Most, if not all, magnets used in motors suitable for slot cars are of the sintered ferrite, or ceramic - whichever you prefer - type. In the 1960's, cast Alnico magnets were mostly used.
Cast magnets are manufactured by pouring a molten metal alloy into a mould and then further processing it through various heat-treat cycles. The resulting magnet has a dark gray exterior appearance. Sintered magnets are manufactured by compacting fine Alnico powder in a press, and then sintering the compacted powder into a solid magnet.
Ferrite magnets are sintered permanent magnets, composed of Barium or Strontium Ferrite. This class of magnets, aside from good resistance to demagnetisation, has the advantage of low cost and are the type used in mass produced motors such as the Mabuchi motors used by Scalextric.
Most of the very powerful "rare earth" or "Cobalt" types of magnets, as used in Eurosport "strap" motors, are a hybrid of casting and sintering but because of the many special steps involved in manufacturing they tend to be somewhat expensive. As with everything, you pay for performance!
All magnets are easily demagnetised if not handled with care. Special care should be taken to ensure that the magnets are not subjected to adverse repelling fields, since this could partially demagnetise the magnets.
Also, bear in mind that magnetic flux density (field strength) is affected by many external forces. Heat is not as serious as one might expect. Typically, a ferrite magnet will lose 3-5% of its strength by being elevated from room temperature to 100 decrees Celsius, the boiling point of water. When it cools, the strength returns to very near normal, so this is a reversible effect.
There is a point where a magnet will lose all of its strength, never to return until re-magnetised. This is called the "Curie temperature." For a ferrite magnet, this is somewhere around 450 degrees C, about 850 degrees Fahrenheit. Cobalt curies at about 800 degrees C.
The silent killer of magnets is called "contact demagnetisation." Every time you allow a magnet to stick to something, the magnet loses energy! The effect is not as serious on the poles as it is on the sides or ends. The worst possible way to package magnets is in a poly bag, stapled to a card where they are allowed to contact everything and stick together. Even sliding the magnets in the can will lose some flux density. By removing the armature, the flux path will be partially opened and the magnets will be weakened to a degree. Impact has an effect on the flux strength, but it really isn't significant.
If magnets are partially demagnetised, they may be easily re-magnetised, or "zapped".
Can the magnetic strength be increased through "zapping"?
With, for example, Mabuchi motor magnets, it is more an issue of "saturation". I have found that the magnets are mostly pretty much saturated, anyway. I've tried "zapping" them but there was no marked difference. When a magnet is saturated, it contains all of the magnetic field it can hold and will be as strong as it can and ever will be.
Kind regards
Russell
