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Saturday, 18 April 2015

Power electronics-Model question paper for Diploma/B.E/B.Tech



1(a) Explain how practical switches deviate from ideal switches in their characteristics. (4 Marks)
(b) With a neat circuit diagram and waveforms, explain the principle of working of a step-down chopper connected to a resistive load. (8 Marks)
(c) Explain peripheral effects caused by power electronic converters. (4 Marks)
(d) The reverse recovery time of a diode is 5μs and the rate of fall of diode current is 80 A/μs. If the softness factor is 0.5, determine storage charge and peak reverse current. (4 Marks)
2(a) Explain the need for isolating the control terminals of a semiconductor switch from the triggering source and how to do this. (8 Marks)
(b) Give reasons why power BJTs have been replaced by power MOSFETs and IGBTs in modern power electronic applications. (8 Marks)
(c) Explain the method of calculation of average power loss in a semiconductor switch. (4 Marks)
3(a) With a neat sketch, explain the two transistor model of an SCR. Obtain the condition for turn ON and turn OFF the device. (8 Marks)
(b) Describe thermal model of a thyristor. (4 Marks)
(c) Distinguish between latching current and holding current of an SCR. (4 Marks)
(d) A 450 A thyristor is operated in parallel with a 550 A thyristor. The ON-stage voltage drops of the thristors at these rated currents are 1.8 V and 1.2 V respectively. Estimate the resistance to be connected in series with each thyristor in order that the y share a total load current of 1000 A in proportion to their current ratings. (4 Marks)
4(a) With necessary circuit diagrams and waveforms explain complementary communication. (8 Marks)
(b) What is commutation? What are the methods of commutation? Explain auxiliary commutation. (8 Marks)
(c) State the conditions to be satisfied for proper turn OFF of an SCR. (4 Marks)
5(a) With necessary circuit diagram and waveforms describe the operation of a single-phase half-wave ac controller. Derive an expression for the RMS value of output voltage and current. (8 Marks)
(b) Explain with relevant circuit diagrams and waveforms the working of a single-phase phase control type AC voltage controller connected to R-L load and obtain a relationship between the rms output voltage and the rms input voltage. (8 Marks)
(c) The input voltage to a single-phase half wave ac voltage controller is 230 V, 50 Hz. If it delivers an output power of 3 kW to a resistor load of 15 Ohm, find the delay angle of the thyristor and input power factor. (4 Marks)
6(a) Explain the working of single-phase half-wave controlled rectifier connected to resistive load and derive a relationship between the average output voltage and input voltage. (8 Marks)
(b) Explain the working of a three-phase full converter connected to a highly inductive load with ripple-free load current. (8 Marks)
(c) A three-phase full converter is operated from a three-phase 50 Hz ac supply and the load current is constant under steady operating conditions. Determine the firing angle for the SCRs to obtain an output average dc voltage of 50% of the maximum. If the output voltage is 270 V, calculate the ac supply line-to-line rms voltage. (4 Marks)
7(a) What is a chopper. Explain different control strategies for choppers. (4 Marks)
(b) With power circuit diagram and waveforms of voltages and currents explain the working of impulse commutated chopper. State clearly all the simplifying assumptions made in the analysis. (8 Marks)
(c) A step-up dc chopper has an input voltage of 200 V and an output voltage of 250 V. The blocking period in each cycle is 0.6 ms. Find the period of conduction in each cycle. (4 Marks)
(d) A thyristor chopper is supplying an inductive load with R= 5 Ohm and L= 500 mH. The dc supply to the chopper is 250 V. If the chopper is operating at a frequency of 500 Hz and ON/OFF time ratio of the chopper is 1:2, calculate average load current and maximum/minimum values of load current in one cycle of chopper operation under steady state conditions. (4 Marks)
8(a) Explain the modified sinusoidal-pulse width modulation technique of varying the magnitude of output voltage in a single-phase inverter. (6 Marks)
(b) Explain the phase displacement technique of varying the magnitude of output voltage in a single-phase inverter. (6 Marks)
(c) Define the performance parameters of inverters. (4 Marks)
(d) A single-phase full bridge inverter employs single-pulse width modulation technique to control the output voltage. What should be the pulse width for the rms value of the fundamental component of the output voltage to be 60% of the dc input voltage? (4 Marks)

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