JNTU
JNTU B.Tech III POWER ELECTRONICS Previous Question Paper (November/December - 2018 - R13)
Note: This question paper contains two parts A and B. Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit. Each question carries 10 marks and may have a, b, c as sub questions.
Part A
25 Marks
Differentiate latching current with holding current of an SCR.
Answer
Latching current is the minimum anode current required to maintain the SCR in the ON state immediately after it has been triggered and the gate signal is removed. In contrast, holding current is the minimum anode current required to keep the SCR in the ON state; if the current falls below this value, the SCR turns OFF.
Typically, latching current is higher than holding current, and both are essential parameters for defining the turn-on and turn-off characteristics of the device.
Define different components of turn on time and turn off times of a thyristor.
Answer
The turn-on time of a thyristor is the interval between the application of the gate signal and the moment the anode current reaches 90% of its final value. It consists of three components:
- Delay time (td): The time taken for the anode current to rise to 10% of its final value after the gate pulse is applied.
- Rise time (tr): The time taken for the anode current to rise from 10% to 90% of its final value.
- Spread time (tp): The time taken for the conduction to spread across the entire cross-section of the semiconductor layers.
The turn-off time is the interval between the anode current reaching zero and the device regaining its forward blocking capability. It consists of two components:
- Reverse recovery time (trr): The time required to remove the excess charge carriers from the outer junctions.
- Gate recovery time (tgr): The time required to remove the excess charge carriers from the inner junction, allowing the device to block forward voltage.
What is the effect of source resistance on the average output voltage of a single phase semi-converter?
Answer
Sign in to view the complete answer paper.
How four quadrant of operation is possible with single phase fully controlled converters?
Answer
Sign in to view the complete answer paper.
What are the advantages and disadvantages of chopper?
Answer
Sign in to view the complete answer paper.
What is the Current limit control of dc-dc converter? Explain briefly.
Answer
Sign in to view the complete answer paper.
List out two difference between TRIAC and thyristor?
Answer
Sign in to view the complete answer paper.
What are the three main limitations of a cycloconverter compared ac voltage controller.
Answer
Sign in to view the complete answer paper.
What is the difference between series inverter and parallel inverter?
Answer
Sign in to view the complete answer paper.
Draw the circuit diagram for parallel inverter.
Answer
Sign in to view the complete answer paper.
Part B
50 Marks
Discuss the switching characteristics of SCR by mentioning its salient features.
Answer
Sign in to view the complete answer paper.
What are "dv/dt" and "di/dt" ratings of SCRs? What happens if these ratings are exceeded?
Answer
Sign in to view the complete answer paper.
Draw and explain the transfer and output characteristics of n-channel enhancement type MOSFET’s.
Answer
Sign in to view the complete answer paper.
Mention the importance of snubber circuit which is connected across SCRs.
Answer
Sign in to view the complete answer paper.
Trace the input and output load voltage waveform of 1- Phase half controlled bridge converter having highly inductive load with firing delay angle of 60 degrees. Justify your comments about the power flow.
Answer
Sign in to view the complete answer paper.
Derive an expression for i) average load voltage ii) average load current iii) RMS load voltage of 1-phase half-controlled converter with inductive load.
Answer
Sign in to view the complete answer paper.
Explain the operation of 3-phase fully controlled Converter with R load for inverter operation.
Answer
Sign in to view the complete answer paper.
The dc voltage from a 1- phase fully controlled bridge converter with RL load is 110 V. The ac source voltage is 220 V rms. The load resistance, R = 0.5 Ω, and load inductance, L is large enough to cause the load current to be essentially constant. i) Determine the delay angle α ii) Estimate the power delivered to the load.
Answer
Sign in to view the complete answer paper.
Explain the operation of step-up chopper with a neat circuit diagram and necessary output waveforms and also derive expression for output voltage.
Answer
Sign in to view the complete answer paper.
The step-down dc chopper has a resistive load, R = 20 Ω and input voltage, VS = 220 V. When the chopper remains on, its voltage drop, Vch = 1.5 V and chopping frequency, f = 10 kHz. If the duty cycle is 80 %, Estimate the: (i) average output voltage (ii) rms output voltage, and (iii) Chopper efficiency.
Answer
Sign in to view the complete answer paper.
Explain the operation of ac chopper with neat circuit diagram and waveforms.
Answer
Sign in to view the complete answer paper.
A chopper circuit is operating on TRC principle at a frequency of 2KHz on 220v dc supply. If the load voltage is 170v, compute the conduction and blocking periods of thyristor chopper in each cycle.
Answer
Sign in to view the complete answer paper.
Discuss the methods of voltage control employed in ac voltage controllers with necessary waveforms.
Answer
Sign in to view the complete answer paper.
A single phase bidirectional controller supplies a resistance load of R = 10 Ω. Determine the output voltage and power consumed by the load for following cases: (i) α = 300 (ii) α = 750 (iii) α = 1200.
Answer
Sign in to view the complete answer paper.
Illustrate the principle of working of a 1-phase to 1-phase bridge type step down cyclo-converter feeding an R load.
Answer
Sign in to view the complete answer paper.
A 1 – phase a.c.regulator feed power to a resistive load of 4 Ω from 230 V, 50 Hz a.c.source. Determine i) the peak value of average and rms thyristor currents for any firing angle α. ii) the minimum circuit turn-off time for any firing angle α.
Answer
Sign in to view the complete answer paper.
A 3-phase bridge inverter is operated in 1800 conduction mode. Derive output line voltage and phase voltage expression.
Answer
Sign in to view the complete answer paper.
A six-step three-phase inverter has an adjustable dc input. The load is a balanced Y connection with a series RL combination in each phase, with R = 5 Ω and L = 50 mH. The output frequency is to be varied between 30 and 60 Hz. Determine the range of the dc input voltage required to maintain the fundamental-frequency component of current at 10 A (rms).
Answer
Sign in to view the complete answer paper.
Describe sinusoidal PWM control of single phase VSI with the help of suitable waveform.
Answer
Sign in to view the complete answer paper.
A square-wave inverter has a dc source of 125 V, an output frequency of 60 Hz, and an RL series load with R = 20 Ω, and L = 25 mH. Determine (i) an expression for load current, (ii) rms load current, and (iii) average source current.
Answer
Sign in to view the complete answer paper.