EE2301 Power Electronics Anna University Question Bank Question Paper Model Question Paper Important Questions 2 marks and 16 marks questions

EE2301 Power Electronics Anna University Question Bank Question Paper Model Question Paper Important Questions 2 marks and 16 marks questions EE 2301 Power Electronics Anna University Question Bank Question Paper Model Question Paper Important Questions 2 marks and 16 marks questions

EE2301-Power Electronics

1.  Why IGBT is very popular nowadays?

a. Lower hate requirements

b. Lower switching losses

c. Smaller snubber circuit requirements

2.  What are the different methods to turn on the thyristor?

a.  Forward voltage triggering

b.  Gate triggering

c.  dv/dt triggering

d.  Temperature triggerin g

e.  Light triggering

3.  What is the difference between power diode and signal diode?

S.No.

Power diode                                                 Signal diode

1.

Constructed with n-layer, called

Drift region is not present.

drift region between p+ layer and

n+ layer.

2.

The voltage, current and power

Lower

ratings are higher.

3.

Power diodes operate at high

Operates at higher switching speed.

speeds.

4.  IGBT is a voltage controlled device. Why?

Because the  controlling parameter is gate-emitter voltage.

5.  Power MOSFET  is a voltage controlled device. Why?

Because the output (drain) current can b e controlled  by   gate-source voltage.

6.  Power BJT  is a current controlled device. Why?

Because the output (collector) current can be controlled  by  base current.

7.  What is the relation between a  and ß?

ß =    a/1-a

a  =    ß/1- ß

8.  What are the different types of power MOSFET?

a.  N-channel MOSFET

b.  P-channel MOSFET

9.  How can a thyr istor turned off?

A thyristor can be turn ed off by making the current flowing through it to zero.

unit-1

10. Define latching current.

The latching current is defined as the minimum value of anode curr ent which it

must attain during turn on process to maintain conduction when gate signal is removed.

11. Define holding current.

The holding cu rrent is defined as the minimum value of anode current below

which it must fall to for turning off the thyristor.

12. What is a snubber circuit?

It consists of a series combination of a resistor and a capacitor in parallel with the

thyristors. It is mainly used for dv / dt protection.

13. What losses occur in a thyristor during working conditions?

a.  Forward conduction losses

b.  Loss due to leakage current during forward and reverse blocking.

c.  Switching losses at turn-on and turn-off.

d.  Gate triggering loss.

14. Define hard-driving or over-drivin gate.

When gate  current is several times higher than the minimum gate cur rent

required, a thyristor is said to be hard-fired or over-driven. Hard-firin g o f a thyristor

reduces its turn-on time and enhances its di/dt capability.

15. Define circuit turn off time.

It is defined as the time during which a reverse voltage is applied  across the

thyristor during its commutation process.

16. Wh y circuit turn off time should be greater than the thyristor turn -off time?

Circuit turn off time should be greater than the thyristor turn-off time for reliable

turn-off, otherwise the device may turn-on at an undesired instant, a process called

commutation failure.

17. What is the turn-off time for converter  gr ade SCRs and inverter grade SCRs?

Turn-off time fo r converter grade SCRs is 50 – 100 ms turn-off time for converter

grade SCRs and inverter  gr ade SCRs and  for inverter grade SCRs is 3 – 50 ms.

18. What are the adv antages  of GTO over SCR?

a.  Elimination of commutation of commutating components in forced commutation,

resulting in redu ction in cost, weight and volume.

b.  Reduction in acoustic noise and electromagnetic noise due to elimination of

commutation chokes.

c.  Faster turn-off, p ermitting high switching frequencies.

d.  Improved efficiency of the converters.

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9. What is meant by phase  controlled rectifier?

It convrts fixed ac voltage into variable dc voltage.

20. Mention some of the applications of controlled rectifier.

a.  Steel rolling mills, printing press, textile mills and paper mills employing dc

motor drives.

b.  DC traction

c.  Electro chemical and electro-metallurgical process

d.  Portable hand tool drives

e.  Magnet power supplies

f.  HVDC transmission system

21. What is the function of freewheeling diodes in controlled rectifier?

It serves two process.

a.  It prevents the output voltage from becoming negative.

b.  The load current is transferred from the main th yristors to the freewheeling diode,

thereby allowing all of its thyristors to regain their blocking states.

22. What are the adv antages  of freewheeling diodes in a controlled in a controlled rectifier?

a.  Input power factor is improved.

b.  Load  current wav eform is improved and thus the load performance is better.

23. What is meant by delay angle?

The delay angle is defined as the an gle between th e zero crossin g of the inp ut

voltage and the instant the thyristor is fired.

24. What are the adv antages  of single phase bridge co nverter ov er single phase mid-point

converter?

a.  SCRs are subjected to a peak-inverse voltage o f 2Vm in a fully controlled bridge

rectifier. Hence for same voltage  and currnt ratin gs of SCrs, power handled  by

mid-point configuration is about

b.  In mid-point converter,  each secondary winding should be able to supply the load

power. As such, the transformer rating in mid-point converter is double the load

rating.

25. What is commutation angle or overlap an gle?

The commutation period when outgoing and incoming thyristors are conducting is

known as overlap p eriod. The angular period, when both devices share con duction is

known as the commutation angle or overlap angle.

26. What are the different methods of firing circuits for line commutated converter?

a.  UJT firing circuit.

b.  The cosine wave crossing pulse timing control.

c.  Digital firing schemes.

27. Give an expression for average voltage of single phase semiconverters.

Average output voltage V

= (V

/ p) (1 + cos a ).

dc

m

28. What is meant by input power factor in controlled rectifier?

The input power factor is defined as the ratio of the total mean input power  to the

total RMS input volt-amperes.

PF = ( V

I

cos f

) / ( V

I

where V

= phase voltage, I

= fundamental

)

1

1

1

rms

rms

1

1

component of the supply current, f

= input

displacement angle,  I

= supply rms current.

1

rms

29. What are the adv antages  of six pulse converter?

a.  Commutation is made simple.

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

c.  Inductance reduced in series is considerably r educed.

30. What is meant by commutation?

It is the process of changing the direction of cur rent flow in a particular path of

the circuit. This process is used in thyristors for tu rning it off.

31. What are the types of  commutation?

a.  Natural commutation

b.  Forced commutation

32. What is meant by natural commutation?

Here the current flowing through the thyristor goes through a natural zero and

enable the thyristor to turn off.

33. What is meant by forced commutation

In this commutation, the current flowing through the thyristor is forced to become zero by ex ternal circuitry.

34. What is meant by dc chopper?

A dc chopper is a high speed static switch used to obtain variable dc voltage from

a constant dc voltage.

35. What are the applications of dc chopper?

a.  Battery operated vehicles

b.  Traction motor control in electric traction

c.  Trolly cars

d.  Marine hoists

e.  Mine haulers

f.  Electric braking.

36. What are the applications of dc chopper?

Chopper provides

a.  High efficien cy

b.  Smooth acceleration

c.  Fast dynamic response

d.  Regen eration

37. What is meant by step-up and step-down chopper?

In a step- down chopper or Buck converter, the av erage output voltage is less than

the input voltage. In  a step- up chopper or Boost converter, the average output voltage is

more than the input voltage.

38. Write down the expression for average output voltage for step do wn chopper.

Average output voltage for step down chopper V

= a V

, a is the duty cycle

0

s

39. Write down the expression for average output voltage for step up  chopper.

Average output voltage for step down chopper V

=   V

, a is the duty cycle

0

s

1- a

40. What is meant by duty- cycle?

Duty cycle is defined as the ratio of the on time of the chopper to the total time

period of the chopp er. It is denoted by

a

.

41. What are the two types of control strategies?

a.  Time Ratio Control (TRC)

b.  Current Limit Control method (CLC)

42. What is meant by TRC?

In TRC, the value of T

/ T is varied in order to change the average output

on

voltage.

43. What are the two types of TRC?

a.  Constant frequency  control

b.  Variable frequ ency control

44. What is meant by FM control in a dc chopper?

In frequency modulation control, the chopping frequency f (or the chopping

period T) is varied. Here two controls are possible.

a.  On-time T

is kept constant

on

unit-3

b.  Off period T

is kept constant.

off

45. What is meant by PWM control in dc chopper?

In this control method, the on time Ton is varied but chopping frequency is kept

constant. The width of the pulse is varied and hence this type of control is known as Pulse

Width Modulation (PWM).

46. Write down the expression for the average output voltage for step down and step up

chopper.

Average output voltage for step down chopper is V

=

a

V

. Average output

O

S

voltage for step up chopper is V

=

a

V

x [1/ ( 1-

a

)].

O

S

47. What are the different types of chopper with respect to commutation process?

a.  Voltage commutated cho pper.

b.  Current commutated chopper.

c.  Load  commutated chopper.

48. What is meant by voltage commutation?

In this process, a charged capacitor momentarily reverse biases the conducting

thyristor and turn it off.

49. What is meant by current commutation?

In this process, a current pulse is made to flow in the reverse direction through

the conducting thyristor  and when the n et thyristor current becomes zero, it is turned off.

50.  What is meant by load commutation?

In this process, the load current flowing through the thyristor  either becomes zero

or is transferred to another device from the  conducting thyristor.

51. What are the adv antages  of current commutated chopper?

a.  The capacitor always remains char ged with the correct polarity.

b.  Commutation is reliable as load current is less than the peak commutation current

I

.

CP

c.  The auxiliary thyristor T

is naturally commutated as its current passes through

A

zero value.

52. What are the adv antages  of load commutated chopper?

a.  Commutating inductor is not required.

b.  It is capable of commutating an y  amount of load current.

c.  It can work  at high  frequencies in the order of kHz.

d.  Filtering requirements are minimal.

53. What are the disadvantages of load commutated chopper?

a.  For high power applications, efficien cy becomes very low because of high

switching losses at high operating frequencies.

b.  Freewheeling diode is subjected to twice the supply voltage.

c.  Peak load voltage is equal to twice the supply voltage.

d.  The commutating capacitor has to carry full load current at a frequency of half

chopping frequency.

e.  One thyristor pair should be turned-on only when the other pair is commutated.

This can be realized by sensing the capacitor current that is alternating.

54. What is meant by inverter?

A device that converts dc power into ac power at desired output voltage and

frequen cy is called  an inverter.

55. What are the applications of an inverter?

a.  Adjustable speed drives

b.  Induction heating

c.  Stand-by aircraft power supplies

d.  UPS

e.  HVDC transmission

56. What are the main classification of inverter?

a.  Voltage Source Inverter

b.  Current Source Inverter

57. Wh y thyristors are not preferred fo r inverters?

Thyristors require extra commutation circuits for turn off which  results in

uncreased complexity of the circuit. For these r easons thyristors are not preferred for

inverters.

58. How output frequen cy is varied in case of a thyristor?

The output frequency is varied by varying the turn off time of the th yristors in the

inverter circuit, i.e. the delay angle of th e thyristors is varied.

59. Give two advantages of CSI.

a.  CSI does not require any feedback diodes.

b.  Commutation circuit is simple as it involves only thyristors.

60. What is the main drawback of a single phase half bridge inverter?

It require a 3-wire dc supply.

61. Wh y diodes should be connected in antiparallel with thethyristors in inverter circuits?

For RL loads, load current will not be in phase with load voltage and the diodes

connected in antiparallel will allow the current to flow when the main thyristors are

turned off. These diodes are called feedback diodes.

62. What types of inverters require feedback diodes?

VSI with RL load.

63. What is meant a series inverter?

An inverter in which the commutating elements are connected in series with the

load is called a series inverter.

64. What is the condition to be satisfied in the selection of L  and C in a series inverter?

2

R

< 4L/C

65. What is meant a parallel  inverter?

An inverter in which the commutating elements are connected in parallel with the

load is called a parallel inverter.

66. What are the applications of a series inverter?

The thyristorised series inverter produces  an approximately sinusoidal waveform

at a high output frequen cy, ranging from 200 Hz to 100kHz. It is commonly used for

fixed output applications such as

a.  Ultrasonic generator.

b.  Induction heating.

c.  Sonar Transmitter

unit-4

d.  Fluorescent lighting.

67. How is the inverter circuit classified based on  commutation circuitry?

a.  Line commutated inverters.

b.  Load  commutated inverters.

c.  Self commutated inverters.

d.  Forced commutated inverters.

68. What is meant by McMurray inverter?

It is an impulse commutated inverter which relies  on LC cir cuit and an aux iliary

thyristor for commutation in the load circuit.

69. What are the applications of a CSI?

a.  Induction heating

b.  Lagging VAR compensation

c.  Speed control of ac motors

d.  Synchronous motor starting.

70. What is meant by PWM control?

In this method, a fixed dc input voltage is given to the inverter and a controlled

ac output voltage is obtained by adjusting the on  and off periods of th e inverter

components. This is the most popular method of controlling the output voltage and this

method is termed as PWM control.

71. What are the adv antages  of PWM control?

a.  The output voltage  can be obtained without any additional components.

b.  Lower order harmonics can be eliminated or minimized along with its output

voltage control. As the higher order harmonics can be filtered easily, the filtering

requirements are minimized.

72. What are the disadvantages of the harmonics present in the inverter system?

a.  Harmonic currents will lead to excessive heating in the induction motors. This

will reduce the load carrying capacity of the motor.

b.  If the control and the regulating circuits are not pr operly shielded, harmonics from

power ride  can affect their operation and malfunctioning can result.

c.  Harmonic currents cause losses in the ac system and can even some time produce

resonance in the system. Under resonant conditions, the instrumentation and

metering  can be affected.

d.  On critical loads, torque pulsation produced b y the harmonic current  can be

useful.

73. What are the methods of reduction of harmonic content?

a.  Transformer connections

b.  Sinusoidal PWM

c.  Multiple commutation in each cycle

d.  Stepped wave inverters

74. Compare CSI and VSI.

S. No.  VSI  CSI

1.

Input voltage is maintained

Input current is constant but

constant

adjustable

2.

The output voltage does not

The output current does not depend

depend on the load

on the load

3.

The magnitude of the output

The magnitude of the output voltage

current and its waveform

and its waveform depends on the

depends on the nature of the

nature of the load impedance

load impedance

4.

It requires feedback diodes  It does not requires feedback diodes

5.    Commutation circuit is

Commutation circuit is simple i.e. it

complicated i.e. it contains

contains only capacitors.

capacitors and inductors.

75. What are the disadvantages of PWM control?

SCRs are expensive as they must possess low turn-on and turn-off times.

76. What does ac voltage controller mean?

It is device which converts fixed alternating voltage into a variable voltage

without change in frequency.

77. What are the applications of ac voltage controllers?

a.  Domestic and industrial heating

b.  Lighting control

c.  Speed control of single phase and three phase ac  motors

d.  Transformer tap changing

78. What are the adv antages  of ac voltage  controllers?

a.  High efficien cy

b.  Flexibility in control

c.  Less mainten ance

79. What are the disadvantages of ac voltage controllers?

The main draw b ack is the introduction of harmonics in the supply current and the

load voltage waveforms particularly at low output voltages.

80. What are the two methods of control in ac voltage controllers?

a.  ON-OFF control

b.  Phase control

81. What is the difference between ON-OFF control  and phase control?

ON-OFF control: In this method, the thyristors are employed as

switches to connect the load circuit to the source for a few cycles of the load voltage and

disconnect it for another few cycles.   Phase control:  In this method, thyristor switches

connect the load to the  ac source fo r a portion of  each half cycle o f input voltage.

82. What is the advantage of ON-OFF  control?

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

generated by the switching action are reduced.

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

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

high thermal time constant.

84. What is the  duty cycle in ON-OFF control method?

Duty cycle K =  n/ (n + m), where n = no. of ON cycles, m = no. of OFF cycles.

85. What is meant by unidirectional or half-wave ac v oltage controller?

Here the power flow is controlled only during the positive half-cycle of the  input

voltage.

86. What are the disadvantages of unidirectional or half-wave ac voltage controller?

a.  Due to the presence of diode on the circuit, the  control range is limited and the

effective RMS output voltage can be varied between 70.7% and 100%.

b.  The input current and output voltage  are asymmetrical and contain a dc

component.If there is an input transformer, sdatur ation problem will occur

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

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87. What is meant by bidirectional or half-wav e ac vo ltage controller?

Here the power flow is controlled during both cycles of the input voltage.

88. What is the control range of firing angle inac voltage controller with RL load?

The control range is

F

<

a

<180

°

, where

F

= load power factor an gle.

89. What type of gating signal is used in single phase ac voltage controller with RL load?

High frequen cy carrier gating signal is used for single ph ase ac voltage controller

with RL load.

90. What are the disadvantages of continuous gating signal?

a.  More heating of the SCR gate.

b.  Increases the size of pulse transformer.

91. What is meant by high  frequency  carrier gating?

Thyristor is turned on by using a train of pulses fro m

a

to

p

. This type of signal

is called as high frequency carrier gating.

92. What is meant by sequen ce control of ac voltage regulators?

It means that the stages of voltage controllers in parallel triggered in a proper

sequence one after the other so as to obtain a variable output with low harmonic content.

93. What are the adv antages  of sequence control of  ac voltage regulators?

a.  System power factor is improved.

b.  Harmonics are reduced in the source  current and the load voltage.

94. What is meant by cyclo-converter?

It conv erts input power  at one frequency to output power at another frequency

with one-stage conv ersion. Cycloconverter is also known as frequency chan ger.

95. What are the two types of cyclo-converters?

a.  Step-up cyclo-conv erters

b.  Step-down cyclo-converters

96. What is meant by step-up cyclo-converters?

In these converters, the output frequency is less than the supply frequ ency.

97. What is meant by step-down cyclo-con verters?

In these converters, the output frequency is more than the supply frequency.

98. What are the applications of cyclo-converter?

a.  Induction heating

b.  Speed control of high power ac drives

c.  Static VAR generation

d.  Power supply in aircraft or ship boards

99. What is meant by positive converter group in a cycloconverter?

The part of the cycloconverter circuit that permits the flow of current during

positive half cycle of output current is called positive converter  group.

100.What is meant by negative converter group in  a cycloconverter?

The part of the cycloconverter circuit that permits the flow of current during negative half

cycle of output current is called negative converter group.

PART-B

1.Draw the two transistor model of SCR and derive an expression for anode current.

Ans:

•

Schematic diagram (2M)

•

Two transistor model diagram (2M)

•

Operation (4M)

2.Explain the characteristics of SCR

Ans:

•

Equivalent circuit

•

V-I characteristics

•

Switching characteristics

3.Describe the various methods of thyristor turn on.

Ans:

•

Forward voltage triggering

•

The dv/dt triggering

•

Gate triggering

•

Temperature triggerin g

•

Light triggering

4.Explain the operation of MOSFET

Ans:

•

Basic structure, symbol

•

Operation

•

V-I characteristics, Switching characteristics

5.Explain the operation of IGBT

Ans:

•

Basic structure, symbol

•

Operation

•

V-I characteristics, Switching characteristics

6.Derive the expressions for average output voltage and rms output voltage of 1

semiconverter.

Ans:

•

1  semiconverter bridge circuit

•

waveforms of v

, i

, i

, i

, V

, V

o

o

fd

s

T1

T2

•

Operation

•

Average output voltage, rms value of output voltage expression

7.Describe the working of 1  fully controlled bridge converter in the Rectifying mode

and inversion mode. And derive the expressions for average output voltage and rms

output voltage.

Ans:

•

1  full converter bridge circuit

•

waveforms of v

, i

, i

, i

, V

, V

o

o

fd

s

T1

T2

UNIT-1

UNIT-2

Operation for a < 90

, a > 90

Average output voltage, rms value of output voltage expression

•

•

UNIT-2

8. Describe the working of 3  fully controlled bridge converter in the Rectif ying mode

and inversion mode. And derive the expressions for average output voltage and rms

output voltage.

Ans:

•

3  full converter bridge circuit

•

waveforms of v

, i

, i

, v

o

o

A

s

•

Operation

•

Average output voltage expression

9.Describe the working of 3  semi converter. And derive the expressions for average

output voltage and rms output voltage.

Ans:

•

3  semi converter bridge circuit

•

waveforms of v

, i

, i

, v

o

o

A

s

•

Operation

•

Average output voltage expression

10. Describe the working of Dual converter.

Ans:

•

3  dual converter bridge circuit

•

waveforms of v

, i

, v

, v

, i

, i

v

i

o

o

o1

o2

1

2 ,

s ,

c

•

Operation for with circulating current  and without circulating current

•

load voltage expression, peak value i

cp

11. FOR A Type A chopper (first quad rant), express the following v ariables as a function

of Vs, R and duty cycle a in case the load is resistive average output voltage and

current.

Ans:

•

chopper circuit

•

output voltage & current waveforms

•

Average load voltage  expression

12.Describe the principle of step-up chopper. Derive an expression  for the average output

voltage in terms of input dc voltage & duty cycle.

Ans:

•

chopper circuit

•

output voltage & current waveforms

•

Average load voltage expression

13.Describe the working of four quad rant chopper. .

Ans:

UNIT-3

chopper circuit

operation

•

•

14.Explain the working of current commutated chopper with aid of cir cuit diagram and

necessary wav eforms. Derive an expression for its output voltage.

Ans:

•

chopper circuit

•

Modes of operation- equivalent circuit diagrams

•

Current & voltage waveforms

•

Design

15. Explain the working of voltage commutated chopper with aid of circuit diagram and

necessary wav eforms. Derive an expression for its output voltage.

Ans:

•

chopper circuit

•

Modes of operation- equivalent circuit diagrams

•

Current & voltage waveforms

•

Design of C & L

16.Describe the operation of series inverter with aid of diagrams. Describe an

expression for output frequency, current and voltages. What are the disadvantages of

basic series inverter?

Ans:

•

series inverter circuit

•

Current & voltage waveforms

•

Operation

•

Expression for output frequency, V

, V

L

C

18. State different methods of voltage control inverters. Describe about PWM control in

inverter.

Ans:

•

External control of ac output voltage

•

External control of dc input voltage

•

Internal control of Inverter

PWM inverter

•

Single pulse modulation

•

Multiple pulse modulation

•

Sinusoidal pulse modulation

0

19. Explain the operation of 3  bridge inverter for 180

degree mode of operation with aid

of relevant phase and line voltage wav eforms.

Ans:

•

Inverter circuit

•

operation- equivalent circuits

•

Waveforms of phase and line voltage

UNIT-4

0

20. Explain the operation of 3  bridge inverter for 120

deeger mode of operation with aid

of relevant phase and line voltage wav eforms.

Ans:

•

Inverter circuit

•

operation- equivalent circuits

•

Waveforms of phase and line voltage

21. Draw the circuit diagram of 1  auto sequential commutated current source inverter

and explain its operation with equivalent circuits for diffrent modes and necessary

waveforms.

Ans:

•

Inverter circuit

•

Modes of operation- equivalent circuits

•

Waveforms of i

, v

i

c

c ,

o

•

Expression for t

, v

, v

c

c

L

22.Draw the circuit diagram of 1  capacitor commutated current source inverter

and explain its operation with equivalent circuits for differ ent modes and necessary

waveforms.

Ans:

•

Inverter circuit

•

Modes of operation- equivalent circuits

•

Waveforms of i

, v

i

i

i

v

v

c

o ,

o,

T1,

T2,

T1,

T2

•

Expression for v

, v

, v

i

t

c

L

o ,

o,

c

23.Explain the operation of multistage control of AC voltage controllers with neat

diagram.

Ans:

•

Circuit diagram

•

Operation

24.Explain the operation of 1  AC voltage controller with RL load.

Ans:

•

Circuit diagram

•

Operation

•

Waveforms

•

25.Explain the operation of sequence control of AC voltage controller..

Ans:

•

Circuit diagram

•

Operation

•

Waveforms

UNIT-4

Ans:

•

Circuit diagram

•

Operation

•

Waveforms

27. For a 1  voltage controller, feeding a resistive load, draw the waveforms of source

voltage, gating signals, output voltage and voltage across the SCR. Describe the

working with reference to wavefo rms drawn.

Ans:

•

Circuit diagram

•

Operation

•

Waveforms

26. Explain the operation of 1  sinusoidal AC voltage controller..