Experience with using soft magnetic composites for electrical machines

The genesis of the work reported is soft magnetic materials formed by compacting and bonding iron powder (plus possibly alloys). The powder particles are (weakly) insulated one from another and hence eddy currents are restricted and good AC performance is achieved. They should be separated from iron powder compacted parts which are subsequently sintered. This process results in contact between the powder grains and hence very poor AC properties. The bonded material has become known as a “soft magnetic composite” or SMC. The nature of the material and the way that it is formed gives new design freedoms. Of particular importance is the fact that the material is isotropic and complex parts can be formed with good tolerance and surface finish. The compaction process cannot achieve full density and hence permeability is nowhere near as good as laminated steel, a figure of 500 for maximum relative permeability is good. This makes the material relatively better for machines with low demand on permeability for example, permanent magnet, universal (and DC) and synchronous machines. It is more difficult to achieve good results for machines with a high demand on permeability-induction, reluctance and stepping motors fall into this category