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SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

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Figure 3.7 – Developed view at zero electrical degrees.<br />

As the reference line in Figure 3.6 moves forward in electrical angular position, the current in<br />

each phase will increase or decrease and the component MMF of that phase will increase or<br />

decrease proportionally. The component MMF of each phase maintains a cosinusoidal shape in<br />

space—that is, with respect to the stationary stator conductors. As the current in a phase varies<br />

cosinusoidally this space distribution does not change in shape. Rather, the magnitude increases<br />

and decreases cosinusoidally with time; that is, the amplitude of the cosinusoidal space<br />

distribution is modulated cosinusoidally in time. For any given electrical angle, the total MMF<br />

will also be cosinusoidally distributed in space (it is a general property that when two or more<br />

cosine waves of the same period are added, the result is another cosine wave of the same period).<br />

Further, as the component MMFs (with their fixed spatial axes) increase and decrease, the peak of<br />

this total cosinusoid will move in space. Figure 3.8 is the same as Figure 3.7 except the electrical<br />

reference line in Figure 3.6 has been moved to 30<br />

<br />

. Here, I 3/2,<br />

I 0 , and<br />

I 3/2.<br />

Again compare (the instantaneous MMF of each phase with the maximum<br />

A<br />

possible) in conjunction with the values of current at this instant.<br />

A<br />

B<br />

69

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