JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech III POWER ELECTRONICS Previous Question Paper (July/August - 2023 - R13)
Note: i) Question paper consists of Part A, Part B. ii) Part A is compulsory, which carries 25 marks. In Part A, Answer all questions. iii) In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions.
Part A
How to turn OFF an SCR?
Answer
An SCR is turned OFF by a process called commutation, which involves reducing the anode current below the holding current for a duration longer than the turn-off time of the device. This is achieved by either natural commutation (in AC circuits) or forced commutation (in DC circuits).
What are the features of parallel connection of SCRs?
Answer
Parallel connection of SCRs is used to increase the total current-carrying capacity of the circuit. Key features include the need for current-sharing resistors or reactors to compensate for unequal voltage drops and the requirement for simultaneous triggering to ensure even load distribution.
What is a six pulse converter?
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What is a phase controlled technique?
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What a step up chopper does?
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What is a current limit control?
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What is a cyclo converter?
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What are the applications of AC voltage controllers?
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What is PWM?
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What are the features of parallel inverter?
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Part B
Describe the characteristics of power IGBT with neat curves.
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Discuss in detail about the turn ON methods of SCR.
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Discuss in detail about the two transistor analogy of SCR.
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Describe the physical structure of BJT with neat sketch.
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Explain the working of single phase fully controlled converter with RLE load using neat sketch.
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A 3-phase full converter, fed from 3 phase supply 400 V, 50 Hz source, is connected to load R = 10 Ω, E = 350 V and large inductance so that output current is ripple free. Calculate the power delivered to load and input power factor for a firing angle of 30 degrees.
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Explain the working of three pulse converter with RL load using neat sketch.
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A single-phase full converter bridge is connected to RLE load. The source voltage is 230 V, 50 Hz. The average load current of 5 A is constant over the working range. For R = 0.4 Ω and L = 2 mH, compute the firing angle delay for E = 120 V.
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Derive the expression for the output voltage of step up chopper.
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For a step down chopper circuit, source voltage is V = 220 V, chopping period T = 2000 µsec, on-period = 600 µsec, load circuit parameters: R = 1 Ω, L = 5 mH and E = 24 V. (i) Find whether the load current is continuous or not and (ii) Calculate the value of average output current.
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What is a Jones chopper? Explain its working with neat circuit diagram.
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A step-up chopper has input voltage of 120 V and output voltage 240 V. If the conducting time of thyristor-chopper is 80 µsec. compute the pulse width of output voltage. In case output-voltage pulse width is halved for constant frequency operation, find the average value of new output voltage.
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Describe the working of single phase AC voltage controller with anti-parallel SCRs with R load in detail.
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A single-phase full-wave ac voltage controller feeds a load of R = 15 Ω with an input voltage of 230 V, 50 Hz. Firing angle of thyristor is 45 degrees. Determine (i) input power factor and (ii) average input current.
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Explain the principle of operation of single phase cyclo converter with mid-point configuration with resistive load.
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A single-phase voltage controller feeds power to a resistive load of 10 Ω from 230 V, 50 Hz. Calculate: (i) the maximum values of average and rms thyristor currents for any firing angle alpha and (ii) the minimum circuit turn-off time for any firing angle alpha.
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Explain the operation of series inverter with neat waveforms.
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How to control the output voltage of inverters? Explain.
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With the help of neat waveforms and switching strategy, explain the 180 degree conduction mode of operation of three phase inverter.
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What are the advantages of PWM techniques in inverters? Explain.
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