Transient Thermal Impedance - Power Electronics - Past Exam Paper, Exams of Power Electronics

Download Transient Thermal Impedance - 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) Summer 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) List the advantages of using an a.c. converter with integral cycle control for regulating the power of an electric heater. [4] (b) An a.c. converter comprising a thyristor pair is connected between a 400 V, 50 Hz supply and a heating element with a resistance of 18 Ω. If the thyristors are triggered using integral cycle control so that they are ON for 40 cycles and OFF for 20 cycles, calculate… (i) the RMS output voltage and power; (ii) the mean current in each thyristor; (iii) the RMS current in each thyristor. [8] (c) Figure 1 shows the variation of output power with delay angle for an a.c. converter operated with phase control. Use this graph to estimate the delay angle required to produce the same output power as in part (b); verify your answer by calculation. [8] 2. (a) A basic step-down (buck) converter has a 60 V input, an inductance of 9 mH and serves a resistive load of 3 Ω. Sketch the circuit diagram for this scheme, ensuring that the semiconductor devices are clearly shown. [4] (b) The converter operates at a frequency of 500 Hz and is set with a control ratio of (i) 0.2 and (ii) 0.8. For each case, sketch the voltage and current waveforms and calculate the mean output voltage, output current and current ripple. [12] (c) With the help of a clear sketch, show how the control ratio signal for the converter may be generated. [4] 2 3. (a) An electrolysis plant requires a high current, low voltage DC supply that is derived from a 12-pulse rectifier and associated transformer having a three phase input. Sketch the circuit diagram for a viable scheme and explain its functioning. [8] (b) State clearly why the 12-pulse circuit is favoured in this type of application. [6] (c) For the circuit proposed in part (a) estimate the rectifier input voltage if the output voltage is 50 V. [6] 4. (a) Briefly compare the MOSFET and IGBT for use in inverters. [4] (b) Draw the circuit diagram for an inverter type electric welder and briefly explain its action. State the benefits of using the inverter in this application. [6] (c) A single phase bridge inverter has a quasi-square wave output with a peak voltage of 200 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 how the concept of transient thermal impedance can simplify thermal calculations in power electronics applications. [4] (b) Figure 2 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 carries three identical current pulses with a peak value of 1000 A, an on-time of 10 ms and an off-time of 10 ms, calculate the temperature rise throughout the cycle. [12] (c) If the ambient temperature is 40ºC, can the device (silicon) tolerate this duty? [4] 6. (a) An inverter has an output voltage represented by the expression… u = 325sinω t +108sin(3ω t) + 65sin(5ω t) This voltage is applied to a series circuit comprising a resistance of 10 Ω and an inductance of 25 mH. Derive an expression for the circuit current if the fundamental frequency is 50 Hz. [14] (b) Calculate the total harmonic distortion of the load current. [6] 7. (a) Draw the main circuit diagram for a separately excited DC motor, the armature of which is supplied by a single phase four-quadrant thyristor converter and the field of which is supplied by a diode bridge rectifier. [10] (b) The motor described in (a) is provided with closed loop speed control having armature current override. Sketch the control loop for this scheme and briefly explain how it functions. [10]