| POWER ELECTRONICS |
| PAPER NO. 2 |
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| PUT ON: June, 2K3 |
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EC POWER ELECTRONICS (B.Tech 4th Semester,2063) Time : 3 Hours Maximum Marks : 60 NOTE:- This paper consist of Three Sections. Section A is compulsory. Do any Four questions from Section B and any two questions from Section C Section-A Marks : 20 1(a) What are the conditions that must be satisfied for turning on an SCR with a gate signal ? (b) What is the significance of short period and long period of voltage regulator ? (c) What is the difference between dc link convertor and cyclo-convertor ? (d) Design a UJT relaxation oscillator for triggering an SCR, the Ujt has the following characteristics intrinsic Stand off ratio =0.7, Peak current = 50 µA, Valley voltage = 2 V, Velley current = 6 mA, Supply voltage VBB = 20 V, Total base resistance RBB = 7 K ohm, Emitter current IEO (e) A single-phase full bridge invertor has a resistive load of 3 ohms and DC input voltage Vdc 50 V. Compute rms output voltage at the fundamental frequency, output power and current ratings of thyristors. (f) Draw schematic of electronic timer using IC. (g) What are the features of frequency controlled inductor motor ? (h) Compare the features of voltage source inverters. (i) A single pahse half-wave converter is operated at a firing angle of 45o. input voltage is 230 V at 50 Hz. For a resistive load of 10 ohms across converter calculate ripple factor and transformer utility factor. (j) Draw forward and reverse VI characteristics of an SCR and label all the regions clearly. Section-B Marks:5 Each 2. (a) Why the de-rating of any SCR is more for sine wave than for a square wave ? (b) For the static V-I characteristics of thyristor given as
Find the average power loss in the thyristor and its rms current 3. Derive the expression for output voltage of a single phase two pulse two pulse full wave AC to DC power convertor feeding RLE type of load. Sketch a suitable converter configuration and output and load current waveform. 4. A slab of insulating material 0.015 m2 in area and 0.01 m in thickness has relative permittivity of 5 and power factor of 0.05 and is to be heated by dielectric heating. The power required is 400 W at 30 MHz. Determine the voltage required and the resulting current that will flow through the material. If the voltage were to be limited to 650 V, what would be the value of frequency for the same power requirement ? 5. What are di/dt and dv/dt ratings of SCR ? How the device can be protected against di/dt and dv/dt ? 6. What is the voltage regulator ? Draw a neat schematic for regulated power supply using serial regulator with negative output w.r.t. ground. Section-C Marks : 10 Each 7. (a) Explain the following with reference to operations of inverters : (i) Output voltage control (ii) Harmonic elimination. (b) Discuss a single-phase to single-phase bridge type step down cyclo-converter supplying RL type load. Draw the output voltage and load current waveforms. Indicate clearly the type of commutation used in circuit and indicate the conduction of various devices. 8. (a) What are the constituents of industrial Timers? Give the schematic of SCR delay timer and discuss. (b) Draw a block diagram of Electronic control unit with devices used as a switch (SCR) for the control of DC shunt motor. Explain all the components of block diagram. 9. (a) What is induction heating ? How does the frequency affects the power requrements for induction heating ? With a neat sketch explain how surface heating of a small cylinderically iron rod may be implemented electrically. (b) Discuss electronic universal motor control for all types of operations of motor. |
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