Power Electronics II - May 2012

Electronics Engineering (Semester 7)

TOTAL MARKS: 80
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Assume data if required.
(4) Figures to the right indicate full marks. 1 (a) Explain regenerative braking for D.C motors.(5 marks) 1 (b) List the merits and demerits of on-line and off-line UPS.(5 marks) 1 (c) Explain how two quadrant type B chopper operates in first and fourth quadrants.(5 marks) 1 (d) Compare voltage source and current source inverters.(5 marks) 2 (a) Draw and explain the operation of single phase semiconverter fed D.C separately excited motor with relevant waveforms and expressions.(10 marks) 2 (b) Explain with neat diagram the operation of rotor resistance control using chopper.(10 marks) 3 (a) Explain with neat diagram the working of parallel inverter. Explain the need of feedback diodes. Draw capacitor, SCR and load voltage waveforms.(10 marks) 3 (b) Describe the principle of step up chopper. Derive an expression for the average output voltage. State the assumptions made.(10 marks) 4 (a) Explain with relevant circuit diagram the static Scherbius drives for obtaining speeds below and above the synchronous speed.(10 marks) 4 (b) Explain in detail the operation of dual converter with circulating current. List the advantages and disadvantages of the same.(10 marks) 5 (a) A 220V, 1500 RPM, 10A separately excited D.C motor has an armature resistance of 0.1 Ω if it is fed from a single phase fully controlled bridge rectifier with an A.C source voltage of 230V, 50 Hz. Assuming continuous load current compute:
i. Motor speed at firing angle of 30_oand torque of 30Nm
ii. Developed torque at the firing angle of 45_o and speed of 1000 RPM.
(10 marks) 5 (b) Describe a single phase McMurray Bedford method full bridge converter with neat diagram and voltage and current waveforms.(10 marks) 6 (a) Draw and explain the working of flyback converter with relevant waveforms.(10 marks) 6 (b) Explain the operation of load commutated chopper with neat circuit diagram and associated current and votage waveforms.(10 marks) 7 (a) V/f control for induction motor.(7 marks) 7 (b) Effect of source inductance on performance of converter.(7 marks) 7 (c) Harmonic reduction in motors.(7 marks)