Download power electronics - SK Engineering Academy

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING.
POWER ELECTRONICS
PART – A Questions and Answers
UNIT - I
1.What is a SCR?
A silicon-controlled rectifier (SCR) is a three terminal, three-junction
semiconductor device that acts as a true electronic switch. It is a unidirectional device. It
control the amount of power fed to the load. Its structure and symbol are shown in the
figure.
2. Define break over voltage of SCR.
Break over voltage is defined as the minimum forward voltage (gate being
open) at which the SCR starts conducting heavily.
3. Draw the two transistor model of a SCR and mention its applications.
List the applications of SCR .
(i) It can be used as a speed controller in DC and AC motors
(ii) It can be used as inverter.
(iii) It can be used as converter.
(iv) It is used in battery charges.
(v) It is used for phase control and heater control.
(vi) It is used in light dimming control circuits.
4. Draw the VI characteristics of a SCR and mark important points.
1
5. What is meant by latching current & holding current?
Latching current is the minimum anode current required to
maintain the thyristor in the on state immediately after a thyristor has been turned on and
gate signal has been removed.
Holding current is the minimum anode current to maintain
the thyristor in the on state.
6. Draw the two transistor analogy of a SCR
7. Mention the various firing circuits for SCR?
(i)
Simple R and RC triggering circuits.
(ii) UJT pulse triggering circuit.
(iii) Digital firing scheme.
8.. If the latching current in the circuit is 5mA. Obtain the minimum width of the gating
pulse required to properly turn on the SCR
2
9. What is a Triac?
Triac is a three terminal bi-directional semiconductor-switching device. It can
conduct in both the directions for any desired period. In operation it is equivalent to two
SCR’s connected in antiparallel. Next to SCR it is the widely used device for power
control.
10. Draw the volt-ampere characteristic of a Triac.
3
11. Why Triac not popular compared to SCR?
commutation of Triac in inductive load is difficult when compared to
SCR. Hence it is not popular
12. .Distinguish between SCR and TRIAC.
S. No
1.
2.
3.
4.
SCR
It is unidirectional
semiconductor device.
It has fast turn-off time.
It can be used to switch AC
supply frequencies up to
few KHz.
It is triggered by positive
voltage applied to the gate.
Triac
It is a bi-directional
semiconductor device.
It has a comparatively
longer turn -off time.
It can be used to switch AC
supply frequencies up to
40Hz only.
It is triggered by either
positive or negative voltage
applied to the gate.
13. Draw the construction, equivalent circuit and symbol of Triac.
14. What is power MOSFET?
A power MOSFET is a voltage-controlled device and requires only a small
input current. The switching speed is very high and the switching times are of the order
of nanoseconds.
15..Explain the importance of threshold voltage in gate circuit
when the voltage Vgs is increased beyond threshold voltage, the device starts
to conduct and the current will be increased from zero .
4
16. Draw the circuit model of power MOSFET in active and ohmic region
17. list the driving circuits for power MOSFET”S
i) driving the mosfet from TTL
ii) driving the mosfet from cmos
iii)driving the mosfet from linear circuit
iv)isolated gate circuits
18. What is the common method used for di / dt protection?
The value of the di / dt Can be maintained below acceptable limit by using a
small inductor called di / dt inductor in series with the anode circuit.
19. Compare the transistor and thyristor
S.NO
1.
2.
3.
TRANSISTOR
Transistor is a three layer ,
two junction device
Commutation circuitry
which is costly and bulky, is
not required
To keep a transistor in the
conducting state, a
continuous base current is
required
THYRISTOR
Thyristor is a four layer,
three junction device
Commutation circuit is
required
Thyristors require a pulse to
make it conducting and
thereafter it remain
conducting
20 What is an IGBT?
Insulated gate bipolar transistor (IGBT) combines the advantages of BJT and
MOSFET. Therefore it has low switching times as well as low power losses.
21What are the types of IGBT?
1. punch through IGBT
2. non punch IGBT
5
22. What are the advantages of IGBTs?
i.
They have high input gate impedance.
ii. They have low conduction loss.
iii. They have fast switching characteristics.
iv. They have very high operating frequency.
23. List the applications of IGBTs.
(i) They are used in low noise, high performance power supplies.
(ii) They are used in inverters.
(iii)They are used in motor speed controls.
UNIT - II
1. What is a controlled rectifier?
The output voltage of thyristor rectifier is varied by controlling the delay or firing
angle of thyristors. Therefore thyristor rectifiers are called as controlled rectifiers.
2. Define delay angle or firing angle.
Delay angle or firing angle is defined as the time after the input voltage starts to
go positive until the thyristor is fired at cot = α.
3. Define the effect of freewheeling diode.
i ) It prevents reversal of load voltage except for a small voltage drop
ii) It transfers the load current away from the main rectifier, thereby allowing all of its
thyristor to regain its blocking states
4. Mention application of phase controlled rectifier
i) battery charges
ii) DC drives
iii) DC power supplies
5. Mention two functions of freewheeling diode
(a) Reduction of ripple voltage in d.c terminals
(b) Prevents reversal of load voltage except for small diode voltage-drop
(c)
It transfers the load current away from the main rectifier, thereby allowing all
of its thyristors to regain their blocking states.
6
6. Draw the power circuit diagram of half controlled thyristor rectifier.
7. What is the difference between symmetric and asymmetric semiconductor
configuration?
In symmetric semiconductor, each leg contains one SCR and one diode. In
asymmetric semi converter, one leg contains two SCRs and the other leg contains two
diodes
8. compare continuous and discontinuous conduction
S.NO
1.
2.
CONTINUOUS
Β=П+α
Inductance value is very
large
DISCONTINUOUS
Β<П+α
Inductance value is
very small
9. Explain about the two quadrant operation?
A full converter is a two -quadrant converter and the polarity of its output voltage can
be either positive or negative.
10. Define form factor and ripple factor
form factor ff = Erms / Edc
ripple factor = ( ff2 – 1)1/2
11. Explain the inversion mode of fully controlled bridge rectifier?
When firing angle α is greater than 90˚ , then the voltage is maximum in negative is
called inversion mode
7
12. Define the input power factor
P. f = ( IS1 / ISrms) cos α
IS1 = fundamental current
ISrm = rms current
13. Mention the effect of source inductance in converters.
The source inductance causes the outgoing and incoming SCRs to conduct
together .When the source inductor increases, the overlap angle increases and as a
consequence the average output voltage decreases.
14. Why 3-pulse converter is not much used?
The 3-pulse converter requires special types of converter transformer to prevent
DC magnetization. Therefore they are not much used.
15. compare single phase converter and three phase converter
S.no
1.
2.
3.
4.
parameter
Ripple content in
output
Supply current
waveform
Control &
complexity
Single phase
more
Three phase
Less
Square wave
Supply & load
derating
Higher
Quasi square
wave
Complex
control &
implementation
Less
Less complex &
easy control
16. Express the average output voltage of 3Ф semi converter
8
17. compare half controlled and fully controlled rectifier
S .No
1.
2.
3.
Half controlled converter
It uses one thyristor.
One quadrant converter
At time one thyristor is
conducting
Fully controlled converter
It uses four thyristor
Two quadrant converter
At a time two thyristor are
conducting
18. Why is circulating current mode prepared over non - circulating current mode in
dual converter?
The circulating current mode is preferred over non - circulating current mode in
Dual converter due to the following advantages.
(a) Both converters are in operation continuously. The time response to
change the operation from one quadrant to the other is faster.
(b) One converter is always working as rectifier and the other as inverter.
Therefore power flow can be in either direction at any time.
(c) The circulating current helps in maintaining continuous conduction on both
converters irrespective of load.
UNIT III
1. What is meant by time ratio (duty cycle) of a DC chopper?
The ratio of a period to the total time period is known as time control ratio (or)
duty ratio. It is given by.
2. What is the effect of load inductance on the load current waveforms in the case of DC
chopper?
If load inductance is high, it will reduce the ripple in the output currents waveforms.
Load current becomes continuous.
3. What is PWM voltage control of output of a chopper?
In PWM, the frequency is kept constant and output voltage is varied by varying
Ton and T off.
4. Differentiate between single quadrant and two quadrant and two quadrant DC
chopper.
S. No
1.
2.
Single quadrant
Load current flows from
source to load.
Both voltage and current
are positive.
Two quadrant
Load current flows from
load to source.
Voltage is positive, current
is either positive or
negative.
9
6. What is the principle of on-off control?
Thyristor switch connects the AC supply to load for a time ton, the switch is turned
off by a gate pulse inhibiting for time t off.
7. What is the principle of step - up DC chopper?
A constant voltage is applied to the inductor. Hence the current through the
inductor increases linearly during the period T on.
After the SCR is turned off, the energy in the inductor is transferred to the
load along with supply voltage.
8. What is the function of free wheeling diode in a chopper?
(i) It protects SCR from high voltage that may be induced when the
inductive circuits is interrupted.
(ii) It helps to maintain constant current though the load.
(iii) It helps to commutate main SCR.
9. Which power electronic circuits is DC equivalent of transformer?
DC chopper connects fixed DC to variable DC. Autotransformer connects
fixed AC in to variable AC.
10. What are the applications of DC Chopper?
(i) Electric locomotives
(ii) Battery operated cars.
(iii) Power supplies.
11. What are the advantages of DC choppers?
(i) Flexible
(ii) Easy to control & compact
(iii) Closed loop control can be implemented
12. What is the disadvantages of frequency modulated chopper?
Design of filter is difficult, interference with commutation signals.
13. What is Time ratio control?
The Time ratio control (δ) is the ratio between t on to the total time period (T)
i.e
δ = ton
T
By varying the ton period the output voltage can be controlled and this sis
Called as time ratio control.
14.
Why thyristor are not preferred for inverter and choppers?
Inverters operating at high frequencies require fast acting switch. Thyristor
Is not preferred since it can operate at a low switching frequency of 2KHz.
Thyristors require forced commutation circuits.
10
15.
What is meant by step down and step up chopper?
If chopper output voltage is less than input voltage it is called step down chopper.
If chopper output voltage is greater than input voltage it is called step up chopper.
16.
Write down the expression for average output voltage for step down chopper.
Average output voltage for step-down chopper V0 =α Vs. α is the duty cycle.
17.
What is meant by duty cycle?
Duty cycle is defined as the ratio of the on time of the chopper to the total time
period of the chopper. It is denoted by α.
18.
What are the two types of control strategies?
a. time ratio control
b. current limit control
19.
What are the two types of TRC?
a. constant frequency control
b. variable frequency control
20. Compare between linear & switched mode regulator.
S.no
1.
2.
3.
4.
Parameter
swihching
Losses
RFI\EMI
Efficiency
Size & cost
linear regulator
zero
switched mode
High
Absent
Low
High
High
High
High
21. Write advantages of buck-boost regulator?
a. provides inverted output
b. high efficiency
c. Both Buck\boost operation simulation
d. short circuit protection can be easily implemented
e.
22. Write the disadvantages of boost regulator
a. High peak current through device
b. Output voltage is highly sensitive to change in duty cycle
c. Large inductance & capacitance are required
23.
Write the expression of Imax, Imin
The expressions of Imax, Imin are
Vs (1  e Ton \Ta ) E
Im ax 

R (1  e Ton \Ta ) R
11
Im in 
24.
Write the expression of p.u. ripple current.
Im ax  Im in 
25.
Vs (e Ton \Ta  1) E

R (e Ton \Ta  1) R
Vs (1  e Ton \Ta )(1  e (1 )T \Ta )
R
(1  e Ton \Ta )
Write the output voltage expression of step up\step down chopper.
Vo 
26.

Vs
(1   )
Write the output voltage expression of step up chopper.
1
Vo 
Vs
(1   )
UNIT – IV
1.
How the output frequency is varied in the case of an inverter?
Output frequency is varied by varying switching frequency.
3. State the necessity of return current diodes in inverter.
If the load connected to the inverter is inductive, then high voltages occur
Across the thyristor when it is switched off. This will damage the thyristor if
there is no path for diverting the current. Feedback diodes (or) return current
diodes are connected across each thyristor in antiparallel. The feed back
diode which is connected across the thyristors feeds the reactive power to
the supply.
4. State one application for series inverter.

Variable speed AC motor drives.

Induction heating

Standby power supplies

UPS
5. Mention the advantages of current source inverter over voltage source inverter.

Commutation circuit is simple since it requires only capacitors.

Slow response to load changes.

No feedback diodes are required

Utilize thyristors very effectively.

Inherent regeneration failure.
12
6. How to change the output voltage of a square wave inverter?
By using the PWM (Pulse Width Modulation) techniques, the output voltage
of an inverter can be varied.
7. What is the necessity of isolation between power circuit and control circuit?
It is preferred to have separate grounds for power circuit and control circuit.If the
round of power circuit is connected to the ground of control circuit, there is no
isolation. If a wise of power circuit touches control circuit, largecurrent flows
through the control circuit and it get damaged. Hence isolation is required
between power circuit and control circuit.
8. What are the difference between return current diodes and freewheeling diodes?
Freewheeling diode
Return current diode
A single diode is used.
Many diodes are connected
across the thyristor.
Connected across the load.
Connected across each thyristor
in antiparallel.
It provide path to ensure
It feeds the reactive power to
continuous conduction.
the source.
10 What are the difference between VSI and CSI?
VSI
Uses voltage source.
Uses current commutation.
Diodes are connected in
parallel.
Can be used for only 1208 and
1808
Output voltage is quasi square
wave.
CSI
Uses current source.
Uses voltage commutation
Diodes are connected in series.
Can be used for only 1208
Output current is quasi square
wave.
11. What are the limitations of series inverter?

It cannot be used for low frequency.

Output frequency has to be less than resonant frequency.

Pure sinewave cannot be obtained due to dead band.
12. Define harmonic distortion factor?
Harmonic distortion factor is the harmonic voltage to the fundamental voltage.
13. What is the difference between voltage commutation and current commutation.
S. No
Voltage commutation
Current commutation
1.
Precharged capacitor
In current commutation, L-
13
2.
pumps large current though
SCR and turn it off.
Capacitor applies reverse
bias to the outgoing SCR.
C circuit pumps smooth
SCR and turn off.
Diode applies reverse bias
to the outgoing SCR.
14. What is the necessity of PWM within an inverter?
To control voltage and harmonics PWM is used. In PWM, output voltage is
controlled by varying the width of pulse. Percentage harmonic can be controlled
by changing number of pulse per half cycle.
15. What are the methods of voltage control in Inverters?

External control of AC output voltage.

External control of DC input voltage.

Internal control of inverter.
16.
What type of Inverter load requires feedback diodes?
Inductive load.
17. How is third harmonic eliminated at the output of single-phase bridges
inverter?
By having a transformer connection in the output.
18. How does a thyristor commutate in series inverters?
By self- commutation.
19. What is the order of output frequencies of series inverters?
5KHz to 30 KHz
20. What is meant by line commutated inverter?
Phase-controlled converters, when operated in the inverter mode, are called linecommutated inverters.
22. Mention any one demerit of a current source inverter.

It requires a relatively large reactor to exhibit current-source characters and an
extra converter state to control the current.

The dynamic response is slower.

Due to current transfer from one pair of switches to another, an output filter is
required to suppress the output voltage spikes.
23. How do you select SCR for inverter?
o
Turn-off time should be minimum. Use inverter grade SCR’s for low frequency
application. Use BJT or MOSFET for high frequencies.
o
Use devices with high dt \ dt ratings.
o
Use factor of safety of 2 to 3.
14
24. What is the function of feedback diodes in bridge inverter?
Feedback diodes pump reactive current to the DC source.
25. What is the simplest method of eliminating 3rd harmonic in a 3- inverter?
Use 1200 mode or connect the system in star.
26. List the advantages and disadvantages of transistor inverter over thyristor inverter.
Advantages:
Disadvantages:

It requires a driving circuit.

Continuous base pulse should be applied since BJT does not have self latching
property.
27. State and explain the merits and demerits of CSI over VSI.
Merits:

Regeneration is possible with CSI.

Simple power circuit.
Demerits:

Large filter inductor is required in the DC line.

Power factor of the source and converter is poor at higher values
of α.
28. What is meant by total harmonic distortion (THD)?
The total harmonic distortion; which is a measure of closeness in shape between a
waveform and its fundamental component, is defined as
29. What is meant by lowest order harmonic (LOH)?
The lowest-order harmonic is that harmonic component whose frequency is closest
to the fundamental one, and its amplitude is greater than or equal to 3% of the
fundamental component.
30. What are the various methods of harmonic control?

Tap changing

Transformer connections

Filters

PWM.
UNIT V
1.
What is an AC voltage regulator?
An AC voltage regulator is an AC to AC converter. It converts a constant AC
voltage into a variable AC voltage of the same frequency.
2.
What are the two types of control normally used in AC voltage controller?
i. On-off control
ii. Phase angle control
15
3.
What is the difference between on-off control and phase angle control?
In on-of control, thyristor switches connect the load to the AC source for a few
cycles of input voltage and then disconnect it for another few cycles. In phase angle
control, thyristor switches connect the load to the AC source for a portion of each cycle
of input voltage.
4. Why is half wave AC voltage regulator not used?
Single-phase half wave voltage regulator consists of a Thyristor and a diode in anti
parallel. Therefore control is possible only in positive half cycle. The output waves are
very much distorted. Therefore half wave regulator is not used.
5. Draw the circuit diagram of single-phase half wave AC voltage controller.
6. Draw the circuit diagram of single phase full wave AC voltage regulator.
7.
Which method of commutation is used in AC voltage regulators?
Line commutation is used in AC voltage regulator. i.e the thyristor will get turned
off when the current through it falls below the holding current value because of
the nature of the source itself.
8. Define duty cycle of AC voltage controller.
16
Duty cycle = n / n + m
Where n = number of on-cycles
m = number of off-cycles
9. Explain the term sequence control of ac voltage regulators.
Sequence control of a.c regulators means the use of two or more stages of voltage
controllers in parallel for the regulation of output voltage. The voltage controllers
in parallel are triggered in proper sequence one after the other so as to obtain a
vaiable output with low harmonic content.
10. What are the advantages of sequence control of AC voltage regulators?
The advantages of sequence control are 1) the reduction of harmonics in the load
voltages and line currents 2)Improvement of power factor.
11. Name some applications of AC regulator.
AC regulators are used in industries heating, illumination level controller on load
transformer tap changing, speed control of induction motor etc.,
12. What is a Cycloconverters?
A Cycloconverter is a direct-frequency changer that converts ac power at one
frequency to ac power at another frequency by ac-ac conversion, without an intermediate
conversion link.
13. List the applications of cycloconverter.
Cycloconverters are used in
i. Variable speed AC motors.
ii. Induction heating
iii. Electric traction
iv. Static var systems etc.
14. List the advantages of cycloconverters.
i. Freqency conversion in single stage is possible
ii. Both frequency and voltage are controllable.
iii. Natural commutation is used. Hence commutation circuits are not needed.
iv. THD is lesser than d.c link converters.
v. Isolation of a defective SCR does not require the cycloconverters to be
switched off.
vi. The power transfer is bidirectional
15. List the disadvantages of cycloconverters.
i.Cyclocoverter operation is possible only for frequencies less than half of the
input frequency.
ii.Number of SCRs required is more than that for d.c link converters.
iii.Complex control circuitry.
iv.At smaller load currents, the cycloconverters may create problem in firing
delay control.
16. What are the factors affecting the harmonics in cycloconverters?
17
i.
ii.
iii.
iv.
v.
vi.
Number of pulses per cycle
Circulating or non-circulating mode of operation
Continuous or discontinuous conduction
Effect of overlap
Effect of load power factor
Control methods
17. Why the output frequency of a cycloconverter is significantly lower than the input
frequency?
Since the cycloconverter is a phase controlled AC-AC converter producing the
desired output AC voltage by selecting segments of the input voltage utilizing
natural commutation, the output frequency becomes significantly lower than the
input frequency.
18. What is a matrix converter?
The matrix converter is a single-stage converter. It uses bi-directional fully
controlled switches for direct conversion from ac to ac. It is an alternative to the
double sided PWM voltage rectifier-inverter.
19. What are the advantages of matrix converters?
i.Inherent bidirectional power flow
ii.Sinusoidal input-output waveforms with moderate switching frequency.
iii.Possibility of compact design due to absence of dc-link reactive components
iv.Controllable input powerfactor.
20. What are the bottlenecks of matrix converters?
i. Non-availability of the bilateral fully controlled switches capable of highfrequency operation.
ii. Complex control law implementation
iii. An intrinsic limitation of the output-input voltage ratio
iv. Commutation and protection of the switches.
21. What are types of control of matrix converters?
i. Venturini method that is based on a mathematical approach of transfer function
analysis
ii. PWM
iii. Space vector Modulation.
22.
Differentiate step-down cycloconverter and step-up cycloconverter.
S.No
1
2
23.
Step-down cycloconverter
It gives an output whose
frequency is lower than that of
output.
It uses line or natural commutation
Step-up cycloconverter
It gives an output whose Frequency is
higher than that of output.
It requires force commutation
Classify Cycloconverters.
 Single phase to single-phase cycloconverter.
18
 Three phase to single phase cycloconverter.
 Three-phase to three-phase cycloconverter.
Cycloconverters can also be classified as

Step-down cycloconverter

Step-up cycloconverter
24.
Draw the circuit diagram of three phase to single phase bridge configuration of
cycloconverter.
25. Explain the purpose of IGR in cycloconverter.
If by any chance both of the converters are enabled, then the supply is shortcircuited.
To avoid this short circuit, an intergroup reactor (IGR) can be connected between the
converters as shown in Fig.
With IGR cycloconverters can be operated in circulating current mode.
26.
What is meant by blocked group operation?
In a cycloconverter if the two groups of thyistors conduct together, the supply is
short circuited. When the operation is to be shifted from one group to the other an
open circuit exists across the load. This is because of the reason that thyristors need a
finite time to turn off. To avoid the possibility of short circuit it is necessary to block
the turning on of one group when the other group is conducting. The firing schemes
19
of both the groups are so designed that any group can not be triggered till the current
of the other group becomes zero. This is known as blocked group operation.
PART B
UNIT I
1. What are the types of power semi conductor devices? Explain them in detail with
relavant circuit diagrams
2. Draw the cross-sectional view of thyristor. Explain steady state and switching
characteristics of it with waveforms.
3. Draw the cross-sectional view of BJT. Explain steady state and switching
characteristics of it with waveforms.
4. Draw the cross-sectional view of MOSFET. Explain steady state and switching
characteristics of it with waveforms.
5. Draw the cross-sectional view of TRIAC. Explain he operation in detail.
6. How is heat dissipated in power semi conductor devices? Explain.
7. Explain V-I characteristics of power diodes and thyristors with diagrams.
UNIT II
1. Draw the basic circuit of controlled rectifier and explain the operation of the same
with neat waveforms. Derive the average and RMS voltage equations.
2. Explain the operation of full wave controlled rectifier with circuit diagram and
actual waveforms.
3. Derive the output equation of single phase half – bridge controlled rectifier with
proper diagrams.
4. Derive the output equation of single phase full-bridge controlled rectifier with R
and RL loads with neat diagrams.
5. Estimate the load voltage, load current and input power factor for single phase full
and half converters.
6. Derive the output equation of three phase half-bridge controlled rectifier with
suitable diagrams.
7. Derive the output equation of three phase full-bridge controlled rectifier with
suitable diagrams.
8. What are the effects of source inductance and load inductance in three phase
rectifiers? Explain with proof,
9. Explain the principle operation of Dual converter with neat diagrams.
10. Explain the operation of converter circuits with Back emf loads.
11. What are phase control and ON-OFF control? How are they applied to A.C
controllers. Explain.
20
12. Draw the circuit diagram of single phase A.C chopper. Explain with neat wave
forms.
13. Estimate the load voltage, load current and input power factor for single phase
A.C controllers.
UNIT III
1. Explain the operation of Basic d.c chopper circuit with neat diagrams
2. What are the two control strategies applied to d.c chopper? Explain them in detail
with waveforms.
3. Classify the chopper circuits and explain them in detail.
4. Draw the circuit diagram of four quadrant chopper. Explain with neat waveforms.
5. Draw the circuit diagram of two quadrant chopper. Explain with neat waveforms.
6. Explain the working of Buck converter with neat waveforms. Derive the
expression of peak-to-peak ripple current and peak-to-peak voltage across the
capacitor.
7. Explain the working of Boost converter with neat waveforms. Derive the
expression of peak-to-peak ripple current and peak-to-peak voltage across the
capacitor.
8. Explain the working of Buck-Boost converter with neat waveforms. Derive the
expression of peak-to-peak ripple current and peak-to-peak voltage across the
capacitor.
9. Explain the working of cuk converter with neat waveforms. Derive the
expressions for peak-to-peak ripple currents in inductors L1 and L2 and peak-to-peak
voltage across the capacitors C1 and C2.
UNIT IV
1. Draw the basic circuit diagram of an inverter. Explain in detail with the neat
diagrams.
2. Explain the principle of operation of series inverter circuit with the neat wave
forms and derive the expressions.
3. Explain the principle of operation of parallel inverter circuit with the neat wave
forms and derive the expressions.
4. Draw the circuit diagram of three phase voltage source inverter and explain the
following modes
i) 1800 conduction mode
ii) 1200 conduction mode.
5. Draw the circuit diagram of 3 – phase current source inverter and explain in
detail.
6. Draw the circuit diagram of single phase current source inverter and explain in
detail.
7. What is meant by pulse width modulation? Explain its use and its effect on
inverters.
8. Explain the working of single phase series resonant inverter with unidirectional
switches.
21
9. Explain the working of single phase series resonant inverter with bidirectional
switches.
UNIT - V
1. (i) Explain the working and derive the expression for average and rms values of
output voltage of a 1φ Half-wave A.C voltage regulator with Resistive load.
(8)
(ii) Draw and explain the working of three possible bidirectional switches. (8)
2. Explain the working and derive the expression for rms values of output voltage of
a full-wave 1φ A.C voltage regulator with (i) Resistive load (ii) RL Load in
discontinuous current mode of operation. (16)
3. (i) Explain the working of single phase ac voltage regulator working on-off
control mode. Derive the expression for rms value of load voltage.(8)
(ii) A single phase ac voltage controller with resistive load of R=10 Ω and the
input voltage is Vs=120V, 60 Hz. The delay angle of thyristor T1 is α =π/2.
Determine (a) the rms value of output voltage Vo, (b) the input PF and (c) the
average input current. (8)
4. Explain the working of single-phase to single phase cycloconverter with bridge
configuration. (16)
5. (i)Compare and contrast cycloconverters with d.c link converters. (8)
(ii)Explain the working of matrix converter with neat sketch. (8)
6. Explain the working of two-stage sequence control for (i) R-Load (ii)R-L Load
with neat circuit and waveforms.(16)
7. (i)With neat sketch explain the working of multi stage sequence control of A.C
regulators. (10)
(ii) Explain the basic working principle of cycloconverters (6)
8. Draw the schematic and explain the working of three phase to single phase
cycloconverter with neat waveforms. (16)
9. Draw the schematic and explain the working of three phase to three phase
cycloconverter with neat waveforms. (16)
10. A single phase full wave controller supplies RL load. The input rms voltage is
Vs=120V, 60Hz. The load is such that L=6.5mH and R=2.5Ω. The delay angles of
thyristors are equal:α1=α2=π/2. Determine (a) the conduction angle of thyristor
T1,δ; (b) the rms output voltage V0, (c) the rms thyristor current IR; (d) the rms
output current I0; (e) the average current of a thyristor IA; and(f) the input PF.
(16)
22