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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICHNEOLAÍOCHT CHORCAÍ
Semester 1 Examinations 2009/
Module title: Power Conversion
Module: ELEC 7013
School: School of Electrical & Electronic Engineering
Programme title: Bachelor of Engineering in Electrical Engineering — Award
Programme Code EELEC_7_Y
External Examiner(s): Mr. G. Beecher, Dr. M. Duffy
Internal Examiner(s): Mr. D. Collins
Instructions: Answer any four questions. All questions carry equal marks.
State any assumption made.
Duration: 2 hours
Sitting: Winter 2009
Requirements for this examination:
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt, please contact an Invigilator.
Q1. (a) Sketch a circuit diagram for a 12-pulse rectifier and explain why this technique is often used for producing a direct current supply in applications of significant rating. (15 marks) (b) The power supply for a light rail system is 750 V DC and is to be provided by a 12-pulse rectifier and related equipment. If the input supply is 400 V AC and device volt drops are neglected, estimate the transformer turns ratio(s). (10 marks)
Q2. (a) A battery charging voltage of 120 V is furnished by way of a delta-star transformer, a three phase diode bridge rectifier and a DC converter having an adjustable control ratio (duty cycle). Sketch the circuit diagram for this scheme. (8 marks) (b) If the secondary phase voltage of the transformer is 66 V estimate the input voltage to the DC converter and the required control ratio for battery charging. (8 marks) (c) If the mean charging current is 40 A, the switching frequency is 10 kHz and the ripple current is 4 A, what inductance should be inserted in the charging circuit? (9 marks)
Q3. (a) Compare the IGBT and MOSFET for power electronic applications. (9 marks)
(b) Figure 1 contains extracts from a manufacturer’s data sheet relating to an IGBT module. Using the data and calculations, determine the following… (i) the device volt drop for I C = 100 A and V GE = 15 V; (ii) the conduction power loss for a continuous I C = 100 A and V GE = 15 V; (iii) the switching loss for these main circuit ratings: 600 V, 100 A, 10 kHz. (iv) the junction temperature for 200 W loss and a heat sink R th = 0.23 K/W (16 marks)
Q4. (a) Outline the negative impact on power quality of producing a DC supply using a single phase rectifier and filter capacitor. (10 marks) (b) Describe the action of a power factor correction scheme using a step-up (boost) converter that overcomes the drawbacks of (a). Support your answer with a circuit diagram and waveforms. (15 marks) Q5. (a) Explain briefly the principle of phase control for regulating the intensity of theatre lighting and state the main drawbacks of the technique. (10 marks)
(b) An AC converter comprising a phase-controlled triac is connected between a 230 V, 50 Hz supply and a resistive load of 50 Ω. Calculate the RMS output voltage, RMS output current and output power for the following triac angles… (i) 120º, (ii) 90º and (iii) 60º. (15 marks)
