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Code No: W0221/R07 Set No. 1

II B.Tech I Semester Regular Examinations, November 2009ELECTRICAL MACHINES-I

(Electrical & Electronic Engineering)Time: 3 hours Max Marks: 80

Answer any FIVE QuestionsAll Questions carry equal marks

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

2. (a) Explain the role of brushes in DC machines. What is the material used forbrushes? 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 thearmature surface and 8 conductors per slot. If the resistance of one conductoris 2 m, find the resistance of armature. Neglect the resistance of overhang.Find the value of EMF induced if flux per pole is 60mwb and generator isrotated 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 thedemagnetization amp-turns/pole if shunt field resistance is 110. Also calcu-

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 theircharacteristics and equations. [8+8]

7. (a) Explain the functions of NVC and OLC in 4 Point starter. What are theadvantages and limitations of this starter on 3 point starter?

(b) The EMF developed in the armature of a shunt generator at 1155 RPM is240 Volts for a field current of 4.5 Amps and 255 V for a field current of 5Amps. The generator is now used as a motor on a 260 V supply and takes anarmature current of 75 Amps. Find the motor speed, when the field currentis adjusted to 4.8 Amps, total armature resistance is 0.12 . [6+10]

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8. (a) A 110 V DC series motor has an armature resistance of 0.25 and series fieldresistance of 0.2 . When the shaft torque is 28 Nm, the speed is 600 RPM. Atthis speed core and mechanical losses together are equal to 350 W. Calculate

i. Torque required to overcome the losses.

ii. Armature current

iii. Total copper losses

iv. over all efficiency.

(b) Explain the various losses taking place in DC compound motor. How theyvary with load? [10+6]


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1. A electromechanical system, which is U shaped, is excited from a voltage source= Vm sin t. The core of the system is made of homogenous material of uniformcross sectional area. Find the expression for average force experienced on thearmature, 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 connectedin lap with 960 conductors. Calculate the generated EMF on open circuitwhen it runs at 400 rpm. If the armature is wave wound at what speed mustthe machine be driven to generate the same voltage. [8+8]

3. (a) Explain about demagnetizing Ampere turn per pole and Cross magnetizingAmpere 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]

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 paralleloperation of DC Series generators. [8+8]

6. (a) Define torque. Derive the expression for torque developed by a D.C. motorfrom fundamentals.

(b) Determine the torque developed when a current of 30A passes through thearmature 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 is running at1000 RPM and takes an armature current of 50 A. If the field flux is suddenlyreduced by 10%, obtain

i. the maximum value of current at this instant and the correspondingtorque.

ii. Ultimate speed and armature current after the transients are over. As-sume constant load.


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(b) Explain the use of diverters in speed control of DC series motors. [10+6]

8. (a) Compare Swinburnes test and Hopkinsons test conducted on DC machines.

List the advantages and limitations of both the methods.

(b) The Hopkinsons test on two DC machines gave the following results for fullload: line voltage 250 V, line current = 45 A excluding field currents, motorarmature current = 385 A, field currents 5 A and 4 A. Calculate the efficiencyof each machine. Armature resistance of each machine is 0.015 . [6+10]


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1. In a rotary device the inductances of coils with rotor poison at 45o are approximatedas L11= L22= 2+

2

1

45o

andL12=

2

1

90o

. Calculate the torque of field origin

if rotor is fixed at 45o and i1= 5 Amp and i2= 0. Also find the average torque, ifcurrent in coil 1 is 5sin 314 t and coil 2 is short circuited. [16]





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

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 and 0.012 and armature resistance of 0.02 and 0.04 respectively. They are operatingin 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 theireffects 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 slotcontaining 6 conductors. The flux per pole is 20 m.wb and the armature hasa resistance of 0.13 ohm when the motor is connected to 240V supply and thearmature 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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7. (a) What are the different speed control methods of DC Shunt motor? Explaineach 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 . It takes 10 Amp from200 V mains and runs at 1000 RPM. Find the resistance to be connected inseries 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 shuntmotor. Spring Balances, 08 kgs and 30 kgs. Diameter of drum = 42 cm. Speedof 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 resistancesof the armature and field circuit are 0.2 and 300 respectively. Find,

the efficiency when loaded and taking a current of 125 A and the percentagechange of speed. [8+8]


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





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

(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 withothers already supplying a load.

(b) Six DC Shunt Generators are running in parallel. Each generator supplies aload current o f 400A at 210V. The shunt field current of each generator is 4Aand the armature resistance of each generator is 0.04 ohm. If one generator issuddenly switched off, determine the % change in terminal voltage, the totalload 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 slotcontaining 6 conductors. The flux per pole is 20 m.wb and the armature hasa resistance of 0.13 ohm when the motor is connected to 240V supply and thearmature 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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7. (a) Explain the above normal speed, speed control of DC motor. How this controlis achieved? Explain why speed is above normal?

(b) A 220 V series motor runs at 700 RPM when operating at full load current of 20Amp. The motor resistance is 0.5 . Assume magnetic path is un-saturable.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 shuntmotor. Spring Balances, 08 kgs and 30 kgs. Diameter of drum = 42 cm. Speedof 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 and 300 respectively. Find,the efficiency when loaded and taking a current of 125 A and the percentagechange of speed. [8+8]


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