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SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

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Table 4.3; there are no ZS terms present. There are a number of conclusions to be drawn, but the<br />

simplest is to recognize that by using the Clarke or SV transform, we are ignoring the ZS<br />

component, therefore it does not matter whether we transform the pole or phase voltages because<br />

they differ only by the ZS component.<br />

Table 4.3 – Line-neutral voltages expressed in complex components.<br />

Inverter voltage components<br />

state α β 0<br />

<br />

S0<br />

0 0 0<br />

<br />

S1<br />

4/3 0 0<br />

<br />

S2<br />

2/3 2/ 3 0<br />

<br />

S3<br />

2/3 2/ 3 0<br />

<br />

S4<br />

4/3 0 0<br />

<br />

S5<br />

2/3 2/<br />

3 0<br />

<br />

S 6 2/3 2/<br />

3 0<br />

<br />

S7<br />

0 0 0<br />

In six-step squarewave operation (at a constant frequency) the inverter spends an equal amount of<br />

time in each of the states. The projection onto the α- and β- axes is plotted against time in Figure<br />

4.27. Taking the inverse αβ0 transform of the waveforms in Figure 4.27 yields exactly the pole<br />

voltage waveforms in Figure 4.23. Taking the inverse Clarke transform (i.e., the inverse αβ0<br />

transform with empty 0-components) of the waveforms in Figure 4.27 yields exactly the phase<br />

voltage waveforms in Figure 4.23. If the reader is uncertain as to why this is, consult Appendix D<br />

before re-reading this subsection.<br />

184

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