An overview of the compactness of a range of axial flux PM machines

This paper begins with a review of the famous "goodness" of electrical machines concept published in 1965 by Laithwaite. Its correctness when applied to induction machines is demonstrated using the metric of torque density, for which we use the word "compactness", measured in Nm/m³. Technical advances in the latter half of the 20th century are discussed, as are properties of axial flux geometry, that have had a significant impact on machine volume. A range of axial flux permanent magnet (PM) machines, some of which are currently in production by Regal Beloit Corporation are presented, studying in particular their compactness. The paper concludes with a discussion of why larger machines can indeed demonstrate an increase in compactness, although much less dramatically than that proposed by Laithwaite, using the air gap shear stress concept. This discussion includes the difference of this relationship for radial and axial flux PM machines.