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Magnetising Fixtures

In order to magnetise modern rare-earth magnets, large magnetic fields are required. The fields required are large enough to present some significant challenges to the designers of the necessary equipment.

Capacitive Discharge Magnetising

In order to magnetise rare earth materials, a very large field must be produced. This field only needs to be sustained for a short time period, long enough for transient field-opposing eddy currents to decay, and enable a fully saturating field to penetrate the entire magnet volume. A pulse discharge method is universally used to achieve this, as it represents the most energy-efficient way of magnetising, and a brief, but large energy discharge is usually the only practical way to produce the field strengths necessary. The use of such high rates of change of flux density can, however, be problematic without a thorough appreciation of all of the physics and engineering occurring during this brief event.

Thermal Design 

Because of the extreme flux densities of up to 5 Tesla, required to magnetise rare earth materials, currents in the conductors of magnetising tooling must be of the order of thousands of amps. The geometry of the magnet or assembly to be magnetised will dictate the maximum space available for these conductors, and so conductor sizes typically range from sub-millimetre to a few millimetres in diameter. The current densities present in these conductors cannot be sustained for more than a few milliseconds without overheating the conductor, damaging insulation, or ultimately fusing. Even in this short time period, the conductor temperature rise will be significant, and for most practical situations, a cooling strategy will be required. A single shot magnetisation process may raise the conductor temperature by over 100C. The heat removal rate when magnetising on a more frequent cycle, like an automated production situation, could be several kilowatts. Heat paths, materials and thermal stresses must all be considered, and much of the design process can be aided by the use of finite element modelling.

Mechanical Design 

Typical pulse durations for a magnetising process range from under one millisecond to several milliseconds. Peak currents of 30kA are not unusual. The resulting force densities are very large and will destroy the fixture if the conductors are not well supported. The origins of the magnetic forces can be from the direct interation of flux density, B and current density, J in the conductors of the fixture, but can also be from field interations with induced currents. This means that transient stresses in parts of the magnet or assembly could also be potentially destructive. The mechanical design of the fixture must consider these effects and must be robust enough to withstand repetitive high loading for perhaps many thousands of magnetising operations. In addition to the loads experienced within the fixture itself, for larger magnetic assemblies it may be necessary to provide some means of removing the magnetised part from the fixture, as the attractive forces may be large.

Magnetic Design 

Single or multiple pole pair magnetisation can be achieved or assemblies with many magnets can be magnetised with a well-designed fixture, thus avoiding the need to assemble or handle parts with magnetised magnets. This can greatly increase the scope for automated handling or assembly processes, and the production cost benefits that could be gained. A full treatment of all of the elements of the magnetic design of magnetising fixtures would include the following areas:
  • Pole arrangement and inter-pole transitions
  • Required unmagnetised regions (e.g. moulded magnets)
  • Fixture impedance
  • Required pulse shape and energy
  • Eddy current decay times
  • Use of laminated or solid iron cores
  • Conductor paths and insulation
  • Cooling and electrical isolation

Best Solution Capabilities 

The Best Solution design process includes a thorough understanding of the coupled physics of magnetising fixtures using sophisticated finite element modelling methods. Magnetisation transients are observed in 3d models which give a comprehensive insight into magnetic, thermal and stress fields. This insight, coupled with sound practical experience and manufacturing know-how gives us the ability to design and deliver effective tooling for development or production applications.

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