The magnetising process
The majority of Hirst Magnetic Instruments Ltd. magnetising systems contain capacitors which are charged to a high voltage (800 or 3,000 volts) and discharged through magnetising fixtures or coils. This capacitive discharge magnetising system allows the end user to magnetise permanent magnets individually or in product assemblies. The right choice of system is influenced by a number of factors, as highlighted in more detail in the contents below.
In order to magnetise an item it is necessary to expose it to a sufficiently strong magnetic field. The magnetic field is generated by passing a large current through a special coil of wire known as a fixture.
Each magnet is created when a suitable piece of material is placed in the centre of the wire coil and subjected to a strong magnetic field generated within the fixture.
The fixture is often designed specifically for the application ensuring it generates the required field over the specific area/volume of the magnet. The fixture needs to be protected against damage caused by high voltages and currents, excess temperatures and mechanical stresses generated during magnetising.
Many types of magnetising pattern can be produced, each one being associated with a specific industry application.
The magnetising system
Capacitive discharge magnetisers are used where the required magnetising current is too high to be achieved using direct power from the mains supply. Depending on the exact requirements of the geometry to be magnetised, the material type and the fixture design, this current is often in the range 5000-10000A but can also be much higher.
A capacitive discharge magnetiser, charges up an internal capacitor bank to a required voltage level over a short period of time. This stored energy is then discharged into the magnetising fixture (in a much shorter period of time), in a controlled manner, producing the large required currents and hence magnetising field strength.
Hirst incorporate microprocessor technology in their magnetisers to safely manage the charging and discharging of the machine during the magnetising operation making it very safe to use.
The required energy of the capacitor bank is calculated from a combination of the required field strength to magnetise the magnet combined with the fixture geometry and application dimensions. Capacitor banks with stored energy from 100J to over 100KJ are available, with operational voltages from 400-5000V.
These are chosen specifically for the magnetising application and matched to customer requirements.
Magnet calibration and stability
It is often required that the magnet maintains a very stable magnetisation long after it has been magnetised and does not “droop”. Although rare earth magnets and ferrite do not lose their magnetisation after saturation as Alnico can, there is still a relaxation after magnetisation that in some situations can be problematic. Alternatively, it may be required that the magnet is calibrated (also known as setting, treating or aging) to a particular level for use in a calibrated instrument.
If the magnet is “set” then it can be calibrated to a particular level with the aid of an Integrating Fluxmeter, Gaussmeter or other measuring technology as appropriate. Alternatively, the magnet can be “knocked back” by applying a pulse in the opposite direction to the sample to “knock back” the magnetisation, which greatly improves the stability of the magnet.
Hirst Magnetic Instruments Ltd. magnetising systems are designed to meet your specific production requirements.
Most Hirst magnetisers are suitable for integration with mechanical handling systems where the parts to be magnetised are fed to the fixture by means of an appropriate loading and unloading system.
Digital inputs and outputs of 5 volts or 24 volts are provided for integration with PLC systems, to directly control a small number of mechanical handling mechanisms or simply drive external start buttons and status lamps.
RS232 serial interface connections are available on most magnetisers. This can be used for integration with other control equipment or downloading measured data for SPC analysis.
Hirst Magnetic Instruments Ltd. design and build mechanical handling systems around their magnetiser systems. These can be manually loaded, semi-automatic or fully automatic systems.
Monitoring and control
It is possible to add a verification system to the magnetising system. This is used to verify that the magnet has magnetised correctly and is useful for spotting bad magnets that would otherwise have found their way into products.
A Fluxmeter or Gaussmeter is integrated into the system (dependent on application) to provide pass/fail signals indicating good and bad parts or to collect statistical data to monitor any variations in the magnet material over time.