SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

Motors can produce torque using two independent principles: the first is to use two interacting
magnetic fields and the second is to use one magnetic field and a magnetic circuit whose
reluctance is a function of rotor position. Some motors use only the first principle, some use only
the second, and some motors use a combination of the two. When discussing torque production
(and hence, motor control) it is necessary to make these distinctions.
Some observations about common existing motors are useful to learn how to categorize them
according to saliency. When torque is produced only by the interaction of two fields (called
mutual torque), one field is produced by the stator and the other is produced by the rotor; these
machines are called non-salient machines. When torque is produced only by the interaction of
one field and a variable reluctance element (called reluctance torque), that field is produced by
the stator; these machines are called reluctance machines. When a motor uses a combination of
these principles (producing both mutual and reluctance torque) it is called a salient-pole machine.
For salient-pole machines an additional observation is that the rotor is always salient and the
stator may or may not be salient. If only the rotor is salient it is called a singly-salient machine;
when both the rotor and stator are salient it is called a doubly-salient machine.
To summarize, non-salient machines produce only mutual torque, reluctance machines produce
only reluctance torque, and the torque produced by salient-pole machines has both mutual and
reluctance components. Induction motors are non-salient; synchronous machines may be salient
or non-salient (this is true for both wound-field and PM types); a salient machine generally has a
salient rotor; and the only common doubly-salient machine is the switched reluctance motor. In
the literature, a cylindrical rotor 1 is one that is non-salient, smooth bore describes a non-salient
stator, and smooth airgap describes a non-salient machine (rotor and stator).
DC motors have been excluded from this discussion because of the concentration on synchronous
machines. There is indeed a difference in saliency between the wound-field and PM types and
this saliency does affect operation, although the nature is different because the saliency is in the
stator field poles. Many of these aspects have direct parallels to AC machines but cannot be
included here. Something else to note is that slotting has been ignored in this discussion. If
slotting is considered it is clear that it contributes to saliency because flux paths through the teeth
will have lower reluctance than those through the winding slots. One must question where to
1 Cylindrical rotor is found in the literature to mean a non-salient rotor but is also used to refer to a radialflux
machine (as opposed to an axial-flux machine, which has a flat “pancake” rotor).
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