Neodymium has magnetic appeal

Even though prices of rare earth magnets are falling and their performance improving, many design engineers are still choosing not to use them in new devices. Dean Palmer investigates

Even though prices of rare earth magnets are falling and their performance improving, many design engineers are still choosing not to use them in new devices. Dean Palmer investigates

"Since neodymium-iron-boron magnets were first introduced more than 20 years ago, their performance has improved considerably and prices continue to fall. Yet many design engineers still struggle to use them in new devices," explained Andrew Myers, general manager of Rotherham-based magnet manufacturer Magnequench. He continued: "Limited, outdated and inaccurate information is often the root cause of this problem, rather than deficiencies in the material itself."

Myers offered several case studies which all demonstrate the kinds of issues designers are faced with when designing electric motors.

The first example involved a customer that wanted to convert a two-pole motor for a fuel pump from ferrite to bonded neodymium, with an increase in energy product from 4 to 10 MGOe. As Myers explained: "The change was needed because the ferrite magnets were unable to meet a performance specification at low temperatures. Uniquely amongst permanent magnet materials, the intrinsic coercivity of ferrite decreases as the temperature falls.

"Initially, the solution appeared to be simply to replace the ferrite with an identically sized bonded neodymium magnet, keeping the original two-pole design. There was a slight increase in flux and the performance specification was met - although marginally - but finite element analysis (FEA) of the magnetic circuit showed that the return path was saturated; a clear sign that there was too much magnet in the circuit."

Myers said that performance could have been improved by making the return magnet thicker so that it carried all the available flux - a straightforward approach, but not the most effective. "Instead, better use of the magnet can be made by reducing the included angle, or by changing to a four-, six- or eight-pole design. From a manufacturing perspective, a single ring is preferable to arc segments - and the geometry is well suited for multi-pole configurations, provided the assembly is properly magnetised [normally by using a special magnetising fixture]. The preferred solution is a multi-pole ring with a thicker return path.

The second scenario involved a motor that used isotropic neodymium magnets but whose performance was less than expected, resulting in excess cogging. FEA revealed magnetising as the problem - a common occurrence with isotropic materials, because it is the magnetising rather than the material itself that determines the flux pattern.

Myers: "It turned out that the field created by the magnetiser was not perpendicular to the surface of the magnet, as was assumed at the design stage, which meant that the flux was off slightly from the intended angle. Once the model was altered to reflect the true pattern of the flux, the excess cogging was obvious."

Altering the direction of the field delivered by the magnetising fixture was not possible in this particular case, so the only option was to magnetise before assembly. It is important to be aware that the tables of magnetic properties published by manufacturers are from samples of their products that have been magnetised to saturation, and that measurements are made parallel to the direction of magnetisation. If one or both of these conditions are not met, the published properties are invalidated, which can result in under-performance of the kind seen in this case.

The third example involved a diametrically magnetised ring of isotropic bonded neodymium intended for a sensing application, for which a relatively uniform magnetic field inside the ring was required. Most of the flux travels within the wall of the ring rather than where it is needed. Magnequench's solution to this problem was to create the field using a dipole Halbach ring - a configuration that allowed the field at the centre to be calculated before manufacture. "Another advantage was that the ring could then be manufactured and magnetised as a single piece, eliminating the complexities of assembling every segment, each of which had a specific direction of magnetisation," explained Myers.

The resulting flux pattern was made possible by special fixturing and achieved an increase in density from 50 to 650 Gauss at the centre of the ring, with very little wasted outside it.

Tom Shelley

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