Question Paper: Power Electronics : Question Paper Jun 2015 - Electronics & Telecomm (Semester 6) | Pune University (PU)
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## Power Electronics - Jun 2015

### Electronics & Telecom Engineering (Semester 6)

TOTAL MARKS: 100
TOTAL TIME: 3 HOURS
(1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks.

### Answer any one question from Q1 and Q2

1 (a) Draw two transistor analogy of SCR and derive an expression for its anode current IA?(7 marks) 1 (b) Draw & explain single phase fully controlled rectifier (full converter) for R-L load with various o/p voltage wavefoRMS.(7 marks) 1 (c) Single phase full bridge inverter is operated from 48V DC supply, it has a resistive load of R = 2.4 Ω Find:
i) RMS o/p voltage at fundamental frequency (VO1)
ii) RMS o/p power
iii) RMS o/p voltages at second & third harmonic (VO2 & VO3)
(6 marks)
2 (a) Draw construction diagram of n-channel enhancement type MOSFET and explain its steady state charACteristics.(7 marks) 2 (b) Draw & explain three phase half controlled bridge converter for R load with o/p voltage wavefoRMS.(7 marks) 2 (c) Compare 120° mode with 180 0 mode in three phase inverter for balanced star R load.(6 marks)

### Answer any one question from Q3 and Q4

3 (a) Explain operation of step up chopper with circuit diagram and derive an expression for its o/p voltage: $V_o= \dfrac {V_s}{(1-D)}$ where D is duty cycle.(6 marks) 3 (b) A DC chopper with R-L load is operated from 220V DC supply. The load parameters are R = 5Ω , L = 7.5 mH and chopping frequency Fc = 1KHz. If peak to peak load ripple current is maximum, calculate:
iii) Peak to peak load ripple current
(6 marks)
3 (c) Explain various control strategies in DC chopper.(6 marks) 4 (a) Explain operation of four quadrant chopper with circuit diagram.(6 marks) 4 (b) Explain with block schematic working of SMPS. What are its advantages over linear power supply.(6 marks) 4 (c) A single phase full wave AC voltage controller has a resistive load of R = 10Ω and the input voltage is Vs = 120 V(RMS), 50 Hz. The delay angles of thyristors T1 and T2 are equal : a1 = a2 = π/2. Determine
i) the RMS output voltage
ii) the RMS output current
iii) the input PF
(6 marks)

### Answer any one question from Q5 and Q6

5 (a) Explain with block schematic working of On-line & off-line UPS.(8 marks) 5 (b) The speed of a separately excited DC motor (armature) is controlled by a 1-¢ semi-converter. The field current is also controlled by a 1-¢ semi converter and is set to its maximum possible value. The AC supply to both armature & field converters is single phase 208V, 60Hz. The armature resistance Ra=0.275Ω. Field resistance Rf=147Ω. The motor voltage constant Kv=0.7032 V/A. rad/s, the armature & field currents are continuous & ripple free. If load torque TL=45 N-m at 1000 RPM, calculate:
i) Field current If
ii) BACk emf Eg
iii) Firing angle of converter in armature circuit
iv) Input power fACtor of armature circuit converter.
(8 marks)
6 (a) Explain voltage & frequency control method for 3-¢ induction motor drive in detail.(8 marks) 6 (b) What are advantages of electronic ballast over conventional ballast? Explain working of electronic ballast with block schematic.(8 marks)

### Answer any one question from Q7 and Q8

7 (a) What is EMI? Explain various sources & minimizing techniques of EMI.(6 marks) 7 (b) For a thyristor, Maximum junction temperature is 125°C. The thermal resistances are ϕJC=0.16, ϕCS=0.08°C/W, for heat sink temperature of 70°C, calculate total average power loss in thyristor-sink combination.
If heat sink temperature is reduced to 60°C, find new total average power loss in thyristor-sink combination.
(4 marks)
7 (c) Write a note on 'over voltage protection' in power electronics.(6 marks) 8 (a) What is the need of resonant converter? Explain ZCS resonant converter with circuit & waveforms.(8 marks) 8 (b) Explain SLR half bridge DC-DC converter in low frequency with suitable waveforms.(8 marks)