Life cycle of magnets

Permanent magnet materials are now being used in electrical machine applications, and a lot of this equipment will be expected to have long lifecycles.

If performance and lifecycle targets are to be met, detailed characterisation at the design stage using advanced electromagnetic and multiphysics simulation technology will be critical, according to Cobham Technical Services in the UK.

High field strength, rare earth permanent magnets, are key components in many of the new electrical machine designs now appearing on the market. Typical long-life applications for such technology include wave and wind power generators, and the traction motors for hybrid and electric vehicles.

Simulation software already plays a vital role in the development of such machines by shortening design-to-manufacture times and reducing the number of test prototypes that need to be produced.

However, many of the modelling approaches and design software tools currently used are unsuitable for capturing and characterising gradual degradation of magnetic performance over time – especially if this is due to a combination of factors.

Cobham has built up considerable expertise in helping designers optimise the performance of electrical machines, and its Opera software is used by companies worldwide to maximise performance of equipment, including permanent magnet (PM) generators.

The software is also being used by a major research project in the UK, which is investigating the technology that will be needed for low carbon vehicles.

A key part of this research will involve examining the factors that cause the performance of PM traction motors to deteriorate with age, with a view to developing better materials, motors or control techniques to overcome the problem.

Although partial demagnetization of a motor’s permanent magnets is believed to be responsible, at present it is not clear whether this is due to natural degradation, overheating caused by excessive power demands, temporary fault conditions that are rectified during the life of the vehicle, or inherent shortcomings in the design of the motor itself.

According to Chris Riley of Cobham: “The need to be able to accurately characterise the demagnetization of high field strength permanent magnets at any stage in their life cycle, with a view to improving long term performance, has never been more acute.

“PM traction motors are a prime example; their environmental operating conditions and duty cycles are far more severe than those of most industrial or consumer applications, yet users expect them to last for the lifetime of the vehicle – up to 14 years – with little or no change in performance.

“And although wind power generators have a more consistent duty requirement, off-shore platforms will need to endure very hostile climatic conditions. They will also be required to have long design lives – of 20 to 25 years – which again raises concerns about magnet longevity.”

At present, most PM-based traction motors for hybrid and all-electric vehicles use neodymium-iron-boron magnets, but their magnetic field strength reduces with increasing temperature, and above about 220-degrees C the demagnetization effects can be irreversible. The amount of demagnetization depends on a number of factors, including the physical shape of the magnet and its magnetic circuit, as well as the grade of material and the shape of its magnetic characteristic.

Some designers are using neodymium-ironboron magnets in which a small percentage of the neodymium is replaced with dysprosium to raise the coercivity and the temperature at which permanent demagnetization occurs.

Cobham’s Dr Dan Ilea says: “At present, nearly all dysprosium comes from China, and is mainly used for nuclear, laser and magnetic data storage applications.

“There is likely to be a severe shortfall of this element within a couple of years, especially if it is taken up by high volume industries like automobile manufacturing. It is therefore critical to maximise the life of neodymiumiron-boron magnets by designing electrical machines that are not susceptible to self-demagnetization.”