Power Electronics (I) ELEC 433/6411 Midterm ALL ANSWERS CORRECT

COURSE : Power Electronics (I) NUMBER: ELEC 433/6411 EXAMINATION : Midterm DATE : October 22, 2018 TIME : 14:45 - 16:15 # OF PAGES : 2 INSTRUCTOR: Luiz Lopes INSTRUCTIONS: - 1) Crib sheets and programmable calculators are not allowed. 2) If you have a difficulty, you may try making REASONABLE assumptions. State the assumption and how that assumption limits your answers. STUDENT: ID # 1) A single-phase 240V/60 Hz source supplies two loads. Load #1 draws 2.4 kVA with a power factor of 0.8 lagging while Load #2 draws 1.44 kW with a power factor of 0.6 leading. A) compute the apparent power supplied by the source. B) Draw the phasor diagram of the system showing: i) source voltage; ii) current in Load#1; iii) current in Load#2 and iv) current supplied by the source. Note: The magnitude and phase of the phasors must be shown. (10 marks). 2) Consider a three-phase thyristor-based AC-DC converter supplying an RL load with a resistance of 4 Ω and inductance of 16 mH. The grid voltage is 220VLL/60Hz. For a firing angle of 30°, compute: A) The average value of the load current. B) The magnitude of the dominant (lowest order) current harmonic at the DC side. For the following calculations, assume that the load current can be represented by its DC value. C) The reactive power absorbed by the converter. D) The distortion current in the AC side considering the four lowest order harmonics. (20 marks). This study source was downloaded by from CourseH on :27:21 GMT -05:00 3) A semi-controlled (thyristor-diode hybrid) AC-DC converter is shown below. Assume that the thyristors are fired with the same firing angle (α) and that the load is “highly inductive”. A) Explain how the converter operates, that is, when each semiconductor turns ON and OFF. B) Draw 2 line cycles of the following waveforms for α = 45°: i) Source voltage, ii) load voltage, iii) load current and iv) source current. C) Derive an expression for the average voltage across the load as a function of the rms value of the source voltage (Vs) and firing angle of the thryristor (α). (15 marks). D1 D2 THY1 THY2 + Vs - + Vd - Is Id 4) Considering the circuit below