ECE320_Chapter_8 - ECE 320 Energy Conversion and Power...

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Unformatted text preview: ECE 320 Energy Conversion and Power Electronics Spring 2009 Instructor: Tim Hogan (Notes from Prof. Elias Strangas) Chapter 8: Power Electronics (Textbook Chapter 10, and Sections 11.2, 11.3, and reserve book: Power Electronics ) Chapter Objectives As we saw in the last chapter, control over the torque and speed of the motor can be gained through voltage and frequency control to the motor. This can be accomplished by converting the input AC source power to a DC source (rectifying it), then filtering it to reduce harmonics, and finally converting it back to an AC source having the desired frequency and amplitude (inverter). 8.1 Line Controlled Rectifiers We start with a description of how to draw power from a 1-phase or 3-phase system to provide DC to a load. The characteristics of the systems here include that the devices used will turn themselves off (commutate) and that the systems draw reactive power from the loads. 8.1.1 One-Phase and Three-Phase Circuits with Diodes If the source is 1-phase, a diode is used and the load is purely resistive, as shown in Figure 1 then it is a relatively simple configuration. When the source voltage is positive, the current flows through the diode and the voltage of the source equals the voltage of the load. R v d + i v s + v diode + i v s , v d v s , v diode v diode t Figure 1. Simple circuit with diode and resistive load. If the load includes an inductance and a source (such as a battery we wish to charge), as in Figure 2, then the diode will continue to conduct even when the load voltage becomes negative as long as the current is maintained. This comes from the characteristics of the inductor: ( ) ( ) 1 3 3 = = i t i dt v L t L (8.1) Thus, the shaded area A in Figure 2 must equal the shaded area B . v d + i v s + v diode + L + v L E d + 00 00 00 00 0s 5ms 10ms 15ms 20ms 25ms 0V 0V 0V 0s 5ms 10ms 15ms 20ms 25ms 0V 0V 0V 0V 0V 0V 0V 0V 0V Figure 2. Simple circuit with diode and inductive load with voltage source. 8.1.2 One-Phase Full Wave Rectifier More common is a single phase diode bridge rectifier such as shown in Figure 3. The load can be modeled with one of two extremes: either as a constant current source, representing the case of a large v s i E d t v diode t t v L t 1 t 2 t 3 A B inductance that keeps the current through it almost constant, or as a resistor, representing the case of minimum line inductance. We will study the first case with AC and DC side current and voltage waveforms shown in Figure 4 for the ideal case of L s = 0. v s + L + v d C d i s s i d Figure 3. One-phase full wave rectifier. Figure 4. Waveforms for a one-phase full wave rectifier with inductive load. v s I d t i s v d t i d = I d If we analyze these waveforms, the output voltage will have a DC component, V do (where the subscript o represents that this is the ideal case with L s = 0): s s do V V V 9 . 2 2 = (8.2) where V s is the RMS value of the input AC voltage. On the other hand the RMS value of the output voltage will be...
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This note was uploaded on 05/30/2009 for the course ECE ECE320 taught by Professor Timhogan during the Spring '09 term at Michigan State University.

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ECE320_Chapter_8 - ECE 320 Energy Conversion and Power...

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