Pulse-Width Modulation Inverters

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Delilah Simmons
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1 SECTION 3.6 INVERTERS 189 Pulse-Widh Modulaion Inverers Pulse-widh modulaion is he process of modifying he widh of he pulses in a pulse rain in direc proporion o a small conrol signal; he greaer he conrol volage, he wider he resuling pulses become. By using a sinusoid of he desired frequency as he conrol volage for a PWM circui, i is possible o produce a high-power waveform whose average volage varies sinusoidally in a manner suiable for driving ac moors. The basic conceps of pulse-widh modulaion are illusraed in Figure 355. Figure 355a shows a single-phase PWM inverer circui using IGBTs. The saes of IGBT 1 hrough IGBT 4 in his circui are conrolled by he wo comparaors shown in Figure 355b. Acomparaoris a device ha compares he inpu volage v in () o a reference signal and urns ransisors on or off depending on he resuls of he es. Comparaor A compares v in () o he reference volage v x () and conrols IGBTs T 1 and T 2 based on he resuls of he comparison. Comparaor B compares v in () o he reference volage v y () and conrols IGBTs T 3 and T 4 based on he resuls of he comparison. If v in () is greaer han v x () a any given ime, hen comparaor A will urn on T 1 and urn off T 2. Oherwise, i will urn off T 1 and urn on T 2. Similarly, if v in () is greaer han v y () aany given ime, hen comparaor B will urn off T 3 and urn on T 4. Oherwise, i will urn on T 3 and urn off T 4. The reference volages v x () and v y () are shown in Figure 355c. L S Three- phase inverer Recifier 2 C T 1 T 2 T 3 D 1 D 2 D 3 a b c 2 T 1 T 5 T 6 D 4 D 5 D 6 (a) FIGURE 354 (a) Ahree-phase volage source inverer using power ransisors.

volage, he wider he resuling pulses become.

2 190 CHAPTER 3 INTRODUCTION TO POWER ELECTRONICS T 1 T 4 T 1 T 5 T 2 T 5 T 2 T 3 T 6 T 3 T 6 v a () VS v b () VS v c () VS v ab () v bc () v ca () (b) FIGURE 354 (concluded) (b) The oupu phase and line volages from he inverer.

VS v ab () 2 0 2 v bc () 2 0 2 v ca () 2 0 2 (b) FIGURE 354

3 SECTION 3.6 INVERTERS 191 v B1 T 1 v T B3 3 v load () Load v B2 T 2 v u () v v () v B4 T 4 FIGURE 355 The basic conceps of pulse-widh modulaion. (a) A single-phase PWM circui using IGBTs. To undersand he overall operaion of his PWM inverer circui, see wha happens when differen conrol volages are applied o i. Firs, assume ha he conrol volage is 0 V. Then volages v u () and v v () are idenical, and he load volage ou of he circui v load () is zero (see Figure 356). Nex, assume ha a consan posiive conrol volage equal o one-half of he peak reference volage is applied o he circui. The resuling oupu volage is a rain of pulses wih a 50 percen duy cycle, as shown in Figure 357. Finally, assume ha a sinusoidal conrol volage is applied o he circui as shown in Figure 358. The widh of he resuling pulse rain varies sinusoidally wih he conrol volage. The resul is a high-power oupu waveform whose average volage over any small region is direcly proporional o he average volage of he conrol signal in ha region. The fundamenal frequency of he oupu waveform is he same as he frequency of he inpu conrol volage. Of course, here are harmonic componens in he oupu volage, bu hey are no usually a concern in moor-conrol applicaions. The harmonic componens may cause addiional heaing in he moor being driven by he inverer, bu he exra heaing can be compensaed for eiher by buying a specially designed moor or by deraing an ordinary moor (running i a less han is full raed power). A complee hree-phase PWM inverer would consis of hree of he single-phase inverers described above wih conrol volages consising of sinusoids shifed by 120 beween phases. Frequency conrol in a PWM inverer of his sor is accomplished by changing he frequency of he inpu conrol volage.

Then volages v u () and v v () are idenical, and he load volage ou of he circui v load () is zero (see Figure 356).

4 192 CHAPTER 3 INTRODUCTION TO POWER ELECTRONICS v in () Comparaor A v B1 () v B1 v B2 v x () v in > v x v in < v x v B2 () Comparaor B v B3 () v B3 v B4 v y () v in > v y v in < v y v B4 () (b) v x () v y () (c) FIGURE 355 (concluded) (b) The comparaors used o conrol he on and off saes of he ransisors. (c) The reference volages used in he comparaors. A PWM inverer swiches saes many imes during a single cycle of he resuling oupu volage. A he ime of his wriing, reference volages wih frequencies as high as 12 khz are used in PWM inverer designs, so he componens in a PWM inverer mus change saes up o 24,000 imes per second. This rapid swiching means ha PWM inverers require faser componens han CSIs or VSIs. PWM inverers need high-power high-frequency componens such as GTO hyrisors, IGBTs, and/or power ransisors for proper operaion. (A he ime of his wriing, IGBTs have he advanage for high-speed, high-power swiching, so hey are he preferred componen for building PWM inverers.) The conrol volage fed o he comparaor circuis is usually imple-

A PWM inverer swiches saes many imes during a single cycle of he resuling oupu volage.

5 SECTION 3.7 CYCLOCONVERTERS 193 v x () v y () v in = 0 v u () v v () v load () V load = v v v u v load () = 0 FIGURE 356 The oupu of he PWM circui wih an inpu volage of 0 V. Noe ha v u () = v v (), so v load () = 0. mened digially by means of a microcompuer mouned on a circui board wihin he PWM moor conroller. The conrol volage (and herefore he oupu pulse widh) can be conrolled by he microcompuer in a manner much more sophisicaed han ha described here. I is possible for he microcompuer o vary he conrol volage o

PWM circui wih an inpu volage of 0 V. Noe ha v u () = v v (), so v load () = 0.

6 194 CHAPTER 3 INTRODUCTION TO POWER ELECTRONICS v x () v y () v in v u () v v () v load () V load = v v v u FIGURE 357 The oupu of he PWM circui wih an inpu volage equal o one-half of he peak comparaor volage. achieve differen frequencies and volage levels in any desired manner. For example, he microcompuer could implemen various acceleraion and deceleraion ramps, curren limis, and volage-versus-frequency curves by simply changing opions in sofware. A real PWM-based inducion moor drive circui is described in Secion 7.10.

achieve differen frequencies and volage levels in any desired manner.

7 v u () v v () v load () v y () v x () v in () FIGURE 358 The oupu of he PWM circui wih a sinusoidal conrol volage applied o is inpu. 195

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