Magnetic Device Designer

 

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faces the decision of which magnetic mateiral to employ in his design. Today we’ll cover several magnetic material characteristics that strongly influence the decision on which material is best-suited for a specific application.

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Key to making that decision is knowing: • What materials are commercially available • Which issues are important in considering the various materials. This requires the design engineer and purchasing personnel to understand the requirements of their application. • Magnetic output changes with temperature. At the extremes, temperature can cause device failure when the incorrect material is selected. • Assuming all else is satisfactory, a design that utilizes a cost-effective magnet is more likely to be successful in the competitive World Market.

We will focus today on SmCo, NdFeB and Ferrite. These represent about 85% of all permanent magnets sold on a cost basis. Ferrite magnets are extensively used in applications requiring a flexible magnet. On a tonnage basis, these are primarily used for sound-deadening and gasketing applications. Although flexible ferrite is used for low energy-low cost motors, our interest today is in motor and actuator devices which benefit from the unique properties associated with fully dense or rigid bonded magnets.

Reviewing the Key Advantages and Disadvantages of each of the products as
defined by the manufacturing process, we find that fully dense (sintered)
permanent magnets offer the highest magnetic output.
Fully dense means there is no dilution effect from a non-magnetic phase.
The highest output is available from NdFeB. However, as we will see later, other
application requirements may suggest using slightly less powerful SmCo magnets.

Injection molded magnets suffer from the greatest dilution effect. However, their shape and magnetic pole configuration possibilities often make them the most desireable choice. Tight tolerances are a result of molding to die dimensions – – secondary finishing operations are almost never required. Furthermore, assembly can be simplified through the use of insert-, over-, or multicomponent injection molding.

Compression bonded magnets represent a compromise of sorts between fully dense
and injection molded magnets. The volumetric loading of magnetic phase is greater
than injeConsidering our choices by material, NdFeB represents the highest magnetic output
material up to about 150 degrees centigrade.
It is limited to use above about 135 K (-138ºC), due to a change in magnetic
alignment at that temperature. But from 135 K to about 150º centigrade, it provides
excellent output.
One concern with NdFeB is corrosion. It is imperative to obtain material from a
quality manufacturer and specify coatings that reduce risk in the application.
Basic patents for compositions and manufacturing techniques are held, in all the
free-world, primarily by two companies: Sumitomo and Magnequench. When
purchasing NdFeB, it is imperative to positively ascertain that the manufacturer is
licensed to produce and export these products.ction molded magnets, but not as high as sintered, fully dense magnets.
Shape is limited to rather simple cross-sections with only a little improvement in
complexity over sintered magnets.
Perhaps the greatest advantage is that thin wall cylinder magnets can be
manufactured using compression bonding. Thin wall rings or cylinders are not
practical with the sintering process due to warpage during sintering and breakage
during grinding.
Except in the pressing direction which varies with die fill and press set-up,
dimensions are very tight, conforming to the tooling dimensions of the die.

Samarium Cobalt was the first widely used rare earth permanent magnet type, starting with the 1-5 composition in the early ’70s and switching mostly to the 2-17 type in the 1990s. When rare earth ore is mined, all the rare earths become available in the refining process, including cerium, lanthanum, misch metal (a combination of rare earths), praeseodymium, neodymium, dysprosium and samarium. As NdFeB usage goes up, more samarium is also mined and available for magnet production. The biggest advantage of SmCo over NdFeB is that of temperature stability.
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Ferrite is the Rodney Dangerfield of permanent magnets. We use it in vast quantities and treat it (without respect) like the “rust” it is – – special rust to be sure, but… First commercially available in 1961, it is still used in greater quantity by weight than any of the other materials, primarily due to its very low cost.

How does an engineer start the process of selecting a magnet?
Most start by ruling out magnets that cannot be used due to one or another limitation
such as temperature, magnetic output or material cost.
Temperature and cost are probably the two predominant selection criteria. Device
size and weight are used in the final decision.
We will see later in this talk that magnet material, size/weight and system cost are
all interrelated.

Before we launch into a discussion on the three highlighted items from the last slide, it is appropriate to focus on a problem endemic in the industry: underspecifying the magnet. It is essential for the design engineer, purchasing personnel and manufacturer /supplier to agree to a specification that includes everything necessary to ensure proper device function over the design life. The list above should be considered as the bare minimum and can serve to initiate dialogue and agreement amoung the parties.

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Most of you are probably familiar with reversible temperature coefficients – – the amount to which the magnetic output changes as a function of temperature. There are two coefficients: one for Br (induction) and one for Hci (intrinsic coercivity). Ferrite is shown here because, unlike rare earths magnets, ferrite (intrinsic) coercivity increases as temperature increases. Conversely, as temperature drops, coercivity becomes less. Where rare earth magnets have a