The Paradox of Flat Block Magnets in Small Motor Design

In many small permanent magnet motors the conventional arc-shaped ferrite magnets can be replaced with flat blocks of high energy ferrites at a substantial economic advantage and with an improvement in motor performance.

Confining itself to magnetic field design only for a given armature, this paper shows by rigorous calculation and illustrations the relative effects of varying the design parameters of air gap length, magnet thickness, pole arc angle, magnet length, and magnet grade characteristics.

Inherent with flat block magnet field configurations is a longer air gap length at the pole tips. This reduces magnetic cogging and contributes to reduction in pole tip demagnetization due to armature reaction. In addition, the higher energy product of fully oriented flat ferrites relative to the peak energy product of most arcuate segments can result in a significantly lower volume of magnet material required.