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Reg. No. :
Question Paper Code : 40494
B.E./B.Tech. DEGREE EXAMINATIONS, NOVEMBER/DECEMBER 2021.
Fifth Semester
Electrical and Electronics Engineering
EE 8552 — POWER ELECTRONICS
(Common to B.E. Mechatronics Engineering)
(Regulations 2017)
(Codes/Tables/Charts to be Permitted. If any may be Indicated)
Time : Three hours Maximum : 100 marks
Answer ALL questions.
PART A — (10 × 2 = 20 marks)
1. What is a Snubber circuit?
2. Define the threshold gate voltage of power MOSFET.
3. What is the relation between α , β and γ in single-phase fully controlled
rectifier when operating with RL load?
4. What is the basic function of an excitation system?
5. What is a time ratio control?
6. What is meant by the regenerative braking in the battery-operated vehicles?
7. Define modulation index.
8. What are harmonics?
9. What is ON-OFF control in ac voltage controllers?
10. A three phase six-pulse, 50 kVA, 415V cycloconverter is operating at a firing
angle of 450 and supplying load of 0.8 power factor. Determine input current to
the converters.
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PART B — (5 × 13 = 65 marks)
11. (a) (i) Discuss the basic structure and working of power IGBT
(ii) Draw the two-transistor model of SCR and derive an expression for
anode current.
Or
(b) (i) Explain in details the different SCR commutation methods.
(ii) Discuss a typical driver circuit suitable for IGBT.
12. (a) Describe the working of 3 − φ fully controlled bridge converter in the
Rectifying mode and inversion mode. And derive the expression for
average output voltage and rms output voltage.
Or
(b) Explain the effect of source inductance in the performance of the
single-phase fully controlled rectifier. (13)
13. (a) (i) With help of circuit diagram and waveforms explain the principle of
working of boost convener (8)
(ii) For a class chopper working with resistive load of R ohms, input
voltage of Vdc and duty cycle α , express the following variables as
functions of R, Vdc and α .
(1) Avenge output voltage and current
(2) Output current at the of commutation
(3) Average and RMS freewheeling diode currents
(4) RMS value of output voltage
(5) Average and RMS load currents. (5)
Or
(b) (i) Describe the working of any one resonant dc to dc converter (5)
(ii) Explain the waveforms of type A chopper. Derive the expression for
current ripple when it feeds RL load. (8)
2 40494
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14. (a) (i) With neat sketches, explain the operation of three phase voltage
source inverter. Draw phase and line voltage waveforms on the
assumption that each thyristor conducts for 120° and the resistive
load is star connected. (10)
(ii) Write short notes on the principle of UPS. (3)
Or
(b) Explain the principle of space vector PWM applied to three phase VSI
using the space vector diagram. (13)
15. (a) (i) Describe the operation of a 3-phase thyristorised AC voltage
controller with neat power diagram and waveforms (8)
(ii) Explain in detail about multistage control in ac voltage controllers.
(5)
Or
(b) (i) With the suitable circuit, discuss about the matrix converter (7)
(ii) (1) Single phage AC voltage controller has, a resistive load of
R =10 Ω and input voltage is Vs = 120 V , 60 Hz the delay
π
angle of thyristor T1 is α =. Determine, the rms value of
2
output voltage V0 , the input PF and the average input
current. (6)
PART C — (1 × 15 = 15 marks)
16. (a) (i) The buck regulator has an input range of Vs=12V. The regulated
average output voltage is Va=5V at R=500 Ω and the peak to peak
output ripple voltage is 20mV. The switching frequency is 25kHz if
the peak to peak ripple current of inductor is limited to 0.8A
determine
(1) The duty cycle, K
(2) The filter inductance, L
(3) The filter capacitance, C and
The critical value of L and C (10)
3 40494
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(ii) A three phase fully controlled converter charges a battery from a
three phase supply of 230V, 50Hz. The battery emf is 200V and its
interval resistance is 0.5 Ω . On account of inductance connected
series with the battery, charging is constant at 20 A. Calculate (5)
(1) firing angle
(2) supply power factor
(3) in case it is desired that power flows from dc source to ac load,
find the firing angle for the same current.
Or
(b) (i) The input to a three phase dual converter is 400V 50Hz If peak
value of circulating current is limited to a value 20 A find the value
of inductance of the reactor for a firing angle of 60° (6)
(ii) Draw the circuit diagram of 1φ auto sequential commutated
current source inverter and explain its operation with equivalent
circuits for different modes and necessary waveforms. (9)
———————
4 40494
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Reg. No.
Question Paper Code : X10401
B.E/B.Tech. DEGREE EXAMINATIONS NOVEMBER / DECEMBER 2020
Fifth Semester
Electrical and Electronics Engineering
EE8552 - POWER ELECTRONICS
(Common to: Mechatronics Engineering)
(Regulations 2017)
Time: 3 Hours Answer ALL Questions Max. Marks 100
PART- A (10 x 2 = 20 Marks)
1. Differentiate holding current from latching current
2. What do you mean by second breakdown in power BJT?
3. Define pulse number
4. Differentiate the device turn off time from the circuit turn-off time.
5. List the uses of class B chopper
6. What are the effects of quick or fast charging (storage) or high –energy recovery (discharge)
from a battery?
7. What are integral body diodes?
8. Why PWM strategies are used in inverters?
9. What is bidirectional switch?
10. What is the disadvantage of ON-OFF control?
PART- B (5 x 13 = 65 Marks)
11. a) (i)Discuss the basic structure and working of power GTO. (6)
(ii) Figure shows a thyristor controlling the power in a load resistance RL. The supply
voltage is 240V dc and the specified limits for di/dt and dv/dt for the SCR are 50A/µsec
and 300V/µsec respectively. Determine the values of the di/dt inductance and the snubber
circuit parameters Rs and Cs.
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(7)
OR
b) (i) Draw the two transistor model of SCR and derive an expression for anode current. (6)
(ii) Develop a UJT based Trigger circuit for SCR (7)
12. a) Show that the performance of a single – phase full converter as effected by source
inductance is given by the relation
cos (α+µ) = cos α -
OR
b) Describe the working of single –fully controlled bridge converter in the Rectifying mode
and inversion mode. And derive the expressions for average output voltage and rms
output voltage.
13. a) (i) With relevant sketches explain the operation of a buck-boost converter (8)
(ii) With neat block diagrams, explain the principle of operation of battery operated
vehicles. (5)
OR
b) (i) The buck regulator has an input range of Vs = 12V. The regulated average output
voltage is Va = 5V at R=500Ω and the peak to peak output ripple voltage is 20mV. The
switching frequency is 25 kHz if the peak to peak ripple current of inductor is limited to
0.8A. Determine ,
(a). The duty cycle
(b). The filter inductance
(c). The filter capacitance C, and
(d). The critical value of L and C (5)
(ii) Describe the working of four quadrant chopper (8)
14. a) (i)With neat sketches, explain the operation of three phase voltage source inverter. Draw
phase and line voltage waveforms on the assumption that each thyristor conducts for 180o
and the resistive load is star connected. (10)
(ii) Write short notes on the principle of induction heating (3)
OR
b) (i) With suitable circuits, mode diagrams and waveforms explain any one of CSI (9)
(ii) Discuss about different modified SPWM strategies. (4)
15. a) (i). Describe the operation of a 3-phase thyristorised AC voltage controller with neat
power diagram and waveforms.
(ii). Explain in detail about multistage control in ac voltage controllers.
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OR
b) (i) With the suitable circuit, discuss about the matrix converter (7)
(ii) For a single – phase voltage controller feeding a resistive load, show that power
factor is given by the expression
1/2
1
{
− + sin 2}
2
(6)
PART- C (1 x 15 = 15 Marks)
16. a) (i) A three phase fully controlled bridge converter operating from a 3 phase 220V, 50 Hz
supply is used to charge a battery bank with nominal voltage of 240V. The battery bank
has an internal resistance of 0.01Ω and the battery bank voltage varies by ±10% around
its nominal value between fully charged and uncharged condition. Assuming continuous
conduction find out.
(a). The range of firing angle of the converter
(b). The range of ac input power factor.
(c). The range of charging efficiency. When the battery bank is charged with a constant
average charging current of 100 Amps through a 250 mH lossless inductor. (10)
(ii)The manufacturer of a selected diode gives the rate of fall of the diode current
di/dt=20A/µs, and its reverse recovery time trr=5µ. What value of peak reverse current do
you expect. (5)
OR
b) (i)With help of suitable diagram explain the dynamic characteristics of Power diode (6)
(ii)With neat circuit diagram explain the working of Class E Resonant Rectifier (9)
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