SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Figure 5.23 – Field weakening controller.
In summary, it has been shown that for a nonsalient rotor, torque control consists of setting
id 0 and controlling the torque according to T KT iq.
For a salient rotor both i d and i q
must be controlled. Separate control of the “torque-producing” and “flux-producing” components
of the stator current is possible using the FOC control structure, and this may be used to achieve
flux weakening. However, for a salient rotor the torque control and the field-weakening control
are not independent of one another. In addition to these fundamental ideas, several have been
overlooked and the subject of flux weakening is far more complicated than presented here.
Indeed, many of the details and interactions are beyond the author’s present understanding.
First, it seems that the control of the two components of airgap flux is coupled in a synchronous
machine (see [78, ch.5]) and it appears that this is the same as armature reaction flux [78, pp.249-
250]. Second, as hinted by the key phrase above, “to the extent that the stator MMF creates flux
in the airgap,” the stator MMF is not always capable of truly weakening the magnet flux. This is
because the magnet has a relative permeability of near unity, thus the airgap is effectively very
large [78, pp.250], [87, p.96], [68, p.103]; in salient machines the effect is more pronounced [69,
p. 4.18]. This also has something to do with the discussion of internal permeance of the magnet,
as discussed in Chapter 2, and it is the same as saying the armature reaction is small or that the
synchronous reactance/inductance (Appendix B) is small. The degree to which field weakening
can occur thus depends on the motor’s airgap, magnets, and saliency.
If a BPMS motor does not experience flux weakening with a negative d-component of stator
current, the question arises as to what happens when the stator current SV is advanced like that. It
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