Set No. 1 Code No: R059211001 II B.Tech I Semester Regular Examinations, November 2007 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) Discuss the process of self- excitation in a DC machine. What conditions must be fulfilled for the machine to self-excite? (b) A shunt generator has a F.L. current of 196A at 220V.The stray losses are 720W and the shunt field coil resistance is 55Ω .If it has a F.L. efficiency of 88%, Find the armature resistance. Also, find the load current corresponding to maximum efficiency. [8+8] 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 resistance in the field or armature circuit, the armature current being 2A. Calculate the resistance required in series with the shunt winding so that the motor may run at 950rpm when taking a line current of 30A. Shunt winding resistance is 160Ω , armature resistance is 0.4Ω . Assume that flux is proportional to field current. [6+10] 3. Explain the working of a transformer at no load and full load conditions with neat diagrams. [16] 4. What are the different losses occuring in a transformer on load. How can these losses be determined experimentallty. [16] 5. (a) Explain why a 3-phase Induction Motor cannot develop torque when running at synchronous speed. Define the slip and deduce how the frequency of rotor currents and magnitude of rotor e.m.f are related to slip. (b) A 3-phase star connected Induction motor has 55V across its slip rings on open circuit when normal stator voltage is applied. The rotor is star connected and has impedance (0.7+j5)Ω per phase. Find the rotor current when the machine is i. at stand still with the slip rings connected to a star connected starter with a phase impedance of (4+j3)Ω and ii. running normally with 5% slip. [10+6] 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] 1 of 2 Set No. 1 Code No: R059211001 7. Explain with neat diagrams the following types of single phase induction motors (a) split phase induction motor (b) Capacitor start induction motor [8+8] 8. Explain with neat sketch the principle of operation of permanent magnet type moving coil instrument. [16] ⋆⋆⋆⋆⋆ 2 of 2 Set No. 2 Code No: R059211001 II B.Tech I Semester Regular Examinations, November 2007 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) Deduce the condition for maximum efficiency of a D.C. Generator. (b) Explain the various losses occur in a D.C. generator (c) A 6-pole, wave connected armature has 250 conductors and runs at 1200rpm. The emf generated on open ?circuit is 600V. Calculate the useful flux per pole. [6+6+4] 2. (a) Sketch the 3-point starter used to start D.C. shunt motor and explain the function of no-volt coin and overload coil? (b) A 4 pole DC Series motor has 944 wave connected armature conductors. At a certain load the flux per pole is 34.6 mWb and the total mechanical torque developed is 209Nm. Calculate the line current taken by the motor and the speed at which it will run with an applied voltage of 500V. The total motor resistance is 3Ω [10+6] 3. With neat diagram discuss the principle of working of a transformer. [16] 4. (a) ) Draw the equivalent circuit of a transformer and explain the procedure to obtain the same from opencircuit and short circuit tests . (b) A 10 KVA 2000/250 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) Show that when a 3-phase supply is given to the stator winding of Induction 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 frequency of rotor e.m.f at full load is 5 Hz, find the Full-Load speed and % Slip. [12+4] 6. (a) Derive EMF equation of an alternator. Also derive expressions for pitch and breadth factor. (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. 2 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. 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 the following with reference to the indicating instruments (a) Deflecting torque (b) Controlling torque (c) Damping torque (d) Scale and pointer ⋆⋆⋆⋆⋆ 2 of 2 [16] Set No. 3 Code No: R059211001 II B.Tech I Semester Regular Examinations, November 2007 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 the working principle of a D.C. generator. Is there any difference in construction between a motor and a generator? (b) A 250V compound generator has armature, series field and shunt field resistance of 0.4 Ω, 0.2 Ω , and 125 Ω respectively. If this generator supplies 10KW at rated voltage, find the e.m.f. generated in the armature when the machine is connected i. Long shunt and ii. Short shunt. Ignore armature reaction and allow 1V per brush for contact drop. [8+8] 2. (a) How torque is developed in a DC motor, Explain with neat diagram and derive an expression for torque developed in DC motor. (b) A Series motor takes 20A at 400V to drive a fan at 200r.p.m. Its resistance is 1Ω If the torque required to drive the fan varies as the square of the speed, find the necessary applied voltage and current to drive the fan at 300 r.p.m [8+8] 3. How do you classify the transformers and also discuss the different parts of transformer in detail. [16] 4. Discuss the procedure for conducting open circuit test and short circuit test on transformer, and also explain the how to obtain efficieny and regulation from these test. [16] 5. (a) Explain the constructional features of a 3-phase slip ring and squirrel cage Induction Motors. Bring out the merits and demerits of each. (b) A 3-phase star connected Induction motor has 55V across its slip rings on open circuit when normal stator voltage is applied. The rotor is star connected and has impedance (0.7+j5)Ω per phase. Find the rotor current when the machine is i. at stand still with the slip rings connected to a star connected starter with a phase impedance of (5+j4)Ω and ii. running normally with 4% slip. [10+6] 6. (a) Derive EMF equation of an alternator. Also derive expressions for pitch and breadth factor. 1 of 2 Set No. 3 Code No: R059211001 (b) Find the number of armature conductors in series per phase required for the armature of a 3-phase, 10-pole, 50Hz, synchronous generator with 90 slots. The winding is to be star-connected so as to have line voltage of 11 KV. The flux per pole is 0.16 Wb. [10+6] 7. Show that a single phase winding when excited by a single phase supply produce two equal and opposite revolving fields. [16] 8. Explain with neat sketch the principle of operation of permanent magnet type moving coil instrument. [16] ⋆⋆⋆⋆⋆ 2 of 2 Set No. 4 Code No: R059211001 II B.Tech I Semester Regular Examinations, November 2007 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) An 8-pole, lap connected armature has 960 conductors, a flux per pole of 40 × 10 −3 Wb/pole, and runs at 400rpm. Calculate the emf generated on open-circuit. (b) The Induced emf in a DC machine is 200V at a speed of 1200rpm. Calculate the electromagnetic torque developed at an armature current of 15A. (c) A Shunt generator delivers 50kW at 250V and 400rpm. The armature resistance is 0.02Ω and field resistance is 50Ω. Calculate the speed of the machine when running as a shunt motor and taking 50kW input at 250V. [4+8+4] 2. (a) Explain how we can implement flux control and Armature rheostat control methods of speed control for a typical D.C. Shunt Motor. Enumerate the differences in the two methods. (b) A 250V shunt motor on no-load runs at 1000rpm and draws 5A from the lines. The armature resistance is 0.2Ω and the field circuit resistance is 250Ω. Calculate the speed of the motor when it is loaded and draws a current of 50A. The armature reaction weakens the field by 3%. [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) 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) Sketch and explain the shape of the Torque-Slip characteristic of a squirrel cage Induction motor and deduce the condition for maximum torque. [8+8] (b) Draw the internal connections of auto transformer starter and explain its operation. 6. (a) Derive EMF equation of an alternator. Also derive expressions for pitch and breadth factor. 1 of 2 Set No. 4 Code No: R059211001 (b) Find the number of armature conductors in series per phase required for the armature of a 3-phase, 10-pole, 50Hz, synchronous generator with 90 slots. The winding is to be star-connected so as to have line voltage of 11 KV. The flux per pole is 0.16 Wb. [10+6] 7. Explain with neat diagrams the principle of operation of (a) AC servo motor (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 ⋆⋆⋆⋆⋆ 2 of 2 [16]