Magnet Processing - Selecting the Right Equipment
Methods employed in the selection of magnetizing and conditioning equipment for a wide range of
permanent magnet materials are provided in a logical and analytical format. This forms a basic guide applicable to any magnet-processing situation. Table 1 - Common Magnetic Materials Material Coercivity (Hc) Required Magnetizing Force Alnico 2 580 Oersteds 2500 Oersteds Alnico 5 650 Oersteds 3000 Oersteds Alnico 8 1650 Oersteds 8000 Oersteds Ceramic 1 1850 Oersteds 10,000 Oersteds 10,000 Oersteds 12,000 Oersteds SmCo 4500 to 12000 Oersteds 20k to 100k Oersteds Neodymium 3500 to 13000 Oersteds 20k to 45k Oersteds
The selection of magnet processing equipment can be as important as the
selection of the materials used in the product being produced. It must not be made on the single fact that a magnet is to be magnetized. Many aspects including production philosophy, material handling methods, and especially
end product performance, all need to be considered. The synergistic relationships of the many factors that go into the selection of magnet processing equipment indicate a process which must be given high priority.
Background
Ceramic 5
Ceramic 8
2400 Oersteds
2950 Oersteds
The subject of magnetizing and conditioning often is not given its proper place in the development cycle of a product. These very basic but important
processes are considered as items that will easily fall into place at a later date. This may be true for applications using the low coercive materials such as alnico or even some of the ferrites
(Table 1). However, with the increased use of rare earth materials, this is no longer the case since they are more difficult to magnetize. Increased sophistication in product designs is another factor to consider.
Within the physical boundaries of any permanent magnet there are many microscopic particles called domains (Figure 1). Each of these has a north and a south pole, but they exhibit a random
orientation which results in no measurable magnetic field. Only when the magnet is subjected to an external magnetic field of sufficient strength to align the domains can we
say the magnet has been magnetized. A fully magnetized magnet can be considered as being saturated.
To assure saturation, it is necessary to apply an
external magnetizing field (referred to as the Peak H) of sufficient strength to fully align the domains which exhibit the highest coercivity. The weaker domains will follow. The intensity of the applied field
required will depend on the material being used. In general, the alnico family will require 3000 to 8000 oersteds (Oe), the ferrites 10000 to 14000 Oe, and the rare earths 20000 to greater than
50000 Oe. By properly saturating the magnet, it will be able to provide the flux levels and resistance to demagnetization indicative of the particular magnet material. By not providing
proper saturation, the magnet will be more susceptible to demagnetization from both internal and external influences. If a magnet is to be used at a level less than saturation, it
must be fully magnetized and then subjected to a secondary process of controlled demagnetization or conditioning. Only through this process can one maintain the stability
of the magnet material and obtain end product consistency.
Magnet Processing Equipment
The magnet to be magnetized is placed into a fixture which derives its energy from a magnetizer. Several types of magnetizers are currently being manufactured.
The capacitive discharge magnetizer contains a basic power supply which is used to charge an energy storage bank of capacitors (Figure 2). These are charged over a number
of seconds and discharged into a fixture in a matter of milliseconds. One of the advantages of this type of charger is the high current output capability for a relatively low input current.
Figure 2. A photo and block diagram of a capacitive discharge magnetizer
A half-cycle magnetizer operates directly from
the 60 Hz power line. Instead of using capacitors, a portion of the AC wave is passed through the charger to the fixture (Figure 3). Such a magnetizer is applicable to high-speed applications where the input
current is available and the peak magnetizing force requirements are not excessive.
Figure 3. Block Diagram of Half-Cycle Magnetizer
A DC magnetizer is basically an electromagnet. This is most suitable for low coercive materials in simple two-pole configurations. The configuration of this device is usually a C or H frame with one or
two coils driven from a DC power source (Figure 4).
Figure 3. Block Diagram of a DC Magnetizer
Figure 5. An example of a magnet conditioner (left); block diagram of a capacitive discharge conditioner with output wave shapes shown (right)
A conditioner may be of a capacitor discharge or half-cycle design (Figure 5). The output, controlled to a higher degree or resolution than in a magnet charger, may be in the form of
a unidirectional pulse or a dampened wave pulse train. The choice of output depends on the magnet material being processed.
The magnetizing fixture is the device that transforms the output of
the magnetizer into the magnetic field used to magnetize the magnet (Figure 6). Fixtures are available in many forms depending on the magnet being processed and other factors. It may be a single
loop of copper, a coil, or a combination of copper conductors with iron to act as a flux carrying media. Certain applications will require that liquid or air cooling provisions be provided in the fixture to
maintain a safe operating temperature. Conditioning fixtures may be similar in construction.
Figure 6. An example of a magnetizing conditioner
Evaluating an Application
The following is a list of questions that need to be addressed before any magnet
processing equipment can be selected.
- What is the magnet material? This will determine the amount of magnetizing force that will be required to assure saturation.
- What is the size of the magnet? This determines the area over which the magnetizing force has to be developed.
- What is the polar configuration? This defines the number of poles that are to be implanted in the magnet.
- At what production rate is the magnet to be processed? This will influence the size and type of magnetizer, as well as the method of cooling for the fixturing.
- Is the magnet supposed to be magnetized separately, or as part of an assembly? This factor can directly affect the complexity of the fixture.
- What materials are included in the assembly? Materials may be present that will provide resistance to the magnetizing field, or in some cases, aid in the magnetization process.
- What is the end product? Certain products dictate specific production techniques which must be observed. The magnet processing equipment selected must be adaptable to these techniques or conditions.
- What affect will the method of magnetization have on the end product performance? Several methods of magnetization are often possible for a given application. One may be preferred over another. The method selected should provide proper end product performance and at the same time support the manufacturing process with suitable production rates and ease of operation.
- Does the magnet need to be conditioned? The flux level of the magnet or magnet assembly being processed may have to be adjusted to obtain proper end product performance.
- How far from saturation must the field be reduced to achieve proper end product performance?
- To what final accuracy must the field be adjusted?
- How is the magnet to be measured?
All of these questions should be considered for every application. Some may not be directly
applicable. They should become points of review for any new product development project involving the use of permanent magnets.
The process of magnetization cannot be the last item to be considered as a product is
developed. If it is not considered at the beginning of the design process, one could be faced with settling for impractical or costly solutions, especially when high coercive
materials are involved. The high fields required to achieve saturation of these materials can preclude the ability to magnetize them after placement in an assembly or end product.
Flux distribution characteristics of a magnet assembly are directly influenced by the method of magnetization and can dramatically affect end product performance. It is also
important to keep the magnetizing techniques in mind from product inception to production. Any methods adopted for production that do not have roots extending from
the prototype stage can be a cause of serious problems. Also, any changes in fixturing for an existing product line must be carefully reviewed.
In situations where conditioning is required, it must be determined if this process can be
incorporated with the magnetizing. A basic two-pole bar magnet can be processed in this manner. If an application dictates that the direction of the demagnetizing field must be
presented perpendicular to the direction of the magnetization, or if the conditioning is performed while the end product is under test, a separate conditioning station may be required.
Fixture Design Considerations
The design of a magnetizing fixture encompasses factors other than the ability to just saturate a magnet or magnet assembly. It must be designed to provide ease of loading and
unloading, be compatible with production methods, provide the proper flux distribution characteristics, and have adequate structural integrity to withstand production duty
cycles. Adequate cooling provisions must be included to assure that the fixture will not operate beyond the temperature limits of the material used in its construction.
Operator safety is a must. Protection to prevent the operator from coming into contact with any high voltage, pinch points, or any other hazard that may be inherent to a particular application, must be considered.
Conclusion
Only through the use of sound engineering concepts, practical judgement, and a full understanding of the application at hand can a magnet processing system be properly
configured. The process of magnetization or conditioning must be considered as an integral part of any product development from inception to production.
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