22326 Fundamentals of Power
Electronic Viva Questions with 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
:
Voltage
Source Inverter
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.
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 :
Fast
Switching Characteristics than MOSFET, BJT, IGBT
Low
cost
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:
the
resistance of the metal and that of the connection to it.
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.