Electric Machine Goodness: Classic results revisited in light of contemporary machine design

Common understanding among machine designers is that, when scaling down to smaller sizes, most performance parameters will degrade. This principle is embodied in the goodness factor presented by Laithwaite. A derivation of the goodness factor for axial flux geometry is presented, confirming that the result is the same as for radial flux geometry. The paper then discusses the factor with respect to permanent magnet machines, where experience shows that the principle does not hold, in that small machines can have very good characteristics. An initial modification of the factor to cover permanent magnet machines is presented.