Soft Magnetic Powder Composites and Potential Applications in Modern Electric Machines and Devices
Powder metallurgical manufacture has the singular ability to produce net shaped products (gear box parts, motor parts) for the automobile industry. Soft magnetic powder composites (SMC) coupled with the P/M production process open new possibilities in the design and manufacture of parts for electrical applications. With increasing values of operating frequency the use of SMC can contribute to a substantial decrease of specific core losses of the machine, increasing at the same time its total efficiency. In contrast to laminated cores, the manufacturing process of soft magnetic composites does not influence their final magnetic properties. These properties are homogenous and do not change after assembly of the motor. New SMC materials can outperform current laminated steel materials when measurements are done in the same conditions with similar samples. Many prejudices existing about the magnetic properties of SMC can thus be eliminated or attenuated if we respect its optimal use in adequate applications.
I. INTRODUCTION
Continual advances in the area of emobility and high power density electric motors bring new challenges to the laminated motor construction. It is well known that traditional laminated motor construction is limited to 2D magnetic flux to minimize losses in the direction perpendicular to the steel lamination. In contrast, the topologies of the transversal flux motors require very complex 3D magnetizing directions where the use of electrical steels is often not practicable [1]. Additionally, the use of conventional electrical steels is generally limited to medium frequency values < 1000 Hz. With a very high specific electric resistivity SMC materials act as insulators to the classical eddy current and thus provide overall low specific core losses. The use of SMC becomes interesting at frequency values >500 Hz [2] and can increase the power density of electric machines. A relevant example is the product line DYNAX® of Compact Dynamics GmbH. These SMCbased transversal flux machines can reach power values of 25...40 kW, torque of approx. 80 Nm and speed of 10.000 rpm. Because of the small size and low weight (1014 kg) these machines can be used as drives for small electric vehicles or as generators in hybrid solutions. The stators of these machines are technically feasible only by the use of SMC, Fig 1.
Powder metallurgical manufacture has the singular ability to produce near net shaped products (gear box parts, motor parts) for the automobile industry. SMC materials coupled with the P/M production process open new possibilities in the design and manufacture of parts for electrical applications.
It is a matter of common knowledge that the magnetic properties of electromagnetic components depend strongly on their manufacturing process [3...5]. In the case of electrical steels this process includes punching, various assembling methods of laminations (automatic stacking, riveting, welding...) and pressing into the motor frame. These processing steps cause an interior deformation of the material resulting in deterioration of magnetic properties [6]. This deterioration can be considerable in dependence on the size, air gap and the operating range of the electric machine.
Manufacturing of SMCbased magnetic components occurs in conventional powder metal processing which is well established for high volume manufacturing of net shape or near net shape complex products [7]. SMC utilize a processing sequence as follow:

Mixing of powder with lubricant or binder

Compacting

Curing at relatively low temperatures (200 – 650°C)
Since the SMCbased magnetic cores get their final shape after compacting and their final magnetic and mechanical properties after curing, there is no relevant deterioration of magnetic properties after manufacturing process.
Fig. 1. SMCstator of the transversal flux machine DYNAX®60i manufactured by Compact Dynamics GmbH.
II. METHODS OF MAGNETIC MEASUREMENTS
For magnetic measurements of electrical steels the Epstein method is widely used. This standardized method is applied on sheet samples which are cut longitudinal and perpendicular to the rolling direction of the steel strip. The Epstein method was developed exclusively for quality reasons and ignores the negative influence of manufacturing process of electric components on their magnetic properties. To correlate these properties measured on Epstein samples with a real motor the designers apply various correction factors for different types of electric machines as a solution to this problem.
In contrast, the magnetic properties of SMC are typically measured on compacted and annealed rings, considering all magnetizing directions.
To ensure a fair comparison between the magnetic properties of electrical steels and SMC, the measurements were done on samples with the same geometry (toroidal samples; OD = 55 mm, ID = 45 mm, thickness = 5 mm). The influence of geometry on magnetic properties of electrical steel grade M33035A is presented in Fig. 2 and Fig. 3. Additional measurements on different grades with thicknesses of 0.200.35mm confirm this tendency.
Fig. 2. Influence of sample geometry (Epstein vs. toroidal) on the magnetizing behavior of electrical steel grade M33035A.
III. MAGNETIC LOSSES OF SMC IN COMPARISON WITH ELECTRICAL STEELS AT ELEVATED FREQUENCIES
The distribution of magnetic losses in SMC cores deviates from the behavior in laminated cores because of a different structure of ferromagnetic material components. Because of specific microscopic structure of SMC (Fig. 4) their hysteresis losses are higher then the hysteresis losses of electrical steels. In contrast, the amount of “dynamic” losses is lower for SMCmaterials. This is a result of high specific electric resistivity p of SMCcores. For high values of p (according to own measurements p of SMC is 3×10^{3}... 1,3×10^{4} higher than p of electrical steels) the amount of classical eddy current losses of SMC in comparison to electrical steels, according to the equation (1) [8]:
where:
B – induction [T], ƒ – frequency [Hz], d – thickness [m], y –
density [kg/m^{3}], p – specific electric resistivity [Ωm],
is very low.
Fig. 3. Influence of sample geometry (Epstein vs. toroidal) on the specific core loss at J =1 T of electrical steel grade M33035A [9].
Fig. 4. Microstructure of soft magnetic composites after compacting and curing [9].
The specific core loss of standard electrical steel grades used traditionally at elevated frequencies: M33035A, M27035A and NO20 were compared with SMCgrades Siron® S280b, Siron® S300b, Siron® S360, Siron® S400b, Siron® S720 manufactured by PMG Füssen GmbH. The results at J=1T are presented in Fig. 5.
Fig. 5. Comparison of frequency behavior between selected SMC grades and electrical steels at J=1T [9].
The frequency value of the transition point depends on the nominal thickness of comparable electrical steel and can vary for the typical commercial grades between 500 Hz...1500 Hz. So, the application of SMC becomes interesting for machines operating at elevated frequency or for machines with a substantial amount of higher harmonics.
IV. MAGNETIZATION PROCESS OF SMC IN COMPARISON ITH ELECTRICAL STEELS
The magnetization process of SMC is hindered by the typical structure of SMC including pores. The electrical steels have a “smooth” microstructure with some minor impurities but without a typical porosity. The porous microstructure of SMC and the resulting lower density of ferromagnetic element iron (Fe) in comparison with electrical steels is a reason for lower permeability of SMC. The designers however have to decide whether this fact is relevant for the calculated magnetic circuit because of the resulting ratio between the iron path and the air gap. Fig. 6 shows the permeability of tested materials as measured” and Fig. 7 under consideration of an air gap (1 mm = 0,6 %) according to the simplified equation (2) [8]:
where:
µ*  overall permeability of the magnetic circuit, µ_{r}  relative permeability of soft magnetic material, l_{L} length of air gap, l_{Fe}  length of (soft) magnetic path (e.g. Fe).
According to this equation the difference between the permeability of SMC and of electrical steel becomes negligible with increasing length of the air gap. For typical transversal flux motors and machines with permanent magnet excitation the resulting air gap is substantially higher then for e.g. asynchronous machines, decreasing the significance of permeability of applied soft magnetic materials.
Fig. 6. Comparison of permeability of selected electrical steels and SMC materials (SIRON^{®} manufactured by PMG Füssen GmbH) at ƒ=1000 Hz [9] .
Additionally, the advantage of higher permeability of electrical steels becomes substantially reduced with increasing frequency. Thus, for the appropriate design of electric machines a combination of all factors have to be considered: operating frequency, permeability and the air gap of the magnetic circuit, to achieve the best possible performance.
Fig. 7. Comparison of permeability of selected electrical steels and SMC materials (SIRON^{®} manufactured by PMG Füssen GmbH) at ƒ=1000 Hz under consideration of 0,6 % airgap in magnetic circuit according to equation (2) [9].
V. SMC AS ALTERNATIVE MATERIAL FOR ELECTRIC APPLICATIONS
In comparison with widely used electrical steels, SMC have advantages making them suitable for special topologies of electric machines. These advantages are:

High power density by 3D magnetic flux conduction

Lower core losses at elevated frequencies in comparison with electrical steel

Good formability; complex shapes can be directly compacted without destroying the material structure and resulting deterioration of magnetic properties.
Since the magnetic cores get their final shape after compacting and their final magnetic and mechanical properties after curing, they can be immediately wound with wires and assembled into the motor frame. This enables the magnetic core manufacturer to scale the design, and simplify both the core winding geometry, and the motor manufacturing process.
The isotropic natures of the SMC material combined with the net shaping possibilities allow us to introduce new threedimensional design solutions with minimal iron losses and optimized copper winding. The ongoing development in the area of soft magnetic composites proceeds as follows:

improvement of magnetizing behavior

improvement of saturation polarization

shifting of transition point of the eddycurrent loss (see Fig. 6) to lower the values of frequency

optimal choice of application according to the relevance of permeability

improvement of mechanical strength.
These improvements are accomplished through optimization of the compacting and curing process as well as the addition of special binders or lubricants.
VI. CONCLUSIONS
Electrical machines with threedimensional magnetic flux are needed for high efficiency motor applications. At the same time, new applications requiring high operating frequencies are becoming more relevant and available. Soft magnetic powder composites are the upcoming development in the powder metallurgy offering optimal magnetic properties at elevated frequencies and contributing to the increase of the power density and to miniaturization of electric machines. This makes SMC perfect for applications with limited space e.g. in the automotive industry, robotics or selected home appliances. In these fields of electrical applications SMC can even outperform the commercially available electrical steels.
Andreas Schoppa and Patrice Delarbre
PMG Füssen GmbH, Hiebelerstr. 4, 87629 Füssen,
VII. REFERENCES
[1] Ola Anderson, Paul Hofecker, “Advances in Soft Magnetic Composites – Materials and Applications”, Advances in Powder Metallurgy & Particulate Materials – 2009; Proceedings of the 2009 International Conference on Powder Metallurgy & Particulate Materials, Las Vegas.
[2] A. Böhm, I. Hahn, “Comparison of Soft Magnetic Composite (SMC) and Electrical Steel”, Proceedings 2012 2nd International Electric Drives and Production Conference, EDPC 2012, Nuremberg, October 1518, 2012, Page 229  234.
[3] A. Schoppa, “Influence of manufacturing process on the magnetic properties of nonoriented electrical steel”, doctoral thesis, Aachen, Germany, 2001.
[4] W. Deprez, J. Schneider, T. Kochmann, F. Henrotte, K. Hameyer, „Influence of the Manufacturing Process on the Magnetic Properties of Electrical Steel in ECores, Proceedings of the SMM Conference, Düsseldorf, Germany, 2003, Page 167171.
[5] H. Harstick, W. Riehemann, “Influence of Punching and Tool Wear on the Magnetic Properties of NonOriented Electrical Steel, Soft Magnetic Materials Conference SMM21, Budapest, Hungary, 2013.
[6] K. Hameyer, D. van Riesen, F. Henrotte, “About the Modelling of the Magnetic Circuit and the Ferromagnetic Materials in Electrical Machines”, Proceedings of the WMM’2004 (Workshop Metallurgy and Magnetism), Freiberg, Germany, 2004.
[7] Y. Guo, J. G. Zhu, J. J. Zhong, W. Wu, „Core Losses in Claw Pole Permanent Magnet Machines with Soft Magnetic Composite Stators”, IEEE Transactions on Magnetics, vol 39, Page 31993201, September 2003.
[8] R. Boll, „Soft Magnetic Materials” (German: „Weichmagnetische Werkstoffe“), Page 87 and 100, ISBN 3800915464, Vacuumschmelze Hanau, Germany, 1990.
[9] A. Schoppa, P. Delarbre, A. Schatz, “Optimal use of soft magnetic powder composites (SMC) in electric machines”, Proceedings of the 2013 International Conference on Powder Metallurgy & Particulate Materials, Chicago.