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Posted Date: 22 Jun 2009 Posted By: sheshank Member Level: Gold
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2006 Jawaharlal Nehru Technological University Electrical and Electronics Engineering POWER SEMICONDUCTOR DRIVES IV-B.Tech. I-Semester Supplementary Examinations, May 2006 Question paper
Code No. 410202
IV-B.Tech I-Semester Supplementary Examinations, May 2006
POWER SEMICONDUCTOR DRIVES
(Electrical and Electronics Engineering)
Time: 3 hours Max. Marks: 80
Answer any FIVE questions
All questions carry equal marks
---
1. Derive an expression for the circulating current of a 3 ? dual converter. What is the purpose of circulating current inductor?
2. Explain how the speed control of a separately excited dc motor is achieved with thyristor control. Draw a neat circuit diagram giving the triggering methods used.
3. a) Describe relative merits and demerits of four-quadrant dc drive employing non-circulating and circulating current dual converters.
b) A 230v, 870rpm, 100A separately excited dc motors has an armature resistance of 0.05?. It is coupled to an overhauling load with a torque of 400 N-m. Determine the speed at which the motor can hold the load by regenerative braking.
4. a) Draw and explain the Torque-Speed characteristics for dynamic braking operation of dc series motor. Why torque becomes zero at finite speed?
b) A 220v, 24A, 1000rpm separately excited dc motor having an armature resistance of 2? is controlled by a chopper. The chopping frequency is 500Hz and the input voltage is 230v. Calculate the duty ratio for a motor torque of 1.2 times rated torque at 500rpm.
5. a) For stator voltage control scheme of a 3 phase Induction motor discuss about speed range, regeneration, harmonics, torque pulsating, power factor, cost, efficiency and applications.
b) Draw a block schematic diagram for automatic speed control of 3 phase cage Induction motor using solid state AC Voltage Controller on stator side.
6. Discuss in detail the role of Cycloconverters for speed control of Induction motor. Draw neat circuit diagram for speed control for 3 phase Induction motor using Cyclo converters. Mention the merits and limitations of the above scheme.
7. A fan with a load characteristic TL = kN2 is to be driven by an SER drive incorporating a 440V, 50Hz, 100kW induction motor. It is required to deliver rated speed of 1440 rpm and to provide smooth speed control down to 750 rpm. The motor equivalent circuit parameters referred to primary turns are R1 = 0.052?, R2'= 0.06?, Xm = 10?, X1 + X2' = 0.29?, R0 = 100?. Stator to rotor turns ratio: 1:2 Calculate the motor efficiency and power factor at 750rpm. Friction and windage effects may be neglected. It is assumed that the motor is started from rest and run upto 750 rpm by the secondary resistance method.
8. Discuss various methods of speed control of a synchronous motor in detail.
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