Magnetisation methods

Magnetisation methods

For a permanent magnet to perform its function, it is necessary to magnetise it after manufacture. The magnetic field intensity should reach at least three times the coercivity of the given material. Materials with a lower Hc value can only be magnetised after assembling the whole circuit. This can ensure the optimisation of the working point of the magnet.

Magnets with great coercivity are easier to magnetise separately, as there is no need to build special fixtures and eliminate forces generated during magnetisation.

The magnetisation of permanent magnets is mainly carried out using special magnetisers working on the principle of an electromagnet, the coils of which are supplied by pulse current of great magnitude, obtained by discharging condenser batteries or from a specially designed pulse source.

Sometimes it is required that all the magnets in a series have an identically set working point. This is achieved by repeated action of an increasing magnetic field of opposite polarity to the magnetic field after magnetisation. The magnitude of this demagnetisation field depends on the actually measured position of the working point in the pauses during demagnetisation.

Demagnetisation

As the term suggests, it is a process during which the overall magnetic polarisation in a magnet decreases. This can be the result of the effect of an external magnetic field in an opposite direction to the direction of polarisation. The process is called demagnetisation. In addition, every magnet is subject to the action of inner demagnetisation Hd, the origin of which is related to the principle of decreasing inner energy. The magnitude of Hd is given by the equation:

Hd = - Kd . J / mo

(3)

where Kd is the demagnetisation factor

The factor Kd depends on the shape of the magnet and the direction of magnetisation and its precise calculation is, in general, complicated. A simple formula is valid only for a rotational ellipsoid which, however, does not occur in practice. It is therefore determined using table values or various mathematical approximations. The general rule is that the greater the ratio of the dimension of the magnet in the direction of magnetisation and its perpendicular dimensions (sometimes called the magnet slenderness ratio), the lower is its demagnetisation factor. The relationship has the form of a hyperbole, i.e. decreasing the ratio to a half, for example, may well mean reducing Kd by a tenth or tenfold.