IPMD 14th Edition 2010-2001, 5 pages, 2371 words

Author: Dr. Oreste Morandi, Assinter Secretary, Casella postale 272, 10015 IVREA (To), Italy

The majority of soft magnets are today made using PM processing techniques. The following review, updated by Dr. Oreste Morandi, outlines the basics of magnetic materials, the manufacturing routes available and the properties characteristics of such magnets. Examples are given of applications in this rapidly expanding area of PM technology.

The function of an electromagnetic device is to produce a force or a torque. Often a magnetic circuit is compared with that of an electrical circuit or that of a fluid flow. In simplest terms, a magnetic circuit consists of an applied field, a path for the magnetic flux to flow and an air gap, which represents the resistance within the circuit. The applied field represents the potential energy that is necessary to create the magnetic field which provides the magnetic force or the torque. This potential energy results from an electric field provided through coils of wire, or from charged permanent magnets.

More and more current technology uses permanent magnets rather than electrical fields supplied through coils of wire. This brings us to the point that there may be two kinds of materials in a magnetic circuit. These materials have been designated as ‘soft’ and as ‘hard’ (permanent) magnetic materials. They are separated by the property that one kind, the hard or permanent magnets, once charged, retain their magnetism. The soft magnets, on the other hand, lose their magnetic charge once the electrical field or the permanent magnet is removed or reduced. Modern motors today are designed with both kinds of magnetic materials within their components.

Magnetic properties of interest in magnetic materials include magnetic induction, or the force or torque that the device is capable of producing, the remnant magnetisation or the retention of the magnetic field, the magnetic permeability, the coercive field and the resistivity. These properties are controlled by the magnetic domains within the material. These domain walls move whenever an applied field induces them to charge. In moving they realign themselves in the direction of the applied field, resulting in force or torque. To move the domains, work is required and energy is expended. Therefore, whenever the applied field is altered, the domains must change direction. The resistance to the change is known as hysteresis.

Each magnetic material traces a hysteresis curve depending on the applied field. If the applied field is strong enough the magnet is saturated, which means that it is incapable of supplying any increase in the magnetic force, regardless of the applied field. The increase of the magnetic field as a result of an increase in the applied field is represented as a trace known as the magnetisation curve. As the field is changed through the four quadrants, the hysteresis curve is generated.....

Further sections of this article include:

• Soft magnetic materials
• Applications
• CASE STUDY 1 Stator core and armature for diesel fuel injectors
• CASE STUDY 2 Rotor for electromagnetic brake used in electrical gear-motor assembly
• CASE STUDY 3 Rotor for a sensor used in car engines

Figures and Tables:

Fig. 1 Schematic of (a) magnetisation curve (b) hysteresis loop. (Source: R. German, Powder Metallurgy of Iron and Steel, John Wiley & Sons 1998; C. Lall, Fundamentals of Magnetism, Chapter I, Soft Magnetism, MPIF, 1992, p 1-27)

Fig. 2 Hysteresis loops for soft magnetic material and hard magnetic material. (Source: William Smith, Principles of Materials Science and Engineering, McGraw-Hill, Inc. 1986; CD Rom: ‘Powder Metallurgy: Materials, Processes and Applications’. EPMA, Shrewsbury, UK, 2001)

Fig. 3 Influence of powder and process parameters on magnetic properties. (Source: ‘Soft Magnetic Materials’, ; CD Rom: ‘Powder Metallurgy: Materials, Processes and Applications’. EPMA, Shrewsbury, UK, 2001)

Fig. 4 Different designs of ABS sensor rings produced by pressing and sintering. (Courtesy; AMES SA, Spain)

Fig. 5 Sintered Fe-P components used in low-speed stepper motors (Courtesy; AMES SA, Spain)

Fig. 6 Sintered Fe-P pole shoe parts used in electric starter motors. (Courtesy; AMES SA, Spain)

Fig. 7 High strength and machinable powder magnetic stator cores having thin wall and high L/D shape. (JPMA Award Winner 2006 – New Materials Category)

Fig. 8 Reactor powder core produced by Hitachi Metals Ltd, Japan, for high voltage transmission system (Courtesy MPIF)

Fig. 9 Soft magnetic cores made from encapsulated iron powder are used in the Bosch common rail diesel engine system

Table 1 Typical Properties of PM soft magnetic materials for DC and low frequency applications

Table 2 Typical properties of soft magnetic composites (SMC) for AC and high frequency applications