Power Electronics II - May 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. 1 (a) Explain constant torque and constant power drive methods with relevant diagrams and waveforms.(5 marks) 1 (b) The speed of a 10Hp separately excited D.C motor is controlled by a single phase full converter. The rated armature current is 40A. R_a=0.3 Ω. The A.C supply voltage is 260V. The motor voltage is constant is 0.182 V/rpm. While in motoring action with back emf of 192 V , the polarity of it is reversed for regeneration action calculate: 1. Firing angle to keep the motor current at its rated value. 2. The power fed back to the supply. (5 marks) 1 (c) Introducing motor speed control with constant supply voltage and reduced suppy frequency is rarely used in practice. Justify the stateent.(5 marks) 1 (d) List the merits and demerits of online UPS and offline UPS(5 marks) 2 (a) With neat circuit diagram explain the working of a load commutated chopper with relevant voltage and current waveforms. Show voltage variation across each pair of SCR?s as a function of time.(10 marks) 2 (b) Describe modified Mc-Murray Bedford method of half bridge inverter circuit with related voltage and current waveforms.(10 marks) 3 (a) Explain the neat diagram the working of parallel inverter employing feedback diodes. Draw the voltage and current waveforms. What care should be taken to avoid commutations failure?(10 marks) 3 (b) A 220V, 1500 RPM, 10A separately excited D.C motor has an armature resistance of 0.1 ? 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) firing angel for rated motor torque at 600 rmp.
(ii) firing angel for rated motor torque at (-500) rmp
(iii) motor speed for ? =150_o and half rated torque (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 armature current modes.(10 marks) 4 (b) Draw the circuit diagram and explain the rotor resistance control method of using chopper for the speed control of 3 phase induction motor.(10 marks) 5 (a) Explain with relevant circuit diagrams the static scherbius drives for obtaining speeds below as well as above the synchronous speeds.(10 marks) 5 (b) Explain with diagram the operation of series inverter. State the limitations. How these limitations are overcome?(10 marks) 6 (a) With the help of circuit diagram and waveforms, explain the operation of isolated flyback converter in discontinuous mode. Also state the advantages and disadvantages.(10 marks) 6 (b) Describe the principle of step- up chopper. Derive an expression for the average output voltage. State the assumptions made.(10 marks)

Write short notes on :-

7 (a) Dual converter(7 marks) 7 (b) PWM inverter(7 marks) 7 (c) Step up chopper(7 marks)