Magnetics Business & Technology - Spring 2016 - (Page 22)

FEATURE ARTICLE Tailoring of Pole Shapes of Multipolar Injection Molded Magnets By Thomas Schliesch, Head of Research & Development | Max Baermann GmbH Innovative pole shapes for permanent magnets are often needed, especially for sensor applications, but also for rotors in electrical machines. In machine applications the interaction between rotor and armature demands specific pole shapes to decrease cogging torque and machine noise or to raise efficiency. In magnetic sensors multipolar magnets are used e.g. for position sensors, which detect angles or numbers of rotation or distances of linear motion. Accuracy of the many sorts of sensors highly depends on the magnets field shape, which can be very different in varying applications. E.g. sensors which measure rotation angles with magnetoresistive elements or Hall elements in most cases demand at least two field components, with sinusoidal shape and low harmonic distortion. Systems which collect magnetic flux and transfer it via soft magnetic parts to a single component sensor often demand trapezoidal field components. Digital sensors which switch at specific thresholds use the rising edge of a field decrease from zero. All these and many other sorts of fields can be tailored more or less easily with injection molded magnets, especially when in mold magnetized magnets are manufactured. Magnetic fields arise from permanent magnets by their magnetic poles, which can be defined as those parts of magnet faces, which are penetrated by the magnetization vector. Another contribution is originated by sources or sinks of magnetization inside the magnetic material. Following formula describes the H field generated by a permanent magnet of general distribution of magnetization M, see e.g. in [1]. This formula can be derived from Maxwell's equations. (1) Here the first integral describes the face integral over the external areas of the magnet. A denotes the area element with normal vector n. The larger the scalar product of magnetization vector and surface normal vector, the higher is the input of pole faces to the total field H(r) at any location r outside or inside the magnet. The dashed vector r' describes the locations of the magnet body itself. The second integral is performed over the magnets volume and takes into account sources and sinks of magnetization by div´M. I.e. magnetic fields are originated both by pole faces as well as by deviations from homogeneity inside the body. The opposing signs of both integrals show, that e.g. sinks of magnetization (div´M <0) increase magnetic fields near poles with positive direction, whereas sources decrease magnetic fields. Figure 1 shows the mechanisms described by above equation pictorially, here for a detail of a multipolar ring magnet with a bow shaped magnetization inside. In case of a homogeneous polarization all magnetization vectors are parallel within one pole segment. This means that div´M vanishes, so that only pole faces generate magnetic fields. As can be seen in eq. (1) the input is highest, when magnetization is parallel to the normal vector of a respective pole face. When multipolar magnets are assembled by homogeneously magnetized segments, also here only pole faces play a role. Integrals along the segment boundaries cancel mutually. A net increase of originated fields, e.g. in front of a pole with positive radial direction, can be managed by creating sinks of mag- 22 Magnetics Business & Technology * Spring 2016 Figure 1. Origins of magnetic fields from magnetic polarization of a ring magnet. Both pole faces as well as spatial distributions inside ad up to the total field of the magnet. netization in the vicinity of that pole. This can be reached amongst others by assembling one pole by two homogenous segments, where both homogenous sectors show a slightly convergent magnetization relative to the other, see e.g. in [2]. A similar effect is reached, when making multipolar ring structures from arc segments with radial magnetization, where the field is now utilized at the inner face of the ring assembly. Due to the different sign of both integrals of eq. (1) also here div´M leads to an increase of field strength. This effect has been used for many decades in rotating electrical machines, where arc magnets with radial magnetization are implemented quite often. For injection molded multipolar magnets the orientation of M at the pole face as well as sources or sinks inside the magnet can be tailored more or less easily, especially in field oriented magnets. These are magnets which are magnetized and particle oriented inside the mold during the injection molding process. Magnetic materials are Ferrites in many cases as here no additional magnetizing process is needed, but field orientation can also be managed with Rare Earth materials. Best results of such field orientation can be achieved, when the magnetizing fields around the mold are originated by assemblies of strong, temperature resistant permanent magnets, like sintered SmCo or NdFeB magnets of special composition. A standard configuration for such assembly is shown in Figure 2, which depicts a 90° detail for manufacturing an eight pole cylinder magnet. The sintered field sources are indicated by arrows. The single bar magnets are separated by brass or other nonmagnetic metals. A bush made of nonmagnetic wear resistant material isolates them from the cavity. The bore of the cylinder magnet can be formed by a core, which can both be of soft magnetic or nonmagnetic material, depending on the magnetic specification of the final product. The whole assembly is surrounded by a soft magnetic back iron ring, which can be cooled by water or air to balance the relatively high temperatures of the injection molding process. The colors in Figure 2 show the distribution of the radial com- www.MagneticsMagazine.com http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Spring 2016

Editor's Choice
Permanent Magnets in a Changing World Market
Design Considerations Using Permanent Magnets
Insulated Iron Powders, SMC, Current State and Future Possibilities
Research & Development
The Design of a Permanent Magnet Motor for a Direct Drive Fan Application
Tailoring of Pole Shapes of Multipolar Injection Molded Magnets
Electromagnetics
Industry News
Marketplace / Advertising Index
Spontaneous Thoughts: Seventeen Mistakes People Make with Permanent Magnets

Magnetics Business & Technology - Spring 2016