400+ TOP POWER ELECTRONICS Interview Questions and Answers pdf

POWER ELECTRONICS Interview Questions with Answers :-

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 are the different turn on methods of SCR?

  • 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.

POWER ELECTRONICS Interview Questions

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?

  1. Infinite open loop voltage gain
  2. Infinite input impedance
  3. Zero output impedance
  4. Infinite Bandwidth
  5. Zero offset voltage

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

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

15. For High frequency applications will you prefer MOSFET or IGBT? Why?

  1. For High frequency applications, MOSFET is the right choice of the device.
  2. Because MOSFET has low switching losses compare to that of IGBT.
  3. General rule of thumb is for low-frequency applications having frequency range upto 20kHz, we have to use IGBT.
  4. For high frequency applications having frequency range of more than 200kHz, we have to use MOSFET.

POWER ELECTRONICS Interview Questions ::

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.
  • The Load current waveform is improved.

17. What are the types of commutation?

  • Natural commutation
  • Forced commutation

18. 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.

19. 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.

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

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

21. 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.

22. What are the types of cyclo-converters?
Step up cyclo-converter
Step down cyclo-converter.

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

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

25. 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.

26. 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.

27. 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)

28. 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. ie, After delay angle(α) these SCRs are triggered.

29. 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.

33. 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.

34. Two Bulbs Of 100w And 40w Respectively Connected In Series Across A 230v Supply Which Bulb Will Glow Bright And Why?

Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the bulb(P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.

35. What Is Meant By Knee Point Voltage?

Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).

36. What Is Reverse Power Relay?

Reverse Power flow relay are used in generating station’s protection. A generating stations is supposed to fed power to the grid and in case generating units are off,there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

37. What Are The Advantage Of Free Wheeling Diode In A Full Wave Rectifier?

It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in a inductive load.

38. What Is The Full Form Of Kvar?

We know there are three types of power in Electrical as Active, apparent & reactive. So KVAR is stand for “Kilo Volt Amps with Reactive component.

39. Definition Of Power Electronics?

Power electronics refers to control and conversion of electrical power by power semiconductor devices wherein these devices operate as switches.

40. What Is The Main Purpose Of Power Electronics?

  • The main task of power electronics is to control and convert electrical power from one form to another.
  • AC to DC conversion: Rectifier is used for converting an AC voltage to a DC voltage.
  • Rectifier applications: Variable speed dc drives, Battery chargers, DC power supplies and Power supply for a specific application like electroplating.
  • DC to AC conversion: Inverter circuit is used to convert DC voltage to an alternating voltage.
  • Inverter applications: Emergency lighting systems, AC variable speed drives, Un-interrupted power supplies and Frequency converters.
  • DC to DC conversion: A dc-to-dc converter circuit was called a chopper.
  • Chopper applications: DC drive, Battery charger and DC power supply.
  • AC to AC conversion: A cycloconverter converts an AC voltage to another AC voltage.
  • Cycloconverter applications: It is rarely used. Can be used for controlling the speed of an AC traction motor

41. What Are The Different Operation Regions Of The Thyristor (scr)?

SCR or thyristor will have three regions of operations based on the mode in which the device is connected in the circuit.

  1. Reverse blocking region: When the cathode of the thyristor is made positive with respect to the anode and no gate signal is applied. In this region SCR exhibits the reverse blocking characteristics similar to diode.
  2. Forward blocking region: In this region the anode of the thyristor is made positive with respect to the cathode and no gate signal is applied to the thyristor. A small leakage current flow in this mode of operation of the thyristor.
  3. Forward conduction region: when the forward voltage applied between the anode and cathode increases at particular break over voltage avalanche breakdown takes place and thyristor starts conducting current in forward direction. By this type of triggering the device damages the scr. Hence a gate signal is applied before the forward break over voltage to trigger the scr.

42. What Are The Losses That Occur In A Thyristor During Working Conditions?

  • Forward conduction losses
  • Loss due to leakage current during forward and reverse blocking
  • Switching losses at turn on turn off
  • Gate triggering loss

43. What Are The Advantages Of Freewheeling Diode In Rectifier Circuit?

The input power factor is improved. It prevents the output voltage from becoming negative. Load current waveform is also improved.

44. Explain The Function Of 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.

45. What Is An Inverter?

A device which converts dc power into ac power at desired output voltage and frequency is called as Inverter.

46. 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.

47. Why The Circuit Turn Off Time Should Be Greater Than Thyristor Turn Off Time?

If the circuit turn off time is less than the thyristor turn off time the device may turn on at an undesired instant resulting in commutation failure.

48. What Is Chopper?

A dc Chopper is equivalent to the transformer in ac circuit. It is a static switch used to get the variable dc voltage from a constant dc voltage.

49. What Are The Types Of Commutation? (or) Turn Off Methods?

  • Natural commutation
  • Forced commutation

50. 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.

51. 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.

52. What Is Step Down Chopper?

In step down chopper, the average output voltage is less than the input supply voltage. It is also known as Buck converter.

53. What Is Step Up Chopper?

In step up chopper, the average output voltage is more than the input supply voltage. It is also known as Boost converter.

54. What Is Voltage Commutation?

The process of a charged capacitor momentarily reverse biases the conducting SCR and turns it off is called as voltage commutation.

55. What Is Current Commutation?

The process of a current pulse is made to flow in the reverse direction through the conducting SCR and thus made the net SCR current becomes zero, consequently turn off the SCR is called as current commutation.

56. What Are The Advantages Of Current Commuted Chopper?

The advantages of current commutated chopper is;

  • Commutation is reliable as load current is less than the peak commutation current
  • The auxiliary SCR is naturally commutated as its current passes through zero value.
  • The capacitor always remains charged with the correct polarity.

57. What Is Load Commutation?

In load commutation, the load current flowing through the thyristor either becomes zero or is transferred to another device from the conducting SCR.

58. Name Some Of The Current Controlled (current Driven) Devices?

SCR, GTO, GTR.

59. Name Some Of The Voltage Driven (voltage Controlled) Devices.

IGBT, MCT, IGCT, SIT.

60. What Is Meant By Pulse Triggered Devices?

To turn on these kind of devices single pulse of short duration is sufficient. Continuous gate voltage of entire on time is not required. It will avoid the hard triggering.

Example: Thyristor, GTO

61. What Is Meant By Level-sensitive Devices?

In order to maintain these kind of devices in on-state, we need to apply continuous gate current /voltage.

Some of the level sensitive devices are: MOSFET, IGBT, MCT, IGCT

62. What Are The Advantages Of Igbt Over Bjt, Mosfet?

IGBT has

  1. Lower turn on and turn off times than BJT
  2. Lower on state conduction losses than MOSFET
  3. Excellent safe operating area

63. Why Igbt Is Very Popular Nowadays?

  1. Lower hate requirements
  2. Lower switching losses
  3. Smaller snubber circuit requirements

64. What Are The Different Methods To Turn On The Thyristor?

  • Forward voltage triggering
  • Gate triggering
  • dv/dt triggering
  • Temperature triggering
  • Light triggering

65. What Is The Difference Between Power Diode And Signal Diode?

Power diode

  • Constructed with n-layer, called drift region between p+ layer and n+ layer.
  • The voltage, current and power ratings are higher.
  • Power diodes operate at high speeds.

Signal diode

  • Drift region is not present.
  • The voltage, current and power ratings are Lower
  • Operates at higher switching speed.

66. Igbt Is A Voltage Controlled Device. Why?

Because the controlling parameter is gate-emitter voltage.

67. Power Mosfet Is A Voltage Controlled Device. Why?

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

68. Power Bjt Is A Current Controlled Device. Why?

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

69. What Is The Relation Between α & β?

 

70. What Are The Different Types Of Power Mosfet?

  • N-channel MOSFET
  • P-channel MOSFET

71. How Can A Thyristor Turned Off?

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

72. Define Latching Current?

The latching current is defined as the minimum value of anode current which it must attain during turn on process to maintain conduction when gate signal is removed.

73. Define Holding Current?

The holding current is defined as the minimum value of anode current below which it must fall to for turning off the thyristor.

74. 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.

75. What Losses Occur In A Thyristor During Working Conditions?

  1. Forward conduction losses
  2. Loss due to leakage current during forward and reverse blocking.
  3. Switching losses at turn-on and turn-off.
  4. Gate triggering loss.

76. Define Hard-driving Or Over-driving?

When gate current is several times higher than the minimum gate current required, a thyristor is said to be hard-fired or over-driven. Hard-firing of a thyristor reduces its turn-on time and enhances its di/dt capability.

77. 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.

78. Why 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.

79. What Is The Turn-off Time For Converter Grade Scrs And Inverter Grade Scrs?

Turn-off time for converter grade SCRs is 50 – 100 ms turn-off time for converter grade SCRs and inverter grade SCRs and for inverter grade SCRs is 3 – 50 ms.

80. What Are The Advantages Of Gto Over Scr?

  • Elimination of commutation of commutating components in forced commutation, resulting in reduction in cost, weight and volume.
  • Reduction in acoustic noise and electromagnetic noise due to elimination of commutation chokes.
  • Faster turn-off, permitting high switching frequencies.
  • Improved efficiency of the converters.

81. What Is Meant By Phase Controlled Rectifier?

It converts fixed ac voltage into variable dc voltage.

82. Mention Some Of The Applications Of Controlled Rectifier?

  • Steel rolling mills, printing press, textile mills and paper mills employing dc motor drives.
  • DC traction
  • Electro chemical and electro-metallurgical process
  • Portable hand tool drives
  • Magnet power supplies
  • HVDC transmission system

83. What Is The Function Of Freewheeling Diodes In Controlled Rectifier?

It serves two process.

  1. It prevents the output voltage from becoming negative.
  2. The load current is transferred from the main thyristors to the freewheeling diode, thereby allowing all of its thyristors to regain their blocking states.

84. What Are The Advantages Of Free Wheeling Diodes In A Controlled In A Controlled Rectifier?

  • Input power factor is improved.
  • Load current waveform is improved and thus the load performance is better.

85. What Is Meant By Delay Angle?

The delay angle is defined as the angle between the zero crossing of the input voltage and the instant the thyristor is fired.

86. What Are The Advantages Of Single Phase Bridge Converter Over Single Phase Mid-point Converter?

  • SCRs are subjected to a peak-inverse voltage of 2Vm in a fully controlled bridge rectifier. Hence for same voltage and current ratings of SCrs, power handled by mid-point configuration is about
  • 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.

87. What Is Commutation Angle Or Overlap Angle?

The commutation period when outgoing and incoming thyristors are conducting is known as overlap period. The angular period, when both devices share conduction is known as the commutation angle or overlap angle.

88. What Are The Different Methods Of Firing Circuits For Line Commutated Converter?

  1. UJT firing circuit.
  2. The cosine wave crossing pulse timing control.
  3. Digital firing schemes.

89. Give An Expression For Average Voltage Of Single Phase Semiconverters?

Average output voltage Vdc = (Vm /Π) (1 + cosα).

90. 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 = ( V1 I1 cos φ1 ) / ( Vrms Irms) where V1 = phase voltage, I1 = fundamental

component of the supply current, φ1 = input displacement angle, Irms = supply rms current.

91. What Are The Advantages Of Six Pulse Converter?

  • Commutation is made simple.
  • Distortion on the ac side is reduced due to the reduction in lower order harmonics.
  • Inductance reduced in series is considerably reduced.

92. 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.

93. What Is Meant By Forced Commutation?

the thyristor is forced to become zero by external circuitry.

94. 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.

95. What Are The Applications Of Dc Chopper?

  • Battery operated vehicles
  • Traction motor control in electric traction
  • Trolley cars
  • Marine hoists
  • Mine haulers
  • Electric braking.

96. What Is Meant By Step-up And Step-down Chopper?

In a step- down chopper or Buck converter, the average 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.

97. Write Down The Expression For Average Output Voltage For Step Down Chopper?

Average output voltage for step down chopper V0 = α Vs, α is the duty cycle.

98. Write Down The Expression For Average Output Voltage For Step Up Chopper?

Average output voltage for step down chopper

Vs
V0 = ——
1 – α
α is the duty cycle.

99. What Is Meant By Duty-cycle?

Duty cycle is defined as the ratio of the on time of the chopper to the total time period of the chopper. It is denoted by α.

100. What Are The Two Types Of Control Strategies?

  1. Time Ratio Control (TRC)
  2. Current Limit Control method (CLC)

101. What Is Meant By Trc?

In TRC, the value of Ton / T is varied in order to change the average output voltage.

102. What Are The Two Types Of Trc?

  • Constant frequency control
  • Variable frequency control

103. 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.

  • On-time Ton is kept constant
  • Off period Toff is kept constant.

104. 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).

105. Write Down The Expression For The Average Output Voltage For Step Down And Step Up Chopper?

Average output voltage for step down chopper is VO = α VS.

Average output voltage for step up chopper is VO = α VS x [1/ ( 1- α)].

106. What Are The Different Types Of Chopper With Respect To Commutation Process?

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

107. What Is Meant By Voltage Commutation?

In this process, a charged capacitor momentarily reverse biases the conducting thyristor and turn it off.

108. 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 net thyristor current becomes zero, it is turned off.

109. 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.

110. What Are The Advantages Of Current Commutated Chopper?

  • The capacitor always remains charged with the correct polarity.
  • Commutation is reliable as load current is less than the peak commutation current ICP.
  • The auxiliary thyristor TA is naturally commutated as its current passes through zero value.

111. What Are The Advantages Of Load Commutated Chopper?

  1. Commutating inductor is not required.
  2. It is capable of commutating any amount of load current.
  3. It can work at high frequencies in the order of kHz.
  4. Filtering requirements are minimal.

112. What Are The Disadvantages Of Load Commutated Chopper?

  • For high power applications, efficiency becomes very low because of high switching losses at high operating frequencies.
  • Freewheeling diode is subjected to twice the supply voltage.
  • Peak load voltage is equal to twice the supply voltage.
  • The commutating capacitor has to carry full load current at a frequency of half chopping frequency.
  • 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.

113. What Is Meant By Inverter?

A device that converts dc power into ac power at desired output voltage and frequency is called an inverter.

114. What Are The Applications Of An Inverter?

  • Adjustable speed drives
  • Induction heating
  • Stand-by aircraft power supplies
  • UPS
  • HVDC transmission

115. What Are The Main Classification Of Inverter?

  • Voltage Source Inverter
  • Current Source Inverter

116. Why Thyristors Are Not Preferred For Inverters?

Thyristors require extra commutation circuits for turn off which results in increased complexity of the circuit. For these reasons thyristors are not preferred for inverters.

117. How Output Frequency Is Varied In Case Of A Thyristor?

The output frequency is varied by varying the turn off time of the thyristors in the inverter circuit, i.e. the delay angle of the thyristors is varied.

118. Give Two Advantages Of Csi?

  1. CSI does not require any feedback diodes.
  2. Commutation circuit is simple as it involves only thyristors.

119. What Is The Main Drawback Of A Single Phase Half Bridge Inverter?

It require a 3-wire dc supply.

120. Why 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 anti parallel will allow the current to flow when the main thyristors are turned off. These diodes are called feedback diodes.

121. What Types Of Inverters Require Feedback Diodes?

VSI with RL load.

122. 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.

123. What Is The Condition To Be Satisfied In The Selection Of L And C In A Series Inverter?

4L
R2 < ——
C

124. 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.

125. What Are The Applications Of A Series Inverter?

The thyristorised series inverter produces an approximately sinusoidal waveform at a high output frequency, ranging from 200 Hz to 100kHz. It is commonly used for fixed output applications such as

  • Ultrasonic generator.
  • Induction heating.
  • Sonar Transmitter
  • Fluorescent lighting.

126. How Is The Inverter Circuit Classified Based On Commutation Circuitry?

  1. Line commutated inverters.
  2. Load commutated inverters.
  3. Self commutated inverters.
  4. Forced commutated inverters.

127. What Is Meant By Mcmurray Inverter?

It is an impulse commutated inverter which relies on LC circuit and an auxiliary thyristor for commutation in the load circuit.

128. What Are The Applications Of A Csi?

  • Induction heating
  • Lagging VAR compensation
  • Speed control of ac motors
  • Synchronous motor starting.

129. 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 the inverter components. This is the most popular method of controlling the output voltage and this method is termed as PWM control.

130. What Are The Advantages Of Pwm Control?

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

131. What Are The Disadvantages Of The Harmonics Present In The Inverter System?

  • Harmonic currents will lead to excessive heating in the induction motors. This will reduce the load carrying capacity of the motor.
  • If the control and the regulating circuits are not properly shielded, harmonics from power ride can affect their operation and malfunctioning can result.
  • 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.
  • On critical loads, torque pulsation produced by the harmonic current can be useful.

132. What Are The Methods Of Reduction Of Harmonic Content?

  • Transformer connections
  • Sinusoidal PWM
  • Multiple commutation in each cycle
  • Stepped wave inverters

133. Compare Csi And Vsi?

VSI:

  1. Input voltage is maintained constant
  2. The output voltage does not depend on the load
  3. The magnitude of the output current and its waveform depends on the nature of the load impedance
  4. It requires feedback diodes
  5. Commutation circuit is complicated i.e. it contains capacitors and inductors.

CSI:

  1. Input current is constant but adjustable
  2. The output current does not depend on the load
  3. The magnitude of the output voltage and its waveform depends on the nature of the load impedance
  4. It does not requires feedback diodes
  5. Commutation circuit is simple i.e. it contains only capacitors.

134. What Are The Disadvantages Of Pwm Control?

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

135. What Does Ac Voltage Controller Mean?

It is device which converts fixed alternating voltage into a variable voltage without change in frequency.

136. What Are The Applications Of Ac Voltage Controllers?

  • Domestic and industrial heating
  • Lighting control
  • Speed control of single phase and three phase ac motors
  • Transformer tap changing

137. What Are The Advantages Of Ac Voltage Controllers?

  • High efficiency
  • Flexibility in control
  • Less maintenance

138. What Are The Disadvantages Of Ac Voltage Controllers?

The main draw back is the introduction of harmonics in the supply current and the load voltage waveforms particularly at low output voltages.

139. What Are The Two Methods Of Control In Ac Voltage Controllers?

  • ON-OFF control
  • Phase control

140. 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 for a portion of each half cycle of input voltage.

141. 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.

142. 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.

143. What Is The Duty Cycle In On-off Control Method?

Duty cycle K = n/ (n + m),
where n = number of ON cycles,
m = number of OFF cycles.

144. What Is Meant By Unidirectional Or Half-wave Ac Voltage Controller?

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

145. What Are The Disadvantages Of Unidirectional Or Half-wave Ac Voltage Controller?

  • 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%.
  • The input current and output voltage are asymmetrical and contain a dc component.If there is an input transformer, saturation problem will occur
  • It is only used for low power resistive load.

146. What Is Meant By Bidirectional Or Half-wave Ac Voltage Controller?

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

147. What Is The Control Range Of Firing Angle In Ac Voltage Controller With Rl Load?

The control range is Φ < α <180°,

where Φ = load power factor angle.

148. What Type Of Gating Signal Is Used In Single Phase Ac Voltage Controller With Rl Load?

High frequency carrier gating signal is used for single phase ac voltage controller with RL load.

149. What Are The Disadvantages Of Continuous Gating Signal?

  • More heating of the SCR gate.
  • Increases the size of pulse transformer.

150. What Is Meant By High Frequency Carrier Gating?

Thyristor is turned on by using a train of pulses from a to p. This type of signal is called as high frequency carrier gating.

151. What Is Meant By Sequence 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.

152. What Are The Advantages Of Sequence Control Of Ac Voltage Regulators?

System power factor is improved.
Harmonics are reduced in the source current and the load voltage.

153. What Are The Two Types Of Cyclo-converters?

Step-up cyclo-converters
Step-down cyclo-converters

154. What Is Meant By Step-up Cyclo-converters?

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

155. What Is Meant By Step-down Cyclo-converters?

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

156. What Are The Applications Of Cycloconverter?

  • Induction heating
  • Speed control of high power ac drives
  • Static VAR generator
  • Power supply in aircraft or ship boards

157. What Is Meant By Positive Converter Group In A Cyclo-converter?

The part of the cyclo-converter circuit that permits the flow of current during positive half cycle of output current is called positive converter group.

158. What Is Meant By Negative Converter Group In A Cyclo-converter?

The part of the cyclo-converter circuit that permits the flow of current during negative half cycle of output current is called negative converter group.

159. Draw The Two Transistor Model Of Scr And Derive An Expression For Anode Current?

  • Schematic diagram (2M)
  • Two transistor model diagram (2M)
  • Operation (4M)

160. What Are The Characteristics Of Scr?

  • Equivalent circuit
  • V-I characteristics
  • Switching characteristics

161. Describe The Various Methods Of Thyristor Turn On?

  • Forward voltage triggering
  • The dv/dt triggering
  • Gate triggering
  • Temperature triggering
  • Light triggering

162. What Are The Operation Of Mosfet?

  • Basic structure, symbol
  • Operation
  • V-I characteristics, Switching characteristics

163. What Are The Operation Of Igbt?

  • Basic structure, symbol
  • Operation
  • V-I characteristics, Switching characteristics

164. Derive The Expressions For Average Output Voltage And Rms Output Voltage Of 1Φ Semi Converter?

1Φ semiconverter bridge circuit
waveforms of vo , io , ifd, is, VT1, VT2
Operation
Average output voltage, rms value of output voltage expression

165. Describe The Working Of 1Φ Fully Controlled Bridge Converter In The Rectifying Mode And Inversion Mode. And Derive The Expressions For Average Output Voltage And Rms Output Voltage?

1Φ full converter bridge circuit
waveforms of vo , io , ifd, is, VT1, VT2
Operation for α < 900, α > 900
Average output voltage, rms value of output voltage expression.

166. Describe The Working Of 3Φ Fully Controlled Bridge Converter In The Rectifying Mode And Inversion Mode. And Derive The Expressions For Average Output Voltage And Rms Output Voltage?

3Φ full converter bridge circuit
waveforms of vo , io , iA, vs
Operation
Average output voltage expression.

167. Describe The Working Of 3Φ Semi Converter. And Derive The Expressions For Average Output Voltage And Rms Output Voltage?

3Φ semi converter bridge circuit
waveforms of vo , io , iA, vs
Operation
Average output voltage expression.

168. Describe The Working Of Dual Converter?

3Φ dual converter bridge circuit
waveforms of vo , io , vo1 , vo2 , i1, i2 , vs , ic
Operation for with circulating current and without circulating current
load voltage expression, peak value icp

169. For A Type A Chopper (first Quadrant), Express The Following Variables As A Function Of Vs, R And Duty Cycle  In Case The Load Is Resistive Average Output Voltage And Current?

chopper circuit
output voltage & current waveforms
Average load voltage expression

170. Describe The Principle Of Step-up Chopper. Derive An Expression For The Average Output Voltage In Terms Of Input Dc Voltage & Duty Cycle?

  • chopper circuit
  • output voltage & current waveforms
  • Average load voltage expression

171. Describe The Working Of Four Quadrant Chopper?

  • chopper circuit
  • operation

172. Explain The Working Of Current Commutated Chopper With Aid Of Circuit Diagram And Necessary Wave Forms. Derive An Expression For Its Output Voltage?

  • chopper circuit
  • Modes of operation- equivalent circuit diagrams
  • Current & voltage waveforms
  • Design

173. Explain The Working Of Voltage Commutated Chopper With Aid Of Circuit Diagram And Necessary Waveforms. Derive An Expression For Its Output Voltage?

  • chopper circuit
  • Modes of operation- equivalent circuit diagrams
  • Current & voltage waveforms
  • Design of C & L

174. 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?

  • series inverter circuit
  • Current & voltage waveforms
  • Operation
  • Expression for output frequency, VL, VC

175. State Different Methods Of Voltage Control Inverters. Describe About Pwm Control In Inverter?

  • 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

176. Explain The Operation Of 3 Bridge Inverter For 1800 Degree Mode Of Operation With Aid Of Relevant Phase And Line Voltage Waveforms?

  • Inverter circuit
    operation- equivalent circuits
    Waveforms of phase and line voltage

177. Explain The Operation Of 3 Bridge Inverter For 1200 Degree Mode Of Operation With Aid Of Relevant Phase And Line Voltage Waveforms?

  • Inverter circuit
  • Operation- equivalent circuits
  • Waveforms of phase and line voltage

178. Draw The Circuit Diagram Of 1Φ Auto Sequential Commutated Current Source Inverter And Explain Its Operation With Equivalent Circuits For Different Modes And Necessary Waveforms?

Inverter circuit
Modes of operation- equivalent circuits
Waveforms of ic, vc , io
Expression for tc, vc, vL

179. Draw The Circuit Diagram Of 1Φ Capacitor Commutated Current Source Inverter And Explain Its Operation With Equivalent Circuits For Different Modes And Necessary Wave Forms?

  • Inverter circuit
  • Modes of operation- equivalent circuits
  • Waveforms of ic, vo , io, iT1, iT2, vT1, vT2
  • Expression for vc, vL , vo , io, tc

180. Explain The Operation Of Multistage Control Of Ac Voltage Controllers With Neat Diagram?

  • Circuit diagram
  • Operation

181. Explain The Operation Of 1Φ Ac Voltage Controller With Rl Load?

  • Circuit diagram
  • Operation
  • Waveforms

182. Explain The Operation Of Sequence Control Of Ac Voltage Controller?

  • Circuit diagram
  • Operation
  • Waveforms

183. Explain The Operation Of 1 Sinusoidal Ac Voltage Controller?

  1. Circuit diagram
  2. Operation
  3. Waveforms

184. For A 1Φ 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 Waveforms Drawn?

  1. Circuit diagram
  2. Operation
  3. Waveforms

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