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SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

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Taking the αβ0 transform (Appendix D) of the pole voltages in Table 4.1 allows the inverter<br />

states to be written in terms of complex components as shown in Table 4.2, where k c<br />

2/3<br />

is used to achieve magnitude invariance. (The αβ0 transform (rather than the Clarke or SV<br />

transforms) must be used because the three-phase voltages produced by each inverter state<br />

contain a ZS component, as clearly evidenced by the 0-column in Table 4.2.)<br />

Table 4.2 – Inverter states (pole voltages) expressed in complex components; k=c=2/3.<br />

inverter pole voltage components<br />

state α β 0<br />

<br />

S0<br />

0 0 1<br />

<br />

S1<br />

4/3 0 1/3<br />

<br />

S2<br />

2/3 2/ 3 1/3<br />

<br />

S3<br />

2/3 2/ 3 1/3<br />

<br />

S4<br />

4/3 0 1/3<br />

<br />

S5<br />

2/3 2/<br />

3 1/3<br />

<br />

S 6 2/3 2/<br />

3 1/3<br />

<br />

S7<br />

0 0 1<br />

The α- and β- components of the voltages in Table 4.2 are space vectors and may be plotted in the<br />

αβ plane, as shown in Figure 4.26. Since these are the only possible states of the inverter at any<br />

instant, they will be called the base SVs. As usual, any time we work with the SV we are working<br />

with its projection of the three phase variables onto the αβ plane and the ZS component (which is<br />

orthogonal to the αβ plane) thus cannot be seen in the figure.<br />

182

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