Power Electronics II - May 2014
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) Compare symmetrical and asymmetrical and semiconverter(5 marks) 1 (b) What is meant by voltage and current commutation?(5 marks) 1 (c) Compare series and parallel inverter.(5 marks) 1 (d) What is four quadrant D.C drive?(5 marks) 2 (a) Explain sinusoidal pulse width modulation as used in PWM inverter.(10 marks) 2 (b) A load commutated chopper fed from 230V D.C source has a constant current of 50A. For a duty cycle of 0.4 and chopping frequency of 2 kHZ calculate:
1. Value of commutating capacitor.
2. Average output voltage.
3. Circuit turn off time for 1 ? pair of SCR?s
4. Total commutation interval
(10 marks) 3 (a) Explain the working of single phase series inverter with appropriate circuit diagrams and waveforms.(10 marks) 3 (b) Explain constant torque and constant power generation of a separately excited D.C motor. Give the schematic diagram of control unit.(10 marks) 4 (a) A single phase fully controlled bridge with 230V, 50 Hz supply feeds continuous ripple free current of 20A. if La=2.5mH calculate:
i. Calculate overlap angle for the firing angle 60o.
ii. What will be the new value of the overlap angle for same firing angle if load is reduced by 50%.(10 marks) 4 (b) Draw and explain the variable voltage and variable frequency control method for 3 ? phase squirrel cage induction motor. What is the significance of v/f control.(10 marks) 5 (a) A single phase full converter is used to drive separately excited D.C shunt motor with Ra=0.25&Omega. Motor rating are 220V, 750 RPM, 50A. If input voltage to converter connected to armature is 250V ac 50 Hz. Calculate:
i. Firing angle delay for 500 RPM rated torque.
Speed of motor for firing angle ?=600 at half load.
(10 marks) 5 (b) Explain the working of Jone?s chopper with the help of waveforms.(10 marks) 6 (a) Give the advantages of flyback converter compared to forward converter. With the help of neat circuit diagram and waveform, explain the operation of isolated forward converter.(10 marks) 6 (b) Using block diagram discuss different configurations of UPS.(10 marks)
Write Short notes on:
7 (a) Dual converter.(10 marks) 7 (b) Mc'Murray Inverter.(10 marks) 7 (c) Four quadrant chopper.(10 marks)