Development of Soft Magnetic Composites for Low Loss Applications
Soft magnetic composites (SMC) are being developed to provide materials with competitive magnetic properties (good relative permeability and magnetic saturation) but with high electrical resistivity. The high resistivity achieved is a major factor in making these materials attractive in low loss applications, particularly at high frequencies. Unlike conventional laminated materials, the electrical resistance is isotropic and the isotropy of properties is seen as removing the constraints on design imposed on conventional electrical machines by lamination [1, 2]. SMCs utilise high purity iron powder the grains of which are bonded with a coating of an organic material which produces the high electrical resistivity of the compact. In addition, the SMC precursor powder is carefully formulated to produce a high density product (circa 7.4 g/cm3) when cold or warm compacted, which is essential to the viability of SMC in practical applications such as permanent magnet motors.
In a current EPSRC programme looking at new ways to use SMC for electrical machines, assembly from sectional elements and co-compaction of core with windings have been investigated. The latter process is particularly attractive because it offers the possibility of reducing the number of process steps in device fabrication and increasing winding densities which can lead to improvements in device performance. A novel machine design has been developed [3,4] which offers substantial improvements in performance (56% increase in torque at thermal limit) and yet is smaller and more efficient than existing designs. This first design was built from teeth sections pressed from iron powder which were wound after pressing. In subsequent experiments it was demonstrated that co-compaction of core and windings is feasible and that the insulation layer on the iron powder can be maintained and hence losses minimised. Furthermore, copper fill factors are very large (>75% as compared to 35% in conventional machines and 55% for the pre-pressed core design).
The above work should be seen as a proof of concept. For SMCs to achieve their maximum potential the technology has to be developed to achieve net shape fabrication or near-net shape fabrication of machine elements. Ideally, the process should be one of co-compaction of coils and SMC in one operation; substantial progress has been made in this field in the current EPSRC project. Figure 1a shows a section taken through a pre-compacted coil. Figure 1b shows a sub-assembly fabricated by co-compaction of coil and SMC. This was achieved using a triple ram compaction method. However, this method of fabrication is close to the limit of viability because of the low green strength of the SMC and the difficulty of achieving high density in complex sections. Further work on processing to achieve high uniformity of product is therefore necessary.
Another possible co-compaction route investigated, in which the end sections were compacted onto a coil with a pre-compacted core, was less successful because of the low strength and higher loss in the core/end-section interface. Further work to understand the nature of this interface is desirable to ascertain if methods can be developed to improve bonding in this process route. This would have the benefit of removing the requirement for the coil insulation to survive the high temperature steel heat treatment process.
Figure 1: (a) Compacted coil cross-section, (b) co-compacted core and coil.
In our current work there are indications that changing the shape, size and surface finish of the powder can lead to considerable improvements in performance with elongated grains aligned in the direction of magnetisation offering significantly higher permeability and lower hysteresis losses. Indicative figures are a doubling of the permeability and up to a three fold reduction in low frequency iron loss. However, the processing of such elongated starting materials to achieve full density after compaction is considerably more difficult than for spherical powders. Work is needed to analyse the mechanics of the compaction process and develop methods for the full compaction of these materials if their excellent magnetic properties are to be exploited.
Finally, the current work on SMC at Newcastle has been strongly oriented to design and fabrication of prototype electrical machines. There is a need for supporting work on characterisation of the material in relation to the effects of processing to underpin the technological development in its application.
With complex sections, control of the pressing process is critical. Die designs need to be optimised and multiple-ram compaction is often necessary to achieve full density in the SMC. Furthermore, residual stresses and strain hardening introduced by the pressing process affect magnetic performance. The relationship between press design, density, breakdown of SMC insulation and residual stress generation is not well understood. This programme aims to address this issue by investigating the effect of processing variables on the properties of the material. A combination of analysis techniques will be used to characterise the structures and properties of SMC resulting from different production routes and processes.
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on 18th January 2002.
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