The magnetic circuit of an axial field D.C. electrical machine

A permanent magnet d.c. machine is described, with an axial field and a disc-shaped armature. The potential distribution is calculated in three-dimensions for high-coercivity magnets, for which a uniform intrinsic magnetisation can be assumed. This shows the importance of not assuming a constant potential over the pole surface, and of not reducing the system to two-dimensions. The field in the air-gap is found, and a comparison of magnet shapes shows that commonly used circular poles produce poor values of useful field. A compacted mixture of iron powder and epoxy resin has been found to be a good substitute for laminations in the magnetic circuit. It is shown that this material carries about the same flux in the machine as does mild steel. However, it offers a significant reduction in the eddy-current and hysteresis losses, compared with laminated nickel iron.