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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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                                Answer any FIVE Questions
                              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
                    vary with load?                                                 [10+6]




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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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                                Answer any FIVE Questions
                              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-
                         sume constant load.
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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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                                Answer any FIVE Questions
                              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
                   each method and enumerate advantages and disadvantages.
                (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
                                Answer any FIVE Questions
                              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
                   others already supplying a load.
                (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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