Download 2013

Code No: R31024
R10
Set No: 1
III B.Tech. I Semester Regular and Supplementary Examinations, December - 2013
POWER ELECTRONICS
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) Describe the different modes of operation of a thyristor with the help of its static V-I
characteristics.
b) What is IGBT? Give its basic structural features and working.
2. a) With the help of a neat circuit, explain the two transistor analogy of an SCR. Also
discuss the two triggering conditions of SCR.
b) Describe the significance of di/dt and dv/dt in SCRs.
3. Explain with the help of neat power-diagrams and associated waveforms, the operation
of a single-phase half-wave controlled converter with (i) Resistive load (ii) Inductive
load.
4. Discuss the effect of source-inductance on the performance of a single-phase fullycontrolled converter, indicating clearly the conduction of various thyristors during one
cycle.
Derive an expression for its output voltage in terms of Maximum voltage E m, firing
angle α and overlap angle µ.
5. For a three-phase, thyristor-controlled half wave rectifier, with resistive load. Show that
he average output voltage is given by
3 3
E dc 
E cos( )
for 0 / 6
2 m
3

Edc 
E ( 1 cos  
) for / 6  
5 
/
6
2 m
6
6. a) What is an AC voltage controller? List some of its industrial applications. Enumerate
its merits and demerits.
b) A single-phase voltage controller has input voltage of 230 V, 50 Hz and a load of
R=15 Ω. For 6 cycles on and 4 cycles off, determine (a) r.m.s output voltage, (b) input pf (c)
average and r.m.s thyristor currents
7. a) Draw the schematics of step-down and step-up choppers and derive an expression for
output voltage in terms of duty cycle for a step up and step down chopper
b) A chopper circuit is operating on TRC principle at a frequency of 1 kHz on a 220 V DC
supply. If the load voltage is 180 V, calculate the conducting and blocking period of thyristor
in each cycle.
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Code No: R31024
R10
Set No: 1
8. Describe the working of a single-phase half-bridge inverter. What is its main drawback?
Explain how this drawback is overcome. Describe how output power in single-phase
full bridge inverter is doubled than that of a single-phase half-bridge inverter
**
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Code No: R31024
R10
Set No: 2
III B.Tech. I Semester Regular and Supplementary Examinations, December - 2013
POWER ELECTRONICS
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) List the different members of the thyristor family. Draw their characteristics and
explain in brief.
b) Define latching and holding currents as applicable to an SCR. Show these currents on its
static V-I characteristics.
2. a) Describe the series and parallel operation of SCRs.
b) SCRs with a rating of 1000V and 200A are available to be used in a string to handle 6kV
and 1kA. Calculate the number series and parallel units required in case derating factor is
(i) 0.1 and (ii) 0.2.
3. a) Derive an expression for the (i) Average load voltage (ii) RMS load voltage and (iii)
Average load current, for single-phase half controlled converter with resistive load and
inductive load.
b) A resistive load of 10Ω is connected through a half-wave SCR circuit to 220 V,
50Hz, single-phase source. Calculate the power delivered to load for a firing angle of 600.
Find also the value of input power factor.
4. A single-phase full converter feeding RLE load has the following data. Source
voltage=230V, 50Hz; R=2.5Ω, E=100V, Firing angle=300. If load inductance is large
enough to make the load current virtually constant, then
(a) Sketch the time variation of source voltage, source current, load voltage, load
current, current through one SCR and voltage across it,
(b) Compute the average value of load voltage and load current
(c) Compute the input pf.
5. A 3-phase full converter bridge is connected to supply voltage of 230 V per phase and a
frequency of 50 Hz. The source inductance is 4 mH. The load current on DC side is
constant at 20 A. If the load consists of a DC voltage of 400 V having an internal
resistance of 1 Ω, then calculates: (a) Firing angle delay and (b) Overlap angle in
degrees.
6. Describe the operating principle of single-phase to single-phase step-up cycloconverter
with the help of mid-point and bridge type configuration. Illustrate your answer with
appropriate circuit and waveforms.
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Code No: R31024
R10
Set No: 2
7. a) With the help of neat circuit diagram and associated waveforms discuss the operation
of a buck converter.
b) An 80 V battery supplies an R-L load through a chopper circuit. The load inductance
is 40 mH and resistance is 6 Ω. The load has a freewheeling diode D f across it. It is
required to the current between the limits 10 A and 12A. Calculate the time ratio of the
chopper?
8. a) Draw and explain the operation of three-phase series-inverter circuit.
b) A single phase full bridge inverter delivers power to RLC load with R=3 Ω and
XL=12Ω. The bridge operates with a periodicity of 0.2ms. Calculate the value of C so
that load commutation is achieved for the thyristors. Turn off time for thyristors is 12µs.
Factor of safety is 2 assume the load current to contain only the fundamental
component.
**
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Code No: R31024
R10
Set No: 3
III B.Tech. I Semester Regular and Supplementary Examinations, December - 2013
POWER ELECTRONICS
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) Give constructional details of a typical thyristor. Sketch its schematic diagram and
the circuit symbol
b) For an SCR, the gate-cathode characteristic is given by a straight line with a gradient
of 20 volts per ampere passing through origin. The maximum turn-on time is 4µs and
the minimum gate current required to quick turn on is 400mA. If the gate source voltage
is 15 V, calculate the resistance to be connected in series and the gate-power dissipation
2. a) Why SCRs are required to be connected in parallel? What are the problems
associated with parallel connection of SCRs? How they are eliminated?
b)Explain the need and necessity of a snubber circuit in thyristors circuits.
3. Describe the operation of single-phase, two-pulse, mid-point converter with relevant
voltage and current waveforms. Discuss how each SCR is subjected to a reverse voltage
equal to double the supply voltage in case turns ratio from primary to each secondary is
unity.
4. Describe the working of single-phase fully-controlled bridge converter in the following
two-modes: (a) Rectifying mode. (b) Inversion mode. Also, sketch the following
waveforms for α =450 and α =1200. (i) Load voltage waveform (ii) Load current
waveform (iii) Thyristor current and voltage waveforms (iv) Supply voltage and current
waveforms.
5. a) Explain the operation of three-phase, half-controlled converter with resistive load,
and inductive load. Sketch the associated waveforms also.
b) A three-phase fully controlled bridge converter is connected to three-phase a.c supply of
400 V, 50 Hz and operates with a firing angle α = π /4. The load current is maintained
constant at 10A and the load voltage is 360 v. Compute: (i) Source inductance, Ls (ii)
Load resistance, R (iii) Overlap angle, µ .
6. a) Describe the operation of a single phase half-wave a.c voltage regulator with the help
of voltage and current waveforms. Also, derive the expression for average value of
output voltage.
b) List the advantages and disadvantages of Traic over SCR.
7. With the help of neat circuit diagrams and associated waveforms, discuss the operation
of Buck-Boost converter. List the advantages and disadvantages of this type of
converter.
8. a) Explain the principle of operation of an inverter
b)How is the output voltage and frequency of a PWM inverter varied?
**
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Code No: R31024
R10
Set No: 4
III B.Tech. I Semester Regular and Supplementary Examinations, December - 2013
POWER ELECTRONICS
(Electrical and Electronics Engineering)
Time: 3 Hours
Max Marks: 75
Answer any FIVE Questions
All Questions carry equal marks
**
1. a) Explain the various types of triggering methods of SCR briefly. Which is the
universal method and why?
b) Discuss the switching characteristics of the IGBT with the help of neat circuit
diagrams and waveforms.
2. a) What is the necessity of connecting SCRs in series? What are the problems
associated with series connection of SCRs? How they are eliminated?
b) Explain the need of commutation in thyristor circuits. What are the different methods of
commutation schemes?
3. A single-phase half-wave SCR circuit feeds power to a resistive load. Draw waveforms
for source voltage, load voltage, load current and voltage across the SCR for a given
firing angle α. Hence obtain expressions for average and rms load voltages in terms of
source voltage and firing angle.
4. A single phase fully controlled bridge is operated with a resistive load R= 10 Ω, the
input voltage to the bridge is 230V. The firing angle is 600. Determine (i) Average load
voltage (ii) Average and rms load current (iii) Form factor and ripple factor
(iv)
Average output power (v) Rectifier efficiency, and (iv) SCR ratings
5. Explain how two three phase full converters can be connected back to back to form a
circulating current type of dual converter. Discuss its operation with the help of voltage
waveforms across (a) each converter (b) load and (c) reactor. Describe how two
circulating current waveform can be obtained from reactor voltage waveform.
If one of the two converters is loaded, sketch the waveform of their load current also.
6. a) Draw and explain the a.c phase controller circuit using Traic and Diac. Which modes
are used here?
b) A single phase half wave AC voltage controller has a resistive load of R=10Ω and
the input voltage is Vs =120V, 60Hz. The Delay angle of thyristor is α=π/2. Determine
(i) the rms value of output voltage V0, (ii) The input power factor, and (iii) the average input
current.
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Code No: R31024
R10
Set No: 4
7. a) Explain the time ratio control and current limit control, and control strategies used for
chopper.
b) A step-down DC chopper has a resistive load of R=15 ohm and input voltage
Edc=200 V. when the chopper remains ON, its voltage drop is 2.5 V. the chopper
frequency is 1 kHz. If the duty cycle is 50 %, determine: (i) average output voltage (ii)
RMS output voltage (iii) Chopper efficiency (iv) effective input resistance of chopper.
8. Explain the operation of single phase bridge inverter with the help of load, voltage and
current waveforms.
**
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