Power Electronics Interview Questions & Answers

Question 1. What Is Holding Current In Scr?

Answer :

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.

Question 2. What Is Latching Current In Scr?

Answer :

It is the minimum current required to latch(turn on) the SCR from forward blocking state to forward conduction state.

Question 3. What Are The Different Turn On Methods Of Scr?

Answer :

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

Question 4. What Is Snubber Circuit?

Answer :

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.

Question 5. What Is Hard Switching Of The Thyristor?

Answer :

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.

Question 6. What Is Firing Angle?

Answer :

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.

Question 7. What Is Meant By Commutation?

Answer :

The process of changing the direction of current flow in a particular path of the circuit. It is used to turn off the SCR.

Question 8. What Are The Advantages Of Free Wheeling Diode In Rectifier Circuit?

Answer :

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

Question 9. What Is Meant By Cyclo-converter?

Answer :

It is also known as frequency changer. It converts input power at one frequency to output power at another frequency with one stage conversion.

Question 10. What Are The Types Of Cyclo-converters?

Answer :

Step up cyclo-converter
Step down cyclo-converter.

Question 11. What Is Step Up Cyclo-converter?

Answer :

It is the converter whose output frequency is less than the input frequency.

Question 12. What Is Step Down Cyclo-converter ?

Answer :

It is the converter whose output frequency is more than the input frequency.

Question 13. What Is Ac Voltage Controller?

Answer :

It is the one which converts fixed alternating voltage to a variable voltage without change in frequency.

Question 14. What Is Inverter?

Answer :

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

Question 15. What Are The Types Of Inverter?

Answer :

1. Voltage Source Inverter
2. Current Source Inverter

Question 16. What Is Duty Cycle?

Answer :

It is the ratio of the on time of the chopper to total time period of the chopper.

D = Ton / [Ton + Toff]

Question 17. Can Fuses With An Ac Voltage Rating Be Used In A Dc Applications?

Answer :

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.

Question 18. What Are The Different Operation Regions Of The Scr?

Answer :

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

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.

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.

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.

Question 19. What Is Latching Current?

Answer :

Gate signal is to be applied to the thyristor to trigger the thyristor ON in safe mode. When the thyristor starts conducting the forward current above the minimum value, called Latching current, the gate signal which is applied to trigger the device in no longer require to keep the scr in ON position.

Question 20. What Is Holding Current ?

Answer :

When scr is conducting current in forward conduction state, scr will return to forward blocking state when the anode current or forward current falls below a low level called Holding current Note: Latching current and Holding current are not same. Latching current is associated with the turn on process of the scr whereas holding current is associated with the turn off process. In general holding current will be slightly lesser than the latching current.

Question 21. Why Thyristor Is Considered As Charge Controlled Device?

Answer :

During the triggering process of the thyristor from forward blocking state to forward conduction state through the gate signal, by applying the gate signal (voltage between gate and cathode) increases the minority carrier density in the p-layer and thereby facilitate the reverse break over of the junction J2 and thyristor starts conducting. Higher the magnitude of the gate current pulse, lesser is the time required to inject the charge and turning on the scr. By controlling the amount of charge we can control the turning on time of the scr.

Question 22. What Is The Relation Between The Gate Signal And Forward Break Over Voltage (vbo)?

Answer :

Thyristor can be triggered by increasing the forward voltage between anode and cathode, at forward break over voltage thyristor starts conducting. However this process may damage the thyristor, so thyristor is advices to trigger on through the gate pulse. When a gate signal is applied thyristor turns on before reaching the break over voltage. Forward voltage at which the thyristor triggers on depends on the magnitude of the gate current. Higher is the gate current lower is the forward break over voltage.

Question 23. What Are The Different Losses That Occur In Thyristor While Operating?

Answer :

Different losses that occur are:

• Forward conduction losses during conduction of the thyristor
• Loss due to leakage current during forward and reverse blocking.
• Power loss at gate or Gate triggering loss.
• Switching losses at turn-on and turn-off.

Question 24. What Are The Advantages Of Speed Control Using Thyristor?

Answer :

Advantages :

1. Fast Switching Characteristics than MOSFET, BJT, IGBT
2. Low cost
3. Higher Accurate.

Question 25. What Happens If I Connect A Capacitor To A Generator Load?

Answer :

Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, other wise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Secondly, don't connect a capacitor across an alternator while it is picking up or without any other load.

Question 26. Why The Capacitors Works On Ac Only?

Answer :

Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). it allows the ac components to pass through.

Question 27. Explain The Working Principal Of The Circuit Breaker?

Answer :

Circuit Breaker is one which makes or breaks the circuit. It has two contacts namely fixed contact & moving contact under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions (when current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby it forms the open circuit Arc is extinguished by the Arc Quenching media like air, oil, vacuum etc.

Question 28. What Is The Difference Between Isolator And Circuit Breaker?

Answer :

Isolator is a off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. it is manually operated and does not contain any solenoid unlike circuit breaker. it should not be operated while it is having load. first the load on it must be made zero and then it can safely operated. its specification only rated current is given. But circuit breaker is onload automatic device used for breaking the circuit in case of abnormal conditions like short circuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit breaking capacity and 3 is instantaneous tripping current.

Question 29. What Is The Difference Between Earth Resistance And Earth Electrode Resistance?

Answer :

Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition: Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away.

The resistance of the electrode has the following components:

1. the resistance of the metal and that of the connection to it.
2. the contact resistance of the surrounding earth to the electrode.

Question 30. What Is Use Of Lockout Relay In Ht Voltage?

Answer :

A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power and is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.

Question 31. What Is The Power Factor Of An Alternator At No Load?

Answer :

At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.

Question 32. How To Determine Capacitor Tolerance Codes?

Answer :

In electronic circuits, the capacitor tolerance can be determined by a code that appears on the casing. The code is a letter that often follows a three-digit number (such as 130Z).The first two are the 1st and 2nd significant digits and the third is a multiplier code. Most of the time the last digit tells you how many zeros to write after the first two digits and these are read as Pico-Farads.

Question 33. Why Most Of Analog O/p Devices Having O/p Range 4 To 20 Ma And Not 0 To 20 Ma?

Answer :

4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation .For example- a pressure instrument gives output 4mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.

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

Answer :

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.

Question 35. What Is Meant By Knee Point Voltage?

Answer :

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).

Question 36. What Is Reverse Power Relay?

Answer :

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.

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

Answer :

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.

Question 38. What Is The Full Form Of Kvar?

Answer :

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.

Question 39. Definition Of Power Electronics?

Answer :

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

Question 40. What Is The Main Purpose Of Power Electronics?

Answer :

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

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

Answer :

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

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.

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.

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.

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

Answer :

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

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

Answer :

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

Question 44. Explain The Function Of Cyclo-converter?

Answer :

It is also known as frequency changer. It converts input power at one frequency to output power at another frequency with one stage conversion.

Question 45. What Is An Inverter?

Answer :

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

Question 46. Define Circuit Turn Off Time

Answer :

It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.

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

Answer :

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.

Question 48. What Is Chopper?

Answer :

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.

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

Answer :

Natural commutation
Forced commutation

Question 50. What Is Natural Commutation?

Answer :

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.

Question 51. What Is Forced Commutation?

Answer :

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

Question 52. What Is Step Down Chopper?

Answer :

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

Question 53. What Is Step Up Chopper?

Answer :

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

Question 54. What Is Voltage Commutation?

Answer :

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

Question 55. What Is Current Commutation?

Answer :

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.

Question 56. What Are The Advantages Of Current Commuted Chopper?

Answer :

The advantages of current commutated chopper is;

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

Question 57. What Is Load Commutation?

Answer :

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

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

Answer :

SCR, GTO, GTR.

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

Answer :

IGBT, MCT, IGCT, SIT.

Question 60. What Is Meant By Pulse Triggered Devices?

Answer :

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

Question 61. What Is Meant By Level-sensitive Devices?

Answer :

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

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

Answer :

IGBT has

• Lower turn on and turn off times than BJT
• Lower on state conduction losses than MOSFET
• Excellent safe operating area

Question 63. Why Igbt Is Very Popular Nowadays?

Answer :

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

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

Answer :

1. Forward voltage triggering
2. Gate triggering
3. dv/dt triggering
4. Temperature triggering
5. Light triggering

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

Answer :

Power diode

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

Signal diode

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

Question 66. Igbt Is A Voltage Controlled Device. Why?

Answer :

Because the controlling parameter is gate-emitter voltage.

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

Answer :

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

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

Answer :

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

Question 69. What Is The Relation Between α






and β?

Answer :

Question 70. What Are The Different Types Of Power Mosfet?

Answer :

1. N-channel MOSFET
2. P-channel MOSFET

Question 71. How Can A Thyristor Turned Off?

Answer :

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

Question 72. Define Latching Current?

Answer :

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.

Question 73. Define Holding Current?

Answer :

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

Question 74. What Is A Snubber Circuit?

Answer :

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.

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

Answer :

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.

Question 76. Define Hard-driving Or Over-driving?

Answer :

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.

Question 77. Define Circuit Turn Off Time?

Answer :

It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.

Question 78. Why Circuit Turn Off Time Should Be Greater Than The Thyristor Turn-off Time?

Answer :

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.

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

Answer :

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.

Question 80. What Are The Advantages Of Gto Over Scr?

Answer :

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

Question 81. What Is Meant By Phase Controlled Rectifier?

Answer :

It converts fixed ac voltage into variable dc voltage.

Question 82. Mention Some Of The Applications Of Controlled Rectifier?

Answer :

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

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

Answer :

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.

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

Answer :

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

Question 85. What Is Meant By Delay Angle?

Answer :

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

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

Answer :

1. 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
2. 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.

Question 87. What Is Commutation Angle Or Overlap Angle?

Answer :

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.

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

Answer :

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

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

Answer :

Average output voltage V_dc = (V_m /Π) (1 + cosα).

Question 90. What Is Meant By Input Power Factor In Controlled Rectifier?

Answer :

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 ) / ( V_rms I_rms) where V₁ = phase voltage, I₁ = fundamental

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

Question 91. What Are The Advantages Of Six Pulse Converter?

Answer :

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

Question 92. What Is Meant By Natural Commutation?

Answer :

Here the current flowing through the thyristor goes through a natural zero and enable the thyristor to turn off.

Question 93. What Is Meant By Forced Commutation?

Answer :

the thyristor is forced to become zero by external circuitry.

Question 94. What Is Meant By Dc Chopper?

Answer :

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

Question 95. What Are The Applications Of Dc Chopper?

Answer :

1. Battery operated vehicles
2. Traction motor control in electric traction
3. Trolley cars
4. Marine hoists
5. Mine haulers
6. Electric braking.

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

Answer :

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.

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

Answer :

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

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

Answer :

Average output voltage for step down chopper

          Vs     
  V0 =  ------  
        1 - α

α
is the duty cycle.

Question 99. What Is Meant By Duty-cycle?

Answer :

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.

Question 100. What Are The Two Types Of Control Strategies?

Answer :

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

Question 101. What Is Meant By Trc?

Answer :

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

Question 102. What Are The Two Types Of Trc?

Answer :

1. Constant frequency control
2. Variable frequency control

Question 103. What Is Meant By Fm Control In A Dc Chopper?

Answer :

In frequency modulation control, the chopping frequency f (or the chopping period T) is varied. Here two controls are possible.

1. On-time Ton is kept constant
2. Off period Toff is kept constant.

Question 104. What Is Meant By Pwm Control In Dc Chopper?

Answer :

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).

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

Answer :

Average output voltage for step down chopper is V_O = α V_S.

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

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

Answer :

1. Voltage commutated chopper.
2. Current commutated chopper.
3. Load commutated chopper.

Question 107. What Is Meant By Voltage Commutation?

Answer :

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

Question 108. What Is Meant By Current Commutation?

Answer :

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.

Question 109. What Is Meant By Load Commutation?

Answer :

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

Question 110. What Are The Advantages Of Current Commutated Chopper?

Answer :

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

Question 111. What Are The Advantages Of Load Commutated Chopper?

Answer :

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.

Question 112. What Are The Disadvantages Of Load Commutated Chopper?

Answer :

1. For high power applications, efficiency becomes very low because of high switching losses at high operating frequencies.
2. Freewheeling diode is subjected to twice the supply voltage.
3. Peak load voltage is equal to twice the supply voltage.
4. The commutating capacitor has to carry full load current at a frequency of half chopping frequency.
5. 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.

Question 113. What Is Meant By Inverter?

Answer :

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

Question 114. What Are The Applications Of An Inverter?

Answer :

1. Adjustable speed drives
2. Induction heating
3. Stand-by aircraft power supplies
4. UPS
5. HVDC transmission

Question 115. What Are The Main Classification Of Inverter?

Answer :

1. Voltage Source Inverter
2. Current Source Inverter

Question 116. Why Thyristors Are Not Preferred For Inverters?

Answer :

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.

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

Answer :

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.

Question 118. Give Two Advantages Of Csi?

Answer :

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

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

Answer :

It require a 3-wire dc supply.

Question 120. Why Diodes Should Be Connected In Antiparallel With Thethyristors In Inverter Circuits?

Answer :

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.

Question 121. What Types Of Inverters Require Feedback Diodes?

Answer :

VSI with RL load.

Question 122. What Is Meant A Series Inverter?

Answer :

An inverter in which the commutating elements are connected in series with the load is called a series inverter.

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

Answer :

      4L 
R2 < ------
       C

Question 124. What Is Meant A Parallel Inverter?

Answer :

An inverter in which the commutating elements are connected in parallel with the load is called a parallel inverter.

Question 125. What Are The Applications Of A Series Inverter?

Answer :

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

1. Ultrasonic generator.
2. Induction heating.
3. Sonar Transmitter
4. Fluorescent lighting.

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

Answer :

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

Question 127. What Is Meant By Mcmurray Inverter?

Answer :

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

Question 128. What Are The Applications Of A Csi?

Answer :

1. Induction heating
2. Lagging VAR compensation
3. Speed control of ac motors
4. Synchronous motor starting.

Question 129. What Is Meant By Pwm Control?

Answer :

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.

Question 130. What Are The Advantages Of Pwm Control?

Answer :

1. The output voltage can be obtained without any additional components.
2. 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.

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

Answer :

1. Harmonic currents will lead to excessive heating in the induction motors. This will reduce the load carrying capacity of the motor.
2. If the control and the regulating circuits are not properly shielded, harmonics from power ride can affect their operation and malfunctioning can result.
3. 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.
4. On critical loads, torque pulsation produced by the harmonic current can be useful.

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

Answer :

1. Transformer connections
2. Sinusoidal PWM
3. Multiple commutation in each cycle
4. Stepped wave inverters

Question 133. Compare Csi And Vsi?

Answer :

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.

Question 134. What Are The Disadvantages Of Pwm Control?

Answer :

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

Question 135. What Does Ac Voltage Controller Mean?

Answer :

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

Question 136. What Are The Applications Of Ac Voltage Controllers?

Answer :

1. Domestic and industrial heating
2. Lighting control
3. Speed control of single phase and three phase ac motors
4. Transformer tap changing

Question 137. What Are The Advantages Of Ac Voltage Controllers?

Answer :

1. High efficiency
2. Flexibility in control
3. Less maintenance

Question 138. What Are The Disadvantages Of Ac Voltage Controllers?

Answer :

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

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

Answer :

1. ON-OFF control
2. Phase control

Question 140. What Is The Difference Between On-off Control And Phase Control?

Answer :

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.

Question 141. What Is The Advantage Of On-off Control?

Answer :

Due to zero-voltage and zero current switching of thyristors, the harmonics generated by the switching action are reduced.

Question 142. What Is The Disadvantage Of On-off Control?

Answer :

This type of control is applicable in systems that have high mechanical inertia and high thermal time constant.

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

Answer :

Duty cycle K = n/ (n + m),

where n = number of ON cycles,
m = number of OFF cycles.

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

Answer :

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

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

Answer :

1. 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%.
2. The input current and output voltage are asymmetrical and contain a dc component.If there is an input transformer, saturation problem will occur
3. It is only used for low power resistive load.

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

Answer :

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

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

Answer :

The control range is Φ < α <180°,

where Φ = load power factor angle.

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

Answer :

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

Question 149. What Are The Disadvantages Of Continuous Gating Signal?

Answer :

1. More heating of the SCR gate.
2. Increases the size of pulse transformer.

Question 150. What Is Meant By High Frequency Carrier Gating?

Answer :

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.

Question 151. What Is Meant By Sequence Control Of Ac Voltage Regulators?

Answer :

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.

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

Answer :

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

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

Answer :

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

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

Answer :

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

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

Answer :

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

Question 156. What Are The Applications Of Cycloconverter?

Answer :

1. Induction heating
2. Speed control of high power ac drives
3. Static VAR generator
4. Power supply in aircraft or ship boards

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

Answer :

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.

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

Answer :

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.

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

Answer :

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

Question 160. What Are The Characteristics Of Scr?

Answer :

• Equivalent circuit
• V-I characteristics
• Switching characteristics

Question 161. Describe The Various Methods Of Thyristor Turn On?

Answer :

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

Question 162. What Are The Operation Of Mosfet?

Answer :

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

Question 163. What Are The Operation Of Igbt?

Answer :

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

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

Answer :

• 1Φ semiconverter bridge circuit
• waveforms of v_o , i_o , i_fd, i_s, V_T1, V_T2
• Operation
• Average output voltage, rms value of output voltage expression

Question 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?

Answer :

• 1Φ full converter bridge circuit
• waveforms of v_o , i_o , i_fd, i_s, V_T1, V_T2
• Operation for α
  < 90⁰, α
  > 90⁰
• Average output voltage, rms value of output voltage expression.

Question 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?

Answer :

• 3Φ full converter bridge circuit
• waveforms of v_o , i_o , i_A, v_s
• Operation
• Average output voltage expression.

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

Answer :

• 3Φ semi converter bridge circuit
• waveforms of v_o , i_o , i_A, v_s
• Operation
• Average output voltage expression.

Question 168. Describe The Working Of Dual Converter?

Answer :

• 3Φ dual converter bridge circuit
• waveforms of v_o , i_o , v_o1 , v_o2 , i₁, i₂ , v_s , i_c
• Operation for with circulating current and without circulating current
• load voltage expression, peak value i_cp

Question 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?

Answer :

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

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

Answer :

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

Question 171. Describe The Working Of Four Quadrant Chopper?

Answer :

• chopper circuit
• operation

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

Answer :

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

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

Answer :

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

Question 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?

Answer :

• series inverter circuit
• Current & voltage waveforms
• Operation
• Expression for output frequency, V_L, V_C

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

Answer :

• 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

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

Answer :

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

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

Answer :

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

Question 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?

Answer :

• Inverter circuit
• Modes of operation- equivalent circuits
• Waveforms of i_c, v_c , i_o
• Expression for t_c, v_c, v_L

Question 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?

Answer :

• Inverter circuit
• Modes of operation- equivalent circuits
• Waveforms of i_c, v_o , i_o, i_T1, i_T2, v_T1, v_T2
• Expression for v_c, v_L , v_o , i_o, t_c

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

Answer :

• Circuit diagram
• Operation

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

Answer :

• Circuit diagram
• Operation
• Waveforms

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

Answer :

• Circuit diagram
• Operation
• Waveforms

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

Answer :

• Circuit diagram
• Operation
• Waveforms

Question 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?

Answer :

• Circuit diagram
• Operation
• Waveforms