chap17 - 17 Line-Commutated Rectifiers Conventional diode...

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17 Line-Commutated Rectifiers Conventional diode peak-detection rectifiers are inexpensive, reliable, and in widespread use. Their shortcomings are the high harmonic content of their ac line currents, and their low power factors. In this chapter, the basic operation and ac line current waveforms of several of the most common single-phase and three-phase diode rectifiers are summarized. Also introduced are phase-controlled three-phase recti- fiers and inverters, and passive harmonic mitigation techniques. Several of the many references in this area are listed at the end of this chapter [1–15]. Rigorous analytical design of line-commutated rectifier and filter circuits is unfeasible for all but the simplest of circuits. Typical peak-detection rectifiers are numerically ill-conditioned, because small changes in the dc-side ripple voltage lead to large changes in the ac line current waveforms. There- fore, the discussions of this chapter are confined to mostly qualitative arguments, with the objective of giving the reader some insight into the physical operation of rectifier/filter circuits. Waveforms, harmonic magnitudes, and power factors are best determined by measurement or computer simulation. 17.1 THE SINGLE-PHASE FULL-WAVE RECTIFIER A single-phase full-wave rectifier, with uncontrolled diode rectifiers, is shown in Fig. 17.1. The circuit includes a dc-side L–C filter. There are two conventional uses for this circuit. In the traditional full-wave rectifier, the output capacitor is large in value, and the dc output voltage v ( t ) has negligible ripple at the second harmonic of the ac line frequency. Inductor L is most often small or absent. Additional small inductance may be in series with the ac source A second conventional use of this circuit is in the low-harmonic rectifiers discussed in the next chapter. In this case, the resistive load is replaced by a dc- dc converter that is controlled such that its power input port obeys Ohm’s law. For the purposes of under- standing the rectifier waveforms, the converter can be modeled by an effective resistance R, as in the cir-
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610 Line-Commutated Rectifiers cuit of Fig. 17.1. In this application, the L–C filter is required to filter the conducted electromagnetic interference (EMI) generated by the converter. The inductor and capacitor element values are typically small in value, and v ( t ) is approximately a rectified sinusoid. More generally, there may be several sec- tions of L–C filter networks, connected to both the dc and ac sides of the diode rectifier, which filter EMI, smooth the dc output voltage, and reduce the ac line current harmonics. The presence of any filter degrades the ac current waveform of the rectifier. With no reactive elements ( L = 0 and C = 0), the rectifier presents a purely resistive load to the ac input. The output volt- age v ( t ) is then a rectified sinusoid, there are no ac line current harmonics, and the power factor is unity. Addition of reactive elements between the rectifier diodes and the load leads in general to ac line current
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This note was uploaded on 01/17/2010 for the course EL 5673 taught by Professor Dariuszczarkowski during the Spring '09 term at NYU Poly.

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chap17 - 17 Line-Commutated Rectifiers Conventional diode...

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