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 RectifiersWe 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 DiodesIf 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. Rvd+ivs+vdiode+ivs, vdvs, vdiodevdiodetFigure 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:
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