Re-magnetising

[john fletcher 1Participant @johnfletcher1]

In former times when motor bikes had magnetos, some magnetos would require re magnetizing to obtain a good spark. You could take the magneto to the motor bike repairer and have it fixed, a ten minute job. Has any readers got details technical or otherwise of re magnetising magnetos or other articles where a permanent magnet is used, such as in an analogue multi meters (Avo) and where its internal magnet may have lost some of its magnetism. I'm interested in making re magnetiser and how its used. John

[john fletcher 1Participant @johnfletcher1]

[Howard LewisParticipant @howardlewis46836]

Model Railway shops used to provide a service to remagnetise the permanent magnet in the motors.

Not suitable for larger items, probably.

In principle, a magnetiser is fed with DC to provide a constant magnetic field which magnetises the object placed across its poles.

From the point of safety, I would suggest using a voltage no higher than 24, with proper earthing of all metal parts.

Probably, a few seconds should suffice, especially if the coil draws a heavy current.

If you could find one, maybe the dynamo from an old commercial vehicle, using the field coils, might be a starting point?

Howard

[Dave WoottonParticipant @davewootton]

Hi John

Have a look at the Brightspark magneto's website, they have loads of info on magneto's on there and they have built there own magneto remagnetising machine. have used them in the past for mag spares and their external condenser conversion, very good to deal with.

They do offer a magneto remagnetising service at reasonable cost, I'm sure if you asked them nicely they would give you more information on their home built machine. The owner seems keen to encourage people to have a go at working on their own mags.

Warning you can lose hours looking at all the information on the site!

Dave

[AdrianRParticipant @adrianr18614]

Magnets are made by using an intense pulse of magmatism. This is done by discharging a high voltage capacitor through a coil. If you google "make your own magnetizer" you will find some ideas.

All can be quite lethal if your fingers are in the wrong place.

[Neil WyattModerator @neilwyatt]

I recall the old way was to wind a few turns of thick wire round the magnet and (very) briefly brush the ends of the wires across a car battery's terminals.

If it doesn't work, connect the wires the other way round.

If this doesn't work don't blame me…

Neil

[not done it yetParticipant @notdoneityet]

To demagnetise, one puts the part to be demagnetised into a strong alternating magnetic field and slowly weaken the field to zero. This may mean moving the part slowly until completely out of the magnetic field.

To magnetise, one arranges the opposite – a strong magnetic field (preferably all around the object) which is suddenly collapsed, leaving the magnetised item with the poles at switch-off – just pot luck which way it ends up.

So both the single high energy pulse from a large high voltage capacitor or a high DC current, through a coil, which is switched off suddenly (don’t want too much inductance in the circuit) will suffice for most purposes.

The simple way to magnetise a bar is by ‘stroking’ with the pole of a strong magnet – not too efficient, but works. Simple demag is by hammering the magnet or heating it (red hot and cooling away from any strong magnetic fields works very well).🙂

[LathejackParticipant @lathejack]

Well this has reminded me that I made a start on a magneto remagnetiser almost six years ago. I bought a roll of insulated copper wire and machined up a pair of mild steel cores and cut a slab of mild steel for the base, I then machined up a second set of cores of a larger diameter to those shown, but I haven't got much further with it yet.

Apparently soft ingot iron should ideally be used for the coil cores because of its low carbon content, so it does not retain any magnetism when power to the remagnetiser is switched off, unfortunately it is quite expensive at this size but mild steel is usable. I downloaded and printed off the instructions and have them filed away somewhere.

Edited By Lathejack on 15/04/2020 22:01:17

Edited By Lathejack on 15/04/2020 22:05:58

[duncan webster 1Participant @duncanwebster1]

There was an article in ME way back, perhaps 2005, about making a re-magnetiser. Someone who is good with indices will be able to find it

[John OlsenParticipant @johnolsen79199]

When magnetising, the field does not need to be collapsed suddenly. The only requirement is that the item to be magnetised should be subjected to a high enough magnetic field. It does not need to be subjected to it for very long, and a very short time will usually suffice. This is convenient, since quite high currents are often required. If you look at the hysteresis loop for permanant magnet materials, it will be almost a square shape. The idea is to apply a magnetic field which will take the material right into the saturation region. When the external field is removed, the item will relax back to the remanent magnetic state. Some energy is actually stored in the material when it becomes magnetised.

The problem is that the permananet magnetic field is actually applying a demagnetising field to itself, so with the older materials the magnet will tend to gradually lose its suck. The keeper helps with that, by providing an external path for the demagnetising field. The newer materials are both stronger and less prone to demagnetising., but putting a keeper on is still not a bad idea, it also protects things like mechanical watches and floppy disks and tapes from damage.

John

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