MCQs and Answers

Thermal and hydro VIVA Questions :-

1.Why efficiency in Thermal Power Plant is Low?
Almost 50% of the heat generated is lost at the condenser as heat rejection. It is unavoidable as with out heat rejection it is not possible to convert heat energy into mechanical energy and drive the turbine without drop in temperature. Therefore majority of the loss takes place in the condenser. Thus efficiency of the thermal power plant is between 30-35%.

2.On What Cycle does Thermal Power Plant operate?
Thermal Power plant works on the principle of Rankine cycle

3.Why Generation Voltage in Thermal Power Plant is between 11kV to 33kV?
The current carrying conductor cross section depends upon the magnitude of the current it is carrying and insulation strength of the conductor depends on the maximum voltage it can withstand. Therefore while designing the generator an optimum value is chosen between the amount of the current and voltage conductor can withstand.

4.What are the different circuits in Thermal Power Plant?
Some of the major circuits in the thermal power plant are:

• Coal and Ash circuit
• Air and gas circuit
• Cooling water circuit
• Feed water and steam flow circuit

5.How efficiency of Thermal Plant can be improved?
Some of the methods by which the efficiency of the thermal plant can be improved are:

• By increasing the temperature and pressure of the steam entering the turbine
• By reducing the pressure in the condenser
• By reheating the steam between different stages between the turbine

6.Advantages and Disadvantages of Thermal Power Plant?

Advantages:

1. Thermal Power Plants can be operated near the load centers unlike hydro and nuclear plants
2. Requires less space compared to hydro plants and cost of construction is less
3. Running or operating costs are less compared to diesel or gas plants
4. Can able to handle over load for certain period of time

Disadvantages:

1. Emits green house gases and causes pollution
2. Coal and Ash handling requires large area
3. Efficiency is low

6.What are the advantages of Hydropower?
• A renewable source of energy – saves scarce fuel reserves.
• Non-polluting and hence environment friendly.
• Long life – The first hydro project of India completed in 1897 is still in operation at Darjeeling.
• Cost of generation, operation and maintenance is lower than the other sources of energy.
• Ability to start and stop quickly and instantaneous load acceptance/rejection makes it suitable to meet peak demand and for enhancing system reliability and stability.
• Has higher efficiency (over 90%) compared to thermal (35%) and gas (around 50%).
• Cost of generation is free from inflationary effects after the initial installation.
• Storage based hydro schemes often provide benefits of irrigation, flood control, drinking water supply, navigation, recreation, tourism, pisciculture etc.
• Being located in remote regions leads to development of interior backward areas (education, medical, road communication, telecommunication etc.)

7.Why hydropower is called renewable source of energy?
Hydropower is called renewable source of energy because it uses and not consumes the water

8.What is the classification of Hydro Projects based on Installed Capacity?
Micro: up to 100 KW
d capacity >= 500 MW

9.How energy is generated in Hydroelectric Power Plant?
for generation of electricity, and the hydropower leaves this vital resource available for other uses
Mini: 101KW to 2 MW
Small: 2 MW to 25 MW
Mega: Hydro projects with installe
Thermal Projects with installed capacity >=1500 MW

10.Which is the largest Hydropower station in the world?
Three Gorges project in China on Yang-Yang river is the largest power station in the world having
installed capacity of around 22,500 MW.

11.How does cost of generation from Hydropower Plant compare with other sources of electricity?
The hydro power generation is highly capital-intensive mode of electricity generation but being
renewable source of energy with no consumables involved; there is very little recurring cost and
hence no high long term expenditure. It is cheaper as compared to electricity generated from
coal and gas fired plants. It also reduces the financial losses due to frequency fluctuations and it
is more reliable as it is inflation free due to not usage of fossil fuel.

12.Which is the oldest Hydropower Plant in India?
The oldest Hydropower power plant is in Darjeeling District in West Bengal. It’s installed capacity
is 130KW and was commissioned in the year 1897.

13.What is the estimated total Hydropower potential of India?
The hydro power potential of India is around 1,48,701 MW and at 60% load factor, it can meet the
demand of around 84,000 MW.

14.How much of the total Hydro power potential has been exploited so far in India?
Around 19.9% of Hydropower potential has been exploited in India.

15.What are the different types of dams?
Different types of dams are conventional concrete dam, Roller compacted concrete dam, rock fill
dam, Concrete Faced Rock fill Dam (CFRD), Earth fill dam, arch dam, barrages etc.

16.Why the unit sizes of hydro generating machines are not standardized as in case of thermal powerplants?
Since the size of hydro generating machines are based on availability of water in river and the
water head available at a particular project site, the size of the machines keeps varying from
location to location and river to river. The sizes are also based on logistics and variation of water
in river during the year.

17.What are the different types of Hydro Schemes?
Different types of Hydro Schemes are:-
1. Purely Run – of – River Power Station.
2. Storage type Power Station.
3. Run – of – River Stations with Pondage.

18.What are the major reasons for balancing Hydro-thermal mix?
Seasonal load curves of our regional grids match with the pattern of hydro power generation.
During summer/monsoon season when the generation at hydro power plants is high, the load
factor of the system is high due to heavy agricultural load. During winter, the thermal stations
operating at base load and hydro stations working as peak load stations will take care of weather
beating loads. Thus the operational needs of hydro & thermal stations are complimentary and the
balanced mix helps in optimal utilization of the capacity.

19.Why Hydropower stations are preferred solution for meeting peak loads in grids?
Due to its unique capabilities of quick starting and stopping of hydro machines, hydropower
stations are found to be economical choice to meet peak load in the grid.

20.What are approaches to tackle sedimentation problem of reservoir?
The following are some approaches to tackle sedimentation problem of reservoir:-

• Catchment Area Treatment (CAT) for reduction of silt load includes afforestation of the
  catchment area and Environmental works such as construction of check dams.
• Effective de-silting arrangements for prevention of silt.
• Silt resistant equipment for withstanding the silt.

THERMAL & HYDRO LAB VIVA Questions and Answers Pdf ::

Electrical Circuits VIVA Questions and Answers :-

1.What is Current?

2.Please define Ampere?

3.Could you measure current in parallel?

4.What is the difference between Voltages or Potential Difference? And what are they?

5.Could you measure Voltage in series?

6.How many Types of Circuit Loads are there in a Common Electrical Circuit?

7.What are the different Sign Conventions used in electric circuits?

8.What do you mean by Passive Circuit Elements and why these are called Passive?

9.State and define Ohm’s Law?

10.Please Define Ohm’s Law for A.C (Alternating Current)?

11.What is the function of Capacitor in Electrical Circuits?

12.Why Inductors are installed in electrical Circuits?

13.Briefly explain the purpose of Inductor in an electric circuit?

14.What do you mean by dependent and independent voltage sources?

15.Differentiate between ideal and non-ideal voltage sources?

16.What does the term “Voltage Regulation” means?

17.What is DC Current source? Differentiate between ideal and non ideal current sources?

18.What id difference between power and energy?

19.What is Voltage Divider Rule?

20.What is Kirchoff ’s Current Law (KCL)?

21.What is Current Divider Rule (CDR)?

22.What is Superposition Theorem?

23.Please elaborate Th´evenin’s Theorem?

24.State and define Norton’s Theorem?

25.What is the method to solve circuit using Source Transformation?

 

Electrical Circuits VIVA Questions and Answers :-

26.What is Maximum Power Transfer Theorem?

27.Define Super node?

28.What does the term Supermesh means?

29.What do you mean by Phasors?

30.What does the term Power Factor shows?

31.Differentiate between Real and Apparent Power?

32.What does Bode Plots shows?

33.Differentiate between Low Pass, High pass and Band Pass filter?

34.What is Corkscrew Rule?

35.What does Ampere turns shows?

36.Differentiate between Magnetic field and Magnetic Flux?

37.What is Force on a conductor in a magnetic field ?

38.What is force on parallel conductors ?

39.What do you mean by Pull of Electromagnet?

40.How would you calculate Power loss in a resistor?

41.At what factor Resistance R of a conductor depends?

42.Differentiate between Kirchhoff’s First law and Kirchhoff’s Second law?

43.Differentiate between Induced emf and Self inductance?

44.How to calculate Energy stored in an inductance?

45.How to calculate Energy stored in a capacitance

46.At what base you will tell the direction of rotation of DC Motors and Generators?

47. Explain bypass capacitor?

48. What is meant by coupling capacitor?

49. Why does amplifier gain reduce?

50. Explain the different regions in frequency response?

51. State the types of distortions in amplifier?

52. What is cross over distortion? How it can be eliminated?

53. Define noise?

54. Define step response?

55. Draw the symbol of JFET and MOSFET.

56. What are the two modes of MOSFET?

57. Define pinch-off voltage

58. What is feed back and what are feed back amplifiers?

59. What is meant by positive and negative feedback?

60. What are the advantages and disadvantages of negative feed back?

61. Differentiate between voltage and current feedback in amplifiers?

62. Define sensitivity?

63. Define De-sensitivity?

64. What is the type of feedback used in an op- amp Schmitt trigger?

65. Give the expression for the frequency of oscillations in an op-amp sine wave

66. oscillator?

67. What are the conditions for sustained oscillator or what is Backhouse criterion?

68. What is Oscillator circuit?

69. What are the classifications of Oscillators?

70. What are the types of feedback oscillators?

71. Define Piezo-electric effect?

72. Draw the equivalent circuit of crystal oscillator?

73. What is Miller crystal oscillator? Explain its operation?

74. State the frequency for RC phase shift oscillator?

75. Give the topology of current amplifier with current shunt feedback?

76. What are gain margin and phase margin?

77. What is the minimum value of hfe for the oscillations in transistorized RC

78. Phase shift oscillator?

79. What is LC oscillator?

80. Draw the circuit of clapp oscillator?

81. How does an oscillator differ from an amplifier?

82. Name two low frequency oscillators?

83. What is class AB operation?

84. Define conversion efficiency of a power amplifier. What is its value for class C

85. power amplifier?

86. What is crossover distortion? How it can be eliminated?

87. What is meant by Harmonic distortion?

88. What is the drawback of class B amplifier? How is this minimized?

89. Define thermal resistance in the context of power amplifier.

90. What are the types of class B amplifier?

Electrical Machines VIVA Questions :-

1.What is the power factor of a transformer at no load?
At no load, the power factor of a transformer is very low and lagging, whereas the power factor on load is nearly equal to the power factor of the load, which it is carrying.

2.What is the normal phase difference between the voltage and the no-load current in a transformer?
The no-load current in a transformer normally lags behind the voltage by about 80°– 85°.

3.What are the essential parts of a transformer?
The essential parts of a transformer are as follows:

1. Magnetic circuit consisting of laminated iron core and clamping structure
2. Primary winding
3. Tank filled with insulating oil
4. HT terminals with bushings
5. LT terminals with bushings
6. Conservator tank
7. Breather
8. Vent pipe
9. Thermometer

4.What is the name of the winding to which supply is given?
Primary winding.

5.What is the name of the winding from which the supply is taken for load connections?
Secondary winding.

6.Which material is used for the core of a transformer and why?
Laminations of specially alloyed silicon steel (silicon content 4–5 per cent) are used due toits high electrical resistance, high permeability,non-ageing characteristics and minimum ironloss.

7.What is the use of iron core in a transformer?
The iron core is used in a transformer to provide continuous easy magnetic path of low reluctance.

8.How is magnetic leakage reduced?
Magnetic leakage is reduced to a minimum by sectionalizing and interleaving the primary and secondary windings.

9.Why are iron cores in transformers made laminated?
Iron cores are made laminated to reduce eddy current loss.

10.What determines the thickness of the lamination or stamping?
Frequency determines the thickness of the lamination or stamping

11.Why are the laminations insulated from each other?
The laminations are insulated from each other by insulating varnish or thin paper to break the path of eddy currents and thus reduce eddy current loss.

12.What is stacking factor? What is its approximate value?
Stacking factor is the ratio of iron content in the laminated varnished core by volume. Its value is about 90 per cent, that is, 10 per cent volume is occupied by varnish and air.

13.What is called grain-oriented laminations?
Grain-oriented laminations are cold rolled laminations specially annealed to orient the iron crystals, that is, the grains in a uniform way in the direction of rolling to get very high permeability and low hysteresis loss.

14.What is the permissible maximum flux density in transformer core?
1.6–1.8 Wb/m2

15.What is the phase relationship between the primary and secondary voltages of a transformer?
The primary and secondary voltages of a transformer are 180° out of phase.

16.What is turn ratio of a transformer?
The ratio of the number of turns in the primary to the number of turns in the secondarywindings is called the turn ratio or the ratio of transformation of the transformer, which isindicated by a constant.

17.What is voltage ratio of a transformer?
Voltage ratio is the ratio of the voltage between the line terminals of one winding to that between the line terminals of another winding at no load.

18.What current flows in the transformer primary when its secondary is open?
When the secondary is open, there is no current in the secondary of the transformer. The primary takes a small current I0) from the source called no-load current which has a magnetizing
component (I0 sin?0) producing the magnetic flux and a working component (I0 cos?0) supplying real power for iron losses.

19.What is the formula for calculating no-load current?
No-load current

20.Why is the frequency not changed during transformation of electrical energy in a transformer?
As the same flux having a definite frequency is responsible for the production of emf on both the primary and secondary windings, there is no question of change in the frequency.

21. What is the emf equation of a transformer?
Erms = 4.44 Fm fN V
where Fm = maximum value of the flux linkage with both the windings
f = frequency of the
supply,N = number of turns.

22.What are the two basic types of transformers?
The two basic types of transformers are:

1. The isolation type in which the two windings are physically isolated and electrically insulated from each other.
2. The auto transformer type in which one coil is used for both the windings.

23.What are the types of transformers according to the arrangement of iron cores?
There are three types: (i) core type, (ii) shell type and (iii) distributed core or Berry type.

24.What magnetic circuit is formed in Berry-type constructions and why?
Distributed magnetic circuits are formed in Berry-type construction because of its distributed cores.

25.What is called limb of a transformer?
The vertical portion of the iron core where the windings are placed is called limb of a transformer.

26.Why are LT windings placed near the core?
LT windings are placed near the core to reduce the total dielectric strength of the insulating materials provided on the winding.

27.What are the types of windings according to the construction?
Sandwich type and cylindrical type.

28.What is the difference between cylindrical-type and sandwich-type winding?
In cylindrical-type winding, the length of the coils is equal to the length of the core limb.
The primary and secondary windings are placed one over the other, placing low-voltage winding nearer the core, whereas in sandwich-type winding HT and LT windings are placed lengthwise one above and other like a sandwich.

29.What are the types of transformers ?
(i) Step up transformer (ii) Step-down transformer.

30 .hat do you mean by step-up and step-down transformers?
When a transformer converts low voltage to high voltage it is called a step-up transformer,and when the transformer converts high voltage to low voltage it is called a step-down transformer.

31.What is an ideal transformer?
A transformer having an overall efficiency of 100 per cent is called an ideal transformer.

32.What do you mean by power transformer?
Transformers that are used on transmission lines for the transmission and distribution of relatively large quantities of energy are called power transformers.

33.What do you mean by distribution transformers?
When transformers are used for distributing the energy from transmission lines as well as net-works for local consumption and the secondaries are directly connected to the consumer’s
load, they are called distribution transformers.

34.What do you mean by lighting transformer?
A transformer used to supply a distribution circuit having no motors connected to it is called lighting transformer.

35.How does a transformer contribute towards the widespread popularity of AC system over DC?
High voltage of AC system can be obtained using a transformer for transmission of electrical power.Using a transformer, AC voltage can be increased or decreased without any power loss.

36.The required thickness of lamination in a transformer decreases when
The applied voltage increases

37.Oil in transformers is used to –
Cool the windings

38.The windings of a transformer are divided into several coils because
It is difficult to wind as one coil

39.The size and construction of bushings in a transformer depend upon the
Size of tank and voltage supplied.

40.What is the resistance of the field winding of a d.c. shunt generator kept low?
If the field resistance of a d.c. generator is more than particular value ( critical resistance), The generator will fail to build up the voltage. For this reason, the field resistance of a d.c. shunt
generator is kept low.

41.What do you understand by external characteristics of a d.c. generator?
The graph between the terminal voltage and load current is known as external characteristics of a d.c. generator, provided speed and field current remain constant.

42.What will happen if the d.c. machine is operated below rated speed?
This will result in overheating due to two reasons; first, more field current has to be maintained in order to produce the rated voltage. Second, decrease in fanning action due to dsecrease in speed.

43.What is the most important precaution in any experiment with d.c. shunt motor?
Before switching on d.c. supply, a sufficient resistance should be put in series with the armature of the d.c. shunt motor.

44.What range of speed can you get with the field control method of speed control of d.c. shunt motor?
Speed higher than rated speed can be obtained by using this method.

45.What range of speed can you get with the armature control method of speed control of d.c. shunt motor?
Speed lower than the rated speed can be obtained by the armature control method.

46.Does the direction of rotation of d.c. shunt motor would get reversed if the armature current and field current both are reversed?
No.

47.If the rated speed of a d.c. shunt motor is1440 r.p.m, which method of speed control would you suggested to obtain a speed of 1500 r.p.m?
Field control method of speed control is suggested.

48.What will happen if the d.c. shunt motor opened accidentally running on no-load has its shunt field winding?

• The field will be reduced to only to the value of residual flux. The speed will be very high.
• The parts of motor may even fly apart.

49.What is the most essential condition for the voltage build up for a d.c. shunt generator?
There should be a residual magnetism in the poles of the DC shunt generator.

50. A d.c. shunt motor is found suitable to drive fans because they require

Small torque at start up and large torque at high speeds

ELECTRICAL MACHINES LAB VIVA Questions and Answers Pdf ::

Electrical Measurements VIVA questions :-

1. what is measurement?
The process of determining the amount, degree, or capacity by comparison (direct or indirect) with the accepted standards of the system units being used.

2. what is accuracy of a measurement?
The degree of exactness (closeness) of a measurement compared to the expected (desired) value.

3. what is Resolution of a measurement?
The smallest change in a measured variable to which an instrument will respond.

4. what is precision of a measurement?
A measure of the consistency or repeatability of measurement, i.e. successive reading do not differ.

5. what is Sensitivity of a measurement?
The ratio of the change in output (response) of the instrument to a change of input or measured
variable.

6. what is Error of a measurement?
The deviation of the change in output (response) of the instrument to a change of input or measured
variable.

7. what are the three major categories of error?
Error may arise from different sources and are usually classified as
1 Gross error 2. Systematic error 3. Random error

8. what is gross error?
These errors are mainly due to human mistakes in reading or in using instruments or error in recording observations. This error also occurs due to incorrect adjustment of instruments and computational mistakes these errors can not treat mathematically.

9. what is Systematic error?
These error occur due1. Instrumental error 2. Environmental error 3. Observational error

10. what is instrumental error?
The instrumental errors arise due to three main reasons:
1. Due to inherent shortcoming in the instrument
2. Due to misuse of the instruments
3. Due to loading effect of instrument

11.what is Cathode ray oscilloscope (CRO)?
Cathode ray oscilloscope is a instrument used for display, measurement and analysis of
waveforms and other phenomenon in electrical and electronic circuit

12.what are the basic component of a CRO?
CRO Circuit consists of following components:
1. Vertical deflection system 2. Horizontal deflection system 3.Synchronization circuit
4. Blanking circuit 5 Intensity modulation 6position control
7. Focus control 8. Cathode ray tube 9. Calibration circuit

13.what is the function of probe in CRO?
The probe performs the very important function of connecting the test circuit to oscilloscope
without altering, loading or otherwise disturbing the test circuit.

14 how many types of probe used in CRO?
There are three types of probe used in CRO:
1 Direct probe 2. Isolation probe 3. Detector probe

15.what are the functions of different probes used in CRO?

• Direct Probe: direct probe avoids stay-pick up which may create problems when low level signals
  are being measured. It is used for low freuency or low impedance circuit.
• Isolation probe: Isolation probe is used in order to avoid the undesirable circuit loading effects of the shielded probe.
• Detector probe: when analyzing the response to modulated signals used in Communications equipment like AM, FM and TV receivers, the detector probe functions to separate the low frequency modulation component from the high freuency carrier.

16.what is the function of Attenuator in CRO?
The voltage in input terminal of the vertical amplifier causes the beam to deflect off the CRT
screen, is unite low in amplitude. So that high amplitude signals may be displayed, an attenuator
network is placed between the vertical input terminals of the vertical amplifier. The main function of the attenuator is to reduce the amplitude of the vertical input signal before applying it to vertical amplifier.

17.which device is used for the source of emission of electrons in a CRT?
A barium and strontium oxide coated cathode is used for the source of emission of electrons in a
CRT.

18.what is the function of Auadag in a CRO?
Ans: An Audage is used in a CRO to collect secondary emission electrons.

19.what is the function of electron gun assembly used in CRT?
The electron gun assembly produces a sharply focused beam of electrons which are accelerated .
to high velocity .this focused beam of electrons strikes the fluorescent screen with sufficient energy to cause a luminous spot on the screen.

20. what is the function of electron gun used in CRT?
The source of focused and accelerated electrons beam is the electron gun. The electron gun
emits electrons and forms them into a beam consist of a heater, a cathode, a grid, a pre-accelerating anode, a focusing anode and an accelerating anode.

21.What do you mean by an instrument transformer?
transformer which is used with measuring instrument is called instrument transformers.
Instrument transformer is used as a protection circuit of power system for the operation of over current, under voltage, Earth fault etc.

22.what are the types of instrument transformer?
there are two types of instrument transformer:

1. Current transformer
2. Voltage transformer

23.What do you mean by a Current transformer?
The transformers which are used for measurement of current is called current transformer.

24.what do you mean by a Voltage transformer?
The transformers used for voltage measurement are called voltage or potential transformer.

25.what are the Advantages of instrument transformer?
Advantages of instrument transformer are:
1. Instrument transformers are moderate in size and used for 5A current and 100 to 200 V
measurement.
2. Instruments using instrument transformer are low cost.
3. Replacement of damage parts is Easy.
4. The metering circuit is isolated the high voltage power circuit hence insulation and safty is
assured for operation.
5. There is low power consumption in metering circuit.

26.what do you mean by a phase Angel of instrument transformer?
The angle between secondary current and primary current of a current transformer is called phase angle of instrument transformer.

27.what do you mean by transformation ratio of instrument transformer?
It is the ratio of primary winding current to the secondary winding current of an instrument
transformer.

28.what do you mean by Nominal ratio of instrument transformer?
It is the ratio of rated primary winding current to the rated secondary winding current of an
instrument transformer.

29.what do you mean by Turns ratio of instrument transformer?
It is the ratio of number of turns in secondary winding to the number of turns in primary winding of an instrument transformer.

30.what do you mean by ratio correction factor of instrument transformer?
It is the ratio of the transformation ratio to the nominal ratio of an instrument transformer

ELECTRICAL MEASUREMENTS LAB VIVA Questions and Answers Pdf ::

Power Electronics LAB VIVA Questions :-

1.What is holding current in SCR?
It is the minimum current required to hold the SCR in forward conduction state.
When the forward current becomes less than holding current, SCR turns from forward conduction state to forward blocking state.

2. What is latching current in SCR?
It is the minimum current required to latch(turn on) the SCR from forward blocking state to forward conduction state.

3.What aturn on methods of SCR?
Herere the different Forward voltage triggering

• Gate Triggering
• dv/dt triggering
• Temperature triggering
• Light triggering

4.What is snubber circuit?
The snubber circuit is used for the dv/dt protection of the SCR. It is a series combination of a resistor and a capacitor in parallel with the SCR.

5.What is hard switching of the thyristor?
When gate current is several times higher than the required gate current, the SCR is said to be hard fired. It reduces the turn ON time and enhances the di/dt capability.

6.What is firing angle?
The angle between the zero crossing of the input voltage and the instant the SCR is fired is called as delay angle or firing angle.

7.What is meant by SOA?
SOA – Safe Operating Area determines the voltage and current boundary within which the Power Device can be operated without destructive failure.

8.What are the main components used for isolating the Power Circuits, Power Semiconductor from the low-power circuit?
Opto-Couplers, Transformers

9.Name some of the current controlled (current driven) devices…
SCR, GTO, GTR

10.Name some of the voltage driven ( Voltage controlled) devices
IGBT, MCT, IGCT, SIT

11.What is duty cycle?
It is the ratio of the ON time of the chopper to total time period of the chopper.
D = Ton / [Ton + Toff]

12.Can fuses with an AC voltage rating be used in a DC applications?

• Fuses must be rated for the voltage AC or DC in which they will be used.
• Generally, fuses have a DC voltage rating that is half of the maximum AC voltage rating.

13.What are the characteristics of ideal Opamp?

• Infinite open loop voltage gain
• Infinite input impedance
• Zero output impedance
• Infinite Bandwidth
• Zero offset voltage

14.For High voltage applications will you prefer MOSFET or IGBT?

1. For High voltage applications we have to use IGBT.
2. Because MOSFETs are low voltage devices. ie, Their voltage rating is lesser than IGBT.
3. General rule is MOSFETs are suitable for applications which has breakdown voltage less than 250V.
4. The IGBTs are suitable for applications which has breakdown voltage upto 1000V

15.For High frequency applications will you prefer MOSFET or IGBT? Why?
For High frequency applications, MOSFET is the right choice of the device.
Because MOSFET has low switching losses compare to that of IGBT.
General rule of thumb is for low-frequency applications having frequency range upto 20kHz, we have to use IGBT.
For high frequency applications having frequency range of more than 200kHz, we have to use MOSFET.

16.What are the advantages of free wheeling diode in rectifier circuit?
The input power factor is improved. It prevents the output voltage from becoming negative. Load current waveform is improved.

17.What is meant by commutation?
The process of changing the direction of current flow in a particular path of the circuit. It is used to turn off the SCR.

18.What are the types of commutation?

• Natural commutation
• Forced commutation

19.What is natural commutation?
The process of the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off is called as natural commutation.

20.What is forced commutation?
The process of the current flowing through the thyristor is forced to become zero by external circuitry is called as forced commutation.

21.What are the types of commutation with respect to commutation process?

• Voltage commutated chopper
• Current commutated chopper
• Load commutated chopper

22.What is meant by cyclo-converter?
It is also known as frequency changer. It converts input power at one frequency to output power at another frequency with one stage conversion.

23.What are the types of cyclo-converters?

1. Step up cyclo-converter
2. Step down cyclo-converter.

24.What is step down cyclo-converter?
It is the converter whose output frequency is less than the input frequency.

25.What is step up cyclo-converter ?
It is the converter whose output frequency is more than the input frequency.

26.What does the Voltmeter in AC mode show? Is it RMS value or peak value?
Multimeter in AC mode shows RMS value of the voltage or current. Also when it is DC mode it will show the RMS value only.

27.What is the necessity to use the special machines?
General purpose motors (Induction motors, synchronous motors) are neither precision speed nor precision position motors. For many automated systems require high precise speed and high precise positioning motors. In such cases special purpose motors like stepper motors, PMDC motors etc. are used.

28.What are the control strategies of chopper?
The control strategies of chopper are
1. Pulse width modulation PWM (Variable TON, Constant frequency)
2. Frequency modulation (Constant TON or TOFF, Variable frequency)
3. Current Limit Control (CLC)

29.What is delay angle or what is firing angle of phase controlled rectifier?
The delay angle is the angle at which thyristors are triggered after zero crossing. After zero crossing of supply voltage, one pair of thyristors is
forward biased. After delay angle(α) these SCRs are triggered.

30.What is Universal Motor?
It is defined as a motor which can be operated either on DC or single-phase AC supply at approximately the same speed and output.
The universal motor is built exactly like a series DC motor. But a series DC motor cannot be run as a universal motor, even though both motors look the same internally and externally.
We cannot use these motors in the industrial applications due to the low efficiency (25% -35%). It has high starting torque and a variable speed characteristic. It runs at dangerously high-speed on no load.

31.Give some examples of power electronics applications in the day-to-day life?
We can list a huge number of power electronics applications. Few of the applications which we can see in our daily life are
UPS – Uninterruptible Power Supply
SMPS – Switch Mode Power Supply
Speed Control of Motors
ICU

32.What is meant by PMDC?
PMDC stands for Permanent Magnet DC Motor
A Permanent Magnet DC Motor is similar to an ordinary dc shunt motor except that its field is provided by permanent magnets instead of salient-pole wound field structure.
There are three types of permanent magnets used for such motors namely;
(i) Alnico Magnets
(ii) Ceramic magnets
(iii) Rare-earth magnets
The major advantages are low noise, small size, high-efficiency, low manufacturing cost.

POWER ELECTRONICS LAB VIVA Questions and Answers :-

NETWORK ANALYSIS VIVA Questions :-

1.What is resistance?
the resistance is the property of a material to oppose the flow of current in a material.its unit is ohm.

2.What are the material used for resistor?
the material used are maganin (alloy of copper magnese and nickel),constantan(alloy of nickel and
copper).

3.what is inductance?
It is the property of a material by virtue of which it opposes any change of magnitude and direction of
current passing through the conductor.

4.what happens to voltage when current through the inductor is constant?
The voltage across inductor is zero.

5.how will you define capacitance?
It is the ability to store electric charge within it.Capacitance is a measure of charge per unit voltage
that can be stored in an element.

6.What happens to voltage when current is zero?
the voltage is constant.

7.When we use 3 terminal resistor?
It is used when resistance is less than 1 ohm.

8.what is the unit of charge and current?
the units are coulomb and ampere.

9.What are the properties of a resistor?
the properties are high resistivity ,resistance to oxidation, corrosion and moisture.

10.what is Q factor?
the Q factor is ratio of inductive reactance to resistance of a coil.

11.What are the material used for inductance coil?
the materials used are marble because it is unaffected by atmospheric conditions.

12.Which capacitor is preferred for high voltage and frequency?
The vaccum and gas filled capacitor are used for high voltage and frequency applications.

13.State Kirchoff current law?
The algebraic sum of currents at any node of a circuit is zero. The sum of incoming current is equal
to sum of outgoing current.

14.What are dependent sources?
When strength of voltage or current changes in the source for any change in the connected network
they are called dependent sources.

15.List examples of voltage source?
The examples of voltage source are battery and generator.

16.List examples of current sources?
semiconductor devices like transistor and diode are treated as current sources.

17.state Kirchoff voltage law?
Kirchoff voltage law states that the algebraic sum of all branch voltages around any closed loop of a
network is zero at all instant of time.

18.State TheveninTheorem?
This theorem states that any linear network with output terminal AB can be replaced by a single
voltage source V in series with a single impedance.

19.How equivalent impedance is calculated in TheveninTheorem?
All independent voltage sources are short circuited and all independent current sources are open
circuited.

20.What is the limitation of Kirchoffs law?
It fails in distributed parameter network.

21.State Nortons theorem?
This theorem states that any linear bilateral network with active network with output terminals AB
Can be replaced by a single current source in parallel with a single impedance Z..

22.Is the theorem applicable to ac sources?
No it is applicable to dc circuits with and without controlled sources.

23.Define Norton equivalent circuit?
The Norton equivalent circuit is a current generator which is placed in parallel to internal resistance.

24.State Superposition theorem?

If a number of voltages or current sources are acting simultaneously in a linear network the resultant
current in any branch is the algebraic sum of current that would be produced in it when each source acts
alone replacing all other independent sources by their internal resistances.

25.Sate Maximum power transfer theorem?
A resistance load being connected to a dc network receives maximum power when load resistance is
equal to internal resistance.

26.What is the efficiency during maximum power transfer?
50%.

27.Define branch?
It is a part of a network which lies between two junction points.

28.Define active and passive network?
The network which has no current or voltage source is called passive network.
The network which either has current or voltage source is called active network.

29.State Ohm’s Law?
The current through any conductor is directly proportional to the applied potential difference across it
keeping physical condition unchanged.

30.Define unilateral circuit?
A10 The circuit whose properties are not same in either direction is known as unilateral circuit.

31.Define filter?
A filter is an electrical network that can transmit signals within a specified frequency range.

32.List the characteristics of filter?
An ideal filter would transmit signals under the passband frequencies without attenuation and
completely suppress the signal with attenuation band of frequencies with a sharp cutt off profile.

33.Define characteristics impedance?
The characteristics impedance of a filter matches with circuit to which it is connected throughout the
pass band.

34.What is the unit of attenuation?
The unit is decibel and neper.

35.What are the application of filter?
the Filter is used in voice frequency telegraphy,multi channel communication, TV broadcasting and telephony.

36.Define active filter?
The active filter contains components like operational amplifier that introduce some gain in the signal.

37.List advantges of active filter over passive filter?
Active filter eliminate bulky components.It offer gain.It can drive low impedance loads.It is easy to
tune.

38.List the disadvantages of constant K filters?
The attenuation does not increase rapidly beyond cutt off frequency.
characteristics impedance varies widely in the pass band from desired value.

39.Define cuttoff frequency?
The frequency that seperates the pass and attenuation band is known as cutt off frequency.

40. How a band pass filter is constructed?
This filter is a combination of two parallel tuned circuit.This is a special type of LC filter alongwith a
particular BW frequency to be allowed through it.

NETWORK ANALYSIS VIVA Questions and answers Pdf