Mosfet Bridge - Power Electronics - Past Exam Paper, Exams of Power Electronics

Download Mosfet Bridge - 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 (National Diploma in Engineering in Electrical Engineering — Award) (NFQ — Level 7) Autumn 2005 Power Electronics (Time: 3 Hours) Answer FIVE questions. All questions carry equal marks. Mr. K. O'Connell Mr. M. Ahern Mr. D. A. Collins 1. (a) Explain briefly the principle of phase control for regulating the intensity of theatre lighting and state the main drawbacks of the technique. [8] (b) An AC converter comprising a phase-controlled triac is connected between a 230 V, 50 Hz supply and a heating element with a resistance of 25 Ω. Calculate the RMS output voltage, RMS output current and output power for the following angles… (i) 120º, (ii) 90º and (iii) 60º. [12] 2. (a) A step-down converter connects a 72 V battery to the armature of a separately excited DC motor having an inductance of 9 mH and resistance of 0.09 Ω. Sketch the circuit diagram for this scheme. [4] (b) The converter operates at a frequency of 200 Hz and is set with a control ratio of (i) 0.4 and (ii) 0.6. For each case, sketch the voltage and current waveforms and calculate the mean output voltage, output current and current ripple. [12] (c) Estimate the control ratio at which the output current becomes discontinuous. [4] 2 3. (a) Explain why a three phase bridge rectifier is preferable to a three phase half-wave rectifier for industrial applications. [6] (b) A three phase thyristor bridge converter has an input line voltage of 400 V and supplies a load having a mean direct voltage of 400 V and a level current of 150 A. Sketch the circuit diagram for this scheme and calculate the following… (i) the firing angle for the thyristors; (ii) the reactive power of the converter. [8] (c) If the source inductance of 20 mH per phase is taken into account, state what effect this will have on commutation and estimate the effect on the output voltage. [6] 4. (a) Sketch the circuit diagram for a single phase MOSFET bridge inverter and indicate the approximate maximum output voltage, current and frequency for the circuit. [6] (b) For the inverter in (a) and an RL load, sketch the waveform of the output voltage and current, indicating clearly the devices acting at each stage [6] (c) A single phase bridge inverter has a quasi-square wave output with a peak voltage of 360 V and a conduction angle of 120º. Calculate the RMS value of the fundamental, third, fifth and seventh harmonics in the output. [10] 5. (a) Explain what is meant by transient thermal impedance of a power electronic device and indicate how it is useful in selecting a device for a particular application. [6] (b) Figure 1 shows the transient thermal impedance characteristic for a particular semiconductor device. If that device has an on-state volt drop of 2.2 V and is subjected to current pulses with a peak value of 500 A, an on-time of 10 ms and an off-time of 10 ms, calculate the temperature rise at the end of two cycles. [10] (c) For the same device, calculate the maximum continuous current that can be tolerated assuming the device is made of silicon and the ambient temperature is 40ºC. [4]