Permanent magnet electric motor having reduced cogging torque

DETAILED DESCRIPTION It is an object of the present invention to provide a permanent magnet motor having a reduced or eliminated cogging torque while having a cost and complexity similar to conventional motors.
A permanent magnet motor is provided comprising a rotor rotatable about a central motor axis and a stator armature supporting the motor windings disposed coaxially with the rotor and separated from the rotor by a circumferential air gap.
The rotor carries a plurality of permanent magnet elements arranged to provide an angular distribution of alternating magnetic polarity about the rotor.
The stator armature has a number of slots formed therein facing the air gap and disposed at equally spaced intervals about the stator as in conventional practice, the slots house the motor windings.
In accordance with one aspect of the present invention, the permanent magnet elements are shaped and arranged such that the distribution of the density of magnetic energy that is stored in the air gap in the absence of stator slots, as a function of a linear coordinate taken along a circumference disposed within the air gap and coaxial to the motor axis, exhibits an increasing transition in magnitude in a first angular position near a first end of a magnetic polarity and a decreasing transition in magnitude in a second angular position near a second end of the magnetic polarity, the transitions being asymmetrical with respect to each other and the density of magnet energy being substantially constant between consecutive transitions.
Additionally, the slots of the stator are disposed in positions whereby during operation, when a first slot is in a first angular position corresponding to the first angular position of the increasing transition in magnitude of the distribution of magnetic energy density, a second slot is in a second angular position that corresponds to the second angular position of the decreasing transition in magnitude of the distribution of magnetic energy density, each slot functioning to modify the distribution of the density of magnetic energy in the air gap according to a modulation function which has a local development symmetrical to a radial plane passing through the center of the slot