Magnetic Properties of SMC

Magnetic Properties of Soft Magnetic Powder Composites at Higher Frequencies in Comparison with Electrical Steels

Soft Magnetic Composites
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I. INTRODUCTION

Driven by requirements of e-mobility, electric motors bring new challenges to the traditional laminated motor construction to reach high power density. 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. Because soft magnetic powder composites (SMC) can be used for a 3D magnetic flux path, it is an ideal solution for applications such as transverse flux motors [1].

 

Also, in high frequency alternating magnetic fields, SMC materials act as an insulator to the eddy current and thus provide overall low iron losses. The optimal use of SMC can increase the power density of the electric machine [2]. For electrical steel based asynchronous and transversal flux machines the power density, expressing power which can be performed by a certain size of the motor, can reach values of 0.1 – 7.0 kW/dm³. According to the own expertise these values can be substantially increased by the use of SMC in transversal flux machines and by optimization of design.

 

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.

 

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 rings, considering all magnetizing directions and thus correlating with real motors.

 

III. MANUFACTURING PROCESS OF MAGNETIC COMPONENTS

It is a matter of common knowledge that the magnetic properties of electromagnetic components depend strongly on their manufacturing process [3].

 


Fig. 1. Influence of manufacturing steps on the magnetizing behavior of 1.1 kW asynchronous motor [3].

 

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. An example for this deterioration after various steps of manufacturing process is presented in Fig. 1 and Fig. 2.


Fig. 2. Influence of manufacturing steps on the specific core loss of 1.1 kW asynchronous motor [3].

 

IV. MAGNETIC LOSSES OF SMC IN COMPARISON WITH ELECTRICAL STEELS AT ELEVATED FREQUENCIES

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 M330-35A is presented in Fig. 3 and Fig. 4. Additional measurements on different grades with thicknesses of 0.20-0.35 mm confirm this tendency.

 

The distribution of magnetic losses in SMC cores deviates from the behavior in laminated cores because of a different structure of ferromagnetic material components. The hysteresis losses of SMC are higher and the eddy-current losses result from the internal losses within the particles. This general characteristic is illustrated in Fig. 5 [4].

 

The specific core loss of standard electrical steel grades used traditionally at elevated frequencies: M330-35A, M270-35A and NO20 were compared with SMC-grades Siron® S280b, Siron® S300b, Siron® S360, Siron® S400b, Siron® S720 manufactured by PMG Füssen GmbH. The results at J=1T are presented in Fig. 6.

 
Fig. 3. Influence of sample geometry (Epstein vs. toroidal) on the magnetizing behavior of electrical steel grade M330-35A.    Fig. 4. Influence of sample geometry (Epstein vs. toroidal) on the specific core loss of electrical steel grade M330-35A.

 

 
Fig. 5. Frequency behavior of SMC and electrical steels; general view [4].   Fig. 6. Comparison of frequency behavior between selected SMC grades and electrical steels at J=1T.

 

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. Additionally, Fig. 7 gives an overview of core losses at various frequencies and at various values of polarization for two selected materials (SMC and electrical steel M330-35A).

 


Fig. 7. Comparison of frequency behavior between electrical steel grade M330-35A and SMC-grade SIRON® S360.

 

Below the transition point of frequency electrical steels have lower specific core losses in the whole range of magnetic polarization then SMC. Above this point (approx. 500 Hz – if compared SIRON® S360 with M330-35A) the specific core losses of SMC become lower. This can be observed in the whole range of magnetic polarization as well.

V. MAGNETIZATION PROCESS OF SMC IN COMPARISON WITH ELECTRICAL STEELS

The magnetizing behavior (J vs. H) was determined as well. The results are presented in Fig. 8.

The magnetization process of SMC is hindered by the typical structure of SMC including pores (Fig. 9) [5]. 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.

 

 
Fig. 8. Comparison of magnetizing behavior between selected SMC grades and electrical steels at f = 5000 Hz.   Fig. 9. Microstructure of soft magnetic composites after compacting and curing [5].

 

 

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. 9 shows the permeability of tested materials “as measured” and Fig. 10 under consideration of an air gap (1 mm) according to the simplified equation (1) [4]:

where:
μ* - overall permeability of the magnetic circuit, μr – relative permeability of soft magnetic material, lL - length of air gap, lFe - 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 than for e.g. asynchronous machines, decreasing the significance of permeability of applied soft magnetic materials.

 


Fig. 10. Comparison of permeability of selected electrical steels and SMC materials (SIRON® manufactured by PMG Füssen GmbH) at f=1000 Hz .

 

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. 11. Comparison of permeability of selected electrical steels and SMC materials (SIRON® manufactured by PMG Füssen GmbH) at f=1000 Hz under consideration of 1 mm air-gap according to equation (1).

VI. SMC AS ALTERNATIVE MATERIAL FOR ELECTRIC APPLICATIONS

In comparison with widely used electrical steels, SMC have advantages making them suitable for special constructions of electric machines. These advantages are:

  • High power density by 3D magnetic flux conduction
  • Lower core losses at elevated frequencies compared 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 three-dimensional design solutions with minimal iron losses and optimized copper winding. The on-going 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 eddy-current loss (see Fig. 6) to lower the values of frequency
  • optimal choice of application according to the relevance of permeability (see Fig. 11)
  • 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.

 

VII. CONCLUSSIONS

Electrical machines with three-dimensional 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.

VIII. 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. 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, to be published
[3] A. Schoppa, “Influence of manufacturing process on the magnetic properties of non-oriented electrical steel”, doctoral thesis, Aachen, Germany, 2001.
[4] R. Boll, „Soft Magnetic Materials” (German: „Weichmagnetische Werkstoffe“), Page 87 and 101, ISBN 3-8009-1546-4, Vacuumschmelze Hanau, Germany, 1990.
[5] P. Delarbre, E. Holzmann, A. Schoppa, PMS500 – a modern soft magnetic composite material for electric machines, 5th International Conference on Magnetism and Metallurgy WMM’12, Proceedings, Page 326 – 334, 2012, Ghent, Belgium.

 

written by: Andreas Schoppa, Patrice Delarbre, Elmar Holzmann, Maximilian Sigl
PMG Füssen GmbH
Hiebelerstr. 4, 87629 Füssen, Germany
e-mail: andreas.schoppa@pmgsinter.com