2301 Power Electronics Qb 2014 Final

8/10/2019 2301 Power Electronics Qb 2014 Final

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Subject Code / Name: EE2301 POWER ELECTRONICS

1 KCE / EEE / III YR / PE

UNIT-I

POWER SEMI-CONDUCTOR DEVICES

PART - A

1. List the advantages of the IGBT. (April - May 2010)a. Lower hate requirementsb. Lower switching lossesc. Smaller snubber circuit requirements

2. What are the different methods to turn on the thyristor? (April May 2011)a. Forward voltage triggeringb. Gate triggeringc. dv/dt triggeringd. Temperature triggeringe. Light triggering

3. IGBT is a voltage controlled device. Why? (April May 2012)Because the controlling parameter is gate-emitter voltage.

4. Define latching current. (Nov- Dec 2010)The latching current is defined as the minimum value of anode current which it must attain duringturn on process to maintain conduction when gate signal is removed.

5. Define holding current. (Nov- Dec 2009 & 2012) (April - May 2010)The holding current is defined as the minimum value of anode current below which it must fall tofor turning off the thyristor.

6. What is a snubber circuit? (April - May 2009 &2012) (Nov- Dec 2008,2010&2013)It consists of a series combination of a resistor and a capacitor in parallel with the thyristors. It is

mainly used for dv / dt protection.

7. Define circuit turn off time. (Nov- Dec 2011 & 2012)It is defined as the time during which a reverse voltage is applied across the thyristor during itscommutation process.

8. What are the factors that influence the turn-off time of a thyristor? (Nov- Dec 2010)Circuit turn off time should be greater than the thyristor turn-off time for reliable turn-off,otherwise the device may turn-on at an undesired instant, a process called commutation failure.

9. State the advantages of IGBT over MOSFET. (Nov-Dec 2008)

a. Smaller snubber circuit requirementsb. Lower switching lossesc. Lower hate requirements


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10. What are the applications of TRAIC? (Nov-Dec 2012)a. Fan regulators.b. Induction Heating.c. AC Voltage regulators.

11. What is the difference between power diode and signal diode?Power diode Signal diode1.Constructed with n-layer, called drift region between p+ layer and n+ layer. Drift region is notpresent.2.The voltage, current and power ratings are higher. Lower3.Power diodes operate at high speeds. Operates at higher switching speed.

12. Power MOSFET is a voltage controlled device. Why?Because the output (drain) current can be controlled by gate-source voltage.

13. Power BJT is a current controlled device. Why?Because the output (collector) current can be controlled by base current.

14. What are the different types of power MOSFET?a. N-channel MOSFETb. P-channel MOSFET

15. How can a thyristor turned off?A thyristor can be turned off by making the current flowing through it to zero.

16. What losses occur in a thyristor during working conditions?a. Forward conduction lossesb. Loss due to leakage current during forward and reverse blocking.c. Switching losses at turn-on and turn-off.d. Gate triggering loss.

17. Define hard-driving or over-driving.When gate current is several times higher than the minimum gate current required, a thyristor issaid to be hard-fired or over-driven. Hard-firing of a thyristor reduces its turn-on time andenhances its di/dt capability.

18. What is the turn-off time for converter grade SCRs and inverter grade SCRs?Turn-off time for converter grade SCRs is 50 100 ms turn-off time for converter grade SCRs andinverter grade SCRs and for inverter grade SCRs is 3 50 ms.

19. What are the advantages of GTO over SCR?a. Elimination of commutation of commutating components in forced commutation, resulting inreduction in cost, weight and volume.b. Reduction in acoustic noise and electromagnetic noise due to elimination of commutationchokes.c. Faster turn-off, permitting high switching frequencies.d. Improved efficiency of the converters.

PART- B

1. Explain the structure, different modes of operation and characteristics of Triac. (16)

(Nov-Dec 2008)

2. Describe about any one driver circuit and snubber circuit for MOSFET. (16)(Nov-Dec 2008)


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4. What is commutation angle or overlap angle? (April-May 2011)The commutation period when outgoing and incoming thyristors are conducting is known asoverlap period. The angular period, when both devices share conduction is known as thecommutation angle or overlap angle.

5. What are the advantages of six-pulse converter over the 2pule convertor?(NovDec 2012)

a. Commutation is made simple.b. Distortion on the ac side is reduced due to the reduction in lower order harmonics.c. Inductance reduced in series is considerably reduced.

6. What are the effects of freewheeling diode on the performance of a converter?(April-May 2011)

The load current is transferred from the main thyristors to the freewheeling diode, therebyallowing all of its thyristors to regain their blocking states. It prevents the output voltage frombecoming negative.

7. Define voltage ripple factor. (Nov-Dec 2010)

It is defined as the ratio of the net harmonic content of the output voltage to the average outputvoltage.

8. What is meant by inversion mode of rectifier? (April-May 2009) (Nov-Dec 2012)In single phase full converter > 90 the voltage at the dc terminal is negative. Therefore, powerflows from load to source & the converter operates as line commutated inverter. Source voltage Vsis negative & Current is positive. This is known as inversion mode or synchronous mode.

9. What is meant by line commutated converter? (April-May 2011)Here the current flowing through the thyristor goes through a natural zero and enable thethyristor to turn off.

10. What is meant by forced commutation? (Nov-Dec 2010)In this commutation, the current flowing through the thyristor is forced to become zero byexternal circuitry.

11. What is meant by phase controlled rectifier?It converts fixed ac voltage into variable dc voltage.

12. Mention some of the applications of controlled rectifier.a. Steel rolling mills, printing press, textile mills and paper mills employing dc motor drives.

b. DC tractionc. Electro chemical and electro-metallurgical processd. Portable hand tool drivese. Magnet power suppliesf. HVDC transmission system

13. What are the advantages of freewheeling diodes in a controlled in a controlled rectifier?a. Input power factor is improved.b. Load current waveform is improved and thus the load performance is better.

14. What is meant by delay angle?

The delay angle is defined as the angle between the zero crossing of the input voltage and theinstant the thyristors is fired.


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15. What are the advantages of single phase bridge converter over single phase midpointconverter?a. SCRs are subjected to a peak-inverse voltage of 2Vm in a fully controlled bridge rectifier. Hencefor same voltage and current ratings of SCRs, power handled by mid-point configuration is about b.In mid-point converter, each secondary winding should be able to supply the load power. As such,the transformer rating in mid-point converter is double the load rating.

16. What are the different methods of firing circuits for line commutated converter?a. UJT firing circuit.b. The cosine wave crossing pulse timing control.c. Digital firing schemes.

17. What is meant by input power factor in controlled rectifier?The input power factor is defined as the ratio of the total mean input power to the total RMS inputvolt-amperes.PF = ( V1 I1 cos 1 ) / ( VrmsIrms) where V1 = phase voltage, I1 = Fundamental component of thesupply current, 1 = input displacement angle, Irms = supply rms current.

18. What is meant by commutation?It is the process of changing the direction of current flow in a particular path of the circuit. Thisprocess is used in thyristors for turning it off.

19. What are the types of commutation?a. Natural commutationb. Forced commutation

PART-B

1. (i) Describe the operation of a single phase two pulse bridge converter using 4 SCR'S withrelevant waveforms. (10) (Nov-Dec 2010)(ii) Discuss the working of above converter in the converter mode with RLE load. (6) (Nov-Dec 2010)

2. (i) A single phase semi converter is operated from 120 V 50 Hz ac supply. The loadcurrent with an average value Idc is continuous and ripple free firing angle = /6. Determine.(a) Displacement factor.(b) Harmonic factor of input current.(c) Input power factor. (10) (Nov-Dec 2010)

(ii) Write a note on battery charger. (6) (Nov-Dec 2010)

3. With necessary circuit waveforms, Explain the principle of operation of 6-pulse converter (fullycontrolled). Derive the expression for average output voltage in it. (16) (Nov-Dec 2008)

4. Explain the effect of source inductance in the operation of single phase fully controlledconverter. (16) (Nov-Dec 2008)

5. Explain the operation of three phase full converter. Also derive the expression for its averageoutput voltage. (16) (April-May 2009)

6. Explain the working of a three phase full converter with R load for the firing angles of 60 o, 90oand 150o. (16) (Nov-Dec 2011)

7. (i) Explain the operation of a single phase full bridge converter with RL load for continuous anddiscontinuous load currents. (10) (Nov-Dec 2011)


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(ii) A single phase full bridge converter is connected to R load. The source voltage is 230 V, 50 Hz.The average load current is 10A. For R=20 find the firing angle. (6) (Nov-Dec 2011)

8. Explain with necessary circuit diagrams, waveforms and working of a 3 phase fully controlledconverter. Derive the expressions for load voltage and load current. (16) (May-June 2012)

9. Explain the operation of three phase semi converter with neat waveforms. (16) (Nov-Dec 2012)

10. A 220 V, 1KW resistive load is supplied by 220 V, 50 Hz source through single phase fullycontrolled rectifier. Determine the following for 800W output,(a) Output voltage(b) RMS value of input current(c) Fundamental component of input current(d) Displacement factor. (16) (Nov-Dec 2012)

UNIT-III

DC TO DC CONVERTER

PART - A1. What is meant by dc chopper? (Nov-Dec 2013)A dc chopper is a high speed static switch used to obtain variable dc voltage from a constant dcvoltage.

2. What is meant by step-up chopper? (Nov-Dec 2010)In a step- up chopper or Boost converter, the average output voltage is more than the inputvoltage.

3. What is meant by line commutation? (Nov-Dec 2011)Here the current flowing through the thyristor goes through a natural zero and enable thethyristor to turn off.

4. What is meant by step-down chopper? (April-May 2011)In a step- down chopper or Buck converter, the average output voltage is less than the inputvoltage.

5. What is meant by duty-cycle? (April-May 2012)Duty cycle is defined as the ratio of the on time of the chopper to the total time period of thechopper. It is denoted by .

6. What is meant by Time Radio Control? (Nov-Dec 2008&2013)(April-May 2010&2011)In TRC, the value of Ton / T is varied in order to change the average output voltage.

7. What are the advantages of current commutated chopper? (April-May 2012)a. The capacitor always remains charged with the correct polarity.b. Commutation is reliable as load current is less than the peak commutation current ICP.c. The auxiliary thyristor TA is naturally commutated as its current passes through zero value.

8. What is the constant frequency control of chopper? (Nov-Dec 2012)Frequency of the chopper remains constant, but ON period is changed to vary the output.

9. What is meant by SMPS? (April-May 2009)SMPS means Switch Mode Power Supply. SMPS is based on the chopper principle. Varying the dutycycle of chopper by PWM techniques controls the output dc voltage.


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10. What are the advantages of SMPS over the phase controlled rectifier? (Nov-Dec 2011)a. For the same power rating,b. SMPS is of smaller size,c. Lighter in weight and processes,d. Higher efficiency,e. High frequency operationf. Less sensitive to input voltage variations.

11. What are the applications of dc chopper?a. Battery operated vehiclesb. Traction motor control in electric tractionc. Trolly carsd. Marine hoistse. Mine haulersf. Electric braking.

12. What are the advantages of dc chopper?Chopper provides

a. High efficiencyb. Smooth accelerationc. Fast dynamic responsed. Regeneration

13. Write down the expression for average output voltage for step down chopper.Average output voltage for step down chopper V0 = Vs, is the duty cycle.

14. What are the two types of control strategies?a. Time Ratio Control (TRC)b. Current Limit Control method (CLC)

15. What are the two types of TRC?a. Constant frequency controlb. Variable frequency control

16. What is meant by FM control in a dc chopper?In frequency modulation control, the chopping frequency f (or the chopping period T) is varied.Here two controls are possible.a. On-time Ton is kept constantb. Off period Toff is kept constant.

17. What is meant by PWM control in dc chopper?In this control method, the on time Ton is varied but chopping frequency is kept constant. Thewidth of the pulse is varied and hence this type of control is known as Pulse Width Modulation(PWM).

18. What are the different types of chopper with respect to commutation process?a. Voltage commutated chopper.b. Current commutated chopper.c. Load commutated chopper.

19. What is meant by voltage commutation?In this process, a charged capacitor momentarily reverse biases the conducting thyristor and turnit off.


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20. What are the advantages of load commutated chopper?a. Commutating inductor is not required.b. It is capable of commutating any amount of load current.c. It can work at high frequencies in the order of kHz.d. Filtering requirements are minimal.

21. What are the disadvantages of load commutated chopper?a. For high power applications, efficiency becomes very low because of high switching losses athigh operating frequencies.b. Freewheeling diode is subjected to twice the supply voltage.c. Peak load voltage is equal to twice the supply voltage.d. The commutating capacitor has to carry full load current at a frequency of half choppingfrequency.e. One thyristor pair should be turned-on only when the other pair is commutated. This can berealized by sensing the capacitor current that is alternating.

PART B1. (i) Discuss the principle of operation of DC-DC step down chopper with suitable waveform.

Derive an expression for its average DC output voltage. (8) (Nov-Dec 2010)

(ii) A step-down dc chopper has a resistive load of R = 15 and input voltage Edc = 200 V. Whenthe chopper remains ON, its voltage drop is 2.5 for a duty cycle of 0.5.Calculate :(1) Average and r.m.s value of output voltage(2) Power delivered to the load. (8) (Nov-Dec 2010)

2. Draw the circuit of CUK regulator and explain its working principle with necessary waveform indetail. (16) (Nov-Dec 2010)

3. With necessary circuit and waveforms, explain the principal of operation of step down dc to dcconverter with RL load. (16) (Nov-Dec 2008)

4. Describe the working principle of a boost converter with necessary circuit andwaveforms. (16) (May-June 2009) (Nov-Dec 2008)

5. (i) A dc chopper has an input voltage of 200V and a load of 15 ohm resistance. When chopper ison, its voltge drop is 1.5 V and the chopping frequency is 10 KHz. If the duty cycle is 80%, find(a) Average output voltage.(b) RMS output voltage.(c) Chopper on time. (12)(May-June 2009)(ii) Prove the output voltage of step down chopper is Vo = DVs. (4) (May-June 2009)

6. Explain the working of Buck-Boost converter with sketch and waveforms and also drive theexpression for Is. (16) (Nov-Dec 2011)

7. With a neat sketch and output voltage waveforms, explain the working of full bridge SMPS.(16) (Nov-Dec 2011)

8. What is DC chopper? Describe various types of chopper configuration with appropriatediagrams. (16) (May-June 2012)

9. Classify the basic topologies of switching regulators and explain the operation of buck regulator

with continuous load current using suitable waveforms. (16) (May-June 2012)

10. Discuss the operation of DC-DC boost converter and prove that its output voltage is alwaysgreater than input voltage. (16) (Nov-Dec 2012)


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UNIT-IV

INVERTERS

PART - A

1. List the applications of an inverter? (April-May 2011)a. Adjustable speed drivesb. Induction heatingc. Stand-by aircraft power suppliesd. UPSe. HVDC transmission

2. List the advantages of Current Source Inverter. (April May 2010)a. CSI does not require any feedback diodes.b. Commutation circuit is simple as it involves only thyristors.

3. Why the series inverter is called so? (Nov-Dec 2011)An inverter in which the commutating elements are connected in series with the load is called aseries inverter.

4. What is meant by PWM control? (April-May 2011)In this method, a fixed dc input voltage is given to the inverter and a controlled ac output voltage isobtained by adjusting the on and off periods of the inverter components. This is the most popularmethod of controlling the output voltage and this method is termed as PWM control.

5. What are the advantages of PWM control? (Nov-Dec 2010, 2012 &2013)a. The output voltage can be obtained without any additional components.b. Lower order harmonics can be eliminated or minimized along with its output voltage control. Asthe higher order harmonics can be filtered easily, the filtering requirements are minimized.

6. What are the methods of reduction of harmonic content? (April-May 2012)a. Transformer connectionsb. Sinusoidal PWMc. Multiple commutation in each cycled. Stepped wave inverters

7. Write the difference between CSI and VSI. (Nov-Dec 2010) & (April-May 2012)

S.No. VSI CSI

1.

Input voltage is maintained constant Input current is constant but adjustable

2.The output voltage does not dependon the load

The output current does not depend on theload

3.The magnitude of the outputcurrent and its waveform dependson the nature of the load impedance

The magnitude of the output voltage and itswaveform depends on the nature of theload impedance

4. It requires feedback diodes It does not requires feedback diodes

5. Commutation circuit is complicatedi.e. it contains capacitors andinductors.

Commutation circuit is simple i.e. itcontains only capacitors.


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8. What is mean by CSI? (Nov-Dec 2008, 2011 & 2012) & (April-May 2009)A current fed inverter or CSI is fed with adjustable current from a dc source of high impedance isfrom a stiff dc current source.

9. What is meant by PWM technique? (April-May2010)In this control method, the on time Ton is varied but chopping frequency is kept constant. Thewidth of the pulse is varied and hence this type of control is known as Pulse Width Modulation(PWM).

10. What is meant by forced commutation? (Nov-Dec 2010)In this commutation, the current flowing through the thyristor is forced to become zero byexternal circuitry.

11. What is meant by inverter?A device that converts dc power into ac power at desired output voltage and frequency is called aninverter.

12. What are the main classifications of inverter?a. Voltage Source Inverterb. Current Source Inverter

13. Why thyristors are not preferred for inverters?Thyristors require extra commutation circuits for turn off which results in increased complexity ofthe circuit. For these reasons thyristors are not preferred for inverters.

14. How output frequency is varied in case of a thyristor?The output frequency is varied by varying the turn off time of the thyristors in the inverter circuit,i.e. the delay angle of the thyristors is varied.

15. What are the disadvantages of PWM control?SCRs are expensive as they must possess low turn-on and turn-off times.

16. What is the main drawback of a single phase half bridge inverter?It require a 3-wire dc supply.

17. Why diodes should be connected in anti-parallel with the thyristors in inverter circuits?For RL loads, load current will not be in phase with load voltage and the diodes connected in antiparallel will allow the current to flow when the main thyristors are turned off. These diodes arecalled feedback diodes.

18. What types of inverters require feedback diodes?VSI with RL load.

19. What is meant a parallel inverter?An inverter in which the commutating elements are connected in parallel with the load is called aparallel inverter.

20. What are the applications of a series inverter?The thyristorised series inverter produces an approximately sinusoidal waveform at a high outputfrequency, ranging from 200 Hz to 100kHz. It is commonly used for fixed output applications such

asa. Ultrasonic generator.b. Induction heating.c. Sonar Transmitterd. Fluorescent lighting.


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21. How is the inverter circuit classified based on commutation circuitry?a. Line commutated inverters.b. Load commutated inverters.c. Self commutated inverters.d. Forced commutated inverters.

PART B1. (i) Describe the working of a 1 full bridge inverter with relevant circuit and waveforms.

(8) (Nov-Dec 2010)(ii) What is PWM? List the various PWM techniques and explain any one of them. (8) (Nov-Dec 2010)

2. Explain the Harmonic reduction by transformer corner lines and stepped wave inverters. (16)(Nov-Dec 2010)

3. Explain the principle of operation of three phase inverter with 180o conduction mode withnecessary waveforms and circuit. (16) (Nov-Dec 2008)

4. (i) Describe any one PWM technique used in inverter in detail. (8) (Nov-Dec 2008)(ii) Explain the working principle of single phase current source inverter. (8) (Nov-Dec 2008)

5. Discuss the functioning of three phase voltage source inverter in 120o degree operating mode.(16) (Nov-Dec 2012) (May-June 2009)

6. Explain the different methods of voltage control adopted in an inverter with suitable waveforms.(16) (May-June 2009)

7. With a neat sketch and output waveforms, explain the working of three phase bridge inverter in

180o

mode of operation. (16) (Nov-Dec 2011)

8. Discuss the different modes of operation of series resonant inverter with unidirectional switchwith neat diagram and waveforms. (16) (Nov-Dec 2011)

9. Explain briefly with circuit diagram, waveforms and working of series and parallel inverters.(16) (May-June 2012)

10. Explain in detail, the various types of PWM methods employed in an inverter. (16)(Nov-Dec 2012)

UNIT-V

AC TO AC CONVERTERSPART - A

1. Write some disadvantages of ac voltage controller. (April-May 2011)The main draw back is the introduction of harmonics in the supply current and the load voltagewaveforms particularly at low output voltages

2. What is meant by cyclo-converter? (April-May 2010) & (Nov-Dec 2010)It converts input power at one frequency to output power at another frequency with one-stageconversion. Cyclo-converter is also known as frequency changer.

3. What are the advantages of six-pulse converter over the 2pule convertor?(NovDec 2012)

a. Commutation is made simple.b. Distortion on the ac side is reduced due to the reduction in lower order harmonics.


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4. What are the applications of ac voltage controllers? (Nov-Dec 2010& 2012)a. Domestic and industrial heatingb. Lighting controlc. Speed control of single phase and three phase ac motorsd. Transformer tap changing

5. State the different types of control in AC voltage controller. (April-May 2011)a. ON-OFF controlb. Phase control

6. What are the disadvantages of unidirectional or half-wave ac voltage controller?(April-May 2012)

a. Due to the presence of diode on the circuit, the control range is limited and the effective RMSoutput voltage can be varied between 70 to 100%.b. The input current and output voltage are asymmetrical and contain a dc component. If there isan input transformer, saturation problem will occur.c. It is only used for low power resistive load.

7. What is the control range of firing angle in ac voltage controller with RL load? (April-May 2012)The control range is <


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15. What are the application of off line UPS?(i) Computers(ii) Printers(iii) Scanners(iv) Emergency power supplies

16. What is the difference between ON-OFF control and phase control?ON-OFF control: In this method, the thyristors are employed as switches to connect the load circuitto the source for a few cycles of the load voltage and disconnect it for another few cycles. Phasecontrol: In this method, thyristor switches connect the load to the ac source for a portion of eachhalf cycle of input voltage.

17. What is the advantage of ON-OFF control?Due to zero-voltage and zero current switching of thyristors, the harmonics generated by theswitching action are reduced.

18. What is the disadvantage of ON-OFF control?

This type of control is applicable in systems that have high mechanical inertia and high thermaltime constant.

19. What is the duty cycle in ON-OFF control method?Duty cycle K = n/ (n + m), where n = no. of ON cycles, m = no. of OFF cycles.

20. What are the two types of cyclo-converters?a. Step-up cyclo-convertersb. Step-down cyclo-converters

21. What is meant by step-up cyclo-converters?In these converters, the output frequency is less than the supply frequency.

22. What is meant by step-down cyclo-converters?In these converters, the output frequency is more than the supply frequency.

23. What is meant by positive converter group in a cyclo converter?The part of the cycloconverter circuit that permits the flow of current during Positive half cycle ofoutput current is called positive converter group.

24. What is meant by negative converter group in a cyclo converter?

The part of the cyclo converter circuit that permits the flow of current during negative half cycle ofoutput current is called negative converter group.

PART-B

1. (i) Describe the operation of single phase full wave a.c voltage controller with the help of voltage

and current waveform. Also derive the expression for average value of output voltage.

(10) (Nov-Dec 2010)

(ii) For a single phase a.c regulator feeding a resistance load, show that power factor is given by

the expression. (6) (Nov-Dec 2010)

2. (i) Describe three-phase to three phase cycloconverter with relevant circuit arrangement using

18 thyristors. (8) (Nov-Dec 2010)


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(ii) Show that the fundamentals RMS value of per phase output voltage of low frequency for an m-

pulse cycloconverter is given by (8) (Nov-Dec 2010)

3. Describe the working principle of single phase ac voltage controller with circuit and waveforms.

(16) (Nov-Dec 2008)

4. A single phase full wave AC voltage controller has an input voltage of 230 V, 50Hz and it is

feeding a resistive load of 10 ohms. If firing angle of thyristors is 110 degree, find the output RMS

voltage, input power factor and average current of thyristor. (16) (May-June 2009)

5. Discuss the working of 2 stage sequence control of AC voltage controller. (16) (Nov-Dec 2011)

6. Discuss the working of a 3 phase to single phase cyclo converter with neat voltage and current

waveforms. (16) (Nov-Dec 2011)

7. Explain briefly about the three phase bidirectional delta connected controllers with neat

diagrams. (16) (May-June 2012)

8. Explain briefly about the three phase full wave controller with neat diagrams.

(16) (May-June 2012)

9. What is AC regulator and draw the configuration of a single phase AC regulator and explain theoperation? (16) (May-June 2012)

10. Explain the necessary circuit diagrams and waveforms of a single phase cyclo-converter.(16) (May-June 2012)

2301 Power Electronics Qb 2014 Final

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