JNTU
JNTU B.Tech III POWER ELECTRONICS Previous Question Paper (August/September - 2024 - 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
What a BJT does?
Answer
A Bipolar Junction Transistor (BJT) is a semiconductor device that acts as an electronic switch or amplifier. It controls a large collector current by a small base current, allowing it to amplify signals or turn circuits on and off.
What is holding current? What is its importance?
Answer
Holding current (IH) is the minimum current that must flow through a thyristor (like an SCR or TRIAC) to maintain its conducting state after it has been triggered on. Its importance lies in ensuring reliable turn-on of the device; if the anode current drops below IH, the thyristor will turn off, even if the gate signal is still present.
What is source inductance?
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What are the advantages of freewheeling diode?
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What is a chopper?
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Enlist the limitations of Jones chopper.
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What is a Triac?
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List out the applications of cyclo converters.
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What is the operation of inverter?
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What are the advantages of PWM techniques?
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Part B
During the turn-off process in a thyristor, the reverse recovery current of 10 A is interrupted in a time interval of 4 µs. The thyristor is connected in series with an inductance of 6 mH with no resistance in the circuit. If the source voltage during turn-off process is -300 V, calculate (a) peak voltage across the thyristor when reverse current is interrupted and (b) the value of snubber circuit resistance in case snubber capacitance Cs = 0.3 µF and damping ratio is 0.65.
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A thyristor string is made up of a number of SCRs connected in series and parallel. The string has voltage and current ratings of 11 kV and 4 kA respectively. The voltage and current ratings of available SCRs are 1800 V and 1000 A respectively. For a string efficiency of 90%, calculate the number of series and parallel connected SCRs. For these SCRs, maximum off-state blocking current is 12 mA. Determine the values of static equalizing resistance for the string.
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Explain principle operation of 1-ϕ bridge converter with neat diagram.
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What is necessity of dual converter?
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A 3-phase full converter is delivering a constant load current of 50 A at 230 V DC when its input is 3-phase, 415 V, 50 Hz. If each thyristor has a voltage drop of 1.1 V when conducting. Calculate (a) the firing angle delay of SCR (b) the rms current of SCRs (c) rms source current (d) the mean power loss in each SCR and (e) Input power factor.
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For step down chopper circuit, source voltage Vs = 220 V, chopping period T = 2000 µs, on-period = 600 µs, load circuit parameters: R = 1 Ω, L = 5 mH and E = 24 V. (a) Find whether load current is continuous or not (b) calculate the value of average output current (c) Compute the maximum and minimum values of steady state output current (d) find rms value of load current (e) compute the average value of supply current.
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A step down chopper operating at 2 kHz from a 100 V DC source has a load time constants of 6 ms and load resistance of 10 Ω. Find the mean load current and the magnitude of current ripple for a mean load voltage of 50. Also calculate the minimum and maximum values of load current.
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With the help of neat circuit, explain the operation of single phase midpoint step down cyclo converter for R load.
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Explain the working of single full wave AC voltage controller with neat circuit and waveforms.
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With the help of neat circuit, explain the operation of single phase bridge type step up cyclo converter for RL load.
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Derive the expression for the RMS load voltage in a Triac for R load.
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Explain the voltage control technique of inverters.
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A three phase 180o mode bridge inverter has star connected load of R = 4Ω and L = 20 mH. The inverter is fed from 220 V DC and its output frequency is 50 Hz. Calculate (a) the RMS values of line and phase voltages (b) Load power (c) Average source current (d) Avergae value of thyristor current.
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