MORE IN Power Electronics II
MU Electronics Engineering (Semester 7)
Power Electronics II
May 2013
Total marks: --
Total time: --
INSTRUCTIONS
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary

1 (a) Explain constant torque and constant power drive methods with relevant diagrams and waveforms.
5 M
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 â„¦. 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 M
1 (c) Introducing motor speed control with constant supply voltage and reduced suppy frequency is rarely used in practice. Justify the stateent.
5 M
1 (d) List the merits and demerits of online UPS and offline UPS
5 M

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 M
2 (b) Describe modified Mc-Murray Bedford method of half bridge inverter circuit with related voltage and current waveforms.
10 M

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 M
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 ? =150o and half rated torque
10 M

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 M
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 M

5 (a) Explain with relevant circuit diagrams the static scherbius drives for obtaining speeds below as well as above the synchronous speeds.
10 M
5 (b) Explain with diagram the operation of series inverter. State the limitations. How these limitations are overcome?
10 M

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 M
6 (b) Describe the principle of step- up chopper. Derive an expression for the average output voltage. State the assumptions made.
10 M

Write short notes on :-
7 (a) Dual converter
7 M
7 (b) PWM inverter
7 M
7 (c) Step up chopper
7 M

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