pe course file 09

EEE DEPARTMENT

Course File for Power Electronics

Prepared by

Mr.K.NAGESHAsst Prof. EEE Dept

POWER ELECTRONICSIII Year B.Tech EEE I - SEM

Course file - 2011

OBJECTIVE of POWER ELECTRONICS SUBJECT:

With the advent of semiconductor devices, revolution is taking place in the power transmission distribution and utilization. This 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 Explanation of POWER DEVICES & Thyristors their characteristics

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

Schedule:

Thyristors – Silicon Controlled Rectifiers (SCR’s) - 1 BJT their characteristics - 1 Power MOSFET their characteristics - 1 Power IGBT and their characteristics and other thyristors - 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.

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

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Assignment Questions:

1) Briefly discuss the operation of Power Devices (a) BJT (b) Power MOSFET

(c)Power IGBT

2) Explain in detail the basic operation of SCR with its characteristics.

3) With neat sketches discuss the TURN ON & TURN OFF methods of an SCR

4) Explain the V-I Characteristics of Thyristors with elaborating the following :(a) Latching current(b) Holding current(c) on-state and off-state condition(d) Turn-on and turn-off times(e) Finger voltage

5) The voltage and current ratings of a particular circuit are 3.3KV and 750 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

6) A rectangular pulse of 30V with 10 us duration is applied at the gate. The 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 the dynamic turn-on and turn-off times of SCR with neat waveforms.(b) Discuss the various thermal ratings of SCR.

8) Explain the operation of series connected and parallel connected SCRs withneat circuit diagrams and their characteristics.

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Objective Questions:

1) Which semiconductor power device out of the following is not a current triggered device? (a) Thyristor (b) GTO (c) Triac (d) MOSFET

2) A power BJT has a high interdigitated base emitter structure

(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 typical value of gain in a power BJT is (a) 100 (b) 1 (c) 10 (d) 1000

4) In a GTO, anode current begins to fall when gate current (a) is negative peaks at time t = 0 (b) is negative peak at t = storage period ts

(c) just begins to become negative and t = 0 (d) none of these

5) Thyristor A has rated gate current of 2 A and thyristor B is a rated gate current of 100 mA

(a) thyristor A is a GTO and B is a conventional SCR (b) thyristor B is a GTO and A is a conventional SCR (c) thyristor B may operate as a transistor (d) none of these

6) As compared to UJT, SUS (a) triggers only in one direction (b) does not have negative resistance characteristics (c) needs definite polarity of the applied voltage (d) triggers only at one particular voltage

7) In its application, an SUS behaves in the same way as (a) UJT(b) SCR (c) Tunnel diode(d) None of these

8) Which of the following PNPN devices has two gates?

(a) Triac (b) SCS (c) SUS (d) Diac

9) Which of the following PNPN devices has a terminal for synchronizing purpose?

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(a) SCS (b) Triac (c) Diac (d) SUS

10) Which of the following devices is a three layer device? (a) SCS (b) SUS (c) Triac (d) 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(d) MOSFET

17) The triac can be used only in (a) inverter (b) rectifier (c) multiquadrant chopper (d) cycloconverter

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

19) When transistors are used in series or parallel, a snubber circuit is used to (a) control the current (b) control the voltage (c) limit di/dt (d) all of these

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

21)Which of the following thyristors are gate turned off device? I. Gate turned off thyristor II. State Induction thyristor III. MOS-controlled thyristor

(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) seconds (b) milliseconds (c) microseconds (d) nanoseconds

23) A switched-mode power-supply operation at 20 kHz to 100 kHz range uses as the main switching element:

(a) Thyristor(b) MOSFET (c) Triac (d) UJT

24) The MOSFET switch in its on-state may be considered equivalent to

(a) resistor (b) inductor (c) capacitor (d) battery

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

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

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

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

29) A GTO like all the other power semiconductor devices requires protection against

(a) rates of change of forward current and forward voltage (b) rate of change of current alone (c) rate of change of voltage alone (d) rates of change of forward current and forward voltage and over

voltages and currents

30) The inductance of snubber circuit and capacitance of snubber of a GTO (a) increase the rate of turn-off (b) make the turn-off 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 gate current at low level gate drive. (b) a poor di/dt rating even at high gate current (c) its di/dt improving only at high gate current (d) very slow spreading velocity

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

33) The temperature coefficient of resistivity for power BJT is

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(a) positive (b) negative (c) zero(d) none

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

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

36) The anti saturation arrangement ensure (a) high switching speed but high on state power loss (b) high switching speed and low on-state power loss (c) high switching speed and high breakdown voltage(d) all

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

38) The SOA of a power device (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) None

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

40) The operating frequency of a power MOSFET is higher than a power BJT because

(a) it is a majority carrier device (b) it has an insulated gate (c) drift layer is absent in it (d) its gain is infinite

41) The on-state voltage drop of a power MOSFET is higher than a power BJT because

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(a) it has no drift layer(b) conductivity modulation is absent (c) its current capacity is 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 circuits are simpler (d) conductivity modulation is absent

43) For a MOSFET, snubber circuits (a) are very much essential to give it a dv/dt protection. (b) are not essential due to large SOA, however are still recommended (c) are never used (d) none of the above

44) A device is said to have a symmetric blocking capability if (a) it blocks forward and reverse voltages of equal or comparable

magnitudes (b) it blocks only reverse voltages (c) it blocks only forward voltages (d) none of the above

45) The turn-off gain of the GTO is of the order of (a) 1.2 (b) (b) 3.5 (c) 10.20 (d) > 100

46) The body layer is connected to source terminal in a MOSFET in order to (a) reduce the on-state power dissipation (b) increase the speed of operation (c) avoid the latch-up in MOSFET (d) none

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

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

49) The temperature coefficient of resistivity of an IGBT is

(a) positive

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(b) negative (c) flat(d) none

50) The SOA of IGBT is better than that of a power transistor because

(a) it is a majority carrier device (b) it is a minority carrier device (c) second breakdown is absent due to its flat temperature coefficient of (d) Temperature

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) conductivity modulation (c) the n drift layer

(d) none

53) The nonpunch through IGBT has a

(a) symmetrical blocking capacity (b) (b) asymmetrical blocking capacity (c) no blocking capacity at all(d) 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 and turn-off (c) It is a minority carrier device (d) It is a majority 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 with the main supply (b) It must use separate power supply

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(c) It should provide a train of pulses (d) None of these

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

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 (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) minimum (b) reverse (c) equal to cathode voltage (d) equal to break-over voltage

65) A SCR is a _________ switch.

(a) two directional (b) unidirectional (c) three-directional (d) four-directional

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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 (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 of each thyristor (b) Anode circuit of each thyristor (c) Gate circuit of only one thyristor (d) Anode circuit of only 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

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(b) Is 1 to 1.5 V & Increases slightly with load 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 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

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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 (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 COMMUTATION 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, IGBT - Numerical problems – Line Commutation and Forced Commutation circuits.

Objectives:

Detailed Explanation on Two Transistor analogy.

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

Schedule:

Two transistor analogy – SCR - 1 UJT firing circuit - 1 Series and parallel connections of SCR’s - 1

Snubber circuit details - 1 Specifications and Ratings of SCR’s, BJT, IGBT - 1 Numerical problems - 1 Line Commutation and Forced Commutation circuits - 1

Reference Books:

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

2) Power Electronics – By P.S Bimbra Khanna Publishers.


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 the parallel operation of SCR’s and define and derive 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 Sketches Explain Line commutation & Forced Commutation Circuits & Operation.


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 of a 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 (d) gate current becomes zero.

8) A forward voltage can be applied to an SCR after its

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(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 (a) short rise time with pulse width = 3 msec. (b) long rise time with pulse width = 6 msec. (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 in series with RL circuit can be reduced by (a) increasing circuit resistance R (b) decreasing R (c) increasing circuit inductance (d) decreasing L

11) Turn-on time of an SCR can be reduced by using a (a) rectangular pulse of high amplitude and narrow width (b) rectangular pulse of low amplitude and wide width (c) triangular pulse (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 than 35 A (c) 35 A (d) None of these

13) Surge current rating of an SCR specifies the maximum (a) repetitive current with sine wave

(b) non-repetitive current with rectangular wave(c) non-repetitive current with sine wave

(d) repetitive current with rectangular wave

14) In the circuit given below, the function of the

transistor is

(a) to provide control signal to trigger SCR(b) to make SCR-ON (c) to make SCR-OFF

(d) to amplify anode-current

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15) In a thyristor, the magnitude of the anode-current will (a) increase if gate-current is increased (b) decrease if gate current is decreased (c) increase if gate-current is decreased (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

(a) decrease the load resistance (b) increase the resistance (c) increase the gate-pulse (d) none of these

17) Most SCRs can be turned-off by voltage reversal during negative half-cycle of the ac supply for

(a) all frequencies (b) frequencies upto 300 Hz (c) frequencies upto 30 kHz (d) frequencies upto 300 kHz

18) 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) reverse the polarity of the applied voltage (d) remove the gate-current of conducting SCR

19) If a diode is connected in antiparallel with a SCR, then (a) both turn-off power loss and turn-off time decrease (b) turn-off power loss decreases but turn-off time increases (c) turn-off power loss increases, but turn-off time decreases (d) none of the above

20) In a commutation circuit, employed to turn-off an SCR, satisfactory turn-off isobtained when

(a) circuit turn-off time < device turn-off time (b) circuit turn-off time > device turn-off time (c) circuit time constant > device turn-off time (d) circuit time constant < device turn-off time

Gate Triggering Circuits

21) UJT oscillators are used for gate-triggering of thyristors for (a) Better phase control (b) Snap action

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(c) Being cheap and simple (d) none of the above

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

23) A device that does not exhibit negative resistance characteristic is (a) FET(b) UJT (c) tunnel diode(d) SCR

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

(a) 1.16 mA (b) 1.28 mA (c) 1.34 mA (d) 1.41 mA

25) A PUT has VBB = 24 V, & RB1 = 3RB2. The value of h will be (a) 1/3(b) 2/3 (c) 3/4(d) 4/3

26) A PUT relaxation oscillator has values VBB = 15 V, R = 22 kW, R2 = 6 kW, IP = 100 mA, VV = 1 V, IV = 7 mA, C= 1 mF, RK = 100 kW, R3 = 12 kW. The value of VPwill be

(a) 0.7 V(b) 10 V (c) 10.7 V(d) 15 V

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

(a) 0.2 (b) 0.4 (c) 0.7 (d) 0.99

28) When a UJT is used for triggering of an SCR, the waveshape of the voltage is a (a) Sine Wave(b) Saw-tooth wave (c) Trapezoidal wave (d) Square wave

29) Optocouplers combine(a) SITs and BJTs(b) IGBTs and MOSFETs (c) Power transformer and silicon transistors (d) Infrared light-emitting diode and silicon phototransistor

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

(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

(a) current distribution (b) voltage distribution (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

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

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

(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(a) 0.5 to 1% (b) 1 to 5%(c) 8 to 20% (d) 25 to 50%

36) To obtain the highest possible string efficiency, the SCRs connected in string must have

(a) different characteristics (b) same characteristics(c) same voltage ratings only (d) same current ratings only

37) String efficiency is used for measuring the(a) voltage rating of SCRs(b) current rating of SCRs(c) temperature rating of SCRs(d) degree of utilization of SCRs

Prepared byMr.K.NAGESH

Asst Prof. EEE Dept

38) In series string, thyristor having the highest leakage resistance or low voltage current will share

(a) larger portion of the applied voltage(b) smaller portion of the applied voltage(c) larger portion of current(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 rise of pressure

40) In optical triggering technique, LASCR is connected in

(a) gate circuit of each thyristor(b) anode circuit of each thyristor(c) gate circuit of only one thyristor(d) anode circuit of only one thyristor.

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) d 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

Syllabus:

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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 Phase Half Controlled Converters Circuits Explanation with different

loads & Free wheeling Diode Solving the Numerical problems

Schedule;

Phase control technique - 1 Single phase Line commutated converters - 1 Mid point and Bridge connections - 1 Half controlled converters with Resistive, RL loads and RLE load - 2 Derivation of average load voltage and current - 1 Active and Reactive power inputs to the converters Without Free wheeling Diode - 1 Active and Reactive power inputs to the converters With Free wheeling Diode - 1 Numerical problems - 1

Reference Books:

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

2) Power Electronics – By P.S Bimbra Khanna Publishers.


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 = 300, α = 450.α = 600.

2) Single phase half controlled bridge converter feeds an 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 the Operation of Single phase Half controlled Converter With R & RL & RLE Loads & Plot the necessary wave forms.

4) Explain the need of Fly Wheel Diode in the Converter operation

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5) Derive the output voltage & current equations for the single phase Half controlled Converter with suitable considerations.


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 average output voltage in volts is

(a) 400/π (b) 300/ π(c) 240/ π(d) 360/ π

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

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6)

7)

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

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(a) 240°(b) 180° (c) 200° (d) 120°

Answers:

1) c 2) a 3) b 4) c 5) a 6) d

7) a 8) a 9) c 10) d

UNIT – IV SINGLE PHASE FULLY CONTROLLED CONVERTERS

Syllabus: 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 and with Free wheeling Diode, Effect of source inductance – Derivation of load voltage and current – Numerical problems.

Objectives:

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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 - 1 Mid point and Bridge connections with Resistive, RL loads

and RLE load - 1 Derivation of average load voltage and current - 1 Line commutated inverters - 1 Active and Reactive power inputs to the converters without

Free wheeling Diode - 1 Active and Reactive power inputs to the converters with Free wheeling Diode - 1 Effect of source inductance - 1 Derivation of load voltage and current - 1 Numerical problems - 1


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 = 300, α = 450.α = 600.

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 on the converter operation.

5) Derive the output voltage & current equation for the Full converter operations with R, RL, RLE loads


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

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(a) 100 V (b) 141.4 V (c) 200 V (d) 282.8 V

2) A 1 – phase full controlled converter uses

a) 2 diodes & 2 SCRsb) 4 SCRsc) Both a & bd) Nonee)

3) In a Single phase full converter Bridge the Output Voltage given by

4) For continuous conduction, in a single-phase full converter each pair of SCRs conducts for

5) For discontinuous load current and extinction angle β > π radians, in a single- phase full converter, each SCR conducts for

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 > π, freewheeling diode conducts for

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

angle β < π, the freewheeling diode conducts for

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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 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 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

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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 Source inductance–Dual converters (both single phase and three phase) - Waveforms –Numerical Problems.

Objective:

Three Phase 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:

Three phase converters - 1 Three pulse and six pulse converters - 1 Mid point and bridge connections average load voltage With R and RL loads - 1 Effect of Source inductance - 1 Dual converters (both single phase and three phase) -Waveforms - 2 Numerical Problems - 1


1) A three phase full converter is supplied from a three phase 230V, 60Hz supply. The load current is continuous and has negligible ripple. If the average load current Idc = 150A and commutating inductance Lc = 0.1mH, determine the overlap angle when(a) α = 100

(b) α = 300 and(c) α = 600

2) Explain the operation of three phase fully controlled bridge converter with RL loads. Describe in detail with discontinuous conduction mode with 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 the average load voltage at firing 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.

4) A six pulse thyristor converter is connected to the mains through a transformer of6% reactance. If the rms value of the voltage at the secondary of the transformers 415V, calculate the voltage regulation. Neglect resistance in converter. The full load dc current is 200A. What is the value of commutation angle.

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5) (a) Explain the operation of three phase, half controlled bridge converter with R load and associated waveforms.(b) Derive the expression for average load voltage for α = 30

.


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 isa. 0.955b. 0.827c. 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. 60b. 120c. 180d. 90

5. In a 3- Φ half wave diode rectifier ,if the per phase input voltage is 200V then the average output voltage is

a. 233.91Vb. 116.95Vc. 202.56Vd. 101.28V

6. In a three phase semi converter ,for firing angle less than or 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 isa. 481.7Vb. 460Vc. 345Vd. 230V

9. In a three phase half wave diode rectifier, the peak inverse voltage in terms of average output voltage is

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a. 1.571b. 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 180b. converter for α =0 to 90c. inverter for α =90 to 180d. 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. 10b. 30c. 50d. 110

13. In a three phase semi converter , for firing angle less than or equal to 600 ,free wheeling diode conducts for a. 30b. 60c. 90d. 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. zerob. source voltagec. Source voltage minus the inductance dropd. 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 isa. 1/2 Ib. 1/3Ic. 1/4Id. I

18. In a three phase semi converter, the three SCRs are fired at an interval of

a. 60b. 90c. 120d. 180

19. In a three phase full converter, the six SCRs are fired at an interval of

a. 30b. 60c. 90d. 120

20. In a three phase full converter, the three SCRs pertaining to one group are fired at an interval of

a. 30b. 60c. 90d. 120

21. In the circulating current mode of a dual converter, _ _ _ _ _ _ _ is inserted between the converters 1 and 2

a. reactorb. resistorc. fused. capacitor

22. The effect of source inductance is to reduce the

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a. average dc output voltageb. dc currentc. ac input voltaged. losses

23. In a single phase full converter, the output voltage due to overlap is

a. zerob. source voltagec. Source voltage minus the inductance dropd. inductance drop

24. The angular period over which both the incoming and outgoing SCRs are conducting together is called

a. Overlap angleb. firing anglec. extinction angled. ignition angle

25. The total number of thyristors conducting simultaneously in a 3-phase full converter with overlap considered has the sequence of

a. 3,3,2,2b. 3,3,3,2c. 3,2,3,2d. 2,2,2,3

26. A four quadrant operation requires

a. two full converters in seriesb. two full converters connected back to backc. 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. oneb. twoc. threed. four

28. Reactor limits the magnitude of _ _ _ _ _ _ _ in a circulating current type dual converter.

a. circulating voltageb. circulating currentc. load voltaged. load current

29. In a dual converter with circulating current mode firing pulses to the two converters are so adjusted that =

a. 120

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b. 150c. 90d. 180

30. Two full converters connected back to back the same dc load is called a _ _ _ _ a. chopperb. inverterc. rectifierd. 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,00b. 26A,00c. 20A ,900d. 26 A,900

32. In the case of AC voltage regulator ,control range of firing angle is given as a. Φ < ∞ < ,1800b. α < Φ < ,1800c. Φ < α < ,3600d. α < Φ < ,3600

33. In a circulating current type dual converter ,the nature of the voltage across reactor is

a. alternatingb. pulsatingc. directd. triangular

34. In a dual converter ,converters 1 and 2 work as under

a. 1 as rectifier ,2 as inverterb. both as rectifiersc. both as invertersd. 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

13) d 14) c 15) b 16) d 17) b 18)c

19) b 20) d 21) a 22) a 23) a 24) a

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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 load voltage, current and power factor wave forms – Firing 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. 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:

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Introduction to AC voltage controllers - 1 Single phase two SCR’s in anti parallel –With R and RL loads - 1 Modes of operation of Triac – Triac with R and RL loads - 1 Derivation of RMS load voltage, current and power factor - 1 Firing circuits & Numerical problems - 1 Introduction to Cyclo converters - 1 Single phase mid point Cyclo converters with Resistive and inductive load (Principle of operation only) - 1 Bridge configuration of single phase cyclo converter (Principle of operation only) – Waveforms - 1

ASSIGNMENT QUESTIONS:

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 controllerusing 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 asingle 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 = 4ohms 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

6) For a single phase mid-point cyclo-converter, explain the operation of the circuitwhen fed to R-load with the help of neat circuit diagram and relevant outputwaveforms 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 forcontinuous 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 form230V, 50Hz source and controlling power to resistive load with the help of neat

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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 = 220V,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


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 angle.

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. 20b. 15c. 10d. 5

3. In a single phase ac voltage regulator with RL load , ac power can be controlled if

a. α > Φ and γ < 1800b. α < Φ and γ = 1800c. α < Φ and γ > 1800

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d. α > Φ and γ = 1800

4. A single phase voltage controller using two SCR's connected in anti parallel acts as controlledrectifie if

a. load is R and pulse gating is usedPower 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 voltagesource 200 sin 314t .For a firing delay of 900 ,the power delivered to load in kW ,is

a. 0.5b. 0.75c. 1d. 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. 1/2b. 1c. 2d. 47. The cycloconverters (CCs ) require natural or forced commutation as under a. natural commutation in both step up and step down CCsb. forced commutation in both step up and step down CCsc. forced commutation in step up CCsd. forced commutation in step down CCs

8. In a single phase voltage controller with RL load ,when

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

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10. A cyclo converter is a a. frequency changer (fc) from higher to lower frequency with one stage conversionb. fc from higher to lower frequency with two stage conversionc. fc from lower to higher frequency with one stage conversiond. 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 ,constantb. vary, graduallyc. increase, at onced. decrease , instantaneously

12. A three phase to three phase cycloconverter requires

a. 18 SCRS for three pulse deviceb. 18 SCRS for six pulse devicec. 36 SCRS for three pulse deviced. 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/πVb. 300 Vc. 300/ π Vd. 6003/ π V

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 commutationc. Load commutated CC works on line commutationd. 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-SCRdevice to that handled by 18- SCR device

a. 4

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b. 2c. 1d. ½

16. The number of thyristors required for single phase to single phase cyclo converters of the midpoint type are

a. 4b. 8c. 10d. 6

17. The number of thyristors required for three phase to three phase 3-pulse type cyclo converters are

a. 6b. 18c. 36d. 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 – VII CHOPPERS

Syllabus: Choppers – Time ratio control and Current limit control strategies – Step

down choppers Derivation of load voltage and currents with R, RL and RLE loads- Step up Chopper – load voltage expressionMorgan’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:

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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 - 1 Step up Chopper – load voltage expression - 1 Morgan’s chopper, Jones chopper - 1 Oscillation chopper (Principle of operation only) Waveforms - 1 AC Chopper & Problems - 1


1) Explain the operation of a basic dc chopper and obtain the following as a function of Edc, R and duty cycle δ.

i. average output voltage and currentii. rms value of the output voltageiii. RMS and average load currents

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. 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(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 of step-up chopper 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.


1. Chopper is a power semiconductor converter, which is used for

a. AC to DC Conversionb. DC to AC conversionc. DC to DC conversiond. AC to AC conversion

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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 sameb. Increasec. Decreased. 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. Zerob. 100 Vc. 400 Vd. 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. Zerob. 100 Vc. 400 Vd. 500 V5. 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 the on-state drops in the power semiconductor devices, the voltage across the thyristor will be

a. Zerob. 40 Vc. 80 Vd. 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 free-wheeling diode connected across the load will be :

a. Zerob. 50 Vc. 100 Vd. 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. Zerob. 40 V

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c. 80 Vd. 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. Zerob. 50 Vc. 100 Vd. 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 motoringb. Forward brakingc. Reverse moteringd. 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 dutyratioof chopper must

a. Remain sameb. Increasec. Decreased. 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 msb. 1.0 msc. 1.6 msd. 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 msb. 20 msc. 30 msd. 40 ms

13. In a step-down chopper, if the on-time is double of the off-time, then the duty-cycle of thechopper is

a. 20 %

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b. 33 %c. 66 %d. 80 %

14. For a step-down chopper working at a chopping frequency of 100 Hz, the on-time is 8 ms. For anaverage output DC voltage of 400 V, what must be the input DC voltage :

a. 100 Vb. 300 Vc. 500 Vd. 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 %

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 Vb. 40 Vc. 60 Vd. 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 ONb. Chopper is switched OFFc. No operation necessaryd. 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 ONb. Chopper is switched OFFc. No operation necessary

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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 sameb. Become halfc. Become 2 timesd. 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 sameb. Become halfc. Become 2 timesd. Become 2.5 times

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 sameb. Become 0.75 timesc. Become 0.5 timesd. Become 04 times

23. The strategy in current limit control of a step-down chopper is

a. Chopper is operated at a fixed chopping frequencyb. Chopper on-time is kept constantc. Chopper off-time is kept constantd. 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 increasedb. On-time is decreasedc. Off-time is decreasedd. 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 constantb. On-time is kept constant, but the chopping period is variedc. Off-time is kept constant, but the chopping period is varied

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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 constantb. Ratio of off-time to chopping period is kept constantc. On-time or off-time is kept constant, but the chopping period is variedd. 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 sameb. Become halfc. Become 2 timesd. Become 2.5 times

28. In a step-up chopper fed from a DC input voltage of 100 V, working at the duty-cycle 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. Zerob. 100 Vc. 200 Vd. 400 V

29. In a step-up chopper fed from a DC input voltage of 100 V, working at the duty-cycle 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 serieswith the input supply will be

a. Zerob. 100 Vc. 200 Vd. 400 V

30. In a step-up chopper fed from a DC input voltage of 100 V, working at the duty-cycle 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. Zerob. 100 Vc. 200 Vd. 400 V

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31. In a step-up chopper fed from a DC input voltage of 100 V, working at the duty-cycle 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. Zerob. 100 Vc. 200 Vd. 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 sameb. Become halfc. Become 1.5 timesd. Become 2 times

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 msb. 4 msc. 6 msd. 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


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


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 Vb. 100 Vc. 150 Vd. 200 V

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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 constantb. Duty cycle must be made 1.0c. Duty-cycle must be increasedd. 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 constantb. Duty cycle must be made 1.0c. Duty-cycle must be increasedd. Duty-cycle must be decreased

39. In an AC chopper feeding a load, the two main thyristors in a line are connected

a. In series with each otherb. In parallel to each otherc. In reverse parallel to each otherd. 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, thenthe RMS value of output AC voltage will

a. Remains sameb. Decreasec. Increased. Become zero

41. In a Jones chopper circuit, when the auxiliary thyristor-T2 is turned ON, then which conditionoccurs for the main thyristor-T1 by the discharge of the capacitor through T1 and T2

a. T1 turns Onb. T1 gets forward biasedc. T1 gets reverse biasedd. 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 timesb. 1.2 timesc. 2 timesd. 4 times

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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 timesb. 1.2 timesc. 2 timesd. 4 times

44. Which particular component forms the special features of the Morgan chopper circuit

a. 2-winding transformerb. 3-winding transformerc. Tapped autotransformerd. Saturable reactor

45. In Morgan chopper circuit, the average output DC voltage can be decreased by

a. Reducing the operating frequencyb. Increasing the operating frequencyc. Using an auxiliary thyristord. Not using an auxiliary diode

46. In Morgan chopper circuit, the average output DC voltage can be increased by

a. Reducing the operating frequencyb. Increasing the operating frequencyc. Using an auxiliary thyristord. Not using an auxiliary diode

47. Which of the class of commutation does the Jones chopper belong to a. Class-Ab. Class-Bc. Class-Cd. Class-D

48. Which particular component forms the special features of the Jones chopper circuit

a. 2-winding transformerb. 3-winding transformerc. Tapped autotransformerd. Saturable reactor

49. In an AC chopper feeding a load, if the RMS value of output AC voltage is to be increased, then the firing angle of thyristors must bea. Kept constantb. Increasedc. Decreasedd. Made 90

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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

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 INVERTERS

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 - 1 Single phase Inverter - 1 Basic series inverter - 1 Basic parallel Capacitor inverter bridge inverter – Waveforms - 1 Simple forced commutation circuits for bridge inverters - 1 Mc Murray and Mc Murray – Bedford inverters - 1 Voltage control techniques for inverters - 1 Pulse width modulation techniques - 1 Numerical problems - 1


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 E1b. the output power

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c. average and peak current of each thyristor

2) A single phase full bridge inverter uses a uniform PWM with two pulses per half cycle for voltage control. Plot the distortion factor, fundamental component, and lower order harmonics against modulation index.

3) Draw and explain the simple SCR series inverter circuit employing class A type commutation. With the help of important waveforms. State the limitations of this inverter.

4) What are the different pulse width modulation techniques used for inverters.

5) Which of the schemes gives better quality of voltage and current.

6) Calculate the output frequency of a series inverter circuit with following parameters = 10mH, C = 0.1 μF, R = 400 ohms, toff = 0.2 msec. Also determine the attenuation factor.

7) A single phase full bridge inverter uses a uniform PWM with two pulses per half cycle for voltage control. Plot the distortion factor, fundamental 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.


1. In a single-phase half (semi)-controlled bridge inverter, the gate signals to the main thyristors are given at an interval of

b. 90c. 120d. 180

2. In a three-phase fully-controlled bridge inverter, the gate signals to the main thyristors are given at an interval of

a. 60b. 90c. 120d. 180

3. The frequency of output AC voltage is increased in an inverter by : a. Decreasing the input DC voltageb. Increasing the input DC voltagec. Decreasing the time period between the triggering of the successive thyristorsd. Increasing the time period between the triggering of the successive thyristors

4. In a three-phase bridge inverter circuit, regarding the peak value of the output AC voltage, it is

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a. The same as the input DC voltageb. Less than the input DC voltagec. More than the input DC voltaged. 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 bea. 1b. 2c. 3d. 6

6. The total number of thyristors in a three-phase fully-controlled bridge inverter will bea. 1b. 2c. 3d. 6

7. In a single-phase bridge inverter, the gate signals to the main thyristors are given at an interval of

a. 60b. 90c. 120d. 180

8. Inverter is a power semiconductor converter, which is used for :

a. AC to DC conversionb. DC to AC conversionc. DC to DC conversiond. AC to AC conversion

9. The total number of thyristors in a single-phase fully-controlled bridge inverter will be a. 1b. 2c. 3d. 4

10. The total number of thyristors in a single-phase half (semi)-controlled bridge inverter will be

a. 1b. 2c. 3d. 4

11. The total number of free-wheeling diodes in a single-phase fully-controlled bridge inverter will be

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a. 1b. 2c. 3d. 412. The total number of free-wheeling diodes in a single-phase half (semi)-controlled bridge inverter will bea. 1b. 2c. 3d. 4

13. Two thyristors T1 and T2 are 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 constantb. Become zeroc. Increased. Decrease

14. Two thyristors T1 and T2 are 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 constantb. Become zeroc. Increased. Decrease

15. Two thyristors T1 and T2 are 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 cycleb. Will remain zeroc. Will build up in negative half cycled. Will remain constant

16. Two thyristors T1 and T2 are 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 cycleb. Will remain zeroc. Will build up in negative half cycled. Will remain constant

17. Two thyristors T1 and T2 are 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 cycleb. Will remain zero

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c. Will build up in negative half cycled. 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

a. When the load is openedb. When the load is shortedc. When the load is purely resistived. 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 ofa. 1b. 2c. -2d. -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. 1b. 2c. -2d. -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. Resistanceb. Inductorc. Capacitord. Diode

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. Resistanceb. Inductorc. Capacitord. Diode

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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 primaryb. In series with T2c. Across the transformer secondaryd. In series with T1

24. In the McMurray - Bedford full-bridge inverter, total how many numbers of thyristors will be required a. 2b. 4c. 6d. 8

25. In the McMurray - Bedford full-bridge inverter, total how many numbers of diodes will be required

a. 2b. 4c. 6d. 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. 1b. 2c. 3d. 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 thyristorsb. 4 auxiliary diodesc. Additional 4 capacitorsd. Additional 4 inductors

28. In the modified McMurray full-bridge inverter, total how many numbers of thyristors (main +auxiliary) will be required

a. 2b. 4c. 6d. 8

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29. In the modified McMurray full-bridge inverter, total how many numbers of diodes (main +auxiliary) will be required

a. 2b. 4c. 6d. 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 frequencyb. Decrease of RMS value of output AC voltagec. Increase of output frequencyd. 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 controlledb. Only frequency of output AC voltage can be controlledc. Both RMS value and the frequency can be controlledd. 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 voltageb. Reduce the frequency of the output AC voltagec. Reduce the harmonic content in the output AC voltaged. Reduce the number of devices in the inverter

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 waveb. Sinusoidal reference wave is compared with triangular carrier wavec. Sinusoidal reference wave is compared with constant (DC) carrier waved. 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 waveb. Comparing a rectangular reference signal with a sinusoidal carrier wavec. Comparing a sinusoidal reference signal with a rectangular carrier waved. Comparing a rectangular reference signal with a triangular carrier wave

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35. In PWM control of inverters, the ratio of carrier frequency to reference or output frequency is called as

a. Amplitude Modulation indexb. Frequency modulation ratioc. Modulation indexd. 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 indexb. Frequency modulation ratioc. Distortion ratiod. 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

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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pe course file 09

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