PE Course File 09

EEE DEPARTMENT

Course File for Power Electronics

Prepared by Mr.K.NAGESH Asst Prof. EEE Dept

POWER ELECTRONICS III Year B.Tech EEE I - SEM Course file - 2011 OBJECTIVE of POWER ELECTRON ELECTRONICS ICS SUBJECT: With the advent of semiconductor devices, revolution is taking place place in the power power transm transmiss ission ion distribu distributio tion n and utilizat utilization ion.. Th This is course course introduces the basic concepts of power semiconductor devices, converters and choppers and their analysis.

UNIT – I POWER SEMI CONDUCTOR DEVICES Syllabus: Thyristors – Silicon Controlled Rectifiers (SCR’s) – BJT – Power MOSFET – Power IGBT and their characteristics and other thyristors – Basic theory of  operation of SCR – Static characteristics – Turn on and turn off methods- Dynamic characteristics of SCR - Turn on and Turn off times -Salient points

Objectives:  History of Thyristors & its generation details –(SCR’s) 

Detailed Detailed Explanation Explanation of



Explanation on operation of SCR – Static characteristics Discussion on Turn on and turn off methods, Dynamic characteristics of SCR



POWER DEVICES DEVICES & Thyristors Thyristors their character characteristic istics s

Schedule:  Thyristors – Silicon Controlled Rectifiers (SCR’s)

-1 -1

 BJT their characteristics  Power MOSFET their characteristics  Power IGBT and their characteristics and other thyristors

-1 -1

 Basic theory of operation of SCR – Static characteristics

-1

 Turn on and turn off methods, Dynamic characteristics of SCR

-1

 Turn on and Turn off times -Salient points

-1

Reference Books: 1. Power Electronics – by M. D. Singh & K. B. Kanchandhani, Tata Mc Graw – Hill Publishing Company, 1998.

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2. Power Electronics : Circuits, Devices and Applications – by M. H. Rashid, Prentice Hall of India, 2nd edition, 1998

Assignment Questions: 1) Briefly Briefly discuss discuss the the operation operation of Power Power Device Devices s (a) BJT BJT (b) Power MOSFET (c)Power IGBT 2) Explain Explain in detail the the basic operation operation of SCR SCR with its charac characteris teristics. tics. 3) With neat neat sketches sketches discuss discuss the TURN TURN ON & TURN OFF methods methods of an SCR SCR 4) Explain Explain the V-I Character Characteristic istics s of Thyristors Thyristors with with elaborating elaborating the following following : (a) Latching current (b) Holding current (c) on-state and off-state condition (d) Turn-on and turn-off times (e) Finger voltage The volt voltag age e and and curr curren entt rati rating ngs s of a part partic icul ular ar circ circui uitt are are 3.3K 3.3KV V and and 750 750 5) The amps.SCRs with rating of 800V and 175 amps are available. The recommended minimum derating factor is 15%. Calculate min. series and parallel units required. Also calculate the values of resistance and capacitance to be used in the static and dynamic equalizing circuits if the max. forward blocking current for the SCRs is 25mA and ΔQmax is 50μC. Where ΔQmax is max. charge stored in thyristor rectangul gular ar pulse pulse of 30V with 10 us duration duration is applie applied d at the gate. The 6) A rectan average gate power dissipation of the thyristor is 0.5W and a peak gate drive power is 5W.Calculate the values of the series resistance to be connected in the gate circuit, the frequency and duty cycle of the triggering pulse. 7) (a) Explain Explain the the dynamic dynamic turn-on turn-on and turnturn-off off times times of SCR SCR with neat neat waveforms waveforms.. (b) Discuss the various thermal ratings of SCR. 8) Explain Explain the operation operation of series series connected connected and parallel parallel connected connected SCRs SCRs with neat circuit diagrams and their characteristics.

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Objective Questions: 1) Which Which semiconduct semiconductor or power device device out of the following following is not a current current triggered device? (a) Thyristor (b) GTO (c) Triac (d) MOSFET 2) A power BJT BJT has a high high interdigitat interdigitated ed base emitter emitter structur structure e (a) To reduce current crowding during turn-on/off and hence avoid second breakdown (b) To increase gain of the transistor (c) To increase the switching frequency (d) To increase its voltage rating. 3) The typic typical al value value of gain gain in a power power BJT BJT is (b) 1 (c) 10 (d) 1000 (a) 100 4) In a GTO, anode anode current current begins begins to to fall when when gate curren currentt (a) is negative negative peaks at at time t = 0 (b) is negative peak at t = storage period t s (c) just begins begins to become become negative negative and t = 0 (d) none of these these 5) Thyristor Thyristor A has has rated gate gate current current of 2 A and thyrist thyristor or B is a rated gate current of 100 mA (a) thyristor A is a GTO and B is a conventional SCR SCR (b) thyristor B is a GTO and A is a conventional SCR SCR (c) thyristor thyristor B may operate operate as a transistor transistor (d) none of these these 6) As compar compared ed to to UJT, UJT, SUS (a) triggers triggers only in one directio direction n (b) does not have negative negative resistance characteristics characteristics (c) needs definite polarity of the applied voltage (d) triggers only at one particular particular voltage 7) In its application, application, an an SUS behaves behaves in the the same same way as as (a) UJT (b) SCR (c) Tunnel diode (d) None of these 8) Which Which of the the following following PNPN PNPN devices devices has has two gates? gates?

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(a) Triac (c) SUS

(b) SC SCS (d) Diac

9) Which of the following PNPN devices has a terminal for synchronizing purpose? (a) SCS (c) Diac

(b) Triac (d) SUS

10)

Which of the following devices is a three layer device? (a) SCS (b) SUS (c) Tr Triac (d) Di Diac

11)

Which of the following methods will turn SCS off? (a) Applying negative pulse to the anode (b) Applying a positive pulse to the anode gate (c) Applying negative pulse to the cathode gate (d) All of these

12)

Which of the following PNPN devices does not have a gate terminal? (a) Triac (b) SCS (c) SUS (d) Complementary SCR

13)

In a GTO, anode current begins to fall when gate current (a) is negative peak at time t = 0 (b) is negative peak at time t = storage time ts (c) just begins to become negative at t = 0 (d) none of these

14)The device which cannot be triggered by voltage of either polarity is (a) Diac (b) Triac (c) Schottkey diode (d) SUS

15)

A triac and SCR are compared (a) Both are unidirectional devices (b) Triac requires more current for turn-on than SCR at a particular voltage (c) Triac has less time for turn-off than SCR (d) Both are available with comparable voltage and current ratings

16)

The uncontrolled electronic switch employed in power-electronic converters is (a) Thyristor (b) Bipolar junction transistor (c) Diode

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(d) MOSFET

17)

18)

The triac can be used only in (a) invert inverter er (b) rectif rectifier ier (c) multiquadra multiquadrant nt chopper chopper (d) cycloconv cycloconverter erter

Power MOSFET is a (a) voltage voltage controlled controlled device (b) current current controlled controlled device (c) frequency frequency controll controlled ed device (d) none of of the above above

19)

When transistors are used in series or parallel, a snubber circuit is

used to (a) control control the current current (b) control control the voltage voltage (c) limit limit di/d di/dtt (d) all of thes these e

20)

Which of the following is preferred for VHF/UHF applications? (a) (a) BJT BJT (b) MOSFET MOSFET (c) (c) SIT (d) (d) IGBT IGBT

21)Which of the following thyristors are gate turned off device? I. Gate turned off thyristor II. State In Induction th thyristor III. III. MOSMOS-co con ntrol trolle led d thy thyris ristor tor (a)I only (b) II only (c) I and II (d) I, II and III

22)

In a power-MOSFET, switching times are of the order of few (a) second seconds s (b) millisecond milliseconds s (c) micros microseco econds nds (d) nanoseconds

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23)

A switched-mode power-supply operation at 20 kHz to 100 kHz range uses as the main switching element: e lement: (a) Thyristor (b) MOSFET (c) Triac (d) UJT

24)

The MOSFET switch in its on-state may be considered equivalent to (a) resist resistor or (b) induct inductor or (c) capaci capacitor tor (d) battery

25)

A triac is effectively (a) antiparallel connection of two thyristors (b) antiparallel connection of a thyristor and a diode (c) antiparallel antiparallel connecti connection on of two diodes (d) two thyristor, in parallel to increase increase the current capacity of the device device

26)

Peak inverse rating of a triac (a) is the same same as that of a thyristor thyristor (b) is greater than that of a thyristor (c) is inferior and very very much less than that a thyristor thyristor (d) is not very significant due to the nature of its application application

27)

A reverse conducting thyristor is effectively (a) two thyristors thyristors in antiparallel antiparallel (b) a diode connected connected antiparallel with a thyristor (c) two diodes diodes in antipar antiparallel allel (d) two thyristors thyristors connected connected in parallel

28)

A Gate-turn-off thyristor (a) requires a special special turn-off circuit like a thyristor (b) can be turned-off by removing the the gate-pulse (c) can be turned-off by a negative current current pulse at the gate (d) can be turned-off by a positive current current pulse at the the gate

29)

A GTO like all a ll the other power semiconductor devices requires protection against (a) rates of change of forward current current and forward voltage (b) rate of change change of current current alone (c) rate of chang change e of voltage voltage alone (d) rates of change of forward current and forward voltage and over voltages and currents

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30)

The inductance of snubber circuit and capacitance of snubber of a

GTO (a) increase increase the rate of turn-of turn-off  f  (b) make the turn-off turn-off very very slow (c) cause over voltages and spikes of voltage during turn-off  (d) cause over voltages and spikes of voltage during turn-on

31)

An amplifying gate thyristor has (a) the advantages of high high gate current at low level gate drive. (b) a poor di/dt rating even at high gate current current (c) its di/dt improvi improving ng only at high gate current current (d) very slow spreading spreading velocity velocity

32)

A BJT operates as a switch (a) under small small signal condition conditions s (b) with no signal signal condition condition (c) in the active region of transfer transfer characteristic characteristic (d) under large signal signal conditions conditions

33) The temperature coefficient of resistivity for power BJT is (a) positi positive ve (b) negati negative ve (c) (c) zero zero (d) none

34)

The main cause of the second breakdown in power BJT is (a) existence existence of the drift drift layer (b) low thicknes thickness s of base (c) current crowding crowding and negative temperature temperature coefficient of resistivity resistivity (d) (d) none none

35)

The turn-off snubber is connected in power BJT (a) to reduce reduce the turn-on turn-on losses (b) to reduce reduce the turn-off turn-off times (c) to divert the switching switching loss from the transistor to the snubber (d) (d) none none

36)

The (a) high (b) high (c) high (d) (d) all all

anti saturation arrangement ensure switching speed speed but high on state power loss switching speed speed and low on-state on-state power loss switching switching speed and high breakdown voltage

37)The conductivity modulation in power BJT (a) reduces reduces the turn-on turn-on time (b) reduces the the on-state voltage drop (c) increase increase the on-state on-state voltage voltage drop (d) (d) all all

38)

The SOA of a power device

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(a) gives the maximum operating temperature (b) specifies the maximum voltage and current (c) is an area in which the operating point of the device must be located for its safe operation. (d) (d) None None

39)

A transistor cannot be protected by a fuse because (a) a fuse of that current current rating is not available (b) its thermal time time constant is very less less (c) over temperature limit of power power transistor is high (d) none of of the above above

40)

The operating frequency of a power MOSFET is higher than a power BJT because (a) it is a majority carrier carrier device device (b) it has an insulated insulated gate gate (c) drift layer layer is absent absent in it it (d) its gain is infinite

41)

The on-state voltage drop of a power MOSFET is higher than a power BJT because (a) it has no no drift drift layer (b) conductiv conductivity ity modulation modulation is absent absent (c) its current current capacity capacity is higher higher (d) both a & b

42)

Paralleling of MOSFET is quite easier because (a) it has a positive temperature coefficient of resistivity (b) its on-state voltage drop is much lesser (c) its gate-drive gate-drive circuits circuits are simpler simpler (d) conductiv conductivity ity modulation modulation is absent absent

43)

For a MOSFET, snubber circuits (a) are very much essential to give it a dv/dt protection. protection. (b) are not essential due to large SOA, however however are still recommended (c) are neve neverr used used (d) none of of the above above

44)

A device is said to have a symmetric blocking capability if  (a) it blocks forward forward and reverse voltages voltages of equal or comparable magnitudes (b) it blocks only reverse reverse voltages (c) it blocks only only forward forward voltages voltages (d) none of of the above above

45)

The turn-off gain of the GTO is of the order of  (a) (a) 1.2 1.2

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46)

(b) (b) 3.5 (c) (c) 10.2 10.20 0 (d) > 100 The body layer is connected to source terminal in a MOSFET in order

to (a) reduce the on-state power power dissipation dissipation (b) increase the the speed of operation (c) avoid the the latch-up latch-up in MOSFET MOSFET (d) (d) none none

47)

MOS devices should be handled by the package, not by leads to (a) avoid the damage damage due to handling handling (b) avoid damage damage due to static charge charge (c) avoid damage damage due due to moisture moisture (d) none of above above

48)

An IGBT structure is obtained by (a) adding an insulated gate to the BJT and adding a p + layer. (b) by combining combining a MOSFET MOSFET and BJT (c) none none of the above above (d) both both a & b

49)

The temperature coefficient of resistivity of an IGBT is (a) positi positive ve (b) negati negative ve (c) (c) flat flat (d) (d) none none

50)

The SOA of IGBT is better than that of a power transistor because (a) it is a majority carrier carrier device device (b) it is a minority carrier device (c) second breakdown is absent due to its flat temperature coefficient of  (d) Temperatur Temperature e

51) The maximum operating frequency of an IGBT is approximately (a) 10 kHz

(b) 50 kHz (c) 100 kHz (d) none

52)

The reduction in the on-state voltage drop in IGBT takes place due to (a) added p + layer in the IGBT structure (b) conductiv conductivity ity modulation modulation (c) the n drift drift laye layerr

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(d) (d) none none

53)

The nonpunch through IGBT has a (a) symmetric symmetrical al blocking blocking capacity (b) (b) asymmetrical asymmetrical blocking blocking capacity capacity (c) no blocking blocking capacit capacity y at all (d) (d) none none

54)

The blocking capacity of a punch-through IGBT is (a) symmetrical (b) asymmetrical (c) linear (d) none of the above

55)

A MOSFET controlled thyristor has a gate-turn-off capability because (a) The structure does not have a latching capability (b) There are separate MOSFET, for turn-on turn-on and turn-off  turn-off  (c) It is a minority minority carrier carrier device device (d) It is a majority carrier carrier device

56)

The turn-off time of an MCT is approximately (a) 0.1 ms (b) 1 ms (c) 2.3 ms (d) 10.20 ms

57)

Which of the following conditions is necessary for triggering system for thyristors? (a) It should be synchronized synchronized with the the main supply (b) It must use use separate power power supply (c) It should should provide provide a train of pulses pulses (d) None of these these

58)

For thyristors, pulse triggering is preferred to dc triggering because (a) Gate dissipati dissipation on is low (b) Pulse Pulse system is simpler simpler (c) Triggering system is required for a very short duration (d) all of thes these e

59) The SCR is turned-off when the anode current falls below (a) forward current rating (b) breakover voltage (c) holding current (d) latching current

60) In a SCR circuit, the angle of conduction can be changed by changing (a) anode voltage (b) anode current (c) forward current rating

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(d) gate current

61) The normal way to close a SCR is by approximate (a) Gate current (b) Cathode current (c) Anode current (d) Forward current

62) If gate current is increased, the anode-cathode voltage at which SCR closes is (a) Increased (b) Decreased (c) Maximum (d) Least

63) A conducting SCR can be opened by reducing __________ to zero. (a) Supply voltage (b) Gate voltage (c) Gate current (d) Anode current

64) With gate open, a SCR can be turned-on by making supply voltage (a) minimu minimum m (b) revers reverse e (c) equal to to cathode cathode voltage voltage (d) equal to break-over break-over voltage voltage

65) A SCR is a _________ switch. (a) two directional (b) unidirecti unidirectional onal (c) three-directional (d) four-directional

66) The turn-off time of thyristor is 30 m sec at 50°C. It.s turn-off time at 100° is (a) same (b) 15 m sec (c) 60 m sec (d) 100 m sec

67) Turn-off time of a thyristor effects its (a) Operating voltage (b) Operating frequency (c) Overload capacity (d) Thermal behavior

68) The di/dt capability of a thyristor increases (a) When the gate current is zero

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(b) When the gate current increases (c) When the gate current decreases (d) When the anode to cathode voltage rating increases. 69) In optical triggering technique, LASCR is connected in (a) Gate circuit circuit of each thyristo thyristorr (b) Anode circuit circuit of each thyristor thyristor (c) Gate circuit circuit of only one thyristo thyristorr (d) Anode circuit of only one one thyristor. 70) Dynamic equalizing networks are used to limit the (a) Rate of rise of current (b) Rate of rise of voltage (c) Rate of rise of temperature (d) Rate of rise of pressure 71) Thermal runaway of a thyristor occurs because (a) Positive resistance coefficient of the junction (b) Negative resistance coefficient of the junction (c) If the latching current is more (d) If the thyristor is loaded with wider current pulses. 72) A positive voltage is applied to the gate of a reverse biased SCR (a) This inject more electrons into junction J1 (b) This increases reverse leakage current into anode (c) Hesting of junction is unaffected (d) b& c 73) At a room temperature of 30°C, minimum voltage and current required to fire a SCR is (a) 3 V, 40 mA (b) 0.6 V, 40 mA (c) No limit (d) 3 V, 100 mA 74) When the SCR conducts, the forward voltage drop (a) Is 0.7 V (b) Is 1 to 1.5 V & Increases slightly slightly with load current current (c) Is 5 V (d) Remains constant with load current 75) The turn-on time of a SCR with inductive load is 20 ms. The pulse train Frequency is 2.5 kHz with a mark/space ratio of 1/10, then (a) The SCR will turn-on (b) The SCR will not turn-on (c) The SCR will turn-on if inductance is removed (d) The SCR will turn-on if pulse frequency is increased to two times. 76) An SCR is rated at 75 A peak, 20 A average. The greatest possible delay in

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the trigger angle if the dc is a rated value is (a) 47.5° (b) 30° to 45° (c) 75.5° (d) 137° 77) In a SCR (a) gate current is directly proportional to forward breakover voltage. (b) as gate-current is raised, forward breakover voltage reduces. (c) gate-current has to be kept ON continuously for conduction. (d) forward-breakover voltage is low in the forward blocking state. 78) There are only silicon controlled rectifiers and not germanium because (a) Si is available as compared to Ge. (b) Only Si has stable off-state. (c) Si only has the characteristic a1 + a2 < 1 at low collector currents and reaches 1 at high currents. (d) Both b & c 79) For normal SCRs, turn-on time is (a) less than turn-off time tq, (b) more than tq (c) equal to tq (d) half of tq 80) The average on-state current for an SCR is 20 A for conduction angle of  120°. The average on-state current for 60° conduction angle will be (a) 20 A (b) 10 A (c) Less than 20 A (d) 40 A 81) The average on-state current for an SCR is 20 A for a resistive load. If an inductance of 5 mH is included in the load, then average on-state current would be (a) more than 20 A (b) less than 20 A (c) 15 A (d) 20 A 82) In a thyristor, anode current is made up of  (a) Electrons only (b) electrons or holes (c) Electron and holes (d) none of these 83)When a thyristor gets turned ON, the gate drive (a) should not be removed as it will turn-off the SCR (b) may or may not be removed (c) Should be removed (d) Should be removed in order to avoid increased losses and higher junction Temperature 84) The forward voltage drop during SCR-on state is 1.5 V. This voltage drop (a) Remains constant and its independent of load current (b) Increases lightly with load current

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(c) Decreases slightly with load current (d) Varies linearly with load current

Answers: 1) d

2) a

3) c

4) b

5) a

6) d

7) a

8) b

9) d

10) d

11) d

12) c

13) b

14) d

15) b

16) c

17) c

18) a

19) c

20) c

21) c

22) d

23) b

24) c

25) a

26) c

27) b

28) c

29) d

30) c

31) a

32) d

33) b

34) c

35) c

36) a

37) b

38) c

39) b

40) a

41) d

42) b

43) b

44) a

45) b

46) c

47) b

48) a

49) c

50) c

51) a

52) b

53) a

54) b

55) b

56) b

57) a

58) d

59) c

60) d

61) c

62) b

63) d

64) d

65) b

66) c

67) b

68) b

69) a

70) b

71) b

72) d

73) a

74) b

75) a

76) b

77) b

78) d

79) a

80) c

81) a

82) c

83) d

84) b

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UNIT – II

DEVICES AND COMMUTATIO COMMUTATION N CIRCUITS

Syllabus: Two transistor analogy – SCR - UJT firing circuit ––– Series and parallel connections of SCR’s – Snubber circuit details – Specifications and Ratings of SCR’s, BJT, BJT, IGBT IGBT - Numeri Numerical cal probl problems ems – Line Line Commut Commutati ation on and Forced Forced Commut Commutatio ation n circuits.

Objectives:  Detailed Explanation on Two Transistor analogy.  Explanation of UJT Firing Circuit operation.  Discussion of Series & Parallel Connection of SCRs.  Explanation of Snubber Circuits for SCRs protection.  Discussion of Specifications & Ratings of Different power Devices.

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 

Explanation of Line Commutation & Forced Commutation Circuits. Solving the Numerical problems.

Schedule:  Two transistor analogy – SCR

-1

 UJT firing circuit

-1

Series and parall parallel el connec connectio tions ns of SCR’s SCR’s  Series

-

 Snubber circuit details

-1

 Specifications and Ratings of SCR’s, BJT, IGBT  Numerical problems

-1 -1

 Line Commutation and Forced Commutation circuits

-1

1

Reference Books: 1) Power Power Electronic Electronics s – by M. D. Singh Singh & K. B. Kanchandh Kanchandhani, ani, Tata Mc Graw Graw – Hill Publishing Company, 1998. 2) Power Electronics – By P.S Bimbra Khanna Publishers.

Assignment Questions: 1) Derive the Static equalizing and dynamic equalizing parameters in case of series and parallel connected SCRs. 2) (a) What is the importance of Surge current rating of a thyristor, explain in detail. (b) A thyristor has half-cycle surge current rating of 1000mA for a 50Hz supply. Calculate its one-cycle surge current rating and I2t rating. 3) (a) Explain the necessity of series and parallel connection of SCRs. (b) What is String efficiency in series and parallel connections. (c) What are the problems arising in series and parallel connections.

4) The latching current of a thyristor with DC voltage source of 100V is 50mA. Calculate the value of minimum width of the gate pulse current when connected to a pure inductive load of 1H. Compute the effect, if a resistance of 10 ohms is connected in series with the load.

5) (a) Draw the equivalent circuit of a UJT and explain its working. (b) Describe the VI characteristics of a UJT. Clearly explain its negative resistance nature. 6) (a) Explain Explain the parallel parallel operation operation of SCR’s SCR’s and and define and and derive string string efficiency. (b) Explain various protection methods for SCR.

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7) Explain the need of Snubber Circuits in SCRs. 8) With neat Sketche Sketches s Explain Explain Line commutation commutation & Forced Forced Commutati Commutation on Circuits Circuits & Operation.

Objective Questions: 1) A Thyristor can be termed as (a) DC switch (b) AC switch (c) Both A or B are correct (d) square-wave switch 2) On-state voltage drop across a thyristor used in a 250 V supply system is of the order of  (a) 100-110 V (b) 240-250 V (c) 1-1.5 V (d) None of these 3) In a thyristor, ratio of latching current to holding current is (a) 0.4 (b) 1.0 (c) 2.5 (d) None of these 4) Gate characteristics characteristics of a thyristor thyristor (a) is a straight line passing through the origin (b) is of the type, V V = a + b. IV (c) is a curve between V g and I g (d) has a spread between two curves of V g = I g . 5) In an SCR, anode current flows over a narrow region near the gate during (a) delay time d (b) rise time tr and spread time tp (c) td and tp (d) td and tr 6) Turn-on time for an SCR is 10 msec. If an inductance is inserted in the anode circuit, then the turn-on time will be (a) 10 msec (b) less than 10 msec (c) more than 10 msec (d) about 10 msec

7) Turn-off time of an SCR is measured from the instant (a) anode current becomes zero (b) anode voltage becomes zero (c) anode current and anode voltage become zero at the same time

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(d) gate current becomes zero. 8) A forward voltage can be applied to an SCR after its (a) anode current reduces to zero (b) gate recovery time (c) reverse recovery time (d) anode voltage reduces to zero 9) For an SCR, with turn-on time of 5 msec, an ideal trigger pulse should have 1 (a) short rise time with pulse width = 3 msec. 2 (b) long rise time with pulse width = 6 msec. 3 (c) short rise time with pulse width = 6 msec. (d) long rise time with pulse-width = 3 msec. 10) Turn-on time of an SCR SCR in series with RL circuit can be reduced reduced by 1 (a) increasing circuit resistance R (b) decreasing R 2 3 (c) increasing circuit inductance 4 (d) decreasing L 5 11) Turn-on time of an SCR can be reduced by using a 6 (a) rectangular pulse of high amplitude and narrow width 7 (b) rect ectangu angula larr pu pulse lse of of low low ampl amplit itud ude e an and wi wide wid width 8 (c) triangular pulse 9 (d) trapezoidal pulse 12) Specification sheet for an SCR gives its maximum rms-on-state current as 35 A. This rms rating for a conduction angle of 120° would be (a) more than 35 A (b) less less than 35 A (c) 35 A (d) None of these 13) Surge current current rating of an SCR SCR specifies the maximum maximum 1 (a) re repetitive cu current wi with si sine wa wave 2 (b) non-repetitive current with rectangular wave (c) non-repetitive current with sine wave 3 (d) repetitive current with rectangular wave

14)

In the circuit given below, the function of the

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transistor is

1 2 3

(a) to provide control signal to trigger SCR (b) to make SCR-ON (c) to make SCR-OFF (d) to amplify anode-current

15) In a thyristor, thyristor, the magnitude of the the anode-current will will 1 (a) increase if gate-current is increased 2 (b) decrease if gate current is decreased 3 (c) increase if gate-current is decreased 4 (d) not change with any variation in gate current 16) An SCR does not conduct for a certain value of load resistance. In order to make it ON, it is necessary to 1 (a) decrease the lo load resistance (b) increase the re resistance 2 (c) increase the gate-pulse (d) none of these 17) Most SCRs can can be turned-off turned-off by voltage voltage reversal reversal during during negative negative half-cycle half-cycle of  the ac supply for 1 (a) all frequencies (b) frequencies upto 300 Hz 2 (c) (c) frequen frequencie cies s upto upto 30 kHz kHz (d) freq frequen uencie cies s upto upto 300 kHz kHz 3 18)

1 2 3 4

In circuit given below, in order to make a conducting SCR off, it is necessary to

(a) make other SCR-off   (b) make other SCR-ON (c) rev reverse erse the the pola polarrity ity o off the the appl applie ied d vol volta tage ge (d) remov emove e the the gate gate-c -cur urrrent ent of cond onducti ucting ng SCR

19) If a diode is connected in antiparallel with a SCR, then 1 (a) (a) both both turn turn-o -off ff powe powerr loss loss and and tur turnn-of offf time time decr decrea ease se 2 (b) (b) tur turnn-of offf pow power er loss loss decr decrea ease ses s but but turn turn-o -off ff time time incr increa ease ses s 3 (c) (c) tur turnn-of offf pow power er loss loss incr increa ease ses, s, but but tur turnn-of offf tim time e dec decreas reases es 4 (d) none of the above

Prepared by Mr.K.NAGESH Asst Prof. EEE Dept

1 2 3 4 5

20) In a commutation circuit, employed to turn-off an SCR, satisfactory turn-off is obtained when (a) circ ircuit tur turn-of n-offf tim time e < dev device ice ttur urnn-of offf tim time (b) circ ircuit uit tu turn-o rn-off ff tim time > devi devic ce ttu urn-o rn-off ff tim time (c) cir circ cuit uit tim time e c con onst stan antt > devi devic ce ttu urn-o rn-off ff tim time (d) circ ircuit uit tim time e c con onst stan antt < devic evice e ttu urn-o rn-off ff time time

Gate Triggering Circuits 21) UJT oscillators are used for gate-triggering of thyristors for for (a) Better phase control (b) Snap action 0 (c) Being cheap and simple 1 (d) none of the above 2 22) It is recommended to use UJT oscillator for gate-triggering of the thyristors mainly because (a) it is fairly simple (b) it provides sharp firing pulses (c) it is less expensive (d) none of the above 0 23) A device that does not exhibit negative resistance characteristic is (a) FET (b) UJT (c) tunnel diode (d) SCR 1 24) A UJT has one base resistance of 5.2 k. Its intrinsic stand of ratio is 0.67. The inter-base voltage of 12 V is applied across the two passes. The value of base current will be 3 (a) 1.16 mA (b) 1.28 mA 4 (c) 1.34 mA (d) 1.41 mA 5 25) A PUT has VBB = 24 V, & R B1 = 3RB2. The value of h will be 6 (a) 1/3 7 (b) 2/3 8 (c) 3/4 9 (d) 4/3 10 26) A PUT relaxation oscillator has values VBB = 15 V, R = 22 kW, R 2 = 6 kW, IP = 100 mA, VV = 1 V, IV = 7 mA, C= 1 mF, R K = 100 kW, R 3 = 12 kW. The value of  VPwill be 11 (a) 0.7 V 12 (b) 10 V 13 (c) 10.7 V

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14 15

(d) 15 V

27) In a UJT, UJT, intrinsic stand off ratio h is typically

1 (a) 0.2 0

(c) 0.7

(b) 0.4 (d) 0.99

2 28) When a UJT is used for for triggering of an SCR, the waveshape waveshape of the voltage is a 3 (a) Sine Wave 4 (b) Sawaw-toot tooth h wave ave 5 (c) Trap Trape ezo zoid idal al wav wave 6 (d) Square wave 7 29) Optocouplers combine 0 (a) SITs and BJTs 8 (b) IGBTs and MOSFETs 9 (c) Powe ower tran transf sfor orm mer and and sil silicon icon tran transi sis stor tors 10 (d) (d) Infr Infrar ared ed lig light ht-em -emit itti ting ng dio diode de and and sili silico con n phot photot otra rans nsis isto torr

2 30) In a UJT, maximum value of charging resistance is associated with

1 2

(a) Peak Point (b) valley point (c) any point between peak & valley point (d) after the valley point

Series and Parallel Operation of Thyristors 31) Equalising circuits are provided across each SCR in series operation to provide uniform 1 2 3

(a) curre urrent nt dis distrib tribu ution tion (b) (b) vo voltag ltage e dis disttrib ributio ution n (c) firing of SCRs (d) all of the above 32) In series connected thyristors (a) L is used for tuning out junction capacitance (b) L&C is used for filtering out the ripple (c) R, C is called a snubber circuit (d) L is intended to increase di/dt at switch on

4

5

6 7 8

33) Two identical SCRs are placed back-to-back in series with a load. If each is fired at 90°, a dc voltmeter across the load will read (a) zero (b) 2/ p ◊ peak voltage (c) 1/ p ◊ peak voltage (c) 4/ p ¥ peak voltage

9 34)In order to obtain static voltage equalisation in series connected SCRs, connections are made of 

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10 11 12

(a) one reactor against the string (b) resistors of different values across each SCR (c) resistors of same value across each SCR (d) one reactor in series with the string

35) Derating factors for parallel connection of thyristors are normally in the range 13 (a) 0.5 to 1% 14 (b) 1 to 5% 8 to 20% 15 (c) 16 (d) 25 to 50% 17 36) To obtain the highest possible string efficiency, the SCRs connected in string must have 18 (a) different characteristics 19 (b) same characteristics 20 (c) same voltage ratings only 21 (d) same current ratings only 22 37) String efficiency is used for measuring the 23 (a) voltage rating of SCRs 24 (b) current rating of SCRs 25 (c) temperature rating o off S SC CRs degree of utilization of SCRs 26 (d) 38) In series string, thyristor having the highest leakage resistance or low voltage current will share 27 (a) larger portion of the applied voltage 28 (b) smaller po portion of of th the ap applied vo voltage 29 (c) larger portion of current 30 (d) smaller portion of current 39) Dynamic equalizing networks are used to limit the (a) rate of rise of current (b) rate of rise of voltage (c) rate of rise of temperature (d) rate of of ri rise of of pressure 40) In optical triggering technique, LASCR is connected in 1 2 3 4

(a) (b) (c) (d)

gate circuit of each thyristor anode circuit of each thyristor gate circuit of only one thyristor anode circuit o off only one thyristor.

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Answers: 1) a

2) c

3) c

4) d

5) d

6) c

7) a

8) b

9) c

10) d

11) a

12) c

13) c

14) c

15)

16) a

17) c

18) b

19) d

20) b

21) a

22) b

23) a

24) a

25) c

26) c

27) c

28) b

29) d

30) a

31) b

32) c

33) a

34) c

35) c

36) b

37) d

38) a

39) b

40) a

UNIT – III

SINGLE PHASE HALF CONTROLLED CONVERTERS

d

Syllabus: Phase control technique – Single phase Line commutated converters – Mid point and Bridge connections – Half controlled converters with Resistive, RL loads and RLE load– Derivation of average load voltage and current -Active and Reactive power inputs to the converters without and with Free wheeling Diode – Numerical problems

Objective   

Detailed Explanation on Phase Control Technique. Single Single Phase Phase Half Controlled Controlled Converters Converters Circuits Circuits Explanation Explanation with different loads & Free wheeling Diode Solving the Numerical problems

Schedule;  Phase control technique

-1 -1

 Single phase Line commutated converters  Mid point and Bridge connections  Half controlled converters with Resistive, RL loads and

-1

 RLE load

-2

 Derivation of average load voltage and current  Active and Reactive power inputs to the converters

-1

 Without Free wheeling Diode

-1

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

Active and Reactive power inputs to the converters  With Free wheeling Diode  Numerical problems

-1 -1

Reference Books: 1) Power Power Electronic Electronics s – by M. D. Singh Singh & K. B. Kanchandh Kanchandhani, ani, Tata Mc Graw Graw – Hill Publishing Company, 1998. 2) Power Power Electronic Electronics s – By P.S P.S Bimbra Bimbra Khanna Khanna Publishers. Publishers.

Assignment Questions: 1) Explain the operation of a singe phase half wave converter for R-load with neat circuit diagram and necessary waveforms. Also derive the output average voltage and current for

α = 30 , α = 45 .α = 60 . 0

0

0

2) Single phase half controlled bridge converter feeds a n inductive load. Determine the average load voltage and load current for a firing angle of 300 and 1200 respectively. The input a.c. voltage is 230V and load resistance is 10 ohms and inductance is 10mH. 3) Explain Explain the Operation Operation of Single Single phase phase Half control controlled led Converter Converter With With R & RL &  RLE Loads & Plot the necessary wave forms. 4) Explain Explain the need need of Fly Wheel Wheel Diode Diode in the the Converter Converter operat operation ion 5) Derive Derive the output output voltage voltage & current current equations equations for for the single single phase Half  Half  controlled Converter with suitable considerations.

Objective Questions: 1) A converter means (a) Rectifier (b) Inverter (c) both a & b (d) none 2) A Rectifier means which converts (a) a.c to d.c (b) d.c to a.c (c) both (d) none 3) A single phase half-wave controlled rectifier has 400 sin 314 t volts as the input voltage and resistor R is the load. For firing angle of 60° for the SCR, the a verage

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output voltage in volts is (a) 400/π

π (c) 240/ π (d) 360/ π (b) 300/

4) A freewheeling diode is placed across the dc load (a) to prevent reversal of load voltage (b) to permit transfer of load current away from the source (c) Both (a) and (b) above (d) None of the above

5) In a Single phase semi converter the avg Output Voltage given by

6)

7)

8) A Single Phase Semi Converter uses (a) 2 diodes & 2 SCRs (b) 4 SCRs (C) 3 Diodes 7 1 SCR

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(d) None. 9) Referring to the Fig.the type of the load is

(a) inductive load (b) resistive load (c) dc motor (d) capacitive load.

10) A half-wave thyristor converter supplies a purely inductive load, as shown in Fig. If the triggering angle of the SCR is 120°, the extinction angle will be

(a) 240° (b) 180° (c) 200° (d) 120°

Answers: 1) c

2) a

3) b

4) c

7) a

8) a

9) c

10) d

5) a

6) d

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UNIT – IV SINGLE PHASE FULLY CONTROLLED CONVERTERS Syllabus: Fully controlled converters, Mid point and Bridge connections with Resistive, Resistive, RL loads and RLE load– Derivation Derivation of average average load voltage voltage and current current – Line Line commut commutate ated d invert inverters ers -Acti -Active ve and Reacti Reactive ve power power inputs inputs to the conver converter ters s without and with Free wheeling Diode, Effect of source inductance – Derivation of  load voltage and current – Numerical problems.

Objectives:  Single Phase Fully Controlled Converters Circuits Explanation with R,RL,RLE loads & Free wheeling Diode with the output quantities Derivations.  Discussion of source inductance effect on converter operation.  Solving the Numerical problems.

Schedule:  Introduction to Fully controlled converters  Mid point and Bridge connections with Resistive, RL loads and RLE load  Derivation of average load voltage and current  Line commutated inverters  Active and Reactive power inputs to the converters without Free wheeling Diode  Active and Reactive power inputs to the converters with  Free wheeling Diode  Effect of source inductance  Derivation of load voltage and current  Numerical problems

-1 -1 -1 -1 -1 -1 -1 -1 -1

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Assignment Questions: 1) Explain the operation of a single phase full wave mid-point converter with R-load with the help of circuit and output waveforms with respect to supply voltages. Derive the output voltage for

α = 30 , α = 45 .α = 60 . 0

0

0

2) A single phase fully controlled bridge is used for obtaining a regulated converter dc output voltage. The rms value of ac input voltage is 230V and firing angle is maintained at 600, so that the load current is 4A. (a) Calculate the d.c. output voltage and active and reactive power input. (b) Assuming load resistance remains same and if free-wheeling diode is used at the output, calculate dc output voltage. The firing angle is maintained at 600.

3) (a) Derive the expression for the input power factor of single phase fully controlled bridge rectifier. (b) Explain the effect of freewheeling diode in detail. Also, justify the statement  “Freewheeling diode improves the power factor the system”. 4) Explain the Effect of Source inductance inductance on the converter converter operation. 5) Derive the output voltage & current equation for the Full converter operations with R, RL, RLE loads

Objective Questions: 1) A single phase full-wave mid-point thyristor converter uses a 230/200 V Transformer with centre taps on the secondary side. The P.I.V. per thyristor is (a) 100 V (b) 141.4 V (c) 200 V (d) 282.8 V 2) A 1 – phase phase full controlled controlled converter converter uses a) 2 diod diodes es & 2 SCRs SCRs b) 4 SCRs c) Both a & b d) None e) 3) In a Single phase full converter Bridge the Output Voltage given by

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4) For continu continuous ous conduction conduction,, in a single-phase single-phase full converte converterr each pair of SCRs conducts for

5) For discontinuous load current and extinction angle single- phase full converter, each SCR conducts for

β > π radians, in a

6) In a single-phase full converter, if  α and β are firing and extinction angles respectively, then the load current is discontinuous if 

7) In a single phase converter with discontinuous conduction and extinction angle β >  β >

π , freewheeling diode conducts for

8) In a single-phase converter with discontinuous conduction and extinction angle β <  β <

π , the freewheeling diode conducts for

9) In a single-phase full converter, if output voltage has peak and average values of  325 V and 133 V respectively, then the firing angle is (a) 40°

(b) 50°

(c) 70°

(d) 130°

10) In a single-phase full-converter, if the load current is I and ripple-free, then the

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average thyristor current is (a) I/2

(b) I/3

(c) I/4

(d) I/5

11) In a single-phase full-converter, the number of SCRs conducting during overlap is (a) 1 (b) 2 (c)3 (d) 4 12) A single-phase, one pulse controlled circuit has resistance and counter emf load and 400 sin 314 t as the source voltage. For a load counter emf of 200 V, the range of firing angle control is (a) 30° to to 150° 150° (b) 30° to 180° (c) 60° to 120° (d) 60° to 180°

13) The effect of the source inductance on the performance of the single-phase and three-phase full-converters is to (a) reduce the ripples in the load current (b) make discontinuous current as continuous (c) reduce the output voltage (d) increase the load voltage

Answers: 1) d

2) b

3) a

4) b

5) b

6) a

7) b

8) d

9) b

10) c

11) d

12) a

13) c

UNIT – V

THREE PHASE LINE COMMUTATED CONVERTERS

Syllabus: Three phase converters – Three pulse and six pulse converters – Mid point and bridge connections average load voltage With R and RL loads – Effect of  Sour So urce ce indu induct ctan ance ce–D –Dua uall conv conver erte ters rs (bot (both h sing single le phas phase e and and thre three e phas phase) e) Waveforms –Numerical Problems.

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Objective:   

Three Three Phase Phase Conver Converter ters s Circui Circuits ts Explan Explanatio ation n with with R,RL,R R,RL,RLE LE loa loads ds & Free Free wheeling Diode with the output quantities Derivations. Discussion of source inductance effect on converter operation. Solving the Numerical problems.

Schedule:  Three phase converters

-1

 Three pulse and six pulse converters  Mid point and bridge connections average load voltage With R and RL loads

-1

 Effect of Source inductance

-1 -1

 Dual converters (both single phase and three phase) -Waveforms

-2

 Numerical Problems

-1

Assignment Questions: 1) A three phase full converter converter is supplied supplied from a three phase 230V, 60Hz 60Hz supply. The load curre current nt is contin continuou uous s and has neglig negligibl ible e ripple ripple.. If the average average loa load d current current Idc = 150A and commutating commutating inductance inductance Lc = 0.1mH, determine the overlap angle when (a) α = 10 0 (b) α = 30 0 and (c) α = 600

2) Explain the operation of three phase fully controlled bridge converter with RL loads. loads. Describe Describe in detail with discontin discontinuous uous conduction conduction mode with associated associated waveforms.

3) A three phase, half wave controlled converter is connected to a 380V (line) supply. The load current is constant at 32A and is independent of firing angle. Find Find the average average load voltag voltage e at firing firing angle angle of 00 and 450, given that the thyristors have a forward voltage drop of 1.2V. What value of current and peak reverse voltage rating will the thyristor require and what will be the average power dissipation in each thyristor. six puls pulse e thyr thyris isto torr conv conver erte terr is conn connec ecte ted d to the the main mains s thro throug ugh h a 4) A six transformer of6% reactance. If the rms value of the voltage at the secondary of  the transformers transformers 415V, calculate calculate the voltage voltage regulation regulation.. Neglect Neglect resistance resistance in converter. The full load dc current is 200A. What is the value of commutation angle.

5) (a) Explain the operation of three phase, half controlled bridge converter with R load and associated waveforms.

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(b) Derive the expression for average load voltage for α = 30

.

Objective Questions: 1. For a three phase six pulse diode bridge rectifier ,the average output voltage in terms of maximum value of line voltage Vm is

’  2. In a three phase half wave diode rectifier ,the ratio of average output voltage to per phase maximum ac voltage is a. 0.955 b. 0.827 c. 1.654’  d. 1.169

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3. Each diode of a 3-Φ half wave diode rectifier conducts for a. 60 b. 120 c. 180 d. 90 4. Each diode of a 3-Φ ,6-pulse bridge diode rectifier conducts for a. 60 b. 120 c. 180 d. 90 5. In a 3- Φ half wave diode rectifier ,if the the per phase input voltage is 200V then the average output voltage is a. 233.91V b. 116.95V c. 202.56V d. 101.28V 6. In a three phase semi converter converter ,for firing angle less than or equal equal to 600 each thyristor anddiode conduct ,respectively for

7. In a three phase full wave diode rectifier , if Vm is the maximum value of line voltage ,then each diode is subjected to a peak inverse voltage of 

8. In a three phase half wave diode rectifier, dc output voltage is 230V .The peak inverse voltage across each diode is a. 481.7V b. 460V c. 345V d. 230V

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9. In a three phase half wave diode rectifier, the peak inverse voltage in terms of  average output voltage is a. 1.571 b. 900 c. 1200 d. 1800 10. In a three phase half wave diode rectifier, if Vm is the maximum value of per phase voltage ,then each diode is subjected to a peak inverse voltage of 

11. A three phase semi converter can work as a. converter for α =0 to 180 b. converter for α =0 to 90 c. inverter for α =90 to 180 d. inverter for α =0 to 90 12. In a three phase semi converter for firing angle equal to 1200 and extinction angle equal to 1110 , freewheeling diode conduct for a. 10 b. 30 c. 50 d. 110 13. In a three phase semi converter , for firing angle less than or equal to 600 ,free wheeling diode conducts for a. 30 b. 60 c. 90 d. zero degree 14. In a three phase semi converter , for firing angle equal to 900 ,and for continuous conduction ,each SCR and diode conducts for

15. In a three phase semi converter for firing angle equal to 1200 and extinction angle equal to 1100 , each SCR and diode conduct ,respectively for

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16. In a three phase full converter , the output voltage during overlap is equal to a. zero b. source voltage c. Source voltage minus the inductance drop d. average value of the conducting phase voltages 17. In a three phase full converter, if the load current is I and ripple free then the average thyristor current is a. 1/2 I b. 1/3I c. 1/4I d. I 18. In a three phase semi converter, the three SCRs are fired at an interval of  a. 60 b. 90 c. 120 d. 180 19. In a three phase full converter, the six SCRs are fired at an interval of  a. 30 b. 60 c. 90 d. 120 20. In a three phase full converter, the three SCRs pertaining to one group are fired at an interval of  a. 30 b. 60 c. 90 d. 120 21. In the circulating current mode of a dual converter, _ _ _ _ _ _ _ is inserted between the converters 1 and 2 a. reactor b. resistor c. fuse d. capacitor

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22. The effect of source inductance is to reduce the a. average dc output voltage b. dc current c. ac input voltage d. losses 23. In a single phase full converter, the output voltage due to overlap is a. zero b. source voltage c. Source voltage minus the inductance drop d. inductance drop 24. The angular period over which both the incoming and outgoing SCRs are conducting together is called a. Overlap angle b. firing angle c. extinction angle d. ignition angle 25. The total number of thyristors conducting simultaneously simultaneously in a 3-phase full converter with overlap considered has the sequence of  a. 3,3,2,2 b. 3,3,3,2 c. 3,2,3,2 d. 2,2,2,3 26. A four quadrant operation requires a. two full converters in series b. two full converters connected back to back c. two full converters connected in parallel; d. two semi converters connected back to back. 27. A dual converter using two full converters can give _ _ _ _ _ quadrant operation a. one b. two c. three d. four 28. Reactor limits the magnitude of _ _ _ _ _ _ _ in a circulating current type dual converter. a. circulating voltage b. circulating current c. load voltage d. load current

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29. In a dual converter with circulating current mode firing pulses to the two converters are so adjusted that = a. 120 b. 150 c. 90 d. 180 30. Two full converters connected back to back the same dc load is called a _ _ _ _ a. chopper b. inverter c. rectifier d. dual converter 31. A 1- Φ voltage controller is employed for controlling the power flow from 260V,50Hz source into a load consisting of .The value of maximum RMS load current and the firing angle are respectively a. 20A,00 b. 26A,00 c. 20A ,900 d. 26 A,900 32. In the case of AC voltage regulator ,control range of firing angle is given as a. Φ < ∞ < ,1800 b. α < Φ < ,1800 c. Φ < α < ,3600 d. α < Φ < ,3600 33. In a circulating current type dual converter ,the nature of the voltage across reactor is a. alternating b. pulsating c. direct d. triangular 34. In a dual converter ,converters 1 and 2 work as under a. 1 as rectifier ,2 as inverter b. both as rectifiers c. both as inverters d. 1 as chopper and two as inverter

Answers: 1) b

2) b

3) b

4) b

5) a

6) c

7) a

8) a

9) c

10) b

11) a

12) c

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13) d

14) c

15) b

16) d

17) b

18)c

19) b

20) d

21) a

22) a

23) a

24) a

25) c

26) b

27) d

28) b

29) d

30) d

31) a

32) a

33) a

34) a

UNIT – VI AC VOLTAGE CONTROLLERS & CYCLO CONVERTERS Syllabus: AC voltage controllers – Single phase two SCR’s in anti parallel – With R and RL loads – modes of operation of Triac – Triac with R and RL loads – Derivation of RMS RMS load load volt voltag age, e, curr curren entt and and powe powerr fact factor or wave wave form forms s – Firing Firing circuits circuits -Numerical problems -Cyclo converters – Single phase mid point cyclo converters with Resistive and inductive load (Principle of operation only) – Bridge configuration of single phase cyclo converter (Principle of operation only) – Waveforms

Objectives:  Explanation of Basic AC Voltage Controller operation.

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   

Discussion of different loads operation of AC Voltage controller. Triac Operation Explanation. Basics of Cycloconverters & its modes of operation. Cycloconverter operations with different loads

Schedule:  Introduction to AC voltage controllers  Single phase two SCR’s in anti parallel –With R and RL loads  Modes of operation of Triac – Triac with R and RL loads  Derivation of RMS load voltage, current and power factor  Firing circuits & Numerical problems

-1 -1 -1 -1 -1 -1

 Introduction to Cyclo converters  Single phase mid point Cyclo converters with Resistive and inductive load (Principle of operation only)  Bridge configuration of single phase cyclo converter (Principle of operation only) – Waveforms

-1 -1

ASSIGNMENT ASSI GNMENT QUEST QUESTION ION S: 1) (a) Explain the principle of ON-OFF control used in a.c. voltage controller. (b) Derive the expression for the input power factor in an a.c. voltage controller using ON-OFF control. (c) Explain its application with the help of a circuit and waveforms.

2) Derive the output rms voltage, output rms current and source power factor for a single phase ac voltage controller fed to R-L load. 3) The ac voltage controller uses on-off control for heating a resistive load of R = 4 ohms and the input voltage is Vs = 208V, 60Hz. If the desired output power is P0 = 3KW, determine the (a) duty cycle δ (b) input power factor (c) sketch waveforms for the duty cycle obtained in (a)

4) A single phase load of resistance of 12 ohms is fed from 240V (rms), 50 Hz supply by a pair of inverse parallel thyristors. Find the mean power in the load at firing angle of  (a) 00 (b) 900 and (c) 1200. Ignore source inductance and device voltage drops. 5) (a) What is a cyclo converter? (b) What are the varieties of single phase cyclo converters. (c) What are the salient features of cyclo converters. (d) What are the major limitations of cyclo converters

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6) For a single phase mid-point cyclo-converter, explain the operation of the circuit when fed to R-load with the help of neat circuit diagram and relevant output waveforms for α = 300 and α = 1200 for f0 = 1/4 fs. 7) Explain the operation of single phase midpoint cyclo converter with R-L load s for continuous conduction with relevant circuit diagram nd necessary output waveforms for f0 = 1/3 fs. 8) Explain the operation of single phase bridge type cyclo converter when fed form 230V, 50Hz source and controlling power to resistive load with the help of neat circuit diagram and output voltage and current waveforms for α = 450 and α = 1600 for f0 = 1/5 fs. 9) For the ideal type A-chopper circuit, following conditions are given, Edc = 2 20V, chopping frequency, = 500 Hz, duty cycle δ=0.3 and R = 1 ohm, L = 3mH and Eb = 23V. Compute the following quantities. (a) Check whether the load current is continuous or not. (b) Average output current (c) maximum and minimum values of steady state output current

Objective Questions: 1. AC voltage controllers is a device which converts _ _ _ _ _ _ _ _ directly to _ _ _ _  _ _ without a change in _ _ _ _ _ a. fixed alternating voltage ,variable alternating voltage ,frequency. b. fixed direct voltage ,variable direct voltage , phase angle. c. variable alternating, fixed alternating voltage , frequency. d. variable direct voltage , fixed direct voltage , phase a ngle.

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2. A single phase voltage controller feeds power to a resistance of 10 Ω .The source voltage is 200V rms. For a firing angle of 900, the rms value of thyristor current in amperes is a. 20 b. 15 c. 10 d. 5 3. In a single phase ac voltage regulator with RL load , ac power can be controlled if  a. α > Φ and γ < 1800 b. α < Φ and γ = 1800 c. α < Φ and γ > 1800 d. α > Φ and γ = 1800 4. A single phase voltage controller using two SCR's connected in anti parallel acts as controlled rectifie if  a. load is R and pulse gating is used Power Electronics - 2nd MID - b. load is RL and pulse gating is used. c. load is RL and continuous gating is used. d. load is R and high frequency carrier gating is used. 5. A load resistance of 10Ω is fed through through a single phase voltage controller from a voltage source 200 sin 314t .For a firing delay of 900 ,the power delivered to load in kW ,is a. 0.5 b. 0.75 c. 1 d. 2 6. Three phase to three phase cycloconverters employing 18 SCRs and 36 SCRs have the same voltage and current ratings for their component thyristors .The ratio of VA rating of 36-SCR device to that of 18 SCR device is a. 1/2 b. 1 c. 2 d. 4 7. The cycloconverters (CCs ) require natural or forced commutation as under a. natural commutation in both step up and step down CCs b. forced commutation in both step up and step down CCs c. forced commutation in step up CCs d. forced commutation in step down CCs 8. In a single phase voltage controller with RL load ,when

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9. In a single phase voltage controller with RL load , α is the firing angle, Φ is the load phase angle and β is the extinction angle .For this voltage controller ,output power can be controlled if and

10. A cyclo converter is a a. frequency changer (fc) from higher to lower frequency with one stage conversion b. fc from higher to lower frequency with two stage conversion c. fc from lower to higher frequency with one stage conversion d. either (a) or (c) 11. For converting 3-phase supply at one frequency to single phase at a lower frequency ,the basic principle is to _ _ _ _ _ the firing angle _ _ _ _ _ a. keep ,constant b. vary, gradually c. increase, at once d. decrease , instantaneously 12. A three phase to three phase cycloconverter requires a. 18 SCRS for three pulse device b. 18 SCRS for six pulse device c. 36 SCRS for three pulse device d. 38 SCRS for six pulse device

13. A three phase to single phase conversion device employs 6-pulse bridge cycloconverter .For an input voltage of 200V per phase ,the fundamental rms value output voltage is a. 600/πV b. 300 V c. 300/ π V d. 6003/ π V

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14. Which of the following statement is incorrect for cycloconverters a. step-down cycloconverter (CC) works on natural commutation. b. Step-up CC requires forced commutation c. Load commutated CC works on line commutation d. Load commutated CC requires a generated emf in the load circuit 15. Three phase to three phase cycloconverters employing 18 SCRs and 36 SCRs have the same voltage and current ratings for their component thyristors. The ratio of power handled by 36-SCR device to that handled by 18- SCR device a. 4 b. 2 c. 1 d. ½ 16. The number of thyristors required for single phase to single phase cyclo converters of the midpoint type are a. 4 b. 8 c. 10 d. 6 17. The number of thyristors required for three phase to three phase 3-pulse type cyclo converters are a. 6 b. 18 c. 36 d. 42

Answers: 1) a

2) c

3) a

4) b

5) c

6) c

7) c

8) a

9) b

10) d

11) b

12) a

13) b

14) c

15) a

16) d

17) a

UNIT UNI T – VII CHO CHOPPE PPERS RS Syllabus:

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Choppers – Time ratio control and Current limit control strategies – Step down choppers Derivation of load voltage and currents with R, RL and RLE loadsStep up Chopper – load voltage expression Morgan’s chopper – Jones chopper and Oscillation chopper (Principle of operation only) Waveforms –– AC Chopper – Problems.

Objectives  Detailed Explanation of Chopper circuits & basic operation & working.  Discussion on different types of choppers (Morgan’s chopper, Jones chopper, Oscillation chopper  Explanation of AC Chopper  Solving the Numerical problems.

Schedule:  Introduction to Choppers

-1

 Time ratio control and Current limit control strategies

-1

 Step down choppers Derivation of load voltage and currents With R, RL and RLE loads  Step up Chopper – load voltage expression

-1 -1

 Morgan’s chopper, Jones chopper

-1

 Oscillation chopper (Principle of operation only) Waveforms

-1

 AC Chopper & Problems

-1

Assignment Questions: 1) Explain the operation of a basic dc chopper and obtain the following as a function of Edc, R and duty cycle δ. i. averag average e outpu outputt voltag voltage e and curr current ent ii. rms value value of of the the outpu outputt volta voltage ge iii. iii. RMS and averag average e load load cur curren rents ts

2) Explain the working of Class-D commutation circuit and also mention its application with neat circuit.

3) For the ideal type A-chopper circuit, following conditions are given, Edc = 220V, chopping frequency, = 500 Hz, duty cycle δ=0.3 and R = 1 ohm, L = 3mH and Eb = 23V. 23 V. Compute the following quantities.

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(a) Check whether whether the load current current is continuous continuous or not. (b) Average Average output output current current (c) maximum and minimum values of steady state output current (d) A step-up chopper with a pulse width of 150 μs operating on 220V, dc supply. 4) Compute the load voltage if the blocking period of the device is 40 μs. (a) What is the necessity necessity of step-up step-up chopper where where do you use.

5) An RL - Eb type load is operating in a chopper circuit from a 400 Volts d.c. source. For the load, L = 0.05 H and R = 0. For a duty cycle of 0.3, find the chopping frequency to limit the amplitude of load current excursion to 8A.

Objective Questions: 1. Chopper is a power semiconductor converter, which is used for a. AC to DC Conversion b. DC to AC conversion c. DC to DC conversion d. AC to AC conversion 2. In a step-down chopper working at a frequency of 500 Hz and supplying a highly inductive load, when the chopper power semiconductor device is changed from OFF state to ON state, the current in the load will a. Remain same b. Increase c. Decrease d. Become zero 3. In a step-down chopper operating from DC input voltage of 400 V, neglecting the on-state drops in the power semiconductor devices, the voltage across load during the on-period of thyristor is : a. Zero b. 100 V c. 400 V d. 500 V 4. In a step-down chopper operating from DC input voltage of 500 V, neglecting the on-state drops in the power semiconductor devices, the voltage across load during the off-period of thyristor is : a. Zero b. 100 V c. 400 V d. 500 V 5. In a step-down chopper fed from a DC input voltage of 100 V and supplying a highly inductive load, when the chopper thyristor is switched OFF, then neglecting

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the on-state drops in the power semiconductor devices, the voltage across the thyristor will be a. Zero b. 40 V c. 80 V d. 100 V 6. In a step-down chopper fed from a DC input voltage of 200 V and supplying a highly inductive load, when the chopper thyristor is switched OFF, then neglecting the on-state drops in the power semiconductor devices, the voltage across freewheeling diode connected across the load will be : a. Zero b. 50 V c. 100 V d. 200 V 7. In a step-down chopper fed from a DC input voltage of 100 V and supplying a highly inductive load, when the chopper thyristor is switched ON, then neglecting the on-state drops in the power semiconductor devices, the voltage across the thyristor will be a. Zero b. 40 V c. 80 V d. 100 V 8. In a step-down chopper fed from a DC input voltage of 200 V and supplying a highly inductive load, when the chopper thyristor is switched ON, then neglecting the on-state drops in the power semiconductor devices, the voltage across free-wheeling diode connected across the load will be : a. Zero b. 50 V c. 100 V d. 200 V 9. A separately excited DC motor is supplied from a fixed input DC voltage through a step-down chopper system. In which quadrant can the DC motor be operated by this system a. Forward motoring b. Forward braking c. Reverse motering d. Reverse braking

10. A separately excited DC motor is operated from a fixed input DC voltage through a step-down chopper system. For changing the mode of operation from discontinuous to continuous, the dutyratio

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of chopper must a. Remain same b. Increase c. Decrease d. become zero 11. A step-down chopper supplies an output DC voltage of 200 V from the input DC voltage of 250V. If the chopping frequency is 500 Hz, the off-time will be a. 0.4 ms b. 1.0 ms c. 1.6 ms d. 2.0 ms 12. A step-down chopper supplies an output DC voltage of 120 V from the input DC voltage of 200V. If the off-time is 4 ms, the chopper period is : a. 10 ms b. 20 ms c. 30 ms d. 40 ms 13. In a step-down chopper, if the on-time is double of the off-time, then the dutycycle of thechopper is a. 20 % b. 33 % c. 66 % d. 80 % 14. For a step-down chopper working at a chopping frequency of 100 Hz, the ontime is 8 ms. For an average output DC voltage of 400 V, what must be the input DC voltage : a. 100 V b. 300 V c. 500 V d. 1000 V 15. In a step-down chopper, if the on-time is 8 ms and off-time is 2 ms, then the duty-cycle ofchopper is a. 20 % b. 40 % c. 60 % d. 80 %

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16. In a step-down chopper, if the on-time is 4 ms and the chopping frequency is 100 Hz, then the duty-cycle of the chopper is a. 20 % b. 40 % c. 60 % d. 80 % 17. For an input DC voltage of 100 V, if the duty-cycle of a step-down chopper is 80 %, then average output DC voltage is a. 20 V b. 40 V c. 60 V d. 80 V 18. In the current limit control of a step-down chopper, when the current reaches the upper limit, what operation must be done a. Chopper is switched ON b. Chopper is switched OFF c. No operation necessary d. Load is decreased 19. In the current limit control of a step-down chopper, when the current reaches the lower limit, what operation must be done a. Chopper is switched ON b. Chopper is switched OFF c. No operation necessary d. Load is increased 20. In the time-ratio control of a step-down chopper with a constant chopper period of 10 ms, if the on-time of 4 ms is doubled, then the average output DC voltage will a. Be the same b. Become half  c. Become 2 times d. Become 2.5 times 21. In the time-ratio control of a step-down chopper, if the on-time is kept constant at 2 ms, but the chopping frequency is decreased from 200 Hz to 100 Hz, then average output DC voltage will be : a. The same b. Become half  c. Become 2 times d. Become 2.5 times

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22. In the time-ratio control of a step-down chopper with an input DC voltage of 100 V, if the off-time is kept constant at 4 ms, but the chopping period is decreased from 12 ms to 8 ms then the average output DC voltage will a. Remain same b. Become 0.75 times c. Become 0.5 times d. Become 04 times 23. The strategy in current limit control of a step-down chopper is a. Chopper is operated at a fixed chopping frequency b. Chopper on-time is kept constant c. Chopper off-time is kept constant d. Chopper is made ON and OFF so that load current is maintained between two limits 24. For a fixed chopping frequency of a step-down chopper fed from a constant input DC voltage, if the average output DC voltage is to be decreased, then which of the following operation for the chopper is done a. On-time is increased b. On-time is decreased c. Off-time is decreased d. Chopping period is decreased 25. The strategy in constant frequency system for time-ratio control of a step-down chopper is a. On-time is varied, but the chopping period is kept constant b. On-time is kept constant, but the chopping period is varied c. Off-time is kept constant, but the chopping period is varied d. Ratio of on-time to chopping period is kept constant 26. The strategy in variable frequency system for time-ratio control of a step-down chopper is a. Ratio of on-time to chopping period is kept constant b. Ratio of off-time to chopping period is kept constant c. On-time or off-time is kept constant, but the chopping period is varied d. Ratio of on-time to off-time is kept constant

27. In the time-ratio control of a step-down chopper, the chopping frequency is kept constant at 50Hz. If the on-time of 10 ms is decreased to 5 ms, then the average output DC voltage will a. Be the same b. Become half  c. Become 2 times d. Become 2.5 times

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28. In a step-up chopper fed from a DC input voltage of 100 V, working at the dutycycle of 25 % and supplying a highly inductive load, when the chopper thyristor is switched ON, then neglecting the on-state drops in the power semiconductor devices, the voltage across the thyristor will be a. Zero b. 100 V c. 200 V d. 400 V 29. In a step-up chopper fed from a DC input voltage of 100 V, working at the dutycycle of 25 % and supplying a highly inductive load, when the chopper thyristor is switched OFF, then neglecting the on-state drops in the power semiconductor devices, the voltage across the diode in series with the input supply will be a. Zero b. 100 V c. 200 V d. 400 V 30. In a step-up chopper fed from a DC input voltage of 100 V, working at the dutycycle of 75 % and supplying a highly inductive load, when the chopper thyristor is switched OFF, then neglecting the on-state drops in the power semiconductor devices, the voltage across the thyristor will be a. Zero b. 100 V c. 200 V d. 400 V 31. In a step-up chopper fed from a DC input voltage of 100 V, working at the dutycycle of 75 % and supplying a highly inductive load, when the chopper thyristor is switched ON, then neglecting the on-state drops in the power semiconductor devices, the voltage across the series diode will be a. Zero b. 100 V c. 200 V d. 400 V

32. A step-up chopper supplies an inductive load from an input DC voltage of 100 V. If the duty-cycle is changed from 75 % to 50 %, the average output DC voltage will a. Be the same b. Become half  c. Become 1.5 times d. Become 2 times

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33. A step-up chopper supplies a load with output DC voltage of 500 V from an input DC voltage of 400 V. If the off-time is 8 ms, then the on-time of the chopper is a. 2 ms b. 4 ms c. 6 ms d. 10 ms 34. In a step-up chopper circuit, when the chopper thyristor is turned ON, then the current in the inductor (connected in series with the input DC supply) will a. Remain same b. Increase c. Decrease d. become zero 35. In a step-up chopper circuit, when the chopper thyristor is turned OFF, then the current in the inductor (connected in series with the input DC supply) will a. Remain same b. Increase c. Decrease d. become zero 36. In a step-up chopper, the power semiconductor device is switched ON for a period of 10 ms and is switched OFF for a period of 10 ms. If the input DC voltage is 100 V, then the average output DC voltage will be a. 50 V b. 100 V c. 150 V d. 200 V 37. In a step-up chopper working on a constant input DC voltage, what should be done about the duty cycle, if the average value of the output DC voltage is to be increased a. Duty-cycle must remain constant b. Duty cycle must be made 1.0 c. Duty-cycle must be increased d. Duty-cycle must be decreased 38. In a step-up chopper working on a constant input DC voltage, what should be done about the duty cycle, if the average value of the output DC voltage is to be decreased a. Duty-cycle must remain constant b. Duty cycle must be made 1.0 c. Duty-cycle must be increased d. Duty-cycle must be decreased

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39. In an AC chopper feeding a load, the two main thyristors in a line are connected a. In series with each other b. In parallel to each other c. In reverse parallel to each other d. In parallel to the load 40. In an AC chopper feeding a load, if the firing angle of thyristors is increased from 10o to 30o, then the RMS value of output AC voltage will a. Remains same b. Decrease c. Increase d. Become zero 41. In a Jones chopper circuit, when the auxiliary thyristor-T2 is turned ON, then which condition occurs for the main thyristor-T1 by the discharge of the capacitor through T1 and T2 a. T1 turns On b. T1 gets forward biased c. T1 gets reverse biased d. Condition for T1 remains the same 42. In a Jones chopper circuit, if the inductance-L1 becomes 4 times while the capacitance-C remains the same, then the off-time of chopper will become a. 0.5 times b. 1.2 times c. 2 times d. 4 times 43. In a Jones chopper circuit, if the maximum load current becomes 2 times, then the off-time of chopper will become a. 0.5 times b. 1.2 times c. 2 times d. 4 times 44. Which particular component forms the special features of the Morgan chopper circuit a. 2-winding transformer b. 3-winding transformer c. Tapped autotransformer d. Saturable reactor

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45. In Morgan chopper circuit, the average output DC voltage can be decreased by a. Reducing the operating frequency b. Increasing the operating frequency c. Using an auxiliary thyristor d. Not using an auxiliary diode 46. In Morgan chopper circuit, the average output DC voltage can be increased by a. Reducing the operating frequency b. Increasing the operating frequency c. Using an auxiliary thyristor d. Not using an auxiliary diode 47. Which of the class of commutation does the Jones chopper belong to a. Class-A b. Class-B c. Class-C d. Class-D 48. Which particular component forms the special features of the Jones chopper circuit a. 2-winding transformer b. 3-winding transformer c. Tapped autotransformer d. Saturable reactor 49. In an AC chopper feeding a load, if the R MS value of output AC voltage is to be increased, then the firing angle of thyristors must be a. Kept constant b. Increased c. Decreased d. Made 90

Answers: 1) c

2) b

3) c

4) a

5) d

6) a

7) a

8) d

9) b

10) b

11) a

12) a

13) c

14) c

15) d

16) b

17) d

18) b

19) a

20) c

21) b

22) b

23) d

24) b

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25) a

26) c

27) b

28) a

29) a

30) d

31) d

32) b

33) a

34) b

35) c

36) d

37) c

38) d

39) c

40) b

41) c

42) c

43) a

44) d

45) a

46) b

47) d

48) c

49) c

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UNIT – VIII INVE INVERTERS RTERS Syllabus: Inverters – Single phase inverter – Basic series inverter – Basic parallel Capacitor inverter bridge inverter – Waveforms – Simple forced commutation circuits for bridge inverters – Mc Murray and Mc Murray – Bedford inverters - Voltage control techniques for inverters Pulse width modulation techniques – Numerical problems.

Objectives: 

Detailed Explanation of Inverters operation.  Discussion of different modes of operation of an Inverters  Commutation techniques of Inverters.  Discussion of different types of inverters (Mc Murray and Mc Murray – Bedford inverters).  Explanation of Voltage control techniques for Inverters  Discussion of PWM technique.  Solving Numerical Problems.

Schedule:  Introduction to Inverters  Single phase Inverter

-1 -1

 Basic series inverter

-1

 Basic parallel Capacitor inverter bridge inverter – Waveforms

-1

 Simple forced commutation circuits for bridge inverters  Mc Murray and Mc Murray – Bedford inverters

-1 -1

 Voltage control techniques for inverters

-1

 Pulse width modulation techniques

-1

 Numerical problems

-1

Assignment Questions: 1) Single phase half bridge inverter has a resistive load of R = 3 ohms and dc input voltage Edc = 50V. Calculate a. rms output voltage at fundamental frequency E1 b. the the outp output ut powe powerr c. averag average e and peak peak curr current ent of each each thyr thyrist istor or

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2) A single phase full bridge inverter uses a uniform PWM with two pulses per half half cycl cycle e for for volt voltag age e cont contro rol. l. Plot Plot the the dist distor orti tion on fact factor or,, fund fundam amen enta tall component, and lower order harmonics against modulation index.

3) Draw and explain the simple SCR series inverter circuit employing class A type type comm commut utati ation on.. With With the the help help of impo import rtan antt wave wavefo form rms. s. Stat State e the the limitations of this inverter. 4) What are the differen differentt pulse width modulat modulation ion techniques techniques used used for inverters. inverters. 5) Which Which of the schemes schemes gives gives better qualit quality y of voltage voltage and current. current. Calculate ate the output output freque frequency ncy of a serie series s invert inverter er circui circuitt with with follow following ing 6) Calcul para parame mete ters rs = 10mH 10mH,, C = 0.1 0.1 μF, μF, R = 400 400 ohms ohms,, toff toff = 0.2 0.2 msec msec.. Also Also determine the attenuation factor.

7) A single phase full bridge inverter uses a uniform PWM with two pulses per half half cycl cycle e for for volt voltag age e cont contro rol. l. Plot Plot the the dist distor orti tion on fact factor or,, fund fundam amen enta tall component, and lower order harmonics against modulation index.

8) The single phase modified Me Murray full-bridge inverter is fed by dc source of 300V. The d.c. source voltage may fluctuate by ±15% . The current during commutation may vary form 20 to 100A. Obtain the value of commutating components, if the thyristor turn-off time is 20 μs. Also compute the value of  R.

Objective Questions: 1. In a single-phase half (semi)-controlled bridge inverter, the gate signals to the main thyristors are given at an interval of  b. 90 c. 120 d. 180 2. In a three-phase fully-controlled bridge inverter, the gate signals to the main thyristors are given at an interval of  a. 60 b. 90 c. 120 d. 180 3. The frequency of output AC voltage is increased in an inverter by : a. Decreasing the input DC voltage b. Increasing the input DC voltage c. Decreasing the time period between the triggering of the successive thyristors thyristors d. Increasing the time period between the triggering of the successive thyristors

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4. In a three-phase bridge inverter circuit, regarding the peak value of the output AC voltage, it is a. The same as the input DC voltage b. Less than the input DC voltage c. More than the input DC voltage d. It is not related to the input DC voltage

5. The total number of free-wheeling diodes in a three-phase fully-controlled bridge inverter will be a. 1 b. 2 c. 3 d. 6 6. The total number of thyristors in a three-phase fully-controlled bridge inverter will be a. 1 b. 2 c. 3 d. 6 7. In a single-phase bridge inverter, the gate signals to the main thyristors are given at an interval of  a. 60 b. 90 c. 120 d. 180 8. Inverter is a power semiconductor converter, which is used for : a. AC to DC conversion b. DC to AC conversion c. DC to DC conversion d. AC to AC conversion 9. The total number of thyristors in a single-phase fully-controlled bridge inverter will be a. 1 b. 2 c. 3 d. 4 10. The total number of thyristors in a single-phase half (semi)-controlled bridge inverter will be a. 1 b. 2 c. 3

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d. 4 11. The total number of free-wheeling diodes in a single-phase fully-controlled bridge inverter will be a. 1 b. 2 c. 3 d. 4 12. The total number of free-wheeling diodes in a single-phase half (semi)-controlled bridge inverter will be a. 1 b. 2 c. 3 d. 4 13. Two thyristors T1 and T2 are a re used in a basic series inverter circuit to supply a series R-L-C load from an input DC supply. If the time-delay between turn-off of T1 and torn-on of T2 is increased, then, the inverter frequency will a. Remain constant b. Become zero c. Increase d. Decrease 14. Two thyristors T1 and T2 are a re used in a basic series inverter circuit to supply a series R-L-C load from an input DC supply. If the time-period for oscillation of R-L-C circuit is increased, then the inverter frequency will : a. Remain constant b. Become zero c. Increase d. Decrease 15. Two thyristors T1 and T2 are a re used in a basic series inverter circuit to supply a series R-L-C load from an input DC supply. When thyristor T1 is triggered, the load current a. Will build up in positive half cycle b. Will remain zero c. Will build up in negative half cycle d. Will remain constant 16. Two thyristors T1 and T2 are a re used in a basic series inverter circuit to supply a series R-L-C load from an input DC supply. When both the thyristors are OFF, the load current a. Will build up in positive half cycle b. Will remain zero c. Will build up in negative half cycle d. Will remain constant

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17. Two thyristors T1 and T2 are a re used in a basic series inverter circuit to supply a series R-L-C load from an input DC supply. When thyristor T2 is triggered, the load current a. Will build up in positive half cycle b. Will remain zero c. Will build up in negative half cycle d. Will remain constant

18. In a basic parallel inverter, two thyristors T1 and T2 are used to switch the dc supply alternately to the two halves of the transformer primary. However, in addition, for which condition of load will two feedback diodes become essential e ssential a. When the load is opened b. When the load is shorted c. When the load is purely resistive d. When the load is inductive 19. In a basic parallel inverter, two thyristors T1 and T2 are used to switch the dc supply alternately to the two halves of the transformer primary. When thyristor T1 is turned on, the capacitor gets charged to the voltage of  a. 1 b. 2 c. -2 d. -1 20. In a basic parallel inverter, two thyristors T1 and T2 are used to switch the dc supply alternately to the two halves of the transformer primary. When thyristor T2 is turned on, the capacitor gets charged to the voltage of  a. 1 b. 2 c. -2 d. -1 21. In a basic parallel inverter, two thyristors are used to switch the dc supply alternately to the two halves of the transformer primary. For this, which of the following component is used to connect the input DC source to the mid point of the transformer a. Resistance b. Inductor c. Capacitor d. Diode

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22. In a basic parallel inverter, two thyristors are used to switch the dc supply alternately to the two halves of the transformer primary. For this, which of the following component is used across the transformer primary a. Resistance b. Inductor c. Capacitor d. Diode

23. In a basic parallel inverter, two thyristors T1 and T2 are used to switch the dc supply alternately to the two halves of the transformer primary. For this, where is the load connected to get the alternating supply a. Across one-half of the transformer primary b. In series with T2 c. Across the transformer secondary d. In series with T1 24. In the McMurray - Bedford full-bridge inverter, total how many numbers of  thyristors will be required a. 2 b. 4 c. 6 d. 8 25. In the McMurray - Bedford full-bridge inverter, total how many numbers of  diodes will be required a. 2 b. 4 c. 6 d. 8 26. In a single-phase McMurray inverter, 4 main thyristors are used in a single-phase bridge configuration to supply a load. How many numbers of auxiliary thyristors will be required for the commutation a. 1 b. 2 c. 3 d. 4 27. In the single-phase modified McMurray full-bridge inverter, which of the following component is added to the single-phase McMurray inverter : a. 4 auxiliary thyristors

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b. 4 auxiliary diodes c. Additional 4 capacitors d. Additional 4 inductors 28. In the modified McMurray full-bridge inverter, total how many numbers of  thyristors (main +auxiliary) will be required a. 2 b. 4 c. 6 d. 8

29. In the modified McMurray full-bridge inverter, total how many numbers of diodes (main +auxiliary) will be required a. 2 b. 4 c. 6 d. 8 30. In a three-phase bridge inverter with sinusoidal PWM control, if the modulation index is increased from 0.5 to 0.8, then it results in a. Decrease of output frequency b. Decrease of RMS value of output AC voltage c. Increase of output frequency d. Increase of RMS value of output AC voltage 31. In a three-phase PWM bridge inverter with PWM control, which of the following statements is true regarding the output AC voltage a. Only RMS value can be controlled b. Only frequency of output AC voltage can be controlled c. Both RMS value and the frequency can be controlled d. No control of RMS value or frequency is possible 32. One of the main purpose of using a PWM control in a three-phase bridge inverter is : a. Reduce the RMS value of the output AC voltage b. Reduce the frequency of the output AC voltage c. Reduce the harmonic content in the output AC voltage d. Reduce the number of devices in the inverter

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33. In sinusoidal PWM technique for a three-phase bridge inverter, for the generation of gating signals of thyristors, which type of carrier wave and reference wave are compared a. Sinusoidal reference wave is compared with sinusoidal carrier wave b. Sinusoidal reference wave is compared with triangular carrier wave c. Sinusoidal reference wave is compared with constant (DC) carrier wave d. DC reference wave is compared with sinusoidal carrier wave 34. In a single-pulse width control, the gating signals are generated by : a. Comparing a rectangular reference signal with a rectangular carrier wave b. Comparing a rectangular reference signal with a sinusoidal carrier wave c. Comparing a sinusoidal reference signal with a rectangular carrier wave d. Comparing a rectangular reference signal with a triangular carrier wave

35. In PWM control of inverters, the ratio of carrier frequency to reference or output frequency is called as a. Amplitude Modulation index b. Frequency modulation ratio c. Modulation index d. Distortion ratio 36. In PWM control of inverters, the ratio of amplitude of reference signal to amplitude of carrier wave is called as: a. Modulation index b. Frequency modulation ratio c. Distortion ratio d. Displacement factor

Answers: 1) d

2) a

3) c

4) a

5) d

6) d

7) d

8) b

9) d

10) b

11) d

12) b

13) d

14) d

15) a

16) b

17) c

18) d

19) b

20) c

21) b

22) c

23) c

24) b

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25) b

26) d

27) b

28) d

29) d

30) d

31) c

32) c

33) b

34) d

35) b

36) a

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