Set No. 1

Code No: R059211001

II B.Tech I Semester Supplimentary Examinations, February 2008 ELECTRICAL TECHNOLOGY ( Common to Electronics & Instrumentation Engineering, Bio-Medical Engineering, Electronics & Control Engineering, Electronics & Computer Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Explain O.C.C. of a D.C. shunt generator. (b) A separately exited generator gave the following data for o.c.c. at 1000 r.p.m.. If in Amps 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Eg in Volts 5 50 100 140 170 190 200 205 The armature resistance including brushes is 0.5Ω . If the generator is now shunt connected and is driven at 1100 r.p.m., then, for a total shunt field resistance of 180Ω i. Calculate the no load e.m.f. ii. Calculate the output current and the shunt field current for the terminal voltage of 190V. [8+8] 2. (a) Derive an expression for torque of a D.C. motor. (b) Explain how the torque of a D.C. shunt and D.C. series motor varies with the speed of the motor. (c) A Series Motor of resistance 1Ω between terminals runs at 800rpm at 200V with a current of 15A. Find the speed at which it will run when connected in series with a 5Ω resistance and taking the same current at the same supply voltage. [5+5+6] 3. (a) What is an ideal transformer Draw its no load phasor diagram (b) The primary winding of a 50 HZ single phase transformer has 480 turns and is fed from 6400 V supply The secondary winding has 20 turns. Find the peak value of flux in the core and the secondary Voltage. [8+8] 4. (a) Define voltage regulation of a transformer. Deduce the expression for the voltage regulation (b) The number of turns on the primary and secondary windings of a single phase transformer are 350 and 35 respectively. If the primary is connected to a 2.2 KV 50 HZ supply determine the secondary voltage. [8+8] 5. (a) Explain the principle of 3-phase Induction Motor with the help of rotating magnetic field. (b) A 3-phase, 6-pole alternator is coupled to an engine running at 1200 r.p.m. The alternator supplies an Induction Motor which has a full-load speed of 1164 r.p.m. Find the slip and number of poles of the motor. [10+6] 1 of 2

Set No. 1

Code No: R059211001 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. Explain with neat diagrams the principle of operation of (a) AC servo motor (b) Stepper motor.

[8+8]

8. (a) Discuss the classification of electrical instruments (b) Explain the significance of controlling torque and damping torque relevant to the operation of indicating instruments. [8+8] ⋆⋆⋆⋆⋆

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Set No. 2

Code No: R059211001

II B.Tech I Semester Supplimentary Examinations, February 2008 ELECTRICAL TECHNOLOGY ( Common to Electronics & Instrumentation Engineering, Bio-Medical Engineering, Electronics & Control Engineering, Electronics & Computer Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) With reference to OCC of a D.C. generator, explain the following: i. Why emf is not zero when field current is zero? ii. Will the residual flux induce a voltage in the machine, if speed of the machine is zero? iii. Why does the slope of curve change, after a certain value of field current? (b) A separately excited DC generator has no-load voltage of 120V at a field current of 2A, when driven at 1500rpm, Assuming that it is operating on the straight line portion of its saturation curve, Calculate (i) the generated voltage when the field current is increased to 2.5; (ii) The generated voltage when the speed is reduced to 1400rpm and the field current is increased to 2.84A.[8+8] 2. (a) Explain the construction and working principle of D.C. motor? (b) A series motor has a resistance of 1Ω between its terminals. The motor runs at 800 r.p.m. at 200V taking a current of 15A. Calculate the speed at which the motor will run when connected in series with a 5Ω resistance and taking the same current at the same supply voltage. [10+6] 3. (a) Draw explain the phasor diagram of transformer under load condition. (b) The primary winding of a 50 HZ single phase transformer has 480 turns and is fed from 5400 V supply The secondary winding has 20 turns. Find the peak value of flux in the core and the secondary Voltage. [8+8] 4. (a) State and prove the condition for maximum efficiency of a transformer. (b) A 30 KVA single phase transformer has an iron loss of 457 watts and copper loss of 125 watts when delivering half the full load. At what percentage of full load will the transformer have maximum efficiency. [8+8] 5. (a) Derive the expression for Torque-Slip characteristics of a 3-phase Induction Motor. Explain how the torque of Induction Motor varies with speed with the help of a diagram. (b) Deduce the condition for maximum torque of a 3-phase Induction Motor. (c) A 12-pole, 3-phase Induction Motor is fed from 50Hz supply. If the frequency of rotor e.m.f at Half full-load is 2 Hz, find the Half Full-load speed and %Slip [8+4+4] 1 of 2

Set No. 2

Code No: R059211001

6. (a) In general, the field winding of an 3-phase alternator is a rotating element and the armature is stationary. Give reasons. (b) A 3-phase, star-connected alternator has the following data: Voltage required to be generated on open-circuit = 4000 V at 50-Hz; speed = 750 r.p.m.; stator slots/pole/phase = 3; conductors/slot=12. Calculate: i. Number of poles and ii. Useful flux/pole. Assume all conductors per phase to be connected in series and coil to be full pitch. [6+10] 7. Using double field revolving theory for single phase induction motors. Give its torque speed characteristic. Why this motor does not have starting torque. [16] 8. Explain with neat sketch the principle of operation of permanent magnet type moving coil instrument. [16] ⋆⋆⋆⋆⋆

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Set No. 3

Code No: R059211001

II B.Tech I Semester Supplimentary Examinations, February 2008 ELECTRICAL TECHNOLOGY ( Common to Electronics & Instrumentation Engineering, Bio-Medical Engineering, Electronics & Control Engineering, Electronics & Computer Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Explain how a shunt generator builds up voltage with magnetization curve. Also give the reasons for not building up voltage. (b) A separately excited DC generator, when running at 1000r.p.m. supplied 200A at 125V. What will be the load current when the speed drops to 800 r.p.m. If If is unchanged? Given that armature resistance is 0.04Ω and brush drop is 2V. Armature reaction may be neglected. [8+8] 2. (a) Derive the expression of back emf in a D.C. motor and briefly describe its role in starting and running of motor? (b) A 250V DC shunt motor has an armature resistance of 0.5Ω and a field resistance of 250Ω . When driving a constant torque load at 600rpm, the motor draws 21A. What will be the new speed of the motor if an additional 250Ω resistance is inserted in the field circuit? [8+8] 3. (a) Derive an emf equation of a single phase transformer. (b) The maximum flux density in the core of 250 /3000 Volts 50 HZ single phase transformer is 1.2 webers per square meter. If the emf per turn is 8 volts determine primary and secondary turns and area of the core. [8+8] 4. (a) What parameters of the equivalent circuit of a transformer can be determined from opencircuit and short circuit tests. Explain. (b) A 10 KVA 2500/250 Volts single phase transformer gave the following test results Open circuit test : 250 V, 0.8 A, 50 W Short circuit test: 60V 3A 45 W Calculate the efficiency and voltage regulation at full load 0.8 power factor lag. [8+8] 5. (a) Explain the rotor resistance starter of a 3-phase Induction Motor. Bring out its merits and demerits. (b) A 3-phase, 50Hz, 6-pole Squirrel Cage Induction Motor has a short circuit current equal to 4 times the full-load current and has full-load speed of 970 rpm. Find the starting torque as a percentage of full-load torque if the motor is started by i. DOL starter 1 of 2

Set No. 3

Code No: R059211001 ii. Star-Delta starter

[8+8]

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. Explain with neat diagrams the principle of operation of (a) synchros (b) Stepper motor.

[8+8]

8. Explain the following with reference to the indicating instruments (a) Deflecting torque (b) Controlling torque (c) Damping torque (d) Scale and pointer ⋆⋆⋆⋆⋆

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[16]

Set No. 4

Code No: R059211001

II B.Tech I Semester Supplimentary Examinations, February 2008 ELECTRICAL TECHNOLOGY ( Common to Electronics & Instrumentation Engineering, Bio-Medical Engineering, Electronics & Control Engineering, Electronics & Computer Engineering and Instrumentation & Control Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Mention the different reasons for the drop in the terminal voltage of a shunt generator when it is loaded? (b) A 4-pole, DC shunt generator, with a shunt field resistance of 100Ω and an armature resistance of 1Ω, has 378 wave connected conductors in its armature. The flux/pole is 0.02Wb. If a load resistance of 10Ω is connected across the armature terminals and the generator is driven at 1000rpm, calculate power absorbed by the load. [6+10] 2. (a) Explain with a neat circuit diagram swinburn’s test on DC shunt motor to find the efficiency of DC machine when if runs as motor and generator? Mention the advantages and disadvantages of this method? (b) A series motor has a resistance of 1Ω between its terminals. The motor runs at 800 r.p.m. at 200V taking a current of 15A. Calculate the speed at which the motor will run when connected in series with a 5Ω resistance and taking the same current at the same supply voltage. [10+6] 3. With a neat diagram explain the constructional details of a transformer.

[16]

4. What is regulation of a transformer. How it can be obtained from equivalent circuit parameters. [16] 5. (a) Deduce the relationship between copper loss, mechanical power developed and air-gap power of an Induction Motor. [10+6] (b) The power input to rotor of a 440V, 50Hz, 3-phase, 6-pole Induction Motor is 60KW. The rotor e.m.f has a frequency of 1.5Hz. Calculate i. ii. iii. iv.

Slip rotor speed rotor copper loss mechanical power developed

6. (a) Briefly explain the significance of pitch factor and distribution factor. Develop the Voltage equation for a 3-phase alternator having fractional pitch and distributed winding. (b) The stator of 3-phase, 8-pole synchronous generator driven at 750 rpm has 72 slots. The winding has been made with 36 coils having 10 turns per coil. 1 of 2

Set No. 4

Code No: R059211001

Claculate the rms value of induced emf per phase if the flux per pole is 0.15 mwb, sinusoidally distributed. Assume that full-pitch coils have been used. [10+6] 7. Explain the constructional features and principle operation of a single phase induction motor. [16] 8. With a neat sketch explain in detail moving iron repulsion type instrument ⋆⋆⋆⋆⋆

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[16]