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Old April 30th, 2014, 05:20 PM
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Default IETE AMIETE-ET old scheme Power Electronics Exam paper

Give me question paper for Institute of Electronics and Telecommunication Engineers Power Electronics examination ??
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Old May 2nd, 2014, 01:23 PM
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Default Re: IETE AMIETE-ET old scheme Power Electronics Exam paper

Institute of Electronics and Telecommunication Engineers organized IETE AMIETE-ET Power Electronics examination for admission in diploma courses .

Here I am giving you question paper for Institute of Electronics and Telecommunication Engineers Power Electronics examination

Q.1 Choose the correct or the best alternative in the following: (2×10)
a. When a thyristor is conducting the voltage drop across it is
(A) Absolutely constant
(B) Decreases with increase in load current
(C) Increase slightly with increase in load current
(D) Any of above

b. Triac can be turned on by applying gate signal which is
(A) Positive (B) Negative
(C) Positive or negative (D) None of the above

c. Power MOSFET is a
(A) Voltage Controlled Device (B) Current Controlled Device
(C) Field Controlled Device (D) A and C both
d. A UJT exhibits a negative resistance region:
(A) Before the peak point (B) Between the peak & valley point
(C) After the valley point (D) Both (A) & (B)

e. The effect of source inductance in a controlled rectifier is to
(A) Increase the load voltage
(B) Decrease the load voltage
(C) Reduce the ripples in the load current.
(D) Increase the ripples in the load current

f. If the duty cycle of the chopper circuit is exactly 50% then the pulse is
considered to be
(A) Sine wave (B) Square Wave
(C) Triangular wave (D) None of these above

g. In a 3- phase half wave rectifier the duration of conduction of each
thyristor is
(A) 30o (B) 60o
(C) 45o (D) 120o
h. A device that converts an dc source into ac is called
(A) Inverter (B) Chopper
(C) Rectifier (D) Cyclo-converter
i. The Natural commutation can be used in
(A) DC circuits (B) AC circuits
(C) Both ac & dc circuits (D) None of above
j. A device that converts a fixed frequency ac source into a lower frequency ac
source is known as
(A) An inverter (B) A PWM circuit
(C) A converter (D) A cyclo-converter
Answer any FIVE Questions out of EIGHT Questions.
Each question carries 16 marks.

Q.2 a. What is a thyristor? Give constructional details of a typical thyristor. Also
describe the necessary conditions for turning on of a thyristor. (8)
b. It is required to operate 250A SCR in parallel with 350A SCR with their
respective on-state voltage drops of 1.6V and 1.2V. Calculate the value of
resistance to be inserted in series with each SCR so that they can share the
total load of 600A in proportion to their current ratings. (8)

Q.3 a. Draw and explain the waveform of supply voltage, load voltage, load
current and current through SCR of a single phase half wave controlled
rectifier feeding R-L load. (8)
b. Explain the following in brief (8)
(i) Inverting mode of a converter
(ii) Transformer ratings for rectifying circuits

Q.4 a. State the principle of chopper operation highlighting the operation of step
down and step up chopper. (10)
b. A step up chopper has supply voltage of 250V while the output voltage is
500V. If the off period of chopper be 100µsec, determine the pulse width
of the output voltage. (6)

Q.5 a. What is an ac voltage controller? Explain briefly on-off and phase control
principles of ac voltage controller.

Q.1 Choose the correct or the best alternative in the following: (210)
a. When a thyristor is conducting the voltage drop across it is
(A) Absolutely constant
(B) Decreases with increase in load current
(C) Increase slightly with increase in load current
(D) Any of above

b. Triac can be turned on by applying gate signal which is
(A) Positive (B) Negative
(C) Positive or negative (D) None of the above

c. Power MOSFET is a
(A) Voltage Controlled Device (B) Current Controlled Device
(C) Field Controlled Device (D) A and C both

d. A UJT exhibits a negative resistance region:
(A) Before the peak point (B) Between the peak & valley point
(C) After the valley point (D) Both (A) & (B)

e. The effect of source inductance in a controlled rectifier is to
(A) Increase the load voltage
(B) Decrease the load voltage
(C) Reduce the ripples in the load current.
(D) Increase the ripples in the load current

f. If the duty cycle of the chopper circuit is exactly 50% then the pulse is
considered to be
(A) Sine wave (B) Square Wave
(C) Triangular wave (D) None of these above

g. In a 3- phase half wave rectifier the duration of conduction of each
thyristor is
(A) 30o (B) 60o
(C) 45o (D) 120o

h. A device that converts an dc source into ac is called
(A) Inverter (B) Chopper
(C) Rectifier (D) Cyclo-converter

i. The Natural commutation can be used in
(A) DC circuits (B) AC circuits
(C) Both ac & dc circuits (D) None of above

j. A device that converts a fixed frequency ac source into a lower frequency ac
source is known as
(A) An inverter (B) A PWM circuit
(C) A converter (D) A cyclo-converter

Answer any FIVE Questions out of EIGHT Questions.
Each question carries 16 marks.
Q.2 a. What is a thyristor? Give constructional details of a typical thyristor. Also
describe the necessary conditions for turning on of a thyristor. (8)

b. It is required to operate 250A SCR in parallel with 350A SCR with their
respective on-state voltage drops of 1.6V and 1.2V. Calculate the value of
resistance to be inserted in series with each SCR so that they can share the
total load of 600A in proportion to their current ratings. (8)

Q.3 a. Draw and explain the waveform of supply voltage, load voltage, load
current and current through SCR of a single phase half wave controlled
rectifier feeding R-L load. (8)

b. Explain the following in brief (8)

(i) Inverting mode of a converter
(ii) Transformer ratings for rectifying circuits
Q.4 a. State the principle of chopper operation highlighting the operation of step
down and step up chopper. (10)

b. A step up chopper has supply voltage of 250V while the output voltage is
500V. If the off period of chopper be 100sec, determine the pulse width
of the output voltage. (6)

Q.5 a. What is an ac voltage controller? Explain briefly on-off and phase control
principles of ac voltage controller. (8)

AE26 / DEC. – 2010 3 AMIETE – ET (OLD SCHEME)
b. The 1- Full wave controller supplies an RL load. The input rms voltage
is 120V, 60 Hz. The load is such that L = 6.5 mH and 5 . 2 R . The
delay angles of thyristors are equal and equal to
2

. Determine
(i) input power factor
(ii) rms thyristor and output current. (8)

Q.6 a. Explain the working of cycloconverter using simple diagram. What is
meant by blocked group operation. Explain. (8)

b. A three phase to single phase blocked group operated cycloconverter
constituted by 6-pulse bridge converters is to supply an inductive load. The
output frequency is 2 Hz and the inductive load reactance at this frequency
is 1.6 _. The load circuit resistance is 2 _. The cycloconverter is directly
supplied from a three phase 440 V rms (line to line), 50 Hz source The
cycloconverter uses cosinusoidal modulating signal and sinusoidal
reference voltage. Calculate (8)

(i) RMS output voltage if modulation factor is 0.5 and
(ii) RMS output current Neglect commutation intervals of thyristors.
Q.7 a. Explain the working principle of the following PWM techniques (8)

(i) Multiple pulse-width modulation
(ii) Sinusoidal pulse-width modulation
b. A 1-phase centre-tapped supply inverter has a resistive load of R = 1 K_
and the dc supply voltage is 230 V. Determine the rms output voltage,
Voltage of the fundamental component, and the output power. (8)

Q.8 a. Discuss the methods by which power semiconductor devices may be used
to control the speed of dc drives. (8)

b. Explain with suitable block diagram the control of an induction motor
using a microprocessor. (8)

Q.9 a. Explain the process of natural commutation. (4)

b. Differentiate between
(i) Load side and line side commutation
(ii) Voltage and current commutation (8)

c. An impulse commutated thyristor shown in Figure. Determine the
available turn off time of circuit if V 200 Vs 10 R , F 5 C and
s o V V . (4)

Q.1 Choose the correct or the best alternative in the following: (210)
a. As compared to power MOSFET, a BJT has
(A) Lower switching losses but higher conduction loss
(B) Higher switching losses and higher conduction loss
(C) Higher switching losses but lower conduction loss
(D) Lower switching losses and lower conduction loss.

b. An SCR can be brought to forward conducting state with gate-circuit open
when the applied voltage exceeds.
(A) The forward break over voltage (B) Reverse breakdown voltage.
(C) 1.5 V (D) Peak non-repetitive off-state voltage

c. A single-phase full wave midpoint thyristor converter uses a 230/200 V
transformer with centre tap on the secondary side. The PIV per thyristor is.
(A) 100 V (B) 141.4 V
(C) 200 V (D) 282.8 V

d. The three-phase ac to dc converter which requires neutral point connection is
(A) 3-phase semi-converter (B) 3-phase full converter
(C) 3-phase half wave converter (D) 3-phase full converter with diodes

e. In dc chopper, for chopping period T, the output voltage can be controlled by
FM by varying
(A) T keeping Ton constant (B) T keeping Toff constant
(C) Ton keeping T constant (D) Both (A) & (B)

f. In a CSI, if frequency of output voltage is f Hz, then frequency of voltage
input to CSI is
(A) f (B) 2f
(C) f/2 (D) 3f

g. In a single-phase voltage controller with RL load, _ is the firing angle, Ø is
the load phase angle and ß is the extinction angle. For this voltage
controller, output power can be controlled if _ > Ø and
(A) (ß-_) = _ (B) (ß-_) < _
(C) ß > _ (D) Both (B) and (C)

h. The cycloconverter (CCs) require natural or forced commutation as under
(A) Natural commutation in both step-up and step down CCs
(B) Forced commutation in both step-up and step down CCs
(C) Forced commutation in step-up CCs
(D) Forced commutation in step-down CCs

i. The number of thyristor required for single-phase to single phase
cycloconverter of the mid-point type and for three to three-phase 3 pulse type
cycloconverter are respectively
(A) 4,6 (B) 8,18
(C) 4,18 (D) 4,36

j. A single phase full bridge inverter can operate in load commutation mode in
case load consists of
(A) RL (B) RLC underdamped.
(C) RLC overdamped (D) RLC critically damped

Answer any FIVE Questions out of EIGHT Questions.
Each question carries 16 marks.
Q.2 a. What are the characteristics of an ideal power-switching device? Compare
the characteristics of IGBT and MOSFET. Draw the transfer characteristics
and switching characteristics of IGBT. (12)
b. Define the following terms for S.C.R.:
(i) Latching current,
(ii) Holding current,
(iii) di/dt rating,
(iv) dv/dt rating. (4)

Q.3 a. An RL load is fed from single-phase supply through a thyristor. Derive an
expression for load current in terms of supply voltage, frequency, R, L etc.
Indicate the time limits during which this solution is applicable. (8)

b. Explain the need of commutation in thyristor circuit. What are the different
methods of commutation schemes? Discuss one of them, with a neat
schematic and waveforms. (8)

Q.4 a. Describe the various types of chopper configurations and control strategies
with appropriate waveforms. Also explain multi-quadrant chopper. (8)

b. What techniques are involved in voltage control of single phase inverters?
Explain any one of them with appropriate waveform. (8)

Q.5 a. In detail, explain three-phase to single-phase cycloconverters with circuit
diagram and appropriate waveform associated. (8)

b. An ac-dc converter steps down the voltage through a transformer and
supplies the load through a bridge rectifier. Design a 60Hz power
transformer of the specifications: primary voltage V 120 1 V , 60Hz
(square wave), secondary voltage output V 40 0 V and secondary output
current A 5 . 6 0 I . Assume transformer efficiency % 95 and window
factor 4 . 0 u K . Use E-Core. (8)

Q.6 a. A converter is feeding an RL load as shown in Fig.1 with
5 R , V 220 s V , L=7.5mH, f = 1kHz, k = 0.5, and E = 0 V. Calculate
(i) The minimum instantaneous load current 1 I
(ii) The peak instantaneous load current 2 I
(iii) The maximum peak-to-peak load ripple current
(iv) The average value of load current a I (42 = 8)

b. What is the principle of AC phase controller with proper circuit diagram?
Give the RMS output voltage and average value of output voltage. (8)

Q.7 a. A single-phase 230 V, 1 kW heater is connected across 1-phase, 230 V, 50
Hz supply through an SCR. For firing angle delays of 45° and 90°,
calculate the power absorbed in the heater element. (8)

b. Explain detailed working of voltage commutated chopper with circuit
diagram and associated waveform. (8)

Q.8 a. Describe the working of two stage sequence (transformer tap) control of
voltage controllers with both R load and RL load. What are the advantages
of this controller over single–phase full wave voltage controller? (8)

b. What is blocked group operation? Explain it with circuit diagram and
waveforms if necessary. (8)

Q.9 Write short note on: (Any TWO)
(i) Commutation circuit design.
(ii) Effects of Source and Load Inductances
(iii) Circulating current mode (8×2)
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