Overview of SMC characterization for their adoption in electromechanical systems

Electrical machines designers have shown in the last decade an increasing interest towards new materials that could represent a valid alternative to those commonly adopted in the realization of magnetic circuits. It is the case of parts or components realized with magnetic powders in place of laminated steels, or bonded magnets in place of sintered ones. The focus of the paper is about the realization of magnetic circuits by means of the powder metallurgy technique, adopting the so called “Soft Magnetic Composites”, or SMC, opening to the possibility of realizing by compression complex shapes otherwise impossible to be even considered. The impact of these new materials and their increasing adoption in the electrical machines scenario, has put in evidence the necessity of analyzing their realization process and providing the characterization of the obtained SMCs. The paper will first of all face the problem of describing the influence of the production process parameters, starting from the selection of the possible and more suitable binders, their percentage, the mold pressure, the curing temperature. Then the attention will move on the analysis of all the tests required to get a complete information of the obtained material. In particular, the characterization regards the magnetic behavior and the energetic performance, together with measurements of the conductivity, and the evaluation of the mechanical characteristics. The magnetic and energetic characterization, realized with the “transformer approach”, requires toroidal specimens with two coils wound on the magnetic structure to provide the magnetizing field and to detect the resulting effect. Considering that these materials will be part of devices always more subjected to power electronic supplies, the tests must be conducted in a wide frequency range (55000 Hz) and with different waveforms, in particular the sinusoidal and the triangular wave. The resulting information is a complete characterization of whatever SMC material under the point of view of the B(H) curve, the hysteresis cycle, the magnetic permeability and the specific losses. The analysis of the cited quantities can be performed as a post process activity, providing interesting results regarding the relationship of specific behaviors as a function of the supply frequency. In a world always more and more oriented to energy-saving procedures, significant importance will be addressed to the dissipation aspects, and the analysis of the losses separation should become a primary parameter to be considered in the choice of the most suitable material. A novel thermographic approach for the analysis of the energetic behavior of magnetic materials is also introduced and presented. The methodology allows the identification of possible structural defects, information useful to introduce process modifications. Last but not least, the mechanical properties. The normally adopted laminated steels do not have any problem of mechanical resistance; a deeper penetration of SMC materials will be possible if they will reach similar mechanical behavior. The tests to be conducted according to typical standards are “Transverse Rupture Strength” (or TRS), with a three point bending test, and tensile strength test. The paper also describes the realization steps of several prototypes of electrical machines, both radial and axial flux: from dimensional calculus to the experimental validation, all the phases are described in details