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Posted Date: 01 May 2009 Posted By: Sunil Reddy Member Level: Platinum
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2007 Jawaharlal Nehru Technological University Electronics and Instrumentation Engineering II B.Tech. I-Semester Examinations , August 2007 ELECTRICAL TECHNOLOGY Question paper
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1. a) State the reasons for a self exited DC shunt generator to fail to build up voltage, and suggest the necessary modifications.
b) Explain the importance of critical field resistance and how it can be determined
c) A 4-pole generator having wave-wound armature winding has 51 slots each slot contains 20 conductors. What will be the voltage generated in the machine when driven at 1,500 rpm assuming the flux per pole to be 7.0 m Wb?
2. a) From the first principles, derive an equation for torque developed in a D.C. Motor.
b) A 20kw, 250V d.c. shunt generator has armature and field resistances of 0.04? and 200? respectively. Determine the total armature power developed when working i) as generator delivering 20kw output and ii) as a motor taking 20kw input.
3. a) Explain with phasor diagram, how the flux in the transformer core remains fairly constant from no-load to full load assuming lagging power factor.
b) The no-load current of a transformer is 15A at a power factor of 0.2 when connected to a 460 v, 50c/s supply. If primary winding has 550 turns. Calculate
1. Magnetizing component of no-load current
2. The iron loss
3. The maximum value of flux in the core.
4. a) Develop from fundamentals, the equivalent circuit representation of a single phase 2 winding Transformer.
b) The maximum efficiency of a 500 KVA, 3300/500v, 50 HZ, single phase transformer is 97% and occurs at 75% of full load at U.P.F. If the impedance is 10%. Calculate the regulation at F.L. at 0.8 P.F. lag.
5. a) Explain the principle of rotating magnetic field and hence prove it is of constant magnitude and rotates at synchronous speed.
b) A 3-phase, 50 Hz induction motor has a full-load speed of 1440 r.p.m. For this motor, calculate the following (i) number of poles (ii) full-load slip and rotor frequency (iii) speed of stator field with respect to (a) stator structure and (b) rotor structure and (iv) speed of rotor field with respect to (a) rotor structure (b) stator structure and (c) stator field
6. a) Why a 3-phase induction motor draws higher no load current compared to transformer
8. The starting current of a delta-connected 3-phase induction motor at rated voltage is 5 times the full-load current and the slip at full load is 5%. The no-load current is negligible. (i) If an auto-transformer starter is used to limit the starting current from mains to 2 times the full-load current, estimate the starting torque then obtained as a percentage, of he full-load torque. (ii) Neglecting stator impedance, determine the slip at which maximum torque occurs in the motor.
7. a) Define voltage regulation of an alternator. Explain synchronous impedance method of determining regulation of an alternator.
b) Calculate the voltage induced per phase in a 3phase 50 Hz, alternator having a flux per pole of 0.1515 wb. The no. of conductors in series are 360. Assume full pitch coil with a distribution factor of 0.96.
8. a) Explain the operation of a single phase induction motor on the basis of double revolving field theory.
b) Draw a typical torque-speed curve of a single phase induction motor on the basis of double revolving field theory.
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