Permanent magnets - sizing up the options

Selecting the right type of rare earth permanent magnets can have a significant influence on the technical and commercial viability of a project, says Greg Erskine of Magnequench.

Determining which kind of permanent magnet material is right for a motor, sensor or other device depends on clear answers to a large number of questions, but in broad terms it is likely that three different kinds of information will be needed.

First consider the magnetic specifications for the device you are building.

In what kind of environment will the magnet be operating?.

What are the requirements (or constraints) on the size and shape of the magnet?.

Are there any special considerations relating to its physical dimensions or mechanical properties?.

How much are you willing to pay for the level of magnetic performance you want? And is there room for manoeuvre within that?.

For instance, could you sacrifice a degree of performance in exchange for savings on material costs?.

The next stage is to compare all of these requirements against the relevant characteristics of the materials on offer.

Assessing the wealth of data that this exercise inevitably generates can be a daunting prospect, which is why we always advise magnet users to tap into the technical and design expertise of the manufacturer from the very earliest stages of a project.

Their input could well extend beyond optimising magnet design into the production phase, building complete ready-to-fit assemblies and playing an integrated role in supply chain management.

Notwithstanding all this, it is generally true that most companies intending to work with permanent magnets are able to do some initial assessment of the different magnet options for themselves, which is what the guidelines below are intended to assist with.

They are taken from the Magnequench website, where much more information about each material is readily available.

If you want to take advantage of the tremendous opportunities that polymer bonded magnets offer in terms of shape-forming capabilities, reduced assembly costs, flexibility in magnetic orientation, a wide range of magnetic performance, and suitability for use in different operating conditions, then you can choose from the following bonded magnet options.

When the highest level of magnetic flux density is needed from a bonded magnet and when you can make use of simple shapes like rings, discs or cylinders, then compression moulded NdFeB (Magnequench's Neomag C) is preferred.

When complex shaping capabilities are more important, or when there are opportunities to reduce assembly costs using insert or overmoulding techniques, then injection moulded NdFeB (Neomag I) is the best choice.

In cases where the advantages of an injection-moulded solution are desired, but cost is more important than magnetic performance (in other words size and weight considerations are not critical), then injected moulded ferrite (Koerox I) should be considered.

If only slightly better magnetic performance is needed compared with what is attainable using ferrite alone, and cost is still a major limitation, then hybrid injection moulded ferrite/NdFeB magnets (Koerox HI) may be the right solution.

Because making informed choices between the many different options offered by bonded magnets is much more than a paper exercise, it is often necessary to call on additional input from the manufacturer.

A case in point is the design service offered by Magnequench, which uses FEA software for modelling complex magnetic fields.

If you need absolutely the highest energy density available from a permanent magnet material, and you seek the best value in terms of flux-per-buck, then sintered NdFeB (Neomag S) is the best choice.

If you need only slightly better magnetic performance than can be achieved using a polymer bonded magnet, or require superior corrosion resistance, but still want to take advantage of the flexibility in magnetic orientation possible from an isotropic magnet, then you can choose hot pressed NdFeB (Neomag H).

In special cases where the application calls for the best magnetic properties available in a radially oriented ring geometry, then hot pressed NdFeB (Neomag H RR) radial rings are the best solution.

In cases where superior temperature stability of magnetic properties is required, especially at elevated operating temperatures, and when size and weight considerations are not critical, then sintered AlNiCo (Koerzit S) is the best choice.

When excellent magnetic performance is required along with elevated temperature magnetic performance superior to that attainable from NdFeB, then sintered SmCo (Recoma S) should be considered.

Whether or not the magnet manufacturer has had a role in selecting the most appropriate material and developing the most appropriate design, its resources and expertise are crucial when the production stage is reached.

To give just one example, Magnequench has its own specialist cutting equipment to produce whatever size and shape of magnet the application needs - from the one-offs that are invaluable during prototyping, to the millions often needed to sustain a full production programme.

And if complete assemblies are required, we are equally well equipped; not just with the hardware, but with staff who have years of experience of working with magnets - often in the magnetised state.

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