SKP Engineering College - eee.skpec.edu.ineee.skpec.edu.in/wp-content/uploads/sites/6/2018/10/EE6503-POWER... · What are different power transistor family devices? (May/June 2010)

S.K.P. Engineering College, Tiruvannamalai V SEM

Department of EEE 1 EE6503 POWER ELECTRONICS

SKP Engineering College

Tiruvannamalai – 606611

Department Of Electrical and Electronics Engineering

Question Bank

on

EE 6503 POWER ELECTRONICS

2018



UNIT – I POWER SEMICONDUCTOR DEVICES

Part - A Questions & Answers

1. Define power electronics (May/June 2007)

Power electronics may be defined as applications of solid state electronics for the control and conversion

of electric power

2. What are the different thyristor family devices? (May/June 2010)

SCR, TRIAC, DIAC, SUS, SCS, LASCR, LASCS and PUT

3. Define SCR. (Nov/Dec2008)

SCR is a four layer, three junction, three terminal, unidirectional device.

4. Draw the VI characteristics of SCR (May/June 2009)

5. Define latching current. (Nov/Dec2008 ,2014, May/June 2012)

Latching current is the minimum anode current required to maintain the thyristor in the on state

immediately after a thyristor has been turned on and the gate signal is removed.

6. Define holding current. (Nov/Dec2012 , May/June 2011)

Holding current is the minimum anode current required to maintain the thyristor in the ON state. Holding

current is less than latching current.

7. What are the different turn-on methods? (Nov/Dec2010)

1. Temperature triggering

2. Light triggering

3. Forward voltage triggering

4. dv/dt triggering



5. Gate triggering

8. Define turn-on time. (May/June 2011)

It is defined as the time during which SCR changes from forward blocking state to forward conduction

state.

9. What are the different turn-on time intervals? (May/June 2007)

1.Delay time-time during which anode current rises from forward leakage current to 0.1 Ia.

2. rise time - time during which anode current rises from 0.1 Ia to 0.9 Ia

3.spread time- time during which anode current rises from 0.9 Ia to Ia.

Where Ia is the maximum anode current

10. Define delay time. (Nov/Dec2010)

Time during which anode current rises from forward leakage current to 0.1 Ia or time during which anode

voltage falls from Va to 0.9Va

11. Define rise time. (May/June 2011)

Time during which anode current rises from 0.1 Ia to 0.9 Ia or time during which anode voltage falls from

0.9Va to 0.1Va

12. Define spread time. (Nov/Dec2009)

Time during which anode current rises from 0.9 Ia to Ia or time during which anode voltage falls from

0.1Va to on state voltage drop (1 to 1.5V)

13. Define turn-off process of SCR. (Nov/Dec2012 , May/June 2011)

Dynamic process of SCR from forward conduction state to forward blocking state is called commutation

process or turn off process.

14. Define turn-off time. (May/June 2010)

It is defined as time between the instant anode current becomes zero and the instant SCR regains forward

blocking capability.

15. What are the different turn-off time intervals? (May/June 2012)

Turn-off time is divided into two intervals:

Reverse recovery time: During this time, stored charges in the outer layers are removed.

Gate recovery time: During this time, stored charges in the inner layers are removed by recombination.

16. What are the difference between SCR and TRIAC? (Nov/Dec2012)

SCR- unidirectional and high power device

TRIAC – bidirectional and low and medium power device

17. What are the modes of operation of TRIAC? and which mode TRIAC is more sensitive?

(Nov/Dec2011 , May/June 2008] Mode1: MT2 is positive and Ig is positive with respect to MT1

Mode2: MT2 is positive and Ig is negative with respect to MT1

Mode3: MT2 is negative and Ig is positive with respect to MT1

Mode4: MT2 is negative and Ig is negative with respect to MT1



Sensitivity of the TRIAC is greatest in the first quadrant when turned on with positive gate current

(mode1) and also in third quadrant when turned on with negative gate current (mode4).

18. What are different power transistor family devices? (May/June 2010)

BJT, MOSFET, SIT and IGBT

19. What are the differences between transistor and thyristor family devices?

Power transistors have controlled turn-on and turn-off characteristics

Switching speed of power transistor is high

Voltage and current ratings are lower than that of thyristors

On state voltage drop (0.3 to 0.8V) is lower than thyristors (1 to 1.5V)

20. What are the different types of power MOSFET? (May/June 2010)

1. Depletion type power MOSFET

2. Enhancement type power MOSFET

21.What are the difference between BJT and power MOSFET? (Nov/Dec2010 , May/June 2014) BJT- current controlled device, secondary breakdown occurs and higher switching losses and lower

conduction losses.

MOSFET- voltage controlled device, no secondary breakdown problemand lower switching losses and

higher conduction losses

22. Draw the V-I characteristics of TRIAC. (May/June 2011)



23. Draw the transfer and output characteristics of BJT. ( Nov/Dec2012)

24. What are the properties of IGBT? (Nov/Dec2012, May/June 2011)

IGBT has high input impedance like MOSFET and low on state power loss like BJT. It is free from

secondary breakdown problem. Voltage and current ratings are higher than MOSFET. Switching

frequency is higher than BJT and lower than MOSFET.

25. Define trans conductance.(May/June 2014)

In BJT, it is defined as the ratio between small change in collector current to small change in base emitter

voltage

In MOSFET, it is defined as the ratio between small change in drain current to small change in gate

source voltage



26. Draw the transfer and output characteristics of MOSFET.( Nov/Dec2012 , May/June 2011)

27. Draw the transfer and output characteristics of IGBT. (May/June 2014)

28. What is the purpose of driver circuit in power MOSFET? (Nov/Dec2012)

Driver circuits are designed to provide the necessary control signal for power MOSFET.



29. What are the purposes of using snubber circuit? (Nov/Dec2010 , May/June 2013)

* To limit the stress on power semiconductor switching devices during switching transitions.

* Used for dv/dt protection

In snubber circuit, capacitor and resistor series combination is connected in parallel with the switching

device.

30. Distinguish between holding current and latching current of SCR. (Nov/Dec2009 , May/June

2012)

Latching current is the minimum anode current required to maintain the thyristor in the on state

immediately after a thyristor has been turned on and the gate signal is removed

Holding current is the minimum anode current required to maintain the thyristor in the ON state. Holding

current is less than latching current.

Latching current is related with turn-on process and holding current is related with turn-off process.

Latching current is two to three times holding current.

31. What are the basic features of power MOSFET? (May/June 2013)

Voltage controlled device,operating frequency is high,lower switching losses and higher conduction

losses,temperature co-efficient is negative, parallel operation is easy and secondary breakdown does not

occur.

32. What is meant by secondary breakdown in power BJTs? (Nov/Dec2012)

Secondary breakdown is the breakdown of the junction due to localized heating effects. If high voltage

and high current occur simultaneously during turn-off, a hot spot is formed and the device fails by

thermal runaway.



Part – B Questions

1. Explain the structure and operation of turn on and turn of characteristics of SCR. (Nov/Dec 2014,

Apr/May 2017)

2. (i) Describe the any two methods of turn-on mechanism of SCR.

(ii) Explain the turn off characteristics of SCR.( Nov/Dec 2015)

3. Discuss the transfer, output and switching characteristics of IGBT. (Nov/Dec 2014, Apr/May

2015, Nov/Dec 2017)

4. Explain the switching performance of BJT with relevant waveforms indicating clearly the turn on,

turn off times and their components.

5. (i) Draw and explain the forward characteristics of SCR using two transistor model of SCR.

(ii) Compare any six salient features of MOSFET with IGBT.

6. (i) Compare the performance characteristics of MOSFET with BJT.

(ii) Briefly discuss the R-C triggering of SCR.

7. Discuss the operation of power MOSFET and explain the transfer, output and switching

characteristics of power MOSFET. (May/Jun 2014, Nov/Dec 2014, Apr/May 2015, Nov/Dec

2017)

8. Differentiate natural commutation and forced commutation.

9. Explain the operation of driver and snubber circuits for power MOSFET. (Apr/May 2017)

10. Explain the operation of driver and snubber circuits for power TRIAC and IGBT and explain its

woring and applications. (Nov/Dec 2016)

11. Explain with diagram the various modes of working of TRIAC. (Nov/Dec 2015, May/Jun 2016,

Nov/Dec 2017)

12. Explain the working of a current commutation technique. ( Nov/Dec 2015, Apr/May 2018)

13. Describe the UJT triggering circuit with neat sketch. (Apr/May 2018)

14. With neat diagram, explain the Turn - off characteristics, switching losses and working of

construction circuit of SCR. (Nov/Dec 2016)

15. Explain the operating principle of a thyristor in terms of the 'two transistor analogy'. (May/Jun

2014, May/Jun 2016)



UNIT II - PHASE CONTROLLED CONVERTERS


1. Define controlled rectifier. (May/June 2010)

It converts fixed AC to variable DC and it uses thyristor as the switching device

2. What are the different types of controlled Rectifier? (Nov/Dec2012)

(i)According to input supply

1. single phasecontrolled Rectifier

2. three phasecontrolled Rectifier

(ii)According to quadrant operation

1.Semi converter 2.Full converter 3.Dual Converter

(iii) According to number of pulses per cycle

1.One pulse converter

a. Single phase half wave controlled rectifier

2. Two pulse converter

a. single phase half controlled rectifier b. Single phase full controlled rectifier

3. Three pulse converter

a. Three phase half wave controlled rectifier b. three phase half controlled rectifier

4. Six pulse converter

a. Three phase full converter

5.Twelve pulse converter

3. Draw the waveform for 2-pulse converter with resistive load



4.Draw the waveform for 2-pulse converter with RL load

5. Draw the circuit diagram for single phase fully controlled converter bridge circuit. (May/June

2011)

6. Define inverter operation or inversion mode. (May/June 2010)

In a bridge controlled converter circuit with RL-load, when the firing angle is greater than 90 degree, the

average output voltage is negative and it acts as an inverter. i.e. output voltage is positive and current is

negative and the converter is operated in the second quadrant.

7. Draw the circuit diagram for three phase fully controlled converter bridge circuit.



8. What are the advantages of three phase converter circuits?

1. Distortion on the AC side is reduced.

2. Inductance required in series is reduced

3. Power handling capacity is high

9. What are the uses of phase controlled rectifier circuits?

Speed control of DC motor drives

10. What are the effects of source inductance? (May/June 2011)

It reduces the average output voltage at the load side and reduces the displacement factor in the source

side.

11. Define overlap period or commutation period. (Nov/Dec2012 , 2011)

During overlap period, both incoming and outgoing thyristors are conducting together and the resultant

output voltage is zero in single phase converter and it decreases the output voltage in three phase

converter.

12. Define pulse number

It is the number of discrete output pulses produced during one cycle of the AC supply waveform.

13. What are the functions of freewheeling diode? (Nov/Dec2012 , May/June 2011)

It prevents reversal of load voltage

It transfers the load current away from the main rectifier thereby allowing all of its thyristors to

regain their blocking states.

14. What are the advantages of freewheeling diode? Or what are the effects of freewheeling diode

in converters?

Input power factor is improved.

Load current waveform is improved. As a result, load performance is better.

15. Define input displacement angle

It is defined as the angular displacement between the fundamental component of the AC input current and

the input voltage.

16. Define Input displacement factor

It is the cosine of the input displacement angle

17. Define input power factor. (Nov/Dec2012 , May/June 2011)

It is the ratio of the total mean power to the total RMS input volt ampere.



18. Define input current distortion factor. (Nov/Dec2012 , May/June 2011)

The distortion factor is defined as the ratio of the RMS amplitude of the fundamental component to the

total RMS amplitude.

19. Define input Harmonic factor. (Nov/Dec2010 , May/June 2008)

It is the ratio of the total harmonic content to the fundamental component.

20. Define ripple factor

Voltage ripple factor is defined as the ratio of the net harmonic content of the output voltage to the

average output voltage.

21. What is the effect of source inductance on the output voltage in a three phase bridge rectifier?

The effect of source inductance reduces the average DC output voltage.

Voltage drop due to overlap is 3ωLsIo/π

22. What is the effect of inductive load in the performance of a three phase bridge rectifier?

Load current is continuous

Average output voltage is reduced

23. What are the parameters that are evaluated in controlled rectifiers?

1. Displacement factor

2. Input power factor

3. Distortion factor

4. Total harmonic factor

5. Ripple factor




1. Explain the operation of three phase half wave controlled converter with inductive load. Sketch

the associated waveforms. (16) (Nov/Dec 2015)

2. With necessary circuit and waveforms, explain the principle of operation of three phase controlled

bridge rectifier feeding R-L load and derive the expression for the average output dc voltage. (16)

3. Explain the operation of three phase semi converter with RLE load. Sketch the associated

waveforms. (16)

4. i) Explain the effect of source inductance in the operation of single phase fully controlled

converter, indicating clearly the conduction of various thyristors during one cycle. (8) (May/Jun

2014)

ii) Explain the working of single phase dual converter with circuit diagram and waveforms. (8)

5. Explain the effect of source inductance in the operation of three phase fully controlled converter,

indicating clearly the conduction of various thyristors during one cycle with relevant waveforms.

(16) (Nov/Dec 2016)

6. i)Derive an expression for harmonic factor, displacement factor and power factor of a single phase

semiconverter from the fundamental principle. (8)

ii)Three phase fully controlled rectifier is connected to three phase ac supply of 230V, 60 Hz. load

current is continuous and has a negligible ripple. If the average load current Idc =150 A and the

commutating inductance Lc = 0.1mH. Determine the overlap angle when α = 10°. (8)

7. A three phase half wave rectifier is operated from three phase star connected 208V, 60Hz supply.

Load resistance =10 Ohm. If it is required to obtain an average output voltage 50 % of max

possible output voltage. Calculate i)delay angle ii)rms value of output current iii)average value of

output current iv)thyristor avg and rms current v) efficiency vi)TUF vii)supply power factor. (16)

( Nov/Dec 2015)

8. Explain the operation of single phase dual converter with circulating and non circulating current

type. (16) (May/Jun 2014, Nov/Dec 2015, Nov/Dec 2015)

9. Explain the operation of three phase dual converter with circulating and non circulating current

type. (16) (May/Jun 2014, Nov/Dec 2015, Apr/May 2015, May/Jun 2016, Apr/May 2017,Nov/Dec

2017)

10. Explain with circuit and output wave form working of single phase two pulse fully controlled

converter with RL load discontinuous current mode of operation. (Apr/May 2015, Apr/May 2017)

11. Explain the operating principle of a single phase full controlled bridge converter. (May/Jun 2016)

12. Write short notes on Dual converters. (Nov/Dec 2016)

13. Compare 3 pulse and 6 pulse converters. (Nov/Dec 2016)

14. Explain the operation of 3 phase fully controlled bridge rectifier with necessary waveforms.

(Nov/Dec 2017)

15. he opera on o a pha e e onver er w h loa an al o raw he o p vol age wave or or 0 an 0 .(Apr/May 2018)

16. Explain the working of a single phase full converter for 'RL' load discontinuous mode of

operation with neat sketch and waveforms. (Apr/May 2018)



UNIT III - DC TO DC CONVERTER


1. Define DC chopper. (Nov/Dec2012 , May/June 2011)

A DC chopper is a static device which converts fixed DC input voltage to a variable DC output voltage

directly and it is considered as DC equivalent to an AC transformer.

2. What are the applications of DC chopper? (May/June 2010)

Traction motor control, trolley cars, marine hoists, forklift trucks etc.

3. What are the different control strategies in DC chopper?

Time ratio control and current ratio control

In time ratio control, either on or off time is varied to get a variable DC output voltage.

In current limit control, the chopper is switched on and off so that load current is maintained

between two limits

4. Define step up and step down chopper

In step up chopper, the average output voltage is greater than the input voltage.

In step down chopper, the average output voltage is less than the input voltage.

5. What are the different switched mode regulators?

Buck regulator - Vo is less than the input voltage Vin

Boost regulator - Vo is greater than the input voltage Vin

Buck-Boost regulator - Vo is greater than or less than the input voltage depending on the duty

cycleand polarity of the output voltage is opposite to that of the input voltage.

Cuk regulator - Vo is greater than or less than the input voltage depending on the duty cycle and

polarity of the output voltage is opposite to that of the input voltage.

6. Why transistor family devices suitable for chopper circuits?

1. No commutation problem

2. High frequency operation is possible

7. Why thyristor family devices are not suitable for inverter circuits?

1. It requires external commutation circuits because of DC input supply

2. High frequency operation is not possible

8. Why resonant converters are required?

To avoid high turn on and turn off losses

9. What are the different resonant converters?

1. Zero current switching converters 2. Zero voltage switching converters

10. What are the different devices used for chopper circuits?

BJT, MOSFET and IGBT and GTO



11.What are the different time ratio control methods?

1. Constant frequency system: In this scheme, on-time is varied but chopping frequency is kept constant

and is also called as pulse width modulation scheme.

2. Variable frequency system: In this scheme, chopping frequency is varied and either on time or off time

is kept constant and is also called as frequency modulation scheme.

12. Define pulse width modulation scheme in chopper circuits.

In this scheme, on-time is varied but chopping frequency is kept constant. Variation of on time adjusts the

width of pulses.

13. What are the disadvantages of frequency modulation scheme?

1. Filer design is difficult because of wide variation of frequency.

2. Interference with signalling and telephone lines is high.

3. Large off time makes the load current discontinuous.

14.What are the different classifications of chopper depending upon the direction of current and

voltage ? (Nov/Dec2012)

Class A chopper- output voltage and current are positive

Class B chopper- output voltage is positive and current is positive

Class C chopper- output voltage is positive and current is either positive or negative

Class D chopper – output voltage is either positive or negative but current is always positive.

Class E chopper- Both output voltage and current are either positive or negative

15. Define duty ratio or duty cycle. (Nov/Dec2012 , May/June 2011)

It is the ratio between on time to total time.

α= Ton/T

16. Define switching mode regulators

Switching mode regulators convert unregulated DC output voltage to a regulated DC output voltage.

17. Why filter is required in DC chopper circuits? (Nov/Dec2012 , 2011)

To reduce the ripple content in the output voltage, LC filter is required.

18. What are the advantages and disadvantages of buck regulator?

Adv: Circuit is simple and efficiency is high.

Disadv: Input current is discontinuous and a smoothing input filter is required.

19.What are the advantages and disadvantages of boost regulator?

Adv: Efficiency is high and input current is continuous

Disadv: Output voltage is very sensitive to changes in duty cycle and it is difficult to stabilize the

regulator.

Large filter inductor and capacitor are required.

High peak current flows through the transistor.



20.What are the advantages and disadvantages of buck-boost regulator?

Adv: Efficiency is high and fault current is limited by the inductor.

Disadv: Input current is discontinuous.

High peak current flows through the transistor.

21.What are the advantages and disadvantages of cuk regulator?

Adv: Input current is continuous, low switching losses and efficiency is high.

Disadv: High peak current flows through the transistor and additional inductor and capacitor are required.

22. Define time ratio control

In time ratio control, either on or off time is varied to get a variable DC output voltage.

23. Define current limit control

In current limit control, the chopper is switched on and off so that load current is maintained between two

current limits.

24. What is the advantage of current limit control? (Nov/Dec2012)

Load current is continuous because current is maintained between two limits.




1. Draw the circuit diagram of buck regulator and explain its working principle with necessary

waveforms. Derive the expression for peak to peak ripple voltage of the capacitor that is present

across the load. (16) (Apr/May 2017)

2. Describe the working principle of boost converter with necessary circuit and waveforms. (16)

(Apr/May 2016)

3. Explain the various control strategies of chopper (10)

4. Design a filter component of a buck converter which has an input voltage of 12 V and output

voltage of 5V. The peak to peak output ripple voltage is 20mV and peak to peak ripple current of

inductor is limited to 0.8A. The switching frequency is 25KHz.(6)

5. A dc chopper has an input voltage of 200V and a load of 15ohm resistance. When the chopper is

on, its voltage drop is 1.5V and the chopping frequency is 10KHz. If the duty cycle is 80%.Find

(i) average and rms output voltage ii)chopper on time. (12)

(ii) prove the output voltage of step down chopper is Vo = D Vs. (4)

6. Describe the working principle of buck-boost converter with necessary circuit and waveforms.

(16)

7. What is SMPS? Mention the types of SMPS. Explain flyback SMPS in detail. (16)

8. Write short notes on Push pull SMPS, half bridge and full bridge SMPS(16)

9. Explain L type zero current switching resonant converters. (16)

10. Explain M type zero current switching resonant converters. (16)

11. Explain zero voltage switching resonant converters. (16)

12. Explain the operation of class-C and class-D type of two quadrant choppers. (May/Jun 2014)

13. Draw the power circuit diagram of CUK regulator and explain its operation with equivalent circuit

for different modes with necessary waveforms. (May/Jun 2014)

14. Discuss the operation of step-up DC chopper. Also derive the expression for its output

voltage.(Nov/Dec 2014, Apr/May 2015)

15. Write short notes on switch mode power supply. (Nov/Dec 2014)

16. With neat sketch explain operation of Buck – boost converter with its wave for continuous current

mode of operation. (Apr/May 2015, Nov/Dec 2015,Apr/May 2016, Nov/Dec 2016)

17. Discuss the principle of operation of DC-DC step down chopper with suitable waveform. Derive

an expression for its average for its average DC output voltage. (Nov/Dec 2015, Nov/Dec 2016,

Nov/Dec 2017)

18. Discuss in detail, the voltage commutated chopper. (Apr/May 2016, Apr/May 2018)

19. What is called Resonant switching? Explain the concept with relevant circuit diagrams. (Nov/Dec

2016)

20. Explain the control stratergies of chopper. (Apr/May 2017)

21. Explain the working of buck converter with neat waveform and also derive the expression of peak

to peak voltage across the capacitor. (Nov/Dec 2017)



UNIT IV - INVERTERS


1. Define Inverter (May/June 2011)

DC to AC converts are known as Inverters. The function of an inverter is to change a DC input voltage to

a symmetrical AC output voltage of desired magnitude and frequency. The output voltage could be fixed

or variable at a fixed or variable frequency.

2. Define inverter gain (Nov/Dec2012)

It is the ratio of AC output voltage to DC input voltage

3. Why short pulses are not suitable for inductive load?

For n ve loa , loa rren an’ hange e a ely w h he o p vol age. I he p l e hor ,

the incoming thyristor or the switching device may fail to conduct. So the pulse duration must be long.

4. Why feedback diodes are required in inverters feeding inductive load?

Feedback diodes are required to feed the energy from load to source in case of inductive load.

5. What are the different voltage control methods? (Nov/Dec2012 , May/June 2011)

1. single pulse width modulation

2. multiple pulse width modulation

3. Sinusoidal PWM

4. Modified sinusoidal PWM

5. Phase displacement control

6. What are the advantages of Sinusoidal PWM?

Distortion factor is reduced

Lower order harmonics are eliminated

7. Define current source inverter (Nov/Dec2012 , May/June 2011)

In a current source inverter, o/p current is maintained constant irrespective of load and the output voltage

is forced to change.

8. Define series resonant inverter

The commutating components and switching devices are placed in series with the load to form an

underdamped circuit.

9. What are the applications of Inverter? (May/June 2011)

The applications of inverter are UPS, HVDC systems, adjustable AC motor drive system and induction

heating.

10. Why transistor family devices suitable for inverter circuits?

1. No commutation problem

2. High frequency operation is possible



11. Why thyristor family devices are not suitable for inverter circuits?

1. It requires external commutation circuits because of DC input supply

2. High frequency operation is not possible

12. What are the types of inverter?

Inverters are generally classified as voltage source inverter and current source inverter.

Depending on the nature of the output voltage, inverters are classified as single phase and three phase

inverters.

13. What are the performance parameters of an inverter?

1. Harmonic factor of nth harmonic 2. Total harmonic distortion 3. Distortion factor 4. Lowest order

harmonic

14. What are the different internal voltage control methods? Or What are the different modulation

techniques in inverter? (May/June 2013)

1. Single pulse width modulation 2. Multiple pulse width modulation

3. Sinusoidal pulse width modulation 4. Modified sinusoidal pulse width modulation

15. Define modulation index and what is its use?

The ratio of peak amplitude of reference signal to the peak amplitude of carrier signal is called

modulation index.

M = Ar/Ac

It is used to control the output voltage of the inverter

16. What are the properties of VSI? (Nov/Dec2010 , May/June 2014)

1. Input voltage is constant

2. Input impedance is low

3. Output current magnitude depends on load and output voltage is maintained constant

17. What are the properties of CSI? (Nov/Dec2008 , May/June 2012)

1. Input current is constant

2. Input impedance is high

3. Output voltage magnitude depends on load and output current is maintained constant.

18. What are the advantages of CSI? (May/June 2014)

1. A CSI does not require any feedback diodes

2. Commutation circuit is simple and it contains only capacitors.

3. Peak current of the power device is limited

4. There is no problem of short circuit.

19. What are the disadvantages of CSI? (Nov/Dec2008)

1. Large reactor is required to maintain the current constant

2. Dynamic response is slower

3. Output filter is required to suppress the output voltage spikes

20. What are the types of series resonant inverter? (May/June 2010)

1. Series resonant inverter with unidirectional switches



2. Series resonant inverter with bidirectional switches

21. What are the advantages of resonant inverter? (May/June 2010)

Power switching devices are turned on or turned off at zero current and zero voltage and thereby reduces

the switching losses.

22. Define resonant inverter(Nov/Dec2012 , May/June 2009)

The voltage and current are forced to pass through zero crossing by creating an LC resonant circuit,

thereby called resonant inverter.

23. What are the types of resonant inverter? (Nov/Dec2012 , 2011)

1. Series resonant inverter

2. Parallel resonant inverter

3. Resonant DC link inverter

24. What are the applications of CSI? (May/June 2011)

1. Speed control of AC motors

2. Induction heating

3. Lagging VAR compensation

4. Synchronous motor heating

27. What are advantages of PWM? (Nov/Dec2012 , May/June 2011)

No additional components are required to vary the output voltage

Lower order harmonics can be easily eliminated

28. Compare VSI and CSI. (Nov/Dec2011 , May/June 2014)

VSI

1. Output current depends on the load impedance and output voltage is independent of load

2. Low input impedance

3. A VSI requires feedback diodes

4. Commutation circuit is complex

5. Peak current of the power device is high

6. Multi motor drive is possible

7. It can be operated in both open loop and closed loop mode

CSI

1. Output voltage depends on the load impedance and output current is independent of load

2. High input impedance

3. A CSI does not require any feedback diodes

4. Commutation circuit is simple

5. Peak current of the power device is limited

6. Multi motor drive is not possible

7. It can be operated in closed loop mode only

29. What is the need for voltage control of an inverter? (May/June 2010)

1. To compensate the variations in the input voltage

2. To compensate for the regulation of inverter

3. To supply some special loads which need variations of voltage with frequency such as an induction

motor.



30 What are the different voltage control methods of an inverter? (Nov/Dec2012 , May/June 2011)

1. External control of AC input voltage

2. External control of DC input voltage

3. Internal control of inverter

31. What are the applications of series resonant inverter? (May/June 2011)

1. Induction heating

2. Fluorescent lighting

3. Sonar transmitter




1. Explain the principle of operation of 3 phase voltage source inverter with 180°conduction mode

with necessary waveforms and circuits. Also obtain the expression for line to line

voltage.(Nov/Dec 2014, Apr/May 2015, Nov/Dec 2017)

2. Discuss the functioning of three phase voltage source inverter in 120 degree operating mode with

relevant waveforms and obtain the expression for voltages. (May/Jun 2014, Nov/Dec 2015,

May/Jun 2016, May/Jun 2017, Apr/May 2018)

3. Explain the following PWM techniques used in inverter.i) Sinusoidal PWMii) Multiple PWM.

(May/Jun 2014, Nov/Dec 2014, Apr/May 2015, May/Jun 2017, Apr/May 2018)

4. Explain the operation of single phase capacitor commutated CSI with R load.

5. Explain the harmonic reduction by transformer corner lines and stepped wave inverters.

6. Explain the different methods of voltage control adopted in an inverter with suitable waveforms.

7. Explain the working of series inverter with the help of circuit diagram and relevant waveforms.

(May/Jun 2014, Nov/Dec 2017)

8. Draw the circuit diagram of current source inverter and explain its operation with relevant

waveforms.(May/Jun 2016)



UNIT V - AC TO AC CONVERTERS


1.What is meant by AC voltage controller? (May/June 2010)

AC voltage controller converts fixed AC into variable AC without change in supply frequency

2.What are applications of AC voltage controller (May/June 2014)

a. Domestic and industrial heating

b. Lighting control

c. Speed control of single phase and three phase motors

d. Transformer tap changing

3. What are the advantages and disadvantages of AC voltage controller (Nov/Dec2012 , May/June

2011)

Advantages

i. High efficiency ii. Flexibility in control iii. Less maintenance

Disadvantages:

It introduces harmonics in the supply current and load voltage waveforms particularly at low output

voltage levels

4. What are the 2 methods of control in AC voltage controllers? (Nov/Dec2010 , May/June 2014)

i. ON-OFF control iii. Phase control

5. What are the difference between On OFF control and phase control? (Nov/Dec2012 , May/June

2011)

ON-OFF control:

In this method thyristors are employed as switches to connect the load circuit to the source for few cycles

and disconnect the load from the source for another few cycles

Phase control:

In this method thyristor switches connect the load to the AC source for a portion of each cycle of input

voltage.

onal or half wave ac voltage controller

iii. Due to the presence of diode D1 on the circuit, the control range is limited and the effective RMS

output voltage can e varied between 70.7% and 100%.

iv. The input current and output voltage are asymmetrical and contain a dc component. If there is an

input transformer, saturation problem will occur.

v. It is only used for low power resistive load.

6. What is the advantages and disadvantages of ON-OFF control method? (Nov/Dec2010 ,

May/June 2013)

Advantages:



Due to zero-voltage and zero current switching of thyristors, the harmonics, generated by switching

actions are reduced.

Disadvantages:

This type of control method is applicable only which have a high mechanical inertia and high thermal

time constant.

7.What is meant by duty cycle is ON-OFF control method? (May/June 2010)

Duty cycle K = n/n+m, where n=no.of on cycles, m= no.of off cycles.

8. What is meant by unidirectional or half-wave ac voltage controller? (Nov/Dec2012 , May/June

2011)

In unidirectional controller the power flow is controlled during the positive half-cycle of the input

voltage.

9. Give an expression for RMS and average output voltages of single phase half wave ac voltage

controller. (Nov/Dec2012 , May/June 2014)

Vs=Supply voltage,α=delay angle of thyristor

10. What is meant by full wave or bidirectional ac voltage controller? ( Nov/Dec2012 , May/June

2011)

In bidirectional ac voltage controller, the power flow is controlled during the both cycles of input voltage.

11.What is the extinction angle in a voltage controller? (Nov/Dec2012 , May/June 2011)

In single Phase ac vol age on roller, w h L loa , he loa rren all a ω = β. The angle β alle

as extinction angle.

12.What is the control range of firing angle in ac voltage controller with RL load? (Nov/Dec2011 ,

May/June 2010)

The control range of firing angle is 0<α < 180◦

Where α = load power factor angle

13.What type gating signal is used single phase ac voltage controller with RL load? (Nov/Dec2008 ,

May/June 2012)

High frequency carrier gating signal is used for single phase ac voltage controller with RL load.

14.What are the disadvantages of continuous gating signal? (Nov/Dec2012 , May/June 2009)

i). More heating of the SCR gate. Ii). Increases the size of pulse transformer.

15.What is the meant by high frequency carrier gating? (May/June 2011)

Thyr or rne on by e a ra n o p l e ro α o π. Th ype o gnal alle a h gh req en y

carrier gating.

16. What is cyclo converter (Nov/Dec2012 , May/June 2011)

In a cyclo converter, a.c power at one frequency is converted directly to a lower frequency in a single

conversion stage




1. Draw the circuit diagram of three phase to single phase cycloconverter and explain its operation

with waveforms. (May/Jun 2014, Nov/Dec 2015, May/Jun 2016, Apr/May 2017)

2. With the necessary circuit diagram and waveforms, explain the principle of operation of single

phase ac voltage controller having only thyristor feeding resistive load by on-off control and

phase control. Derive the expression for rms value of output voltages in both cases.

3. Describe the operation of single phase full wave AC voltage controller with the help of voltage

and current waveform. Also derive the expression for average value of output voltage.(May/Jun

2014, Apr/May 2015, Nov/Dec 2015, May/Jun 2016)

4. Explain sinusoidal and multiple PWM techniques used in inverter.

5. Explain the operation of the step down cycloconverter both bridge and midpoint configuration

with necessary waveforms.

6. With aid of circuit diagram, explain the operation of three phase to three phase cycloconverter

employing three phase half wave circuits and list few of its applications.

7. Explain the operation of single phase half wave phase controller and single phase full wave phase

controller with circuit diagrams and waveforms.

8. Explain the principle of working of single phase to single phase step up cycloconverter. List the

factors that affect the performance of cycloconverters.( Apr/May 2015, Nov/Dec 2017)

9. Discuss the working of a single phase AC voltage controller with RL load when its firing angle is

more than the load power factor angle. Illustrate with waveforms.( Nov/Dec 2014)

10. A ngle pha e vol age on roller ee power o a re ve loa o Ω ro 2 0V, 50 Hz source.

Calculate 1) The maximum values of average and RMS thyristor currents for any firing angle α.

2) The minimum circuit tiurn off time for any firing angle α. 3)the ratio of third harmonic voltage

to fundamental voltage for α=60 º

11. Explain Matrix converter. (Nov/Dec 2014, Nov/Dec 2015, Nov/Dec 2016, Nov/Dec 2017)

12. What is meant multi sequence control? Explain it with relevant circuit diagram. (Nov/Dec 2016,

Apr/May 2018)

13. Compar single phase and three phase cyclo converter. (Nov/Dec 2016)

14. What is the importance of power factor control in a converter? (Nov/Dec 2016)

15. Explain the operation of two stage sequence control of AC voltage regulator. (Apr/May 2017)

16. Explain the operation of three phase to three phase cyclo converter. (Apr/May 2018)

Documents

SKP Engineering College - eee.skpec.edu.ineee.skpec.edu.in/wp-content/uploads/sites/6/2018/10/EE6503-POWER... · What are different power transistor family devices? (May/June 2010)