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SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

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Figure D.11 – 3-D base vectors of 180° inverter.<br />

In inverter state S0 all legs are low and the neutral is at VDC<br />

; in S7 all legs are high and the<br />

neutral is at VDC<br />

. In S1, A is high, B and C are low, and the neutral voltage is therefore<br />

(1 / 3) VDC<br />

. Since the pole voltage of phase-A is 1.0V DC , the line-neutral voltage is<br />

1 ( 1/3) V (4/3) V . Figure D.10 had already shown that fact but as discussed in the<br />

DC DC<br />

inverter chapter, the SV transform of either the pole or phase voltage gives the same result<br />

(namely, Figure D.10). Figure D.11 shows graphically what was explained in the inverter<br />

chapter—that the SV (Figure D.10) could not capture the ZS component (which is shown in<br />

Figure D.11). It is known that in the active states the neutral voltage alternates between<br />

(1 / 3) VDC<br />

. This means the 3-D vector would alternately be (1 / 3) V DC below and above the αβ<br />

plane; this is evident in Figure D.11. Visualizing the ZS component in this way proves that the<br />

projection of the ABC phase variables onto the αβ plane is unique only when no ZS component is<br />

present. Likewise it clearly shows that unless the αβ0 transform is used, the ZS component will<br />

be lost when transforming back.<br />

Finally, a brief warning will be issued. It has been shown that the αβ0 transform is simply the<br />

Clarke transform (Equation D.26), augmented with terms for the ZS component. If we are<br />

working with a system in which no ZS component can possibly be present (such as current in an<br />

327

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