POKHARA UNIVERSITY

Level: Bachelor Semester – Spring Year : 2010Programme: BE Full Marks: 100Course: Power Electronics Pass Marks: 45

Time : 3hrs.

Candidates are required to give their answers in their own words as far as practicable.The figures in the margin indicate full marks.Attempt all the questions.

1. a) Explain, with the help of a circuit diagram, the pulse triggering of an SCR using UJT based oscillator.

b) A single phase bridge rectifier employing two SCRs and two diodes is fed from an ac source of 230 V(rms), 50Hz. The rectifier is connected to an R-L load. The load has R=10Ω and L sufficiently large so as to render a constant load current. For the firing angle of 30o, draw the waveforms for

i. The output voltage across the load terminals

ii. The current flowing in each SCR

iii. The current flowing in each diode.

Also calculate the value of the average dc voltage across the load terminals.

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2. a) What is a three-phase semi-converter? A three-phase semi-converter is connected to an R- L load. Deduce an expression for the average dc output voltage across the load terminals. If the firing angle is 60o, draw the waveform of the dc output voltage.

b) What is meant by forced commutation of SCR? Explain Class A commutation technique in detail.

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3. a) What is an inverter? In what respect, is a single phase inverter different from a three-phase inverter?

b) A single-phase full-bridge inverter produces a square wave of 100 V (peak-to-peak) and the frequency of the output voltage is 50 Hz .If the inverter is connected to an R-L load having R=10 Ω and L =50 mH,find the peak value of the load current, considering up to ninth harmonics.

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4. a) A dc shunt motor is to be operated in two quadrants. Draw a circuit using converters for it and explain the working of the circuit.

b) The speed of a separately excited dc motor is controlled by using two single phase semi-converters, one for the field circuit and another for the armature circuit. The converter used for the field current continuous. Following data are furnished for the system:

Field resistance =200Ω

Armature resistance =0.5Ω

Voltage constant = 0.8 V/A-rad/sec

Torque constant =0.8 N- m/ A2

Input ac supply to the converters = 220 V, 50 Hz.

If the load torque is 50 N-m at 1500 rpm, obtain the field current and the firing angle of the converter in the armature circuit.

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5. a) Draw a circuit for a single phase cycloconverter using bridge circuits and explain the working of the circuit for different output frequencies.

b) Using inverter, draw a circuit to control the speed of a single phase induction motor and explain the working mechanism of the circuit.

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6. a) Name the various types of HVDC links and explain the functions of the converters placed at either end of the HVDC transmission system.

b) Give reasons to the necessity of connecting converters in series and parallel in a HVDC transmission system. What are the factors to be taken in consideration in firing these converters?

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7. Write short notes on any two:

a) Light operated relays and their applications

b) Resistance firing circuit

c) Three phase inverter

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