Power Electronics : Question Paper Dec 2015 - Electronics & Telecomm (Semester 7) | Visveswaraya Technological University (VTU)
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Power Electronics - Dec 2015

Electronics & Communication (Semester 7)

(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.
1 (a) Give symbol, characteristics features of the following devices:
(10 marks)
1 (b) Explain briefly the different types of thyristor power converters and mention two applications of each.(10 marks) 2 (a) With the necessary waveforms, explain the switching characteristics of a power transistor.(8 marks) 2 (b) Give the comparison between MOSFET and IGBT.(6 marks) 2 (c) The collector clamping circuit of Fig. Q2 (c) has VCC=100 V, RC=1.5 Ω. Vd1=2.1 V, Vd2=0.9 V, VBE=0.7 V, VB=15 V and RB=2.5 Ω and β=16. Calculate
i) the collector current without clamping.
ii) the collector-emitter clamping voltage VCE.
iii) the collector current with clamping.

(6 marks) 3 (a) Define the following: i) latching current ii) holding current iii) I2t rating. Derive expression for Anode current using two-transistor model in case of SCR.(10 marks) 3 (b) What is the need for protection of thyristor? Explain how thyristors are protected against high $ \dfrac {di}{dt} \text{ and high } \dfrac {dv}{dt}. $(6 marks) 3 (c) A SCR has a $ \dfrac {di}{dt} = 120 \text{A /}\mu\text{ s and a} \dfrac {dv}{dt} $ of 300 V/μs it operates on a 250 V DC source with a load resistance of 10 Ω. Find the suitable values for the components of the snubber circuit.(4 marks) 4 (a) Explain the working of single phase dual converter with neat circuit diagram. Draw relevant waveforms.(10 marks) 4 (b) Explain the working of single phase semiconvert with neat circuit and waveforms. Derive expression for the average output voltage.(6 marks) 4 (c) A single phase full converter supplies an RLE load form a 230 V, 50 Hz supply. The load is highly inductive, so that load current is continuous and ripple free. If R=1 Ω and the load current is IO=10A. Calculate the delay angle α for E=120.(4 marks) 5 (a) Explain the operation of single phase bidirectional controller with resistive load. Obtain the expression for RMS output voltage. Show the waveforms.(10 marks) 5 (b) A single phase full wave AC voltage controller has an RL load. The input voltage is 230 V, 50 Hz and the load is R=2 Ω and XL= 2Ω α1= α2= π/2. Calculate the following:
i) Angle until which the thyristor conducts.
ii) Conduction angle of thyristor.
iii) RMS voltage of output.
(6 marks)
5 (c) What are the advantages and disadvantages of ON-OFF control and phase control of ac voltage controller?(4 marks) 6 (a) Explain the resonant pulse communication with neat circuit and waveforms.(10 marks) 6 (b) Explain the working of complementary communication circuit. Draw relevant waveforms. Derive expression for toff.(6 marks) 6 (c) In the circuit of Fig. Q6 (c) the capacitor is initially charged to a voltage of VC(0)=500 V. If L=15 μH and C=50 μF and the SCR is turned ON at t=0. Calculate
i) the peak value of resonant current and (ii) the conduction time of thyristor.

(4 marks) 7 (a) Give the classification of chopper. Explain briefly each one of them.(10 marks) 7 (b) Explain the principle of operation of a step up chopper.(6 marks) 7 (c) A DC chopper has an input voltage of 200 V and a load of 8 Ω resistance. The voltage drop across thyristor is 2 V and the chopper frequency is 800 Hz. The duty cycle K=0.4. Find
i) Average output voltage
ii) RMS output voltage
iii) Chopper efficiency.
(4 marks)
8 (a) Explain the performance parameters of inverters.(6 marks) 8 (b) Explain the working of transistorized current source inverter.(8 marks) 8 (c) Calculate the RMS values of the fundamental and the two lower order harmonics of a single-phase full bridge inverter employing single-pulse width modulation for output voltage control. The modulation index is 80% and the dc input voltage is 230 V.(6 marks)

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