Electrical isolation of coils in Soft Magnetic Composite applications

Soft Magnetic Composites (SMC) based on iron powder have a strong potential for improving both the performance and the manufacturing of various electromagnetic components and devices, such as inductors, induction heaters and electrical machines. A new casting or moulding method we have developed makes use of a modified composite, termed SM2C, or soft magnetic mouldable composite. Use of it makes it possible to design very complex shapes of magnetic flux conductors and also to achieve a better thermal coupling between electromagnetic components, primarily coils and flux conductors. The latter also creates a new type of problem, concerning the electrical isolation between the coil and the flux conductor. Use of SM2C, increases the demands on the isolation layer, since the molding compound penetrates every single pinhole, potentially creating leak currents or short circuiting. A comprehensive study of suitable coating materials and processes were carried out, an evaluation of the electrical strength of different configurations being made. A model of the wetting of rough surfaces is introduced here and is applied in the test analysis. This model makes it possible to explain why a certain isolation layer can be insufficient, even if the nominal requirements are fulfilled. Multi layer coatings and certain application adjustments are shown to be necessary, in order for the technical demands placed on isolation to be met.