DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING.

POWER ELECTRONICS

PART – A Questions and Answers

UNIT - I

1.What is a SCR? A silicon-controlled rectifier (SCR) is a three terminal, three-junction semiconductor device that acts as a true electronic switch. It is a unidirectional device. It control the amount of power fed to the load. Its structure and symbol are shown in the figure.

2. Define break over voltage of SCR. Break over voltage is defined as the minimum forward voltage (gate being open) at which the SCR starts conducting heavily.

3. Draw the two transistor model of a SCR and mention its applications.

List the applications of SCR . (i) It can be used as a speed controller in DC and AC motors

(ii) It can be used as inverter. (iii) It can be used as converter. (iv) It is used in battery charges. (v) It is used for phase control and heater control. (vi) It is used in light dimming control circuits.

4. Draw the VI characteristics of a SCR and mark important points.

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5. What is meant by latching current & holding current?

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 gate signal has been removed.

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

6. Draw the two transistor analogy of a SCR

7. Mention the various firing circuits for SCR?(i) Simple R and RC triggering circuits.(ii) UJT pulse triggering circuit.(iii) Digital firing scheme.

8.. If the latching current in the circuit is 5mA. Obtain the minimum width of the gating pulse required to properly turn on the SCR

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9. What is a Triac? Triac is a three terminal bi-directional semiconductor-switching device. It can conduct in both the directions for any desired period. In operation it is equivalent to two SCR’s connected in antiparallel. Next to SCR it is the widely used device for power control.

10. Draw the volt-ampere characteristic of a Triac.

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11. Why Triac not popular compared to SCR? commutation of Triac in inductive load is difficult when compared to SCR. Hence it is not popular

12. .Distinguish between SCR and TRIAC.

S. No SCR Triac1. It is unidirectional

semiconductor device.It is a bi-directional semiconductor device.

2. It has fast turn-off time. It has a comparatively longer turn -off time.

3. It can be used to switch AC supply frequencies up to few KHz.

It can be used to switch AC supply frequencies up to 40Hz only.

4. It is triggered by positive voltage applied to the gate.

It is triggered by either positive or negative voltage applied to the gate.

13. Draw the construction, equivalent circuit and symbol of Triac.

14. What is power MOSFET? A power MOSFET is a voltage-controlled device and requires only a small input current. The switching speed is very high and the switching times are of the order of nanoseconds.

15..Explain the importance of threshold voltage in gate circuit when the voltage Vgs is increased beyond threshold voltage, the device starts

to conduct and the current will be increased from zero .

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16. Draw the circuit model of power MOSFET in active and ohmic region

17. list the driving circuits for power MOSFET”S i) driving the mosfet from TTL

ii) driving the mosfet from cmosiii)driving the mosfet from linear circuitiv)isolated gate circuits

18. What is the common method used for di / dt protection? The value of the di / dt Can be maintained below acceptable limit by using a small inductor called di / dt inductor in series with the anode circuit.

19. Compare the transistor and thyristor

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What is an IGBT? Insulated gate bipolar transistor (IGBT) combines the advantages of BJT and MOSFET. Therefore it has low switching times as well as low power losses.

21What are the types of IGBT? 1. punch through IGBT

2. non punch IGBT22. What are the advantages of IGBTs?i. They have high input gate impedance.ii. They have low conduction loss.

S.NO TRANSISTOR THYRISTOR1. Transistor is a three layer ,

two junction deviceThyristor is a four layer, three junction device

2. Commutation circuitry which is costly and bulky, is not required

Commutation circuit is required

3. To keep a transistor in the conducting state, a continuous base current is required

Thyristors require a pulse to make it conducting and thereafter it remain conducting

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iii. They have fast switching characteristics.iv. They have very high operating frequency.

23. List the applications of IGBTs.(i) They are used in low noise, high performance power supplies.(ii) They are used in inverters.(iii) They are used in motor speed controls.

UNIT - II1. What is a controlled rectifier? The output voltage of thyristor rectifier is varied by controlling the delay or firing angle of thyristors. Therefore thyristor rectifiers are called as controlled rectifiers.

2. Define delay angle or firing angle. Delay angle or firing angle is defined as the time after the input voltage starts to go positive until the thyristor is fired at cot = α.

3. Define the effect of freewheeling diode. i ) It prevents reversal of load voltage except for a small voltage drop ii) It transfers the load current away from the main rectifier, thereby allowing all of its thyristor to regain its blocking states

4. Mention application of phase controlled rectifier i) battery chargesii) DC drivesiii) DC power supplies

5. Mention two functions of freewheeling diode (a) Reduction of ripple voltage in d.c terminals (b) Prevents reversal of load voltage except for small diode voltage-drop (c) It transfers the load current away from the main rectifier, thereby allowing all of its thyristors to regain their blocking states.

6. Draw the power circuit diagram of half controlled thyristor rectifier.

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7. What is the difference between symmetric and asymmetric semiconductor configuration? In symmetric semiconductor, each leg contains one SCR and one diode. In asymmetric semi converter, one leg contains two SCRs and the other leg contains two diodes

8. compare continuous and discontinuous conduction

S.NO CONTINUOUS DISCONTINUOUS1. Β = П + α Β < П + α2. Inductance value is very

large Inductance value is very small

9. Explain about the two quadrant operation? A full converter is a two -quadrant converter and the polarity of its output voltage can be either positive or negative.

10. Define form factor and ripple factor form factor ff = Erms / Edcripple factor = ( ff2 – 1)1/2

11. Explain the inversion mode of fully controlled bridge rectifier? When firing angle α is greater than 90˚ , then the voltage is maximum in negative is called inversion mode

12. Define the input power factor

P. f = ( IS1 / ISrms) cos α

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IS1 = fundamental current ISrm = rms current

13. Mention the effect of source inductance in converters. The source inductance causes the outgoing and incoming SCRs to conduct together .When the source inductor increases, the overlap angle increases and as a consequence the average output voltage decreases.

14. Why 3-pulse converter is not much used? The 3-pulse converter requires special types of converter transformer to prevent DC magnetization. Therefore they are not much used.

15. compare single phase converter and three phase converter

S.no parameter Single phase Three phase1. Ripple content in

outputmore Less

2. Supply current waveform

Square wave Quasi square wave

3. Control & complexity

Less complex & easy control

Complex control & implementation

4. Supply & load derating

Higher Less

16. Express the average output voltage of 3Ф semi converter

17. compare half controlled and fully controlled rectifier

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S .No Half controlled converter Fully controlled converter1. It uses one thyristor. It uses four thyristor 2. One quadrant converter Two quadrant converter3. At time one thyristor is

conductingAt a time two thyristor are conducting

18. Why is circulating current mode prepared over non - circulating current mode in dual converter? The circulating current mode is preferred over non - circulating current mode in Dual converter due to the following advantages. (a) Both converters are in operation continuously. The time response to change the operation from one quadrant to the other is faster. (b) One converter is always working as rectifier and the other as inverter. Therefore power flow can be in either direction at any time.

(c) The circulating current helps in maintaining continuous conduction on both converters irrespective of load.

UNIT III

1. What is meant by time ratio (duty cycle) of a DC chopper? The ratio of a period to the total time period is known as time control ratio (or) duty ratio. It is given by.

2. What is the effect of load inductance on the load current waveforms in the case of DC chopper? If load inductance is high, it will reduce the ripple in the output currents waveforms. Load current becomes continuous.

3. What is PWM voltage control of output of a chopper? In PWM, the frequency is kept constant and output voltage is varied by varying Ton and T off.

4. Differentiate between single quadrant and two quadrant and two quadrant DC chopper.

S. No Single quadrant Two quadrant1. Load current flows from

source to load.Load current flows from load to source.

2. Both voltage and current are positive.

Voltage is positive, current is either positive or negative.

6. What is the principle of on-off control?

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Thyristor switch connects the AC supply to load for a time ton, the switch is turned off by a gate pulse inhibiting for time t off.

7. What is the principle of step - up DC chopper? A constant voltage is applied to the inductor. Hence the current through the inductor increases linearly during the period T on. After the SCR is turned off, the energy in the inductor is transferred to the load along with supply voltage.

8. What is the function of free wheeling diode in a chopper? (i) It protects SCR from high voltage that may be induced when the inductive circuits is interrupted. (ii) It helps to maintain constant current though the load. (iii) It helps to commutate main SCR.

9. Which power electronic circuits is DC equivalent of transformer? DC chopper connects fixed DC to variable DC. Autotransformer connects fixed AC in to variable AC.

10. What are the applications of DC Chopper? (i) Electric locomotives (ii) Battery operated cars. (iii) Power supplies.

11. What are the advantages of DC choppers? (i) Flexible (ii) Easy to control & compact (iii) Closed loop control can be implemented

12. What is the disadvantages of frequency modulated chopper? Design of filter is difficult, interference with commutation signals.

13. What is Time ratio control? The Time ratio control (δ) is the ratio between t on to the total time period (T) i.e δ = ton T By varying the ton period the output voltage can be controlled and this sis Called as time ratio control.

14. Why thyristor are not preferred for inverter and choppers? Inverters operating at high frequencies require fast acting switch. Thyristor Is not preferred since it can operate at a low switching frequency of 2KHz. Thyristors require forced commutation circuits.

15. What is meant by step down and step up chopper?If chopper output voltage is less than input voltage it is called step down chopper.

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If chopper output voltage is greater than input voltage it is called step up chopper.

16. Write down the expression for average output voltage for step down chopper. Average output voltage for step-down chopper V0 =α Vs. α is the duty cycle.

17. What is meant by duty cycle? Duty cycle is defined as the ratio of the on time of the chopper to the total time period of the chopper. It is denoted by α.

18. What are the two types of control strategies?a. time ratio control b. current limit control

19. What are the two types of TRC?a. constant frequency controlb. variable frequency control

20. Compare between linear & switched mode regulator.

S.no Parameter linear regulator switched mode1. swihching zero High

Losses2. RFI\EMI Absent High3. Efficiency Low High4. Size & cost High High

21. Write advantages of buck-boost regulator?a. provides inverted outputb. high efficiencyc. Both Buck\boost operation simulationd. short circuit protection can be easily implementede.

22. Write the disadvantages of boost regulatora. High peak current through deviceb. Output voltage is highly sensitive to change in duty cyclec. Large inductance & capacitance are required

23. Write the expression of Imax, IminThe expressions of Imax, Imin are

24. Write the expression of p.u. ripple current.

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25. Write the output voltage expression of step up\step down chopper.

26. Write the output voltage expression of step up chopper.

UNIT – IV

1. How the output frequency is varied in the case of an inverter? Output frequency is varied by varying switching frequency.

3. State the necessity of return current diodes in inverter. If the load connected to the inverter is inductive, then high voltages occur Across the thyristor when it is switched off. This will damage the thyristor if there is no path for diverting the current. Feedback diodes (or) return current diodes are connected across each thyristor in antiparallel. The feed back diode which is connected across the thyristors feeds the reactive power to the supply.

4. State one application for series inverter. Variable speed AC motor drives. Induction heating Standby power supplies UPS

5. Mention the advantages of current source inverter over voltage source inverter. Commutation circuit is simple since it requires only capacitors. Slow response to load changes. No feedback diodes are required Utilize thyristors very effectively. Inherent regeneration failure.

6. How to change the output voltage of a square wave inverter? By using the PWM (Pulse Width Modulation) techniques, the output voltage of an inverter can be varied.

7. What is the necessity of isolation between power circuit and control circuit? It is preferred to have separate grounds for power circuit and control circuit.If the round of power circuit is connected to the ground of control circuit, there is no

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isolation. If a wise of power circuit touches control circuit, largecurrent flows through the control circuit and it get damaged. Hence isolation is required between power circuit and control circuit.

8. What are the difference between return current diodes and freewheeling diodes? Freewheeling diode Return current diode A single diode is used. Many diodes are connected

across the thyristor.Connected across the load. Connected across each thyristor

in antiparallel.It provide path to ensure continuous conduction.

It feeds the reactive power to the source.

10 What are the difference between VSI and CSI?

VSI CSIUses voltage source. Uses current source.Uses current commutation. Uses voltage commutationDiodes are connected in parallel.

Diodes are connected in series.

Can be used for only 1208 and 1808

Can be used for only 1208

Output voltage is quasi square wave.

Output current is quasi square wave.

11. What are the limitations of series inverter? It cannot be used for low frequency. Output frequency has to be less than resonant frequency. Pure sinewave cannot be obtained due to dead band.

12. Define harmonic distortion factor? Harmonic distortion factor is the harmonic voltage to the fundamental voltage.

13. What is the difference between voltage commutation and current commutation.S. No Voltage commutation Current commutation 1. Precharged capacitor

pumps large current though SCR and turn it off.

In current commutation, L-C circuit pumps smooth SCR and turn off.

2. Capacitor applies reverse bias to the outgoing SCR.

Diode applies reverse bias to the outgoing SCR.

14. What is the necessity of PWM within an inverter? To control voltage and harmonics PWM is used. In PWM, output voltage is

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controlled by varying the width of pulse. Percentage harmonic can be controlled by changing number of pulse per half cycle.

15. What are the methods of voltage control in Inverters? External control of AC output voltage. External control of DC input voltage. Internal control of inverter.

16. What type of Inverter load requires feedback diodes? Inductive load.

17. How is third harmonic eliminated at the output of single-phase bridges inverter? By having a transformer connection in the output.

18. How does a thyristor commutate in series inverters? By self- commutation.

19. What is the order of output frequencies of series inverters? 5KHz to 30 KHz

20. What is meant by line commutated inverter? Phase-controlled converters, when operated in the inverter mode, are called line-commutated inverters.

22. Mention any one demerit of a current source inverter. It requires a relatively large reactor to exhibit current-source characters and an extra converter state to control the current. The dynamic response is slower. Due to current transfer from one pair of switches to another, an output filter is required to suppress the output voltage spikes.

23. How do you select SCR for inverter?o Turn-off time should be minimum. Use inverter grade SCR’s for low frequency application. Use BJT or MOSFET for high frequencies.o Use devices with high dt \ dt ratings.o Use factor of safety of 2 to 3.

24. What is the function of feedback diodes in bridge inverter? Feedback diodes pump reactive current to the DC source.

25. What is the simplest method of eliminating 3rd harmonic in a 3- inverter? Use 1200 mode or connect the system in star.

26. List the advantages and disadvantages of transistor inverter over thyristor inverter. Advantages:

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Disadvantages: It requires a driving circuit. Continuous base pulse should be applied since BJT does not have self latching property.

27. State and explain the merits and demerits of CSI over VSI. Merits: Regeneration is possible with CSI. Simple power circuit.Demerits: Large filter inductor is required in the DC line. Power factor of the source and converter is poor at higher values of α.

28. What is meant by total harmonic distortion (THD)? The total harmonic distortion; which is a measure of closeness in shape between a waveform and its fundamental component, is defined as

29. What is meant by lowest order harmonic (LOH)? The lowest-order harmonic is that harmonic component whose frequency is closest to the fundamental one, and its amplitude is greater than or equal to 3% of the fundamental component.30. What are the various methods of harmonic control? Tap changing Transformer connections Filters PWM.

UNIT V

1. What is an AC voltage regulator?An AC voltage regulator is an AC to AC converter. It converts a constant AC

voltage into a variable AC voltage of the same frequency.

2. What are the two types of control normally used in AC voltage controller?i. On-off control

ii. Phase angle control

3. What is the difference between on-off control and phase angle control?In on-of control, thyristor switches connect the load to the AC source for a few

cycles of input voltage and then disconnect it for another few cycles. In phase angle control, thyristor switches connect the load to the AC source for a portion of each cycle of input voltage.

4. Why is half wave AC voltage regulator not used?

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Single-phase half wave voltage regulator consists of a Thyristor and a diode in anti parallel. Therefore control is possible only in positive half cycle. The output waves are very much distorted. Therefore half wave regulator is not used.

5. Draw the circuit diagram of single-phase half wave AC voltage controller.

6. Draw the circuit diagram of single phase full wave AC voltage regulator.

7. Which method of commutation is used in AC voltage regulators?Line commutation is used in AC voltage regulator. i.e the thyristor will get turned off when the current through it falls below the holding current value because of the nature of the source itself.

8. Define duty cycle of AC voltage controller.

Duty cycle = n / n + mWhere n = number of on-cyclesm = number of off-cycles

9. Explain the term sequence control of ac voltage regulators.Sequence control of a.c regulators means the use of two or more stages of voltage controllers in parallel for the regulation of output voltage. The voltage controllers in parallel are triggered in proper sequence one after the other so as to obtain a

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vaiable output with low harmonic content.

10. What are the advantages of sequence control of AC voltage regulators?The advantages of sequence control are 1) the reduction of harmonics in the load voltages and line currents 2)Improvement of power factor.

11. Name some applications of AC regulator.AC regulators are used in industries heating, illumination level controller on load transformer tap changing, speed control of induction motor etc.,

12. What is a Cycloconverters?A Cycloconverter is a direct-frequency changer that converts ac power at one

frequency to ac power at another frequency by ac-ac conversion, without an intermediate conversion link.

13. List the applications of cycloconverter.Cycloconverters are used in

i. Variable speed AC motors.ii. Induction heating

iii. Electric tractioniv. Static var systems etc.

14. List the advantages of cycloconverters.i. Freqency conversion in single stage is possible

ii. Both frequency and voltage are controllable.iii. Natural commutation is used. Hence commutation circuits are not needed.iv. THD is lesser than d.c link converters.v. Isolation of a defective SCR does not require the cycloconverters to be

switched off.vi. The power transfer is bidirectional

15. List the disadvantages of cycloconverters.i. Cyclocoverter operation is possible only for frequencies less than half of the

input frequency.ii. Number of SCRs required is more than that for d.c link converters.

iii. Complex control circuitry.iv. At smaller load currents, the cycloconverters may create problem in firing

delay control.16. What are the factors affecting the harmonics in cycloconverters?

i. Number of pulses per cycleii. Circulating or non-circulating mode of operation

iii. Continuous or discontinuous conductioniv. Effect of overlapv. Effect of load power factor

vi. Control methods

17. Why the output frequency of a cycloconverter is significantly lower than the input

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frequency?Since the cycloconverter is a phase controlled AC-AC converter producing the

desired output AC voltage by selecting segments of the input voltage utilizing natural commutation, the output frequency becomes significantly lower than the input frequency.

18. What is a matrix converter?The matrix converter is a single-stage converter. It uses bi-directional fully controlled switches for direct conversion from ac to ac. It is an alternative to the double sided PWM voltage rectifier-inverter.

19. What are the advantages of matrix converters?i. Inherent bidirectional power flow

ii. Sinusoidal input-output waveforms with moderate switching frequency.iii. Possibility of compact design due to absence of dc-link reactive componentsiv. Controllable input powerfactor.

20. What are the bottlenecks of matrix converters?i. Non-availability of the bilateral fully controlled switches capable of high-

frequency operation.ii. Complex control law implementation

iii. An intrinsic limitation of the output-input voltage ratioiv. Commutation and protection of the switches.

21. What are types of control of matrix converters?i. Venturini method that is based on a mathematical approach of transfer function

analysisii. PWM

iii. Space vector Modulation.

22. Differentiate step-down cycloconverter and step-up cycloconverter.

S.No Step-down cycloconverter Step-up cycloconverter1 It gives an output whose

frequency is lower than that of output.

It gives an output whose Frequency is higher than that of output.

2 It uses line or natural commutation It requires force commutation

23. Classify Cycloconverters. Single phase to single-phase cycloconverter. Three phase to single phase cycloconverter. Three-phase to three-phase cycloconverter.Cycloconverters can also be classified as Step-down cycloconverter Step-up cycloconverter

24. Draw the circuit diagram of three phase to single phase bridge configuration of cycloconverter.

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25. Explain the purpose of IGR in cycloconverter.If by any chance both of the converters are enabled, then the supply is short-circuited.To avoid this short circuit, an intergroup reactor (IGR) can be connected between the converters as shown in Fig.

With IGR cycloconverters can be operated in circulating current mode.

26. What is meant by blocked group operation? In a cycloconverter if the two groups of thyistors conduct together, the supply is short circuited. When the operation is to be shifted from one group to the other an open circuit exists across the load. This is because of the reason that thyristors need a finite time to turn off. To avoid the possibility of short circuit it is necessary to block the turning on of one group when the other group is conducting. The firing schemes of both the groups are so designed that any group can not be triggered till the current of the other group becomes zero. This is known as blocked group operation.

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PART B

UNIT I 1. What are the types of power semi conductor devices? Explain them in detail with relavant circuit diagrams2. Draw the cross-sectional view of thyristor. Explain steady state and switching characteristics of it with waveforms.3. Draw the cross-sectional view of BJT. Explain steady state and switching characteristics of it with waveforms.4. Draw the cross-sectional view of MOSFET. Explain steady state and switching characteristics of it with waveforms.5. Draw the cross-sectional view of TRIAC. Explain he operation in detail.6. How is heat dissipated in power semi conductor devices? Explain.7. Explain V-I characteristics of power diodes and thyristors with diagrams.

UNIT II1. Draw the basic circuit of controlled rectifier and explain the operation of the same with neat waveforms. Derive the average and RMS voltage equations.2. Explain the operation of full wave controlled rectifier with circuit diagram and actual waveforms.3. Derive the output equation of single phase half – bridge controlled rectifier with proper diagrams.4. Derive the output equation of single phase full-bridge controlled rectifier with R and RL loads with neat diagrams.5. Estimate the load voltage, load current and input power factor for single phase full and half converters.6. Derive the output equation of three phase half-bridge controlled rectifier with suitable diagrams.7. Derive the output equation of three phase full-bridge controlled rectifier with suitable diagrams.8. What are the effects of source inductance and load inductance in three phase rectifiers? Explain with proof,9. Explain the principle operation of Dual converter with neat diagrams.10. Explain the operation of converter circuits with Back emf loads.11. What are phase control and ON-OFF control? How are they applied to A.C controllers. Explain.12. Draw the circuit diagram of single phase A.C chopper. Explain with neat wave forms.13. Estimate the load voltage, load current and input power factor for single phase A.C controllers.

UNIT III

1. Explain the operation of Basic d.c chopper circuit with neat diagrams

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2. What are the two control strategies applied to d.c chopper? Explain them in detail with waveforms.3. Classify the chopper circuits and explain them in detail.4. Draw the circuit diagram of four quadrant chopper. Explain with neat waveforms.5. Draw the circuit diagram of two quadrant chopper. Explain with neat waveforms.6. Explain the working of Buck converter with neat waveforms. Derive the expression of peak-to-peak ripple current and peak-to-peak voltage across the capacitor.7. Explain the working of Boost converter with neat waveforms. Derive the expression of peak-to-peak ripple current and peak-to-peak voltage across the capacitor.8. Explain the working of Buck-Boost converter with neat waveforms. Derive the expression of peak-to-peak ripple current and peak-to-peak voltage across the capacitor.9. Explain the working of cuk converter with neat waveforms. Derive the expressions for peak-to-peak ripple currents in inductors L1 and L2 and peak-to-peak voltage across the capacitors C1 and C2.

UNIT IV1. Draw the basic circuit diagram of an inverter. Explain in detail with the neat diagrams.2. Explain the principle of operation of series inverter circuit with the neat wave forms and derive the expressions.3. Explain the principle of operation of parallel inverter circuit with the neat wave forms and derive the expressions.4. Draw the circuit diagram of three phase voltage source inverter and explain the following modesi) 1800 conduction modeii) 1200 conduction mode.5. Draw the circuit diagram of 3 – phase current source inverter and explain in detail.6. Draw the circuit diagram of single phase current source inverter and explain in detail.7. What is meant by pulse width modulation? Explain its use and its effect on inverters.8. Explain the working of single phase series resonant inverter with unidirectional switches.9. Explain the working of single phase series resonant inverter with bidirectional switches.

UNIT - V1. (i) Explain the working and derive the expression for average and rms values of

output voltage of a 1φ Half-wave A.C voltage regulator with Resistive load.(8)

(ii) Draw and explain the working of three possible bidirectional switches. (8)

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2. Explain the working and derive the expression for rms values of output voltage of a full-wave 1φ A.C voltage regulator with (i) Resistive load (ii) RL Load in discontinuous current mode of operation. (16)

3. (i) Explain the working of single phase ac voltage regulator working on-off control mode. Derive the expression for rms value of load voltage.(8)(ii) A single phase ac voltage controller with resistive load of R=10 Ω and the input voltage is Vs=120V, 60 Hz. The delay angle of thyristor T1 is α =π/2. Determine (a) the rms value of output voltage Vo, (b) the input PF and (c) the average input current. (8)

4. Explain the working of single-phase to single phase cycloconverter with bridge configuration. (16)

5. (i)Compare and contrast cycloconverters with d.c link converters. (8)(ii)Explain the working of matrix converter with neat sketch. (8)

6. Explain the working of two-stage sequence control for (i) R-Load (ii)R-L Load with neat circuit and waveforms.(16)

7. (i)With neat sketch explain the working of multi stage sequence control of A.C regulators. (10)(ii) Explain the basic working principle of cycloconverters (6)

8. Draw the schematic and explain the working of three phase to single phase cycloconverter with neat waveforms. (16)

9. Draw the schematic and explain the working of three phase to three phase cycloconverter with neat waveforms. (16)

10. A single phase full wave controller supplies RL load. The input rms voltage is Vs=120V, 60Hz. The load is such that L=6.5mH and R=2.5Ω. The delay angles of thyristors are equal:α1=α2=π/2. Determine (a) the conduction angle of thyristor T1,δ; (b) the rms output voltage V0, (c) the rms thyristor current IR; (d) the rms output current I0; (e) the average current of a thyristor IA; and(f) the input PF.

(16)

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