Power Electronics interview questions

POWER ELECTRONICS
INTERVIEW QUESTIONS
Engineering Funda YouTube Channel
By. Prof. Hitesh Dholakiya
Engineering Funda Android APP Power Electronics YT Playlist
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ENGINEERING FUNDA YOUTUBE CHANNEL BY PROF. HITESH DHOLAKIYA
POWER ELECTRONICS INTERVIEW QUESTIONS
Q-1 Why IGBT is very popular nowadays?
Ans a. Lower hate requirements
b. Lower switching losses
c. Smaller snubber circuit requirements
Q-2 What are the different methods to turn on the thyristor?
Ans a. Forward voltage triggering
b. Gate triggering
c. dv/dt triggering
d. Temperature triggering
e. Light triggering
Q-3 What is the difference between power diode and signal diode?
Ans 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. Lower
3. Operates at higher switching speed.
Q-4 IGBT is a voltage controlled device. Why?
Ans Because the controlling parameter is gate-emitter voltage.
Q-5 Power MOSFET is a voltage controlled device. Why?
Ans Because the output (drain) current can be controlled by gate-source voltage.
Q-6 Power BJT is a current controlled device. Why?
Ans Because the output (collector) current can be controlled by base current.
Q-7 What are the different types of power MOSFET?
Ans a. N-channel MOSFET
b. P-channel MOSFET
Q-8 How can a thyristor turned off?
Ans A thyristor can be turned off by making the current flowing through it to zero.
Q-9 Define latching current.
Ans 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.
Q-10 Define holding current.
Ans The holding current is defined as the minimum value of anode current below
which it must fall to for turning off the thyristor.
Q-11 What is a snubber circuit?
Ans 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
Q-12 What losses occur in a thyristor during working conditions?
Ans a. Forward conduction losses
b. Loss due to leakage current during forward and reverse blocking.
c. Switching losses at turn-on and turn-off.
d. Gate triggering loss.
Q-13 Define hard-driving or over-driving.
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Ans 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.
Q-14 Define circuit turn off time.
Ans It is defined as the time during which a reverse voltage is applied across the
thyristor during its commutation process.
Q-15 Why circuit turn off time should be greater than the thyristor turn-off
time?
Ans 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
Q-16 What is the turn-off time for converter grade SCRs and inverter grade
SCRs?
Ans 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.
Q-17 What are the advantages of GTO over SCR?
Ans a. Elimination of commutation of commutating components in forced
commutation, resulting in reduction in cost, weight and volume.
b. Reduction in acoustic noise and electromagnetic noise due to elimination of
commutation chokes.
c. Faster turn-off, permitting high switching frequencies.
d. Improved efficiency of the converters.
Q-18 What is meant by phase controlled rectifier?
Ans It converts fixed ac voltage into variable dc voltage.
Q-19 Mention some of the applications of controlled rectifier.
Ans a. Steel rolling mills, printing press, textile mills and paper mills employing dc
motor drives.
b. DC traction
c. Electro chemical and electro-metallurgical process
d. Portable hand tool drives
e. Magnet power supplies
f. HVDC transmission system
Q-20 What is the function of freewheeling diodes in controlled rectifier?
Ans It serves two process.
a. It prevents the output voltage from becoming negative.
b. The load current is transferred from the main thyristors to the freewheeling
diode, thereby allowing all of its thyristors to regain their blocking states.
Q-21 What are the advantages of freewheeling diodes in a controlled in a
controlled rectifier?
Ans a. Input power factor is improved.
b. Load current waveform is improved and thus the load performance is better.
Q-22 What is meant by delay angle?
Ans The delay angle is defined as the angle between the zero crossing of the input
voltage and the instant the thyristor is fired.
Q-23 What are the advantages of single phase bridge converter over single phase
mid-point converter?
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Ans a. 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
b. In mid-point converter, each secondary winding should be able to supply the
load power. As such, the transformer rating in mid-point converter is double the
load rating.
Q-24 What is commutation angle or overlap angle?
Ans 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.
Q-25 What are the different methods of firing circuits for line commutated
converter?
Ans a. UJT firing circuit.
b. The cosine wave crossing pulse timing control.
c. Digital firing schemes.
Q-26 What is meant by input power factor in controlled rectifier?
Ans The input power factor is defined as the ratio of the total mean input power to the
total RMS input volt-amperes.
Q-27 What are the advantages of six pulse converter?
Ans a. Commutation is made simple.
b. Distortion on the ac side is reduced due to the reduction in lower order
harmonics.
c. Inductance reduced in series is considerably reduced.
Q-28 What is meant by commutation?
Ans It is the process of changing the direction of current flow in a particular path of
the circuit. This process is used in thyristors for turning it off.
Q-29 What are the types of commutation?
Ans a. Natural commutation
b. Forced commutation
Q-30 What is meant by natural commutation?
Ans Here the current flowing through the thyristor goes through a natural zero and
enable the thyristor to turn off.
Q-31 What is meant by forced commutation?
Ans In this commutation, the current flowing through the thyristor is forced to
become zero by external circuitry.
Q-32 What is meant by dc chopper?
Ans A dc chopper is a high speed static switch used to obtain variable dc voltage
from a constant dc voltage.
Q-33 What are the applications of dc chopper?
Ans a. Battery operated vehicles
b. Traction motor control in electric traction
c. Trolly cars
d. Marine hoists
e. Mine haulers
f. Electric braking.
Q-34 What are the advantage of dc chopper?
Ans Chopper provides
a. High efficiency
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b. Smooth acceleration
c. Fast dynamic response
d. Regeneration
Q-35 What is meant by step-up and step-down chopper?
Ans 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.
Q-36 What is meant by duty-cycle?
Ans Duty cycle is defined as the ratio of the on time of the chopper to the total time
period of the chopper.
Q-37 What are the two types of control strategies?
Ans a. Time Ratio Control (TRC)
b. Current Limit Control method (CLC)
Q-38 What is meant by TRC?
Ans In TRC, the value of (Ton/T) is varied in order to change the average output
voltage.
Q-39 What are the two types of TRC?
Ans a. Constant frequency control
b. Variable frequency control
Q-40 What is meant by FM control in a dc chopper?
Ans In frequency modulation control, the chopping frequency f (or the chopping
period T) is varied. Here two controls are possible.
a. On-time Ton is kept constant
b. Off period Toff is kept constant.
Q-41 What is meant by PWM control in dc chopper?
Ans 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).
Q-42 What are the different types of chopper with respect to commutation
process?
Ans a. Voltage commutated chopper.
b. Current commutated chopper.
c. Load commutated chopper.
Q-43 What is meant by voltage commutation?
Ans In this process, a charged capacitor momentarily reverse biases the conducting
thyristor and turn it off.
Q-44 What is meant by current commutation?
Ans 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.
Q-45 What is meant by load commutation?
Ans In this process, the load current flowing through the thyristor either becomes
zero or is transferred to another device from the conducting thyristor.
Q-46 What are the advantages of current commutated chopper?
Ans a. The capacitor always remains charged with the correct polarity.
b. Commutation is reliable as load current is less than the peak commutation
current
ICP.
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c. The auxiliary thyristor TA is naturally commutated as its current passes
through zero value.
Q-47 What are the advantages of load commutated chopper?
Ans a. Commutating inductor is not required.
b. It is capable of commutating any amount of load current.
c. It can work at high frequencies in the order of kHz.
d. Filtering requirements are minimal.
Q-48 What are the disadvantages of load commutated chopper?
Ans a. For high power applications, efficiency becomes very low because of high
switching losses at high operating frequencies.
b. Freewheeling diode is subjected to twice the supply voltage.
c. Peak load voltage is equal to twice the supply voltage.
d. The commutating capacitor has to carry full load current at a frequency of
half chopping frequency.
e. One thyristor pair should be turned-on only when the other pair is
commutated. This can be realized by sensing the capacitor current that is
alternating.
Q-49 What is meant by inverter?
Ans A device that converts dc power into ac power at desired output voltage and
frequency is called an inverter.
Q-50 What are the applications of an inverter?
Ans a. Adjustable speed drives
b. Induction heating
c. Stand-by aircraft power supplies
d. UPS
e. HVDC transmission
Q-51 What are the main classification of inverter?
Ans a. Voltage Source Inverter
b. Current Source Inverter
Q-52 Why thyristors are not preferred for inverters?
Ans Thyristors require extra commutation circuits for turn off which results in
uncreased complexity of the circuit. For these reasons thyristors are not preferred
for inverters.
Q-53 How output frequency is varied in case of a thyristor?
Ans 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.
Q-54 Give two advantages of CSI.
Ans a. CSI does not require any feedback diodes.
b. Commutation circuit is simple as it involves only thyristors.
Q-55 What is the main drawback of a single phase half bridge inverter?
Ans It require a 3-wire dc supply.
Q-56 Why diodes should be connected in antiparallel with the thyristors in
inverter circuits?
Ans For RL loads, load current will not be in phase with load voltage and the diodes
connected in antiparallel will allow the current to flow when the main thyristors
are turned off. These diodes are called feedback diodes.
Q-57 What types of inverters require feedback diodes?
Ans VSI with RL load.
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Q-58 What is meant a series inverter?
Ans An inverter in which the commutating elements are connected in series with the
load is called a series inverter.
Q-59 What is meant a parallel inverter?
Ans An inverter in which the commutating elements are connected in parallel with
the load is called a parallel inverter.
Q-60 What are the applications of a series inverter?
Ans 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
a. Ultrasonic generator.
b. Induction heating.
c. Sonar Transmitter
d. Fluorescent lighting.
Q-61 How is the inverter circuit classified based on commutation circuitry?
Ans a. Line commutated inverters.
b. Load commutated inverters.
c. Self-commutated inverters.
d. Forced commutated inverters.
Q-62 What is meant by McMurray inverter?
Ans It is an impulse commutated inverter which relies on LC circuit and an auxiliary
thyristor for commutation in the load circuit.
Q-63 What are the applications of a CSI?
Ans a. Induction heating
b. Lagging VAR compensation
c. Speed control of ac motors
d. Synchronous motor starting.
Q-64 What is meant by PWM control?
Ans 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.
Q-65 What are the advantages of PWM control?
Ans a. The output voltage can be obtained without any additional components.
b. Lower order harmonics can be eliminated or minimized along with its output
voltage control. As the higher order harmonics can be filtered easily, the filtering
requirements are minimized.
Q-66 What are the disadvantages of the harmonics present in the inverter
system?
Ans a. Harmonic currents will lead to excessive heating in the induction motors. This
will reduce the load carrying capacity of the motor.
b. If the control and the regulating circuits are not properly shielded, harmonics
from power ride can affect their operation and malfunctioning can result.
c. Harmonic currents cause losses in the ac system and can even some time
produce resonance in the system. Under resonant conditions, the instrumentation
and metering can be affected.
d. On critical loads, torque pulsation produced by the harmonic current can be
useful.
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Q-67 What are the methods of reduction of harmonic content?
Ans a. Transformer connections
b. Sinusoidal PWM
c. Multiple commutation in each cycle
d. Stepped wave inverters
Q-68 Compare CSI and VSI.
Ans 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.
Q-69 What are the disadvantages of PWM control?
Ans SCRs are expensive as they must possess low turn-on and turn-off times.
Q-70 What does ac voltage controller mean?
Ans It is device which converts fixed alternating voltage into a variable voltage
without change in frequency.
Q-71 What are the applications of ac voltage controllers?
Ans a. Domestic and industrial heating
b. Lighting control
c. Speed control of single phase and three phase ac motors
d. Transformer tap changing
Q-72 What are the advantages of ac voltage controllers?
Ans a. High efficiency
b. Flexibility in control
c. Less maintenance
Q-73 What are the disadvantages of ac voltage controllers?
Ans The main drawback is the introduction of harmonics in the supply current and
the load voltage waveforms particularly at low output voltages.
Q-74 What are the two methods of control in ac voltage controllers?
Ans a. ON-OFF control
b. Phase control
Q-75 What is the difference between ON-OFF control and phase control?
Ans 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.
Q-76 What is the advantage of ON-OFF control?
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Ans Due to zero-voltage and zero current switching of thyristors, the harmonics
generated by the switching action are reduced.
Q-77 What is the disadvantage of ON-OFF control?
Ans This type of control is applicable in systems that have high mechanical inertia
and high thermal time constant.
Q-78 What is the duty cycle in ON-OFF control method?
Ans Duty cycle K = n/ (n + m), where n = no. of ON cycles, m = no. of OFF cycles.
Q-79 What is meant by unidirectional or half-wave ac voltage controller?
Ans Here the power flow is controlled only during the positive half-cycle of the input
voltage.
Q-80 What are the disadvantages of unidirectional or half-wave ac voltage
controller?
Ans a. Due to the presence of diode on the circuit, the control range is limited and
the effective RMS output voltage can be varied between 70.7percent and
100percent.
b. The input current and output voltage are asymmetrical and contain a dc
component. If there is an input transformer, sdaturation problem will occur
c. It is only used for low power resistive load.
Q-81 What is meant by bidirectional or half-wave ac voltage controller?
Ans Here the power flow is controlled during both cycles of the input voltage.
Q-82 What is the control range of firing angle inac voltage controller with RL
load?
Ans The control range is(α) = Φ to 180° , where Φ = load power factor angle.
Q-83 What type of gating signal is used in single phase ac voltage controller with
RL load?
Ans High frequency carrier gating signal is used for single phase ac voltage controller
with RL load.
Q-84 What are the disadvantages of continuous gating signal?
Ans a. More heating of the SCR gate.
b. Increases the size of pulse transformer.
Q-85 What is meant by high frequency carrier gating?
Ans Thyristor is turned on by using a train of pulses from α to π. This type of signal
is called as high frequency carrier gating.
Q-86 What is meant by sequence control of ac voltage regulators?
Ans 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.
Q-87 What are the advantages of sequence control of ac voltage regulators?
Ans a. System power factor is improved.
b. Harmonics are reduced in the source current and the load voltage.
Q-88 What is meant by cyclo-converter?
Ans It converts input power at one frequency to output power at another frequency
with one-stage conversion. Cycloconverter is also known as frequency changer.
Q-89 What are the two types of cyclo-converters?
Ans a. Step-up cyclo-converters
b. Step-down cyclo-converters
Q-90 What is meant by step-up cyclo-converters?
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Ans In these converters, the output frequency is less than the supply frequency.
Q-91 What is meant by step-down cyclo-converters?
Ans In these converters, the output frequency is more than the supply frequency.
Q-92 What are the applications of cyclo-converter?
Ans a. Induction heating
b. Speed control of high power ac drives
c. Static VAR generation
d. Power supply in aircraft or ship boards
Q-93 What is meant by positive converter group in a cycloconverter?
Ans The part of the cycloconverter circuit that permits the flow of current during
positive half cycle of output current is called positive converter group.
Q-94 What is meant by negative converter group in a cycloconverter?
Ans The part of the cycloconverter circuit that permits the flow of current during
negative half cycle of output current is called negative converter group.
Q-95 What is a Thyristor?
Ans Thyristor is derived from the properties of a Thyratron tube and a Transistor. It is
used as another name for SCR’S. They are power Semiconductor devices used
for power control applications.
Q-96 What are SCR’s?
Ans SCR’s is Silicon controlled Rectifiers. They are basically used as Rectifier
Q-97 What is Forward break over voltage?
Ans The voltage Vak at which the SCR starts conducting is called as Forward Break
over voltage Vbo. This happens when the junction J2 undergoes Avalanche
breakdown due to high reverse bias on junction J2.
Q-98 What is Reverse break over voltage?
Ans If the reverse voltage is increased more than a critical value, avalanche
Breakdown will occur at J1 and J3 increasing the current sharply. This is
Reverse break over voltage VBO.
Q-99 Why is Vbo greater than Vbr?
Ans Region at junction J2 is higher during forward bias than that of J1 and J3 under
reverse bias.
Q-100 What are modes of working of an SCR?
Ans Reverse blocking mode, Forward blocking mode and Forward conduction mode
are the modes of working of an SCR.
Q-101 Why does high power dissipation occur in reverse blocking mode?
Ans High power dissipation occurs because as voltage increases beyond Vbr current
increases rapidly.
Q-102 Why shouldn’t positive gate signal be applied during reverse blocking?
Mode?
Ans If we apply positive gate signal J3 becomes forward biased. Reverse leakage
current increases and Thyristor gets damaged due to large power dissipation.
Q-103 Explain reverse current Ire?
Ans When cathode voltage is positive, J2 is forward biased; J1 and J3 are reverse
biased. The thyristors will be in reverse blocking state and reverse leakage
current Ire flows.
Q-104 What happens when gate drive is applied?
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Ans When gate drive is applied avalanche breakdown occurs at J2 causing excessive
flow of charges and hence current surge. This turns the SCR into conduction
state faster i.e. the Thyristor turns on at lower and lower anode to cathode
voltages, which are less than Vbo.
Q-105 Differentiate between holding and latching currents
Ans Holding current is the minimum amount of current below, which SCR does not
conduct. It is associated with the presence of gate terminal and concerns turn off
condition. Latching current is the minimum amount of current required for the
SCR to conduct. It is associated with absence of gate terminal and concerns turn
on process. It is greater than holding current.
Q-106 Why is dv/dt technique not used?
Ans As this causes false triggering even when gate or voltage Vak is not applied,
dv/dt technique is not used. Snubbed circuit, which is combination of a C, avoids
this and R .The capacitor is placed in parallel with SCR.
Q-107 What sided?
Ans At the time of turn on, anode current increases rapidly. This rapid variation is not
spread across the junction area of the thyristors. This creates local hotspots in the
junction and increases the junction temperature and hence device may be
damaged. This is avoided by connecting an inductor in series with an SCR.
Q-108 Why should the gate signal be removed after turn on?
Ans This prevents power loss in the gate junction.
Q-109 Is a gate signal required when reverse biased?
Ans No, otherwise SCR may fail due to high leakage current.
Q-110 What are different types of firing circuits to trigger SCR?
Ans *R firing circuit.
*RC firing circuit.
*UJT firing circuit.
*Digital firing circuit.
Q-111 What type of triggering is used in SCR?
Ans Pulse triggering.
Q-112 What is offset current?
Ans When anode voltage is made positive, J1 and J3 are forward biased, J2 is reverse
biased. The Thyristor is in forward blocking or off state condition and the
leakage current is known as offset current Io.
Q-113 What are the advantages of SCR?
Ans *Very small amount of gate drive is required since SCR is regenerative device.
*SCR’s with high voltage and current ratings are available.
*On state losses are reduced.
Q-114 What are the disadvantages of SCR’s?
Ans *Gate has no control once the SCR is turned on.
*External circuits are required to turn off the SCR.
*Operating frequencies are very low.
*Snubber (RC circuits) is required for dv/dt protection.
Q-115 What are applications of SCR?
Ans *SCR’s are best suitable for controlled rectifiers.
*AC regulators, lighting and heating applications.
*DC motor drives, large power supplies and electronic circuit breakers.
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Q-116 What is the difference between an IGBT and SCR?
Ans IGBT comprises of a BJT and a MOSFET where as an SCR comprises of two
BJT’s.
Q.117 Can we replace a SCR by a microprocessor by writing a program to
exhibit characteristics of SCR?
ans. No, we can verify or test the working of SCR using microprocessor but we
cannot replace it practically.
Q-117 What are MOSFET’s?
Ans Metal oxide silicon di-oxide field effect transistor is a voltage-controlled device.
The parts of MOSFET are gate, drain and source.
Q-118 What is the difference between MOSFET and BJT?
Ans The MOSFET is a voltage controlled device where as BJT is a current controlled
device.
Q-119 What is the difference between JFET and MOSFET?
Ans There is no direct contact between the gate terminal and the n-type channel of
MOSFET.
Q-120 What are the two types of MOSFET?
Ans *Depletion MOSFET
- N channel in p substrate.
-P channel in n substrate.
*Enhancement mosfet
-virtual n channel in p substrate
-Virtual p channel in n substrate
Q-121 What is the difference between depletion and enhancement MOSFET?
Ans The channel in the centre is absent for enhancement type MOSFET but the
channel is present in depletion type MOSFET.
The gate voltage can either be positive or negative in depletion type MOSFET’s
but enhancement MOSFET responds only for positive gate voltage.
Q-122 How does n-drift region affect MOSFET?
Ans The n- drift region increases the onstage drop of MOSFET and also the thickness
of this region determines the breakdown voltage of MOSFET.
Q-123 How are MOSFET’s suitable for low power high frequency applications?
Ans MOSFET’s have high on state resistances due to which losses increase with the
increase in the power levels. Their switching time is low and hence suitable for
low power high frequency applications.
Q-124 What are the requirements of gate drive in MOSFET?
Ans *The gate to source input capacitance should be charged quickly.
*MOSFET turns on when gate source input capacitance is charged to sufficient
level.
*The negative current should be high to turn off MOSFET.
Q-125 What is rise time and fall time?
Ans The capacitor Cgs charges from threshold voltage to full gate voltage Vgsp. The
time required for this charging is called rise time. During this period, drain
current rises to full value.
The capacitor Cgs keeps on discharging and its voltage becomes equal to
threshold voltage Vt.The time required for this discharge Cgs from Vgsp to Vt is
called fall time.
Q-126 What is pinch off voltage?
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Ans The voltage across gate to source at which the drain to source current becomes
zero is called pinch off voltage.
Q-127 In which region does the MOSFET used as a switch?
Ans In the linear region.
Q-128 Which parameter defines the transfer characteristics?
Ans The Tran conductance Gm=Id/Vgs
Q-129 Why are MOSFET’s mainly used for low power applications?
Ans MOSFET’s have high on state resistance Rds. Hence for higher currents; losses
in the MOSFET’s are substantially increased. Hence MOSFET’s are
substantially increased. Hence, MOSFET’s are mainly used for low power
applications.
Q-130 How is MOSFET turned off?
Ans To turn off the MOSFET quickly, the negative gate current should be
sufficiently high to discharge gate source input capacitance.
Q-131 What are the advantages of vertical structure of MOSFET?
Ans *On state resistance of MOSFET is reduced.
*Width of the gate is maximized. Hence, gain of the device is increased.
Q-132 What are the merits of MOSFET?
Ans * MOSFET’s are majority carrier devices.
* MOSFET’s have positive temperature coefficient, hence their paralleling is
easy.
*MOSFET’s have very simple drive circuits.
*MOSFET’s have short turn on and turn off times; hence they operate at high
frequencies.
*MOSFET’s do not require commutation techniques.
*Gate has full control over the operation of MOSFET.
Q-133 What are demerits of MOSFET?
Ans *On state losses in MOSFET are high.
*MOSFET’s are used only for low power applications.
*MOSFET’s suffer from static charge.
Q-134 What are the applications of MOSFET?
Ans *High frequency and low power inverters.
*High frequency SMPS.
*High frequency inverters and choppers.
*Low power AC and DC drives.
Q-135 What is IGBT?
Ans Insulated gate bipolar transistor is the latest device in power Electronics .It is
obtained by combining the properties of BJT And MOSFET.
Q-136 In what way IGBT is more advantageous than BJT and MOSFET?
Ans *It has high input impedance of the MOSFET and has low on-state voltage
drop.
*The turn off time of an IGBT is greater than that of MOSFET.
*It has low onstage conduction losses and there is no problem of second
Breakdown as in case of BJT.
*It is inherently faster than a BJT.
Q-137 What are on state conduction losses? How is it low in IGBT?
Ans A high current is required to break the junctions in BJT. This results in On state
conduction losses. The conduction losses in IGBT are proportional To duty cycle
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of the applied voltage. By reducing the duty cycle conduction losses can be
reduced.
Q-138 What is second breakdown phenomenon?
Ans As the collector voltage drops in BJT there is an increase in collector Current
and this substantially increase the power dissipation. This Dissipation is not
uniformly spread over the entire volume of the device but is concentrated in
highly localized regions where the local temperature may grow and forms the
black spots. This causes the destruction of BJT. This is second breakdown.
Q-139 What is switching speed?
Ans The time taken to turn on or turn off a power device is called switching Speed.
Q-140 Can we observe the transfer and collector characteristics of IGBT on CRO?
Ans No. Because the waveform which is to be observed on the CRO should Vary
with respect to time otherwise we can see only a straight line on the CRO.
Q-141 What is punch through IGBT?
Ans The IGBT’s which have n+ buffer layer present are called punch through
IGBT.They have asymmetric voltage blocking capabilities and have faster turn
off times. Hence they are used in choppers and inverters.
Q-142 What is non-punch through IGBT?
Ans The IGBT’S without n+ buffer layer are called non-punch through IGBT’s. They
have symmetric voltage blocking capabilities and are used for rectifier
applications.
Q-143 What are merits of IGBT?
Ans *The drive is simple.
*Onstage losses are reduced.
*No commutation circuits are required.
*Gate has full control.
*Switching frequencies are higher.
*It has flat temperature coefficient.
Q-144 What are demerits of IGBT?
Ans *They have static charge problems.
*They are very costly.
Q-145 What are the applications of IGBT’s?
Ans *Ac motor drives. (Inverters)
*Dc to Dc power supplies. (Choppers)
*UPS systems.
*Harmonic compensators.
Q-146 Why is silicon used in all power semiconductor devices and why not?
Germanium?
Ans The leakage current in silicon is very small compared to germanium. The
germanium is also more sensitive compared to silicon.
Q-147 What is pinch off voltage?
Ans When Vge is made negative, electrons in the n-channel get repelled Creating a
depletion region resulting in a narrower effective channel. If Vge is made
negative enough so as to completely eliminate the channel (High resistance, low
current state), that value is called the pinch off Voltage.
Q-148 What is threshold voltage?
Ans Threshold voltage is the voltage Vge at which IGBT begins to conduct.
Q-149 How is IGBT turned off?
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Ans An IGBT can turn off by discharging the gate by means of short circuiting it to
the emitter terminal.
Q-150 What is the rating of IGBT?
Ans The current rating can be up to 400A, 1200V with switching frequency of
20KHz.
Q-151 What is the maximum firing angle of R-triggering circuit and why?
Ans The maximum firing angle is 90°. This is because the source voltage reaches
maximum value of 90° point and the gate current has to reach Ig(min) some
where between 0-90°. This limitation means that load voltage waveform can
only be varied from α = 0° to α = 90°.
Q-152 What are the disadvantages of R triggering?
Ans Trigger angle a is greatly dependent on the SCR’s Ig(min) and this value varies
between SCR’s and it is also temperature dependent. · Maximum triggering
angle achievable is 90°.
Q-153 In R-triggering circuit why is Rmin is connected in series with variable
resistor?
Ans The limiting resistor Rmin is placed between anode and gate so that the peak
gate current of the thyristor Igm is not exceeded.
Q-154 What is the maximum firing angle of RC-triggering and why?
Ans Maximum firing angle is 180°. This is because capacitor voltage and AC line
voltage differ in phase. By adjusting the value of R it is possible to vary the
delay in turning on the SCR from 0 to 10 msec and hence vary the firing angle
from 0° to 180°.
Q-155 What is an UJT and draw its equivalent circuit?
Ans UJT-uni junction transistor. It has only one type of charge carriers. It has three
terminals emitter, base 1 and base 2. (‘Duo base’ as it has 2 bases)
Q-156 Why is an UJT used in SCR firing circuit?
Ans The voltage at base 1 of UJT is smaller than the voltage needed to trigger the
Scrim the voltage is high, then it will trigger the SCR as soon as the ac supply is
on.
Q-157 Why is the isolation needed between Thyristor and firing circuit?
Ans The trigger circuit operates at low power levels (5-20 volts) whereas thyristors
operate at high voltage levels (250 volts). Hence if the Thyristor acts as a short
the entire 250volts get applied across the firing circuit causing damage. Hence
isolation is needed.
Q-158 How is a pulse transformer different from other transformer?
Ans A pulse transformer is one in which the input at the primary is current which is
transformed into a pulse at the secondary. Thus it does not step-up and step-
down as other transformers.
Q-159 What are the features of pulse transformer?
Ans The primary magnetizing inductance is high, coupling efficiency is high, and
interwinding capacitance is low and has greater insulation.
Q-160 What are the advantages of using pulse transformer?
Ans *Multiple secondary windings allow simultaneous gating signals to series and
parallelconnected thyristors.
*Control circuit and power circuit can be isolated.
Q-161 Why is UJT used in SCR firing circuit?
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Ans As the UJT works in a mode called as a relaxation oscillator i.e. UJT turns on or
off depending on the charging and discharging of the capacitor. Time constant
can be varied with Chance delay angle can be varied .The UJT thus gives a firing
angle range of 0- 180.Vz is supply to UJT, the discharging current when passed
through the pulse transformer triggers SCR with pulses.
Q-162 Why is the sneer diode used?
Ans The sneer diode provides a constant supply voltage for UJT. It enables
synchronization with zero crossings. Sneer diode acts as a regulator. The sneer
clamps the rectified voltage to vs. to prevent erratic firing. This sneer voltage
acts as a supply for UJT relaxation oscillator.
Q-163 What is meant by ramp control, open loop control or manual control with
respect to UJT firing circuit?
Ans Ramp control-The graph of time period in milliseconds with the firing angle in
degrees is a ramp. The ramp slope can be controlled by the potentiometer.
Manual control-The potentiometer in the kit can be used to get various firing
angles. This is manual control.
Q-164 What is a firing circuit?
Ans It is a circuit, which is used to trigger a device at various instants of time.
Q-165 Why a bridge rectifier is used?
Ans The bridge rectifier gives a full wave rectified output, which is high in efficiency
and least ripple factor.
Q-166 What is the load used?
Ans Load is high power dissipation resistor.
Q-167 What is time constant of a circuit?
Ans Time constant of a circuit=RC where R=resistance C=capacitance It gives the
time of charging and discharging of a capacitor.
Q-168 What are the merits of UJT firing circuit over RC triggering circuit?
Ans * Firing angle remains stable.
*Advantages of pulse transformer.
Q-169 What are the advantages of UJT pulse trigger circuit?
Ans The resistors, capacitors depend heavily on the trigger characteristics of the
Thyristor used. The power dissipation is high due to prolonged pulse. But the
pulse triggering can accommodate wide tolerances in triggering characteristics
by instantaneously overdriving the gate. The power level in such circuits is lower
as the triggering energy can be stored slowly and discharged rapidly when the
triggering is required.
Q-170 Why is UJT used as relaxation oscillator?
Ans The UJT is used as a relaxation oscillator to obtain sharp, repetitive pulses with
good rise time. Also it has good frequency stability against variation in the
supply voltage and temperature.
Q-171 What are the applications of UJT trigger circuits?
Ans *Used to trigger SCR’s in single-phase converters, single-phase ac regulators.
*Used in oscillators
*Used in timing circuits
Q-172 What is valley voltage?
Ans It is the voltage at which the UJT turns off and the capacitor starts charging
again.
Q-173 What is the discharging path if the capacitor?
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Ans The capacitor discharges through emitter, base and primary of the pulse
transformer.
Q-174 What is relaxation oscillator?
Ans When the capacitor discharges to a valley voltage, the UJT turns off and
capacitor starts charging again. This mode of working of UJT is called relaxation
oscillator.
Q-175 What is negative resistance?
Ans After the capacitor charges to Vp it starts discharging. During this period the
voltage V decreases with increase in current, hence this portion of V-I
characteristics is called negative resistance.
Q-176 What is interring base resistance?
Ans Inter base resistance is the resistance between 2 bases
Q-177 What is intrinsic standoff ratio?
Ans Intrinsic standoff ratio=Rb1/(Rb1+Rb2). Its value ranges between .52 to .81.
Q-178 What is the width of the triggering pulse?
Ans TG=Rb1.C
Q-179 Why are the capacitors CIF and C used?
Ans Capacitor CIF is used to minimize the ripples and C is used for charging and
discharging so that the trigger is eventually formed.
Q-180 What is ac voltage controller?
Ans If a Thyristor switch is connected between ac supply and load, the power flow
can be controlled by varying the rms value of ac voltage applied to the load and
this type of power circuit is known as an ac voltage regulator
Q-181 What are the applications of ac voltage controllers?
Ans The most common applications of ac voltage controllers are: industrial heating,
on-load transformer tap changing, light controls, speed control of polyphase
induction motors and ac magnet controls.
Q-182 What do you mean by sequence control?
Ans The use of two or more stages voltages controllers in parallel for the regulation
of output voltage.
Q-183 Give the classification of ac voltage regulators.
Ans They are classified as:
1.single phase controllers
2.three phase controllers
Each type can be subdivided into unidirectional and bi-directional control.
Q-184 What are the two types of control?
Ans *on off control: Here Thyristor switches connect the load to the ac source for a
few cycles of input voltage and then disconnect it for another few cycles.
*phase angle control: Here Thyristor switches connect the load to the ac source
for a portion of each cycle of input voltage.
Q-185 Why are extra commutation components not required?
Ans The ac voltage controllers have main supply as input. The SCR’s in these
controllers are turned off by natural commutation. Hence extra commutation
components are not required. Therefore ac voltage controllers are simple and
easy to implement if SCR’s are used.
Q-186 What is the difference between cycloconverters and ac voltage controllers?
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Ans In cycloconverters (ac to variable ac) frequency of output can be varied. In ac
voltage controller’s frequency of output is kept constant, just the output average
value is controlled (on and off times varied).
Q-187 What is diac firing circuit?
Ans A diac firing circuit consists of a diac that is used to generate trigger pulses for
the Thyristor diac can conduct in both directions and it does not have any control
terminal in the form of a gate.
Q-188 What are the merits and demerits of voltage controllers?
Ans The merits are that they are simple without commutation circuits, high efficiency
and less maintenance.
The demerits are that the load current is asymmetric (phase control) and hence
harmonics are present and intermittent supply of power in on-off control.
Q-189 Why is the trigger source for the two Thyristor isolated from each other in a
single-phase voltage controller?
Ans When one Thyristor is on, the other should be off. Both the Thyristor should not
conduct at a time.
Q-190 What is a full controlled rectifier?
Ans It is a two-quadrant ac to dc converter. It has 4 thyristors and hence all of them
can be controlled for rectification purpose. In a full converter the polarity of the
output voltage can be either positive or negative but the output current has only
one polarity.
Q-191 What is a semi converter?
Ans A semi converter is a one-quadrant converter and it has only one polarity of
output voltage and current.
Q-192 What is a dual converter?
Ans A dual converter can operate in all 4 quadrants and both output voltage and
current can be either positive or negative.
Q-193 How can we control the output voltage of a single-phase full converter?
Ans By varying the trigger angle.
Q-194 What is MCB?
Ans MCB-Miniature circuit breaker. This is used as switch, which opens or switches
off when the voltage or current is above the rated value of that of MCB.
Q-195 How many lines are there in single-phase system?
Ans Two lines- 1line
1neutral
Q-196 What is rectification mode and inversion mode?
Ans During the period alpha to 180° the input voltage Vs and input current Is are
positive and the power flows from supply to the load. The converter is said to be
operating in rectification mode. During the period 180 to 180°+alpha the input
voltage Vs and the input current Is positive and there will be reverse power flow
from load to supply. The converter is said to be operating in inversion mode.
Q-197 Where is full bridge converter used?
Ans It is mainly used for speed control of dc motors.
Q-198 What is the effect of adding free wheeling diode?
Ans Free wheeling action does not takes place in single-phase full converter
inherently as there are 4 thyristors and no diodes. From 180 to 180+alpha( ) free
wheeling diode starts conducting. It is more forward biased compared to T1 and
T2.Hence freewheeling diode conducts. The freewheeling diode is connected
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across the output Vo. Hence Vo=0 during freewheeling. The energy stored in the
load inductance is circulated back to the load itself.
Q-199 What are choppers?
Ans A dc chopper converts directly from dc to dc and is also known as dc-dc
converter.
Q-200 What does a chopper consist of?
Ans It can be a power transistor, SCR, GTO, power MOSFET, IGBT or a switching
device.
Q-201 On what basis choppers are classified in quadrant configurations?
Ans The choppers are classified depending upon the directions of current and voltage
flows. These choppers operate in different quadrants of V-I plane. There are
broadly following types of choppers: class a chopper (first quadrant); class B
(second quadrant) Class C and class D (two quadrant choppers), class C in II
quadrant and I whereas class D in IV quadrants, and I class E is four quadrant
operator.
Q-202 What are different control strategies found in choppers?
Ans The different control strategies are pulse width modulation, frequency
modulation and current limit control, variable pulse width and frequency.
Q-203 Explain the principle of operation of a chopper?
Ans A chopper acts as a switch, which connects and disconnects the load, hence
producing variable voltage.
Q-204 What are the advantages of DC choppers?
Ans * High ripple frequency, so small filters are required.
*Power factor is better.
*Efficiency is better.
*Small and compact.
*The dynamic response of choppers is fast due to switching nature of the
device.
Q-205 Define duty cycle.
Ans The duty cycle of chopper controls its output voltage. The value of duty cycle
lies between 0 and 1 and is given by Ton/(Ton+Toff).
Q-206 How can ripple current be controlled?
Ans Ripple current is inversely proportional to the frequency and hence can be
controlled by having higher frequency.
Q-207 What is step up chopper?
Ans If the output average voltage is greater than the supply voltage, then the chopper
is called step up chopper.
Q-208 On what does the commutating capacitor value depend on?
Ans It depends on the load current.
Q-209 What are the disadvantages of choppers?
Ans *They can operate only at low frequencies.
*The commutation time depends on the load current.
*The output voltage is limited to a minimum and maximum value beyond which
we cannot get the output voltage.
Q-210 How do they have high efficiency?
Ans DC choppers uses switching principle, hence they have high efficiency.
Q-211 What are the applications of dc choppers?
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Ans Battery operated vehicles, switched mode power supplies, traction devices,
lighting and lamp controls, trolley cars, marine hoists, and forklift trucks. Mine
haulers etc.
Q-212 What is principle of dc motor?
Ans An electric motor is a machine, which converts electrical energy into mechanical
energy. Its action is based on the principle that when a current carrying
conductor is placed in a magnetic field it experiences a mechanical force whose
direction is given by flemings left hand rule and whose magnitude is given by
F=BIL When the field magnets of a multipolar dc motor are excited and its
armature conductors are supplied with current from supply mains they
experience a force tending to rotate the armature .By Fleming’s left hand rule it
is noted that each conductor experiences a force which tends to rotates the
armature in anticlockwise direction. These forces collectively produce a driving
torque (or twisting moment), which sets the armature rotating.
Q-213 How can the speed of the series motor controlled?
Ans *flux control method
-field divertors
-Armature divertor
*variable resistance in series with the motor.
Q-214 What are the advantages of field method?
Ans *economical,more efficient
*It gives speeds more /above the normal speed.
Q-215 What arethe disadvantages of field method?
Ans Commutation becomes unsatisfactory.
Q-216 What are the factors controlling speed?
Ans Speed can be controlled by controlling flux, resistance, voltage.
Q-217 What is the significance of back emf?
Ans When the motor armature rotates the conductors also rotates and hence cut flux.
Therefore emf is induced and direction is in opposition with the applied voltage
(Fleming’s right hand rule). Because of its opposing direction it is referred to as
back emf Eb. V has to drive Ia against the opposition of Eb.The power required
to overcome this opposition is EbIa.
Q-218 What is torque?
Ans Torque is twisting or turning moment of a force about an axis. The torque
developed by the armature of a motor is armature torque. The torque available
for useful work is known as shaft torque (available at the shaft).
Q-219 How can dc motors be classified?
Ans separately excited
*self-excited.
Q-220 What are the main losses in motors?
Ans *stator losses
*rotor losses
*mechanical losses
Q-221 Why are starter used in dc motor?
Ans Initially Eb =0 and R is usuallly very small,therefore the armature current is very
high which could damage the motor.Hence starters which is basically a
resistance connected in series with the motor.
Q-222 What is the parameter that is being varied by varying the firing angle?
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Ans The armature voltage is varied which inturn varies the speed of the motor by
varying the firing angle.
Q-223 What are the operating modes of dc motor?
Ans Motoring, regenerative braking, dynamic braking, plugging.
Q-224 Why is it called TRIAC and what are its applications?
Ans The word TRIAC is abbreviated from TRI from Triode and AC. The TRIAC is
the best device for AC phase control. The input and load is AC. The power is to
be controlled in positive as well as negative half cycles. The TRIAC is triggered
in every half cycle. Thus, the necessity of anti-parallel SCR’s is eliminated by
TRIAC. Thus, it is used mainly in AC power controllers, heater, fan, triggering
device for SCR’s.
Q-225 How many terminals does a TRIAC have?
Ans A TRIAC has three terminals, main terminal (MT 1), and main terminal (MT 2),
gate (G).
Q-226 What are the two modes of operation of a TRIAC?
Ans *Junction gate operation
*Remote gate operation
Q-227 What are merits of TRIAC?
Ans 1. TRIAC is a bi-directional device that is it conducts in both directions.
2. TRIAC turns-off when voltage is reversed.
3. Single gate controls conduction in both directions.
4. TRIAC’s with high voltages and current ratings are available.
Q-228 What are de-merits of TRIAC?
Ans *TRIAC’s are latching devices like SCR; hence they are not suitable for DC
power applications.
*Gate has no control over the conduction once Triac is turned on.
*TRIAC’s have very small switching frequencies.
Q-229 When is the sensitivity of TRIAC greatest?
Ans The sensitivity of TRIAC is greatest in I quadrant (mode 1) when MT2 and gate
are positive with respect to MT 1 and it triggers for a low value of latching
current.
Q-230 When is the sensitivity of TRIAC low?
Ans The sensitivity is less in mode III (3rd quadrant) when MT 2 is negative and
positive gate current and it requires larger value of latching current to trigger.
Q-231 Why should we use TRIAC for bi-directional flow of current, instead we
can use an ordinary conductor (wire)?
Ans The ordinary conductor does not provide the facility to switch on or off that is,
we cannot control the flow of current. We have to manually switch on or off the
supply.
Q-232 How is the TRIAC different from RCT?
Ans The TRIAC is equivalent to two SCR’s connected in antiparallel where as the
RCT is a SCR and a diode in antiparallel direction.
Q-233 What happens when gate voltage is increased?
Ans The forward break over voltage starts decreasing.
Q-234 What is stepper motor?
Ans A stepper motor is one in which the motor rotates step by step in steps of 1.8
degree.
Q-235 Why stepper motor is also called digital motor?
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Ans A stepper motor is driven by square pulses and hence does not require pulse
width modulation technique. It can be driven by simple digital circuits and hence
is called digital motor.
Q-236 How many steps are covered in one revolution?
Ans 200 steps with step angle of 1.8 degree.
Q-237 What are merits of stepper motor?
Ans It is easy to interface with digital controller, low cost solution for position
control and also zero feedback error in servo system.
Q-238 What are its applications?
Ans Teleprinters,typewriters,plotters,X ray machinery, valve control, servo
mechanism, photo printing and developing, film projectors ,cameras ,carburettor
adjusting, electronic gear box, blood analysers etc
Q-239 What is the advantage of stepper motor?
Ans The major advantage is that its speed can be varied by the digital input. We can
use a microprocessor to control the speed .It can also be rotated in steps. They
can be used for precision control of speed without using closed loop feedback.
Q-240 What is step angle?
Ans The angle through which the motor shaft rotates for each command pulse is
called the step angle. It can be 1.8, 2.5, 7.5 degrees.
Q-241 What is stepping frequency?
Ans F is the stepping frequency in pulses per second.
Q-242 What are the moving parts in stepper motor?
Ans The only moving part in stepper motor is rotor which has no winding,
commutatore, brushes.
Q-243 What are types of stepper motor?
Ans *permanent magnet motor.
*variable reluctance motor-when stator is excited ,the rotor is pulled into the
nearest
Minimum reluctance position.
Q-244 What does stepping motor consist of?
Ans *The indexer is a microprocessor capable of generating step pulses and
direction signals for the driver.
*The driver (amplifier) converts the indexer command signals into the power
necessary to energise the motor windings.
Q-245 What are its advantages?
Ans The step motor is an electromechanical device that converts digital pulses into
mechanical shaft rotation. Advantages are low cost, high reliability, high torque
at low speeds and a simple, rugged construction that operates almost in any
environment.
Q-246 What are its disadvantages?
Ans The main disadvantage in using stepper motor is the resonance effect often
exhibited at low speeds and decreasing torque with increasing speed.
Q-247 What are inverters and what are its applications?
Ans DC to AC converters is known as inverters. The function of an inverter is to
change a DC input voltage into AC output voltage of desired magnitude and
frequency. Inverters are widely used in industrial applications like variable speed
AC motor drives, induction heating, stand-by power supplies and uninterrupted
power supplies.
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Q-248 Why the circuit is called parallel inverter?
Ans The circuit is called parallel inverter because the commutating capacitor is in
parallel with the primary winding of the output transformer whose secondary is
fed to the load.
Q-249 What is the main classification of inverters?
Ans Inverters can be broadly classified into two types namely, Single-phase inverters
and three phase inverters. Each type can use controlled turn-on and controlled
turn-off devices (eg. BJT’s and MOSFET’s etc) or forced commutation thyristers
depending on application.
Q-250 What is VFI and CFI?
Ans An inverter is called a Voltage Fed Inverter (VFI) if the input voltage remains
constant, a Current Fed Inverter (CFI) if the input current is maintained constant.
Q-251 What is variable DC linked inverter?
Ans An inverter is called variable DC linked inverter if the input voltage is
controllable.
Q-252 What is inverter gain?
Ans The inverter gain may be defined as the ratio of the AC output to DC input
voltage.
Q-253 What are the advantages and disadvantages of variable DC linked inverter?
Ans Advantages:
1. Harmonic content does not change with output voltage.
2. Control circuit of an inverter is simple.
Disadvantages:
1. Additional chopper or control rectifier is required.
2. Efficiency of a circuit is reduced due to double conversion.
3. Transistors have to handle variable input voltages.
Q-254 Compare between Voltage source and Current source inverters
Ans Voltage source inverters
1. Input is constant voltage.
2. Short circuit can damage the circuit.
3. Peak current of power-device depends on load.
4. Current wave forms depend on load.
5. Free wheeling diodes are required in case of inducted load.
Current source inverters
1. Input is constant current.
2. Short circuit cannot damage the circuit.
3. Peak current of power-device is limited.
4. Voltage wave forms depend on load.
5. Free wheeling diodes are not required.
Q-255 Explain the principle of variable DC linked inverter?
Ans Harmonic content of the signal also changes if pulse width is varied. This
problem is taken care by DC link inverter. Instead of varying the pulses of
inverter, an input DC voltage is varied. Therefore rms value of output voltage is
varied.
Q-256 What is the commutation technique used in the parallel inverter?
Ans Complementary commutation technique.
Q-257 What is the role of the diodes D1 and D2?
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Ans Diodes D1 and D2 act as free wheeling diodes, they conduct when both SCR’s
turn off. They also provide a path for conduction.
Q-258 Why is the inductor used?
Ans The inductor does not allow drastic changes in current and hence provide di/dt
protection.
Q-259 From where does the inverter derive its dc power input?
Ans It derives the dc power input from the inverter specific external VRPS.
Q-260 Why the output voltage of an inverter is to be controlled?
Ans The output voltage of the inverter is to be varied as per the load requirement.
Whenever the input DC varies the output voltage can change. Hence, these
variations need to be compensated. The output voltage and frequency of an
inverter is adjusted to keep voltage and frequency constant. Thus, the output
voltage of an inverter is to be controlled.
Q-261 What are series inverters?
Ans Inverters in which the commutating elements are permanently connected in
series with the load resistance.
Q-262 What is the condition for selecting commutating element?
Ans They are selected in such a way that the current flow through series connected
elements R, L, C is under damped
Q-263 What are the drawbacks of a basic series inverter?
Ans *If the inverter frequency exceeds the circuit ringing frequency the dc source
will be short-circuited.
*For output frequencies much smaller than the circuit ringing frequency, the
load voltage is di started.
*The source current flows only during the period when the Thyristor T1 is
conducting. This results in large ripple in the source current and peak current
rating of the source inverters.
Q-264 What are the applications of series inverters?
Ans *Induction heating
*Fluorescent lighting
*Variable speed ac motor drives
*Aircraft power supplies
*UPS
*High voltage dc transmission lines
Q-265 Why are the inductors L1, L2 and why are two capacitors needed?
Ans *The resonant frequency, which is, if it is nearby inverter output frequency,
commutation failure will take place. Hence it should be ensured that the
capacitor and inductor are so chosen that it be not near to resonant frequency.
*Equal values of L1, L1’ or C1, C1’ to be chosen so that the uniform inverter
output is maintained.
Q-266 What are the waveforms (output) obtained in inverter?
Ans The output voltage waveforms of ideal inverters are sinusoidal. But for practical
inverters they are non-sinusoidal and contain harmonics due to which the
waveforms may be square wave or quassi square wave.
Q-267 Why can’t we see current waveforms on CRO?
Ans The resistance of CRO is very high. Therefore the current measurement is
incorrect. An attempt to reduce the resistance of CRO reduces the input
impedance, which draws heavy current from the source.
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