r05220401 Electrical Technology

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Code No: R05220401 Set No. 1

II B.Tech II Semester Regular Examinations, Apr/May 2007ELECTRICAL TECHNOLOGY

( Common to Electronics & Communication Engineering and Electronics &Telematics)

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

All Questions carry equal marks⋆ ⋆ ⋆ ⋆ ⋆

1. (a) Why do Iron losses occur in a D.C. Machine? [4+6+6]

(b) Mention the factors on which Iron losses in the armature of a D.C. Machine

depend upon. How these losses are reduced?(c) Draw the power flow diagram of D.C. shunt generator.

2. (a) Discuss the applications of shunt, series and compound motors.

(b) A DC series motor draws a line current of 100A from the mains while runningat 1000rpm. Its armature resistance is 0.15Ω and field resistance is 0.1Ω .Assuming that the flux corresponding to a current of 25A is 40% of thatcorresponding to 100A, find the speed of the motor when it is drawing 25Afrom 230V supply. [8+8]

3. Explain the working of a transformer at no load and full load conditions with neatdiagrams. [16]

4. (a) Explain different lossess that occur in a transformer

(b) A 150 KVA transformer has an iron loss of 1400 watts and full load copperloss of 1600 watts. Find the Find the efficiency of a transformer at 50% fullload (i) upf I (ii) 0.8 pf lagging [8+8]

5. (a) Explain why a starter is necessary for starting of 3-phase Induction Motor?Name the starters used for starting 3-phase Induction Motor.

(b) The power input to rotor of a 440V, 50Hz, 3-phase, 12-pole Induction Motoris 75W. The rotor e.m.f has a frequency of 2Hz. Calculate

i. Slip

ii. rotor speed

iii. rotor copper loss

iv. mechanical power developed [8+8]

6. (a) What is meant by Voltage regulation? Explain in detail the SynchronousImpedance method for obtaining Voltage regulation of 3-phase Alternator.

(b) A 3-Phase star connected alternator is rated at 2000 KVA, 12 KV. The ar-

mature effective resistance and synchronous reactance are 2Ω and 35Ω respec-tively per phase. Calculate the percentage regulation for a load of 1200 kWat power factor of 0.6 lagging. [16]

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7. Explain the principle operation of a shaded pole single phase induction motor anddraw its torque speed characteristic. [16]

8. With a neat sketch explain in detail moving iron repulsion type instrument [16]

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1. (a) The following data pertain to the magnetization curve of a D.C. shunt gener-ator at 300 r.p.m.

I f in Amps 0 0.2 0.3 0.4 0.5 0.6 0.7E G in Volts 7.5 93 135 165 186 202 215

The field resistance of the machine is adjusted to 354.5Ω and the speed is 300prm.For this generator, obtain

(a) Determine graphically the no-load voltage.

(b) Determine the critical value of the shunt field resistance.

(c) Determine the critical speed for the given field resistance

(d) What additional resistance must be added in the field circuit to reduce theno-load voltage to 175V [16]

2. (a) What is the significance of the back e.m.f. of a D.C. Motor?

(b) Deduce the condition for maximum power for a D.C. Motor?

(c) A 230V DC shunt motor takes an armature current of 20A on a particularload. The armature circuit resistance is 0.5Ω . Find the resistance required inseries with the armature to reduce the speed by 50% if (i) the load torque isconstant and (ii) the load torque is proportional to the square of the speed.

[4+4+8]

3. (a) What is an ideal transformer . Derive an expression for induced emf in a

transformer in terms of frequency the maximum value of flux, and the numberof turns on the winding. [16]

4. (a) Define voltage regulation of a transformer. Deduce the expression for thevoltage regulation

(b) The number of turns on the primary and secondary windings of a single phasetransformer are 350 and 35 respectively. If the primary is connected to a 2.2KV 50 HZ supply determine the secondary voltage. [8+8]

5. (a) Show that when a 3-phase supply is given to the stator winding of Induc-tion Motor, it produces a rotating magnetic field of constant magnitude and

rotating at synchronous speed.

(b) A 12-pole, 3-phase Induction Motor is fed from 100Hz supply. If the frequencyof rotor e.m.f at full load is 5 Hz, find the Full-Load speed and %Slip [12+4]

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6. (a) Explain the constructional features of alternator.

(b) How e.m.f is induced in an 3-phase alternator? Derive the expression for e.m.f induced in an alternator in terms of pitch and distribution factors. [8+8]

7. Show that a single phase winding when excited by a single phase supply producetwo equal and opposite revolving fields [16]

8. (a) Discuss the classification of electrical instruments

(b) Explain the significance of controlling torque and damping torque relevant tothe operation Of indicating instruments. [8+8]

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1. (a) What purpose is served by the pole shoe in a D.C. machine? [4+4+4+4]

(b) What are the advantages and disadvantages of carbon brushes?

(c) Why do we use slotted armature in a D.C. machine?

(d) Why is armature winding placed on the rotor of a D.C. machine?

2. (a) Name different types of D.C. motors and state their application?

(b) A 240V DC shunt motor runs on no-load at 800rpm with no extra resistancein the field or armature circuit, the armature current being 2A. Calculate theresistance required in series with the shunt winding so that the motor mayrun at 950rpm when taking a line current of 30A. Shunt winding resistance is160Ω , armature resistance is 0.4Ω . Assume that flux is proportional to fieldcurrent. [6+10]

3. (a) What is an ideal transformer . Derive an expression for induced emf in atransformer in terms of frequency the maximum value of flux, and the numberof turns on the winding. [16]

4. (a) What are the different losses occuring in a transformer on load. How can theselosses be determined experimentallty. [16]

5. (a) Explain the power stages of a 3-phase Induction Motor and hence deducethe relationship between copper loss, mechanical power developed and air-gappower of an Induction Motor.

(b) A 11KW, 3-phase, 6-pole, 50Hz, 400V Induction Motor runs at 960 r.p.m. onfull-load. If it takes 80A on Direct On Line switching, find the ratio of startingtorque to full-load torque when started by star-delta starter, taking efficiency95.6% and p.f. 0.834. [12+4]

6. (a) Explain the pessimistic method of finding voltage regulation of an alternator.

(b) A 600V, 60KVA, 1 phase alternator has an effective resistance of 0.2Ω . A

field current of 10A produces an armature current of 210A on short-circuitand e.m.f. of 480 V on open-circuit. Calculate Full-load regulation with 0.8

p.f. lagging. [8+8]7. Show that a single phase winding when excited by a single phase supply produce

two equal and opposite revolving fields [16]

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1. (a) Discuss the internal and external characteristics of a D.C. shunt generator

(b) A Separately excited generator, when running at 1200rpm supplies 200A at

125V to a circuit of constant resistance. What will be the current when thespeed is dropped to 1000rpm If the field current is unaltered? Armatureresistance is 0.04Ω. Total drop at brushes is 2V. Ignore change in armaturereaction. [10+6]

2. (a) Explain the principle of operation of a DC motor. Derive the equation for thetorque developed by a DC motor.

(b) A 220V shunt motor has am armature circuit resistance of 0.2Ω and fieldcircuit resistance of 110 Ω . At no load the motor takes 5A and runs at 1500r.p.m. if the motor draws 52A at rated voltage and rated load, calculate the

motor speed and its rated shaft torque in N-m. The rotational losses at noload and full load are the same. Neglect armature reaction. [8+8]

3. How do you classify the transformers and also discuss the different parts of trans-former in detail [16]

4. (a) What are the different losses occurring in a transformer. How can these lossesbe determined experimentally. [16]

5. (a) Show that when a 3-phase supply is given to the stator winding of Induc-tion Motor, it produces a rotating magnetic field of constant magnitude androtating at synchronous speed.

(b) A 12-pole, 3-phase Induction Motor is fed from 100Hz supply. If the frequencyof rotor e.m.f at full load is 5 Hz, find the Full-Load speed and %Slip [12+4]

6. (a) Explain the constructional features of alternator.

(b) A 3-phase, star-connected alternator has the following data: Voltage requiredto be generated on open-circuit = 4000 V at 50-Hz; speed = 500 r.p.m.; statorslots/pole/phase = 3; conductors/slot=12. Calculate the useful flux/pole.Assumeall conductors per phase to be connected in series and coil to be full pitch.

[8+8]

7. Explain with neat diagrams the following types of single phase induction motors

(a) split phase induction motor

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(b) Capacitor start induction motor [8+8]



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