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SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

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voltage will be below (above) the bus midpoint by (1 / 3) V DC . In the null states the neutral<br />

voltage will either be high or low V or V <br />

. This is summarized in Figure 4.21.<br />

DC DC<br />

Figure 4.21 – Inverter states and neutral voltages.<br />

The nonzero neutral voltage can be viewed from a bus perspective as being the result of voltage-<br />

divider action, or it can be viewed from the load neutral perspective as being the result of<br />

averaging the three terminal voltages. The latter viewpoint will be adopted because it emphasizes<br />

the fact that the neutral voltage is the common-mode voltage as shown by Equation (4.4).<br />

1<br />

vNM vAM vBM vCM<br />

<br />

(4.4)<br />

3<br />

Recall the voltage relationships for an isolated neutral load connected to a three-phase source.<br />

This is given by Equation (4.5) and is summarized in Figure 4.22 (only phase-A is shown for<br />

clarity).<br />

vAM vAN vNM vAN vAM vNM<br />

(4.5)<br />

Figure 4.22 – Voltage relationships.<br />

179

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