POWER ELECTRONICS

Code No: R05310206 Set No. 1
III B.Tech I Semester Regular Examinations, November 2008
POWER ELECTRONICS
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
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1. (a) What is the importance of Surge current rating of a thyristor, explain in detail.
(b) A thyristor has half-cycle surge current rating of 1000mA for a 50Hz supply.
Calculate its one-cycle surge current rating and I2t rating. [10+6]
2. A single phase fully controlled converter is connected to a load comprised of 2 ohms
resistance and 0.3H inductance. The supply voltage is 230V at 50Hz. Estimate the
average load voltage, average load current and input power factor for a firing angle
of 200. Assume continuous and ripple free load current, draw load voltage waveform.
[5+5+4+2]
3. A three phase, half wave controlled converter is connected to a 380V (line) supply.
The load current is constant at 32A and is independent of firing angle. Find the
average load voltage at firing angle of 00 and 450, given that the thyristors have
a forward voltage drop of 1.2V. What value of current and peak reverse voltage
rating will the thyristor require and what will be the average power dissipation in
each thyristor. [4+4+4+4]
4. A single phase ac voltage controller feeds power to a resistive load of 4 ohms from
230V, 50Hz source. Determine
(a) The max. values of average and rms thyristor currents for any firing angle α.
(b) The minimum circuit turn-off time for any firing angle α.
(c) the max. value of di/dt occurring in the thyristors. [6+6+4]
5. Explain the operation of single phase midpoint cyclo converter with R-L load s for
continuous conduction with relevant circuit diagram nd necessary output waveforms
for f0 = 1/3 fs. [4+6+6]
6. A load commutated chopper, fed from a 230V dc source has a constant load current
of 50A. For a duty cycle of 0.4 and a chopping frequency of 2 KHz, Calculate
(a) the value of commutating capacitance
(b) average output voltage
(c) circuit turn-off time for one SCR pair
(d) total commutation interval [4+4+4+4]
7. Draw and explain the simple SCR series inverter circuit employing class A type
commutation. With the help of important waveforms. State the limitations of this
inverter. [8+4+4]
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Code No: R05310206 Set No. 1
8. (a) What are the different pulse width modulation techniques used for inverters.
(b) Which of the schemes gives better quality of voltage and current. [10+6]
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Code No: R05310206 Set No. 2
III B.Tech I Semester Regular Examinations, November 2008
POWER ELECTRONICS
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
1. The latching current of a thyristor with d.c. voltage source of 100V is 50mA.
Calculate the value of minimum width of the gate pulse current when connected
to a pure inductive load of 1H. Compute the effect, if a resistance of 10 ohms is
connected in series with the load. [8+8]
2. Explain the operation of a singe phase half wave converter for R-load with neat
circuit diagram and necessary waveforms. Also derive the output average voltage
and current for α = 300. [12+4]
3. A three phase, fully controlled converter is connected to a resistive load. Show that
the average output voltage is given by
Vdc=3p3
2 VmCosα for 0 < α < π/3 and
Vdc = 3p3
2 Vm [1 + Cos(α + π/3)] for π/3 < α < 2π/3 [8+8]
4. Explain the operation of a single phase ac voltage controller with neat circuit dia-
gram and output waveforms with respect to source voltage waveforms at α = 600
for Resistive load. [4+8+4]
5. Discuss the working of a single phase mid point cyclo converter with R-L loads and
for discontinuous operation with neat circuit diagram and output rms voltage and
current waveforms for f0=1/3 fs. [4+7+5]
6. Derive the expression for minimum and maximum values of load current for a type
- A chopper and also derive the current ripple. [10+6]
7. Draw and explain the simple SCR series inverter circuit employing class A type
commutation. With the help of important waveforms. State the limitations of this
inverter. [8+4+4]
8. A single phase full bridge inverter uses a uniform PWM with two pulses per half
cycle for voltage control. Plot the distortion factor, fundamental component, and
lower order harmonics against modulation index. [5+5+6]
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Code No: R05310206 Set No. 3
III B.Tech I Semester Regular Examinations, November 2008
POWER ELECTRONICS
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
1. A 200A thyristor operates in parallel with a 300A thyristor. Their ON state voltage
drops are respectively 1.5V and 1.0V. Calculate the value of the resistance to be
inserted in series with each thyristor so that they share a load of 500A in proportion
to their respective current ratings. [16]
2. (a) Compare mid-point converters and bridge type converters and bring out im-
portant features.
(b) Compare discontinuous and continuous current modes of operation of convert-
ers and bring out salient features. [8+8]
3. A three phase, fully controlled converter is connected to a resistive load. Show that
the average output voltage is given by
Vdc=3p3
2 VmCosα for 0 < α < π/3 and
Vdc = 3p3
2 Vm [1 + Cos(α + π/3)] for π/3 < α < 2π/3 [8+8]
4. The ac voltage controller uses on-off control for heating a resistive load of R = 4
ohms and the input voltage is Vs = 208V, 60Hz. If the desired output power is P0
= 3KW, determine the
(a) duty cycle δ
(b) input power factor
(c) sketch waveforms for the duty cycle obtained in (a) [5+6+5]
5. For a single phase mid-point cyclo-converter, explain the operation of the circuit
when fed to R-load with the help of neat circuit diagram and relevant output
waveforms for α = 300 and α = 1200 for f0 = 1/4 fs. [4+8+4]
6. Explain the time ratio control and current limit control strategies used for choppers,
with necessary waveforms and circuit. [8+8]
7. Draw and explain the simple SCR series inverter circuit employing class A type
commutation. With the help of important waveforms. State the limitations of this
inverter. [8+4+4]
8. What are the methods for voltage control within the inverters. Explain in detail
with waveforms. [16]
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Code No: R05310206 Set No. 4
III B.Tech I Semester Regular Examinations, November 2008
POWER ELECTRONICS
(Electrical & Electronic Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
⋆ ⋆ ⋆ ⋆ ⋆
1. Explain in detail various voltage ratings and current ratings of a thyristor. [16]
2. (a) Compare mid-point converters and bridge type converters and bring out im-
portant features.
(b) Compare discontinuous and continuous current modes of operation of convert-
ers and bring out salient features. [8+8]
3. A six pulse thyristor converter connected on the secondary of the 6.6KV/415V,
50Hz transformer is supplying to 460V, 200A a.c, load. Calculate
(a) Converter firing angle
(b) DC power delivered by the converter
(c) ac terminal power
(d) ac line current [4+4+4+4]
4. Two SCRs are connected back-to-back have a load resistance of 400 ohms and a
supply of 110V ac. If firing angle is 600, find
(a) the rms output voltage
(b) average power. [10+6]
5. Discuss the working of a single phase bridge type cyclo converter with RL loads and
for discontinuous operations with relevant output waveforms and circuit diagram
for f0 = 1/2 fs. [4+6+6]
6. For the ideal type A-chopper circuit, following conditions are given, Edc = 220V,
chopping frequency, = 500 Hz, duty cycle δ=0.3 and R = 1 ohm, L = 3mH and Eb
= 23V. Compute the following quantities.
(a) Check whether the load current is continuous or not.
(b) Average output current
(c) maximum and minimum values of steady state output current [6+4+6]
7. Draw and explain the simple SCR series inverter circuit employing class A type
commutation. With the help of important waveforms. State the limitations of this
inverter. [8+4+4]
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Code No: R05310206 Set No. 4
8. Calculate the output frequency of a series inverter circuit with following parameters,
L = 10mH, C = 0.1 μF, R = 400 ohms, toff = 0.2 msec. Also determine the
attenuation factor. [16]
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