RMS Output Voltage - Power Electronics - Past Exam Paper, Exams of Power Electronics

Download RMS Output Voltage - 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) Autumn 2007 Power Electronics (Time: 3 Hours) Answer FIVE questions. All questions carry equal marks. Book of formulae supplied 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 25 Ω. If the thyristors are triggered using phase control with a firing angle of 40º, (i) sketch the waveform for one full cycle and calculate… (ii) the RMS output voltage; (iii) the output power; (iv) the input power factor. [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 80 V input and a 100 V output. If the mean load current is 5 A and the switching frequency is 30 kHz, calculate... (i) the required control ratio (duty cycle) (ii) the input inductance for a current ripple of 0.25 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 15 kV DC source having an internal resistance of 1 Ω supplies a (smooth) current of 800 A to a 11 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] 2 4. (a) With the help of a circuit diagram and switching waveforms for one cycle, describe the action of a single phase voltage source H-bridge inverter supplying an RL load with a square wave output. [10] (b) Explain how the output frequency and voltage of the inverter in (a) is varied. [5] (c) An inverter has a quasi-square wave output voltage with a peak value of 200 V, a frequency of 2 kHz and a conduction angle of 90° in each half cycle. Sketch this waveform and determine the RMS value of the output voltage. [5] 5. (a) Briefly explain how power losses arise in power semiconductor devices. [8] (b) A semiconductor device has a steady-state power loss of 200 W and a thermal resistance of 0.5°C/W. If the ambient temperature is 40°C, calculate the virtual temperature of the device. How should the scheme be modified in order to limit the virtual temperature to 125ºC with the same power loss? [12] 6. (a) A single diode bridge rectifier is used as a simple battery charger. If the battery voltage is equal to half the peak input voltage of the rectifier, and there is no output inductance, sketch the output voltage and current waveforms. [10] (b) A 24 V battery has an internal resistance of 2 Ω and is charged from a single phase diode bridge rectifier having an RMS input voltage of 24 V. Neglecting diode volt drop, calculate the mean charging current and charging power. [10] 7. (a) A speed regulator for a three phase induction motor comprises a single phase diode bridge rectifier, a reservoir capacitor and a three phase IGBT inverter. Provision is also made for controlled switching of a braking resistor. Sketch the circuit diagram for this scheme and briefly describe its action. [10] (b) Assuming a PWM switching strategy, sketch the motor voltage and current waveforms for the following conditions… (i) 1 × rated speed (ii) 0.5 × rated speed [10]