VIVA QUESTIONS
PEDC (EX-504)
[Q.1] What is AC voltage controller?
It is defined as the power electronics converter which converts the fixed AC voltage into a
variable AC voltage without change in the frequency.
[Q.2] What is inverter? What are the types of Inverter?
A device which converts dc power into ac power at desired output voltage and frequency is
called as Inverter.
Inverters are broadly classified into
Voltage Source Inverter (VSI)
Current Source Inverter (CSI)
[Q.3] What is chopper ?
It is equivalent to a dc transformer in ac circuit. The Chopper is a static switch which is used to
get the variable dc output voltage from a constant dc input voltage.
Step down chopper:
In this circuit, the average output voltage [VO = D . VS] is less than the input supply voltage. It
is also known as Buck converter.
Step up chopper:
It is also known as Boost converter. Here the average output voltage [VO = VS / 1 - D] is more
than the input supply voltage.
[Q.4] What is load commutation?
In load commutation, the load current flowing through the thyristor either becomes zero or is
transferred to another device from the conducting SCR.
[Q.5] What is meant by pulse triggered devices?
As the name indicates, to trigger these devices just a pulse is sufficient. Continuous gate voltage
of entire on time is not required. The advantage of pulse triggering is, it will avoid the hard
triggering. Ex. Thyristor, GTO
[Q.6] What is level-sensitive devices? give some examples
Some of the level sensitive devices are
MOSFET
IGBT
MCT
IGCT
In order to keep these devices in the ON state condition, we have to apply gate current/voltage
continuously to these devices.
EX Deptt. SAIT JBP (D. S. Rathore)
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[Q.7] What is meant by GTO?
It is a three terminal, four layer PNPN Power Semiconductor device that can be turned on by a
positive gate current and can be turned off by a reverse gate current.
[Q.8] What are the types of GTO?
Asymmetrical GTO
Symmetrical GTO
[Q.9] What does 10V AC mean? Is it the RMS voltage or Peak voltage or Average voltage?
In general, AC voltages and currents are mentioned in RMS values only. It is sensible to
compare with steady DC voltages and currents.
For example 230V AC supply means, 230Vrms AC Supply.
[Q.10] When we have to connect the SCRs in a parallel manner?
To meet the high current demand we will connect the SCRs in a parallel manner.
Similarly to meet the high voltage demand we have to connect the SCRs in series.
[Q.11] Define Gate Charge in MOSFET?
The gate charge is defined as the amount of charge required for the device during turn-on and
turn-off time. It is the most important parameter we have to consider while selecting the
MOSFET. The switching speed of the MOSFET depends on the speed at which the gate driver
can charge or discharge the input gate charge.
[Q.12] What are the advantages of free wheeling diode in rectifier circuit?
The input power factor is improved. It prevents the output voltage from becoming negative. Load
current waveform is improved.
[Q.13] What is meant by commutation?
The process of changing the direction of current flow in a particular path of the circuit. It is used
to turn off the SCR.
[Q.14] What are the types of commutation?
Natural commutation
Forced commutation
[Q.15] What is natural commutation?
The process of the current flowing through the thyristor goes through a natural zero and enable
the thyristor to turn off is called as natural commutation.
[Q.16] What is forced commutation?
The process of the current flowing through the thyristor is forced to become zero by external
circuitry is called as forced commutation.
[Q.17] What are the types of commutation with respect to commutation process?
Voltage commutated chopper
Current commutated chopper
Load commutated chopper
EX Deptt. SAIT JBP (D. S. Rathore)
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[Q.18] What is meant by cyclo-converter?
Cycloconverter is AC to DC converter. It has 1ф or 3 ф AC input. Output is variable voltage and
variable frequency. Cycloconverters are used for induction motor drives. They have an
advantages of variable frequency output over other AC voltage.
It is also known as frequency changer. It converts input power at one frequency to output power
at another frequency with one stage conversion.
[Q.19] What are the types of cyclo-converters?
Step up cyclo-converter
Step down cyclo-converter.
[Q.20] What is step down cyclo-converter?
It is the converter whose output frequency is less than the input frequency.
[Q.21] What is step up cyclo-converter ?
It is the converter whose output frequency is more than the input frequency.
[Q.22] What does the Voltmeter in AC mode show? Is it RMS value or peak value?
Multimeter in AC mode shows RMS value of the voltage or current. Also when it is DC mode it
will show the RMS value only.
[Q.23] What is the necessity to use the special machines?
General purpose motors (Induction motors, synchronous motors) are neither precision speed nor
precision position motors. For many automated systems require high precise speed and high
precise positioning motors. In such cases special purpose motors like stepper motors, PMDC
motors etc. are used.
[Q.24] What are the control strategies of chopper?
The control strategies of chopper are
1. Pulse width modulation PWM (Variable TON, Constant frequency)
2. Frequency modulation (Constant TON or TOFF, Variable frequency)
3. Current Limit Control (CLC)
[Q.25] What is delay angle or what is firing angle of phase controlled rectifier?
The delay angle is the angle at which thyristors are triggered after zero crossing. After zero
crossing of supply voltage, one pair of thyristors is
forward biased. After delay angle(α) these SCRs are triggered.
[Q.26] What is Universal Motor?
It is defined as a motor which can be operated either on DC or single-phase AC supply at
approximately the same speed and output.
The universal motor is built exactly like a series DC motor. But a series DC motor cannot be run
as a universal motor, even though both motors look the same internally and externally.
We cannot use these motors in the industrial applications due to the low efficiency (25% -35%).
It has high starting torque and a variable speed characteristic. It runs at dangerously high-speed
on no load.
EX Deptt. SAIT JBP (D. S. Rathore)
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[Q.27] Give some examples of power electronics applications in the day-to-day life?
We can list a huge number of power electronics applications. Few of the applications which we
can see in our daily life are
UPS - Uninterruptible Power Supply
SMPS - Switch Mode Power Supply
Speed Control of Motors
ICU
[Q.28] What is meant by PMDC?
PMDC stands for Permanent Magnet DC Motor
A Permanent Magnet DC Motor is similar to an ordinary dc shunt motor except that its field is
provided by permanent magnets instead of salient-pole wound field structure.
There are three types of permanent magnets used for such motors namely;
(i) Alnico Magnets
(ii) Ceramic magnets
(iii) Rare-earth magnets
The major advantages are low noise, small size, high-efficiency, low manufacturing cost.
[Q.29] What is holding current in SCR?
It is the minimum current required to hold the SCR in forward conduction state.
When the forward current becomes less than holding current, SCR turns from forward
conduction state to forward blocking state.
[Q.30] What is latching current in SCR?
It is the minimum current required to latch(turn on) the SCR from forward blocking state to
forward conduction state.
[Q.31] What are the different turn on methods of SCR?
Forward voltage triggering
Gate Triggering
dv/dt triggering
Temperature triggering
Light triggering
[Q.32] What is snubber circuit?
The snubber circuit is used for the dv/dt protection of the SCR. It is a series combination of a
resistor and a capacitor in parallel with the SCR.
[Q.33] What is hard switching of the thyristor?
When gate current is several times higher than the required gate current, the SCR is said to be
hard fired. It reduces the turn ON time and enhances the di/dt capability.
[Q.34] What is firing angle?
The angle between the zero crossing of the input voltage and the instant the SCR is fired is called
as delay angle or firing angle.
EX Deptt. SAIT JBP (D. S. Rathore)
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[Q.35] What is meant by SOA?
SOA - Safe Operating Area determines the voltage and current boundary within which the Power
Device can be operated without destructive failure.
[Q.36] What are the main components used for isolating the Power Circuits, Power
Semiconductor from the low-power circuit?
Opto-Couplers, Transformers
[Q.37] Name some of the current controlled (current driven) devices...
SCR, GTO, GTR
[Q.38] Name some of the voltage driven ( Voltage controlled) devices
IGBT, MCT, IGCT, SIT
[Q.39] What is duty cycle?
It is the ratio of the ON time of the chopper to total time period of the chopper.
D = Ton / [Ton + Toff]
[Q.40] Can fuses with an AC voltage rating be used in a DC applications?
Fuses must be rated for the voltage AC or DC in which they will be used.
Generally, fuses have a DC voltage rating that is half of the maximum AC voltage rating.
[Q.41] What are the characteristics of ideal Opamp?
Infinite open loop voltage gain
Infinite input impedance
Zero output impedance
Infinite Bandwidth
Zero offset voltage
[Q.42] What are the advantages and disadvantages of AC voltage controller?
Advantages:
1. They use line communication, hence no extra communication circuits are required.
2. They have high efficiencies since device losses are reduced.
Disadvantages:
1. Large ripple and harmonics are present in the output.
2. Output waveforms are not sinusoidal.
[Q.43] For High frequency applications will you prefer MOSFET or IGBT? Why?
 For High frequency applications, MOSFET is the right choice of the device.
 Because MOSFET has low switching losses compare to that of IGBT.
 General rule of thumb is for low-frequency applications having frequency range upto
20kHz, we have to use IGBT.
 For high frequency applications having frequency range of more than 200kHz, we have
to use MOSFET.
EX Deptt. SAIT JBP (D. S. Rathore)
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[Q.44] What are the difference between transistor and SCR?
1. SCR is four layer PNPN device, whereas transistor is three layer (PNP or NPN) device.
2. SCR turns on by regeneration but there is no regeneration in BJT.
3. Gate has no control over the conduction once SCR turns on. BJT can be turned off by base
drive.
4. BJT’s have higher switching frequencies than SCR’s.
[Q.45] Explain latching current and holding current of the thyristor?
Latching current: It’s the minimum forward current that flows through the thyristor to keep it
in forward conduction mode at the time of triggering. If the forward current is less than latching
current, thyristor doesn’t turn on.
Holding Current: It is the minimum forward current that flows through the thyristor to keep it
in forward conduction mode. When forward current reduces below holding current, thyristor turn
off.
[Q.46] What are the advantages of MOSFET’s over BJT’s?
i) MOSFET’s have simpler gate drive circuits.
ii) They have high switching frequencies.
iii) MOSFET’s are majority carrier devices.
iv) MOSFET’s have positive temperature coefficients.
[Q.47] Why pulsed gate drive is used for SCR?
Once SCR is turned on, gate drive is no more required. Hence if pulsed gate drive is used, SCR
turns on at particular pulse. The remaining pulses are not used. But pulsed drive reduces losses
in gate cathode of the SCR.
[Q.48] Define the delay angle of phase-controlled rectifier.
It is the angle at which thyristors are triggered after zero crossing. After zero crossing of supply
voltage , one pair of thyristor is forward biased. After delay angle (α), the SCR’s are triggered.
[Q.49] What are the differences between freewheeling diode and feedback diode?
Freewheeling diode is used to circulate the load energy in the load itself. Feedback diode is
used to transfer the load energy to the source. Feedback diode is normally used in inverters.
Freewheeling diode is normally used in controlled rectifiers.
[Q.50] What is inverting operation of the converter ?
Converters are basically AC to DC controlled rectifiers. The power flows from AC supply to DC
load. In inverting operation, the power flows from DC load to AC supply. Under such situation,
the DC load is normally a battery. Thus the power of DC battery load is given to AC supply
through controlled rectifier. For this operation, firing angle is greater than 90˚.
EX Deptt. SAIT JBP (D. S. Rathore)
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[Q.51] What are control strategies of chopper?
Choppers have following control strategies :
1. Fixed frequency operation: in this type of control, the frequency of chopper remains fixed , but
on and off periods of chopper change as per the duty cycle.
2. Variable frequency operation: in this type of control, the on or off period remains fixed .
Hence frequency of the chopper changes when duty cycle is changed.
3. Current limit control (CLC) : in this type of control, the output is sensed, when output current
falls below reference level, chopper is turned on. And when output current rises above reference
level, chopper is turned off.
[Q.52] Explain the use of step-up chopper.
Step-up chopper is normally used for energy transfer. The load energy is transferred to source
by step-up operation.
[Q.53] What is the main difference between voltage source inverter and current source inverter?
In the voltage source inverter, input voltage remains fixed and current varies according to load
variations. But in current source inverter, input current remains fixed and voltage varies
according to the load.
[Q.54] What are PWM inverters?
PWM inverters are basically pulse width modulated inverters. Voltage control in such inverters
is achieved by varying width of the pulse. In one half cycle there can be single or multiple
pulses. Number of pulses determine lowest order harmonic present in inverter output voltage.
Thus PWM inverters provide voltage control as well as harmonic reduction simultaneously.
EX Deptt. SAIT JBP (D. S. Rathore)
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