Magnetisation is the process whereby a permanent magnet material is exposed to a high intensity magnetising field, considerably higher than the intrinsic properties of the material, in order to drive the material into saturation and develop the full permanent magnetic properties.
This is usually the last process in the manufacture of a magnet and would normally be carried out by the manufacturer of the permanent magnet. However, in some cases it can be beneficial for magnets to be supplied un-magnetised. This could be to minimise problems in the handling or storage of high energy magnetised components, or because the final application would benefit from building the assembly with un-magnetised magnets and then magnetising the magnets “post assembly”. Not all magnetic assemblies can be magnetised post assembly, but where possible, considerable savings in assembly time and reliability can be achieved.
After magnetisation, magnets or assemblies should be tested to verify their performance.
There are a number of ways for testing a magnet.
- Full BH Characterisation.
- Total magnet flux or magnetic moment measurement
- Single point Flux density measurements
Full BH characterisation is where the demagnetisation (BH) curve and key magnetic properties are measured. This requires specialised equipment such as a Hysteresisgraph, Vibrating Sample Magnetometer (VSM), or a Pulsed Field Magnetometer. This measurement is restricted to specific sample sizes and shapes and is destructive in the sense that the sample will be demagnetised during the process. It is normally only used to verify material properties of magnet batches rather than magnetisation. However, most magnet manufacturers have this type of equipment and can provide the data if requested.
Total Flux measurements are usually measured with a search coil and an integrating fluxmeter. If a magnet or assembly is placed inside a coil connected and then quickly removed from that coil, a voltage will be induced in the coil. The integrated voltage is proportional to the total flux produced by the magnet. A calibrated coil and fluxmeter can therefore be used to measure the total flux. This is a useful method for testing if a magnet is fully magnetised as it is quantifying the magnetisation of the total magnet volume and less susceptible surface effects particularly if the magnet is an unusual shape. It can be used for comparative measurements against a known standard or for determining actual flux values.
Single point measurements can be made with a Gaussmeter and Hall Probes. These measure the magnitude and of the flux density at the centre of the “hall element” inside the probe. They only measure the field in one direction, normal to the active face of the Hall element, although 3-axis hall probes are becoming more common which will measure the field, in all 3 planes. This measurement is particularly use for sensor applications where a magnet is required to generate a particular field at a certain distance in order to activate a sensor or for testing multipole magnets where the total flux can be zero except on the surface of each pole. In such cases Gaussmeters can be used to map the surface flux density of a magnet or rotor assembly.
Magnet Calibration is required in applications where a high tolerance on the magnitude of the field or force produced by a magnet is necessary. Standard magnetising equipment is designed only to generate a high enough magnetic field to drive a magnet into saturation. The properties produced by the magnet are then dependent on the material composition and magnet shape. Partially magnetising a magnet in order to “set” a magnet property to a specific value is not recommended, partly because the properties are very difficult to control during initial magnetisation, but also because the resulting can be unstable and more susceptible to further demagnetisation. The recommended process is to first fully saturate the magnet to ensure all regions of the magnet are magnetised, then “knock it back” with a small demagnetising field by reversing the field in the magnetising coil, resulting in a much more stable magnet. This process can also be referred to as Magnet Stabilisation. It is more commonly used for older magnet grades such as Alnico’s which have a non-linear demagnetisation curve and are inherently less stable, but the process can also be applied to ferrites and rare earth magnets. Custom made magnetising equipment can be made to allow the setting process to be carried out at the same time as magnetising.