Power Electronics II - Dec 2013
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.
Attempt the following :-
1 (a) Why voltage control is needed in inverter circuit? State the various methods of voltage control in inverter circuits(5 marks)
1 (b) In a single phase full converter the reduction in the output voltage due to the effect of source inductance is 3 Volts. The load current is continuous and ripple free and equal to 100 Amp. Find the value of source inductance and overlap angle if supply voltage is 230 V, 50Hz and ?= ?/6(5 marks)
1 (c) Explain the 'Pluggeing' for D.C. motors.(5 marks)
1 (d) A step up chopper has output voltage of 2 to 4 times the input voltage. For a chopping frequency of 2000 Hz, determine the range of off periods for the gate signal(5 marks)
2 (a) Explain the working of two quadrant type B chopper with the help of neat circuit diagram and waveforms(10 marks)
2 (b) Explain volts/hertz control for a 3 phase induction motor for its speed control. Emumerate its advantages(10 marks)
3 (a) Explain the operation of Dual Converter with neat circuit diagram and waveforms, and give the comparison between non circulating and curculating current mode(10 marks)
3 (b) With the help of neat circuit diagram and waveforms, explain the operation of single phase capacitor commutated current source inverter with R load(10 marks)
4 (a) Draw and explain the power circuit of semiconverter feeding a separately excited D.C motor. Explain with typical voltage and current waveforms the operation in both continuous and discontinuous current modes.(10 marks)
4 (b) Derive the expression for commutating components L and C for a voltage commutated chopper. Also discuss the assumptions made for designing the components(10 marks)
5 (a) Explain the operation of basic series inverter and draw the waveforms for
(i) Voltage across the capacitor
(ii) Voltage across the inductor
(iii) Voltage across the load resistance
How will tou modify it so that the supply current flows in both thehalf cycles of output voltage?(10 marks) 5 (b) Explain with neat diagram and waveforms, the operation of half bridge switching regulator(10 marks) 6 (a) Describe static rotor resistance control method for the speed control of a 3 phase induction motor. Show the effect on developed torque and stator current.(10 marks) 6 (b) A separately excited D.C. moto is supplied from 203 V, 50Hz source through a single phase half wave controlled converter. Its fild is fed through single phase semiconverter with zero degree firing angle delay. Motor resistance Ra=0.7 Ω and Motor constant=0.5 V-sec/ rad. For rated load torque of 15 Nm at 1000 rpm and for constant ripple free current, determine
(i) Firing angle delay of the armature, determine
(ii) RMS vale of thyristor current
(iii) Input power factor of the armature converter.(10 marks)
Write short notes on :-
7 (a) Offline UPS system(7 marks) 7 (b) Need for reduction of Harmonics in inverters(7 marks) 7 (c) Slip power recovery scheme(7 marks)