SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

torque component, sturdily constructed rotors that can operate at very high speed, and some
advantages over non-salient-rotor designs when used in certain sensorless applications.
Magnetic Saliency
Most aspects of machine operation (such as torque production, inductance, and armature reaction)
are influenced by the magnetic structure of the machine. The most general aspect of this structure
is its saliency, which describes the reluctance of the main flux paths in the machine. Before
introducing saliency it is necessary to describe some assumptions used in magnetic modeling
(these will be used throughout the report).
Flux in a motor travels through the rotor, across the airgap, through the stator, then across the
opposite airgap to arrive at the point from which it started. As a particular flux path is traced one
will encounter various media such as air, steel (traditionally called “iron” [33, p.11]), permanent
magnet material, nonferrous winding material (copper, insulation, varnish), and nonferrous
structural material (such as aluminum, polymer, composite). To a certain approximation, all of
the nonferrous material in the motor has a relative permeability of air, which in elementary
machine analysis is taken to be the permeability of free space (μ0). The steel in the motor has a
relative permeability of several thousand. Since everything in the flux path except the steel is
assumed to have a permeability of μ0, often the reluctances of these components are lumped and
considered to be the “effective” airgap reluctance. When this is done the only other permeability
of concern is that of the steel. In basic motor analysis the permeability of the steel is assumed to
be so much larger (sometimes infinitely larger) than μ0 that the reluctance of the steel is
negligible compared to that of the effective airgap. Ignoring leakage, the MMF generated by the
stator winding drives a flux (called the magnetizing flux) through the airgap and the steel. Since
the reluctance of the steel is assumed negligible, the MMF developed across the steel is negligible
and it is therefore ignored in the magnetic circuit. (The electrical analogue is that voltage
produced by a source drives current through the resistance of the supply conductors and the load,
but the supply conductors have a resistance much smaller than the load, thus a negligible voltage
is developed across them and they may generally be ignored.) Further, since there is a negligible
amount of MMF in across the steel it is assumed that the field strength is negligible as well.
Assuming that the airgap is “small” compared to the radius of the rotor the boundary conditions
are then such that it can be assumed that the field is perpendicular to the steel surfaces at the
airgap [27]. There are always nonlinear fringing effects and this assumption is perhaps not a good
one for PM motor analysis since the effective airgap of a PM synchronous machine is much
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