JNTU
JNTU B.Tech III POWER ELECTRONICS Previous Question Paper (July - 2023 - R16)
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 is the two-transistor model of SCRs?
Answer
The two-transistor model represents an SCR as an interconnection of one PNP transistor (Q1) and one NPN transistor (Q2). In this configuration, the collector of Q1 is connected to the base of Q2, and the collector of Q2 is connected to the base of Q1. This regenerative feedback structure explains the latching action of the device: when a gate current is applied, it triggers Q2, which in turn drives Q1, creating a self-sustaining conduction state even after the gate signal is removed.
What is forced commutation? What are the advantages of forced commutation for the ac-dc converters?
Answer
Forced commutation is the process of turning off a thyristor (SCR) by externally applying a reverse voltage across its terminals to reduce the anode current below its holding current level. Unlike natural commutation, which occurs in AC circuits when the supply voltage naturally reverses, forced commutation is required in DC circuits where the current does not naturally cross zero.
The advantages of forced commutation in AC-DC converters include:
- It enables the control of power flow in circuits where the supply is inherently DC or where natural zero-crossing is not available.
- It allows for the implementation of advanced control techniques, such as Pulse Width Modulation (PWM), to improve the harmonic profile of the input current and the quality of the output voltage.
- It provides the ability to operate at higher switching frequencies, which reduces the size and cost of passive filtering components required for the converter.
What are line commutated converters?
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Does the input power factor of converters depend on the load power factor?
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What are the advantages of cyclo converter?
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What is the discontinuous mode of operation of A. C. voltage regulator?
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What is an AC chopper?
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What is step down chopper? Draw its diagram.
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How can a PWM control signal be obtained using a carrier wave and reference wave with the help of diagram?
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Write are the applications of parallel inverter?
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PART – B
Explain in detail about the series connection of SCRs.
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Following are the specifications of a Thyristor operating from a peak supply of 400V Repetitive peak current Ip= 200A, (di/dt)max= 50 A/µs, (dv/dt)max=180 V/µs. Desing Snubber circuit if the minimum load resistance is 15 ohms. Take damping ratio as 0.7 and the time factor of safety as 2.
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What are the main specifications and ratings of SCR to be considered while designing? Explain.
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A thyristor is triggered by a train of pulses of frequency 1 KHZ and duty cycle of 0.4. Calculate the pulse width if average power dissipation is 5 W. Estimate the maximum allowable gate power loss.
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Describe with neat circuit diagram and associated waveforms, operation of a single phase half wave-controlled converter with Inductive load.
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A single-phase full bridge converter using four SCRs feeds power to RLE load with R=10 ohms, L=100mH, and E=100V. The ac source voltage is 230V at 50Hz. Assuming continuous conduction. Determine the average value of load current for firing delay angle 45 degrees.
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Draw the output voltage waveform of 3-phase fully controlled rectifier for a firing angle of 60 degrees. Indicate firing sequence. Also derive expression for output voltage.
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A 3 phase fully controlled bridge rectifier is operating from a 400V, 50 Hz supply. The load is highly inductive and current constant and continuous. Find the load voltage at firing angle of 45 deg.
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A single phase full wave ac voltage controller has a resistive load of R=10Ω and the input Voltage is Vs=120V (rms), 60Hz. The delay angle of thyristors T1andT2 are equal to 90 Degrees. Determine the rms output voltage Vo, the input PF, the average current of thyristors IA, and the rms current of thyristors.
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Draw the circuit of single-phase voltage controller with antiparallel connection of two thyristors and an R-L load. Explain its working. Sketch load voltage and load current waveforms. Derive an expression for output voltage.
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Explain in detail about the time ratio control used in choppers.
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Explain the working of Jones chopper.
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Derive expressions for minimum and maximum values of load current in a step down chopper with R-L-E load.
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A step-up chopper has a source of 250 V (dc) in series with inductance of 0.1 H. If the semiconductor switch is operated with different values of duty ratio, plot output voltage vs duty ratio.
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Draw the circuit diagram and explain the working of series inverter.
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With the help of neat circuit diagram and associated waveforms, explain operation of single phase full bridge voltage source inverter with RL load.
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Explain sinusoidal pulse modulation as used in PWM inverters. Write the important features of the same.
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Explain the principle of operation of parallel inverter.
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