Thyristor Mean Current - Power Electronics - Past Exam Paper, Exams of Power Electronics

Download Thyristor Mean Current - Power Electronics - Past Exam Paper and more Exams Power Electronics in PDF only on Docsity! Cork Institute of Technology Bachelor of Engineering in Electrical Engineering — Award (NFQ — Level 7) Summer 2007 Power Electronics (Time: 3 Hours) Answer FIVE questions. All questions carry equal marks. Prof. E. McQuade Mr. M. Hennessy Mr. D. A. Collins 1. (a) For a phase-controlled direct AC converter, state the effect of delaying the firing angle on the output voltage, power factor and total harmonic distortion. [8] (b) An AC converter comprising a thyristor pair is connected between a 230 V, 50 Hz supply and a resistive load of 15 Ω. If the thyristors are triggered using integral cycle control so that they are ON for 3 cycles and OFF for 7 cycles, (i) sketch the waveform for one full process cycle and calculate… (ii) the RMS output voltage; (iii) the input power factor; (iv) the thyristor mean current. [12] 2. (a) With the help of a circuit diagram and switching waveforms for one cycle, describe the action of a basic step-up (boost) DC converter. [8] (b) The converter at (a) has a 100 V input and a 160 V output. If the mean load current is 10 A and the switching frequency is 40 kHz, calculate... (i) the required control ratio (duty cycle) (ii) the input inductance for a current ripple of 0.5 A (iii) the output capacitance to limit the load voltage ripple to 10% of nominal. [12] 3. (a) With the help of a clear circuit diagram, explain how a three phase fully controlled thyristor converter may be used to transfer power from a DC to an AC supply. [6] (b) A suitably arranged 16 kV DC source having an internal resistance of 1 Ω supplies a (smooth) current of 900 A to a 12 kV three phase supply through a fully controlled thyristor converter. Calculate… (i) the mean and RMS current in each thyristor; (ii) the converter output voltage and firing angle. (iii) the power factor and reactive power of the converter [14]