Combined radial and axial permanent magnet motors using soft magnetic composites

This paper is concerned with reporting the design, construction and testing of a permanent magnet machine which has both axial and radial magnets and is the drive motor for an electric bicycle. The armature of a machine with multiple axes of magnetisation must be capable of carrying alternating flux in all three coordinate directions. Bonded iron powder has isotropic properties and its use for the core makes the motor possible. The result is that a very large magnet surface area is possible within the machine envelope, leading to high specific output per unit volume or weight. Bonded iron powder has been used as a core material for a very long time but it is only recently that magnetic properties have improved to the extent that the inherent advantages are not outweighed by the inferior magnetic properties (in comparison to laminations). The improvement has come about by thinner and higher temperature powder coatings, improved compaction and improved post compaction heat treatment. The paper is mainly concerned with reporting the motor, but the bicycle application also imposes specific design constraints. The concept has far wider application however and an attempt is made to set the motor's design in a wider reference frame.