. Armature Torque and
iii. shaft Torque.                                                      [10+6]
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Code No: W0221/R07                                             Set No. 4
7. (a) Explain the ’above normal speed’, speed control of DC motor. How this control
is achieved? Explain why speed is above normal?
(b) A 220 V series motor runs at 700 RPM when operating at full load current of 20
Amp. The motor resistance is 0.5 Ω. Assume magnetic path is un-saturable.

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What will be the speed if:
i. Load torque is increased by 44%
ii. Motor current = 10 Amp.                                            [6+10]

8. (a) The following readings are obtained when performing a brake test on DC shunt
motor. Spring Balances, 08 kgs and 30 kgs. Diameter of drum = 42 cm. Speed
of the motor = 1000 rpm. Applied voltage = 220 V. Line current = 50 A.
Calculate out put power and eﬃciency.
(b) A 500 V DC shunt motor takes a current of 5 A on no-load. The resistances
of the armature and ﬁeld circuit are 0.2 Ω and 300 Ω respectively. Find,
the eﬃciency when loaded and taking a current of 125 A and the percentage
change of speed.                                                    [8+8]

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