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SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

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Figure 3.4 – Developed view showing superimposed MMF distributions of all phases.<br />

Figure 3.4 shows three plots of MMF (one for each phase) superimposed on a common axis.<br />

These are the MMFs that would be produced if each phase were separately excited by a DC<br />

current of unit-amplitude. The MMFs shown are therefore the maximum that each phase can<br />

contribute. 11 In actual operation the component MMFs will sum together and since all three<br />

phases cannot simultaneously carry full positive current, the per-phase components of MMF will<br />

never peak at the same time; the lines are dashed to serve as a reminder of this. The purpose of<br />

this figure is to graphically illustrate the MMF’s sinusoidal distribution in space. Although the<br />

amplitude of the per-phase MMF peak will scale linearly with the current, the spatial distribution<br />

will remain sinusoidal. The per-phase MMF components can be represented by Equation (3.1),<br />

where Ne represents the effective number of winding turns (as defined in Appendix C) and θ<br />

represents the angle around the periphery of the machine, as shown in Figure 3.1.<br />

11 A current of unit-amplitude (1.0[A]) would produce an MMF of amplitude ( /2)<br />

e<br />

N [A-turn]; since this<br />

is the peak value of MMF produced for unit-amplitude current it is called the unit-amplitude value of<br />

MMF, as shown in the figure. Thus “unit-amplitude” does not refer to a numerical value of 1.0. This<br />

terminology is used to emphasize the relationship between current and MMF. In addition, it is not meant to<br />

indicate a value in the “per-unit” measurement system, which is not used in this report.<br />

66

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