Restoring a magnet's strength

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>>> That's actually one from a Scalextric car, the train ones had a worm

There's a company mentioned upthread that offers a drop-in replacement. They work fine as long as the armature bearings are perfect, but they are sintered bronze, and never get oiled by most users, so wear a lot, which can lead to jamming. Luckily, all spares are still available.

Reply to
John Williamson
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I was merely describing what happens, not endorsing whether it is necessary or not.

Reply to
Tim Streater

Why pis about? Just follow the link I posted and get the correct size.

MBQ

Reply to
Man at B&Q

yes. a coil across the mains with a fuse is ideal. The field/current goes up, the fuse blows and everything inside is magnetized.

HOWEVER best is to replace old ferrite s**te with neodymium magnets if you can.

Reply to
The Natural Philosopher

Hmmm, but are you at the top of the sine wave at that instant?

Reply to
Tim Streater

It depends on the metallurgy and the magnetic behaviour. If you heat to the Curie point, then you _don't_ demagnetise it. Instead you convert it temporarily from a ferromagnet to a paramagnetic material (which doesn't behave as a magnet). However when the temperature reduces back below the Curie point, this effect reverses and the magnetism is restored.

If you heat the steel high enough, then you can also change its metallurgy (any text on hardening & tempering) and these changes will also permanently destroy any magnetism. This is _not_ the same as Curie point behaviour. Nor (as the material is still ferromagnetic, not paramagnetic) can you usefully magnetise it in this state by applying a low field to it and cooling.

As to which happens first, that's a question of alloy. Part of the unique (at the time) behaviour of Alnico was that it had a high Curie point, the only one above a red heat. You can do what you describe to some low-Curie alloys (and without getting to red), but not practically to Alnico.

Yes, this is precipitation magnetisation. It's how Alnico magnets are made in the factory.

In practice though, you're going to get spalling unless you have something like a muffle furnace with a controlled slow heating. Alnico is a pig mechanically, it's too brittle. No-one re-magnetises Alnico magnets in this way. When they're first made, it's for cooling from an initially unmachined billet and they're machined afterwards.

It's also a party trick for making impromptu magnets, but it's not something you'd want to mix with the use of an induction magnetiser. Nor is it easy to do - you not only have to apply a magnetic field, but you have to apply it continuously while cooling below the Curie point. This usually means a long bar magnet, just so that you can apply a field to it sensibly. I know it's sometimes done for forging horseshoe magnets, but it's a fussy process and very sensitive to the alloy.

Reply to
Andy Dingley

Bollocks. Capacitor banks are the usual way, to achieve a unidirectional field.

If you have such a supply handy, or it's 1900 and capacitors aren't usefully available, then a motor-generator set for electroplating (low voltage, huge current) can also be used. Slightly later on (although still quite ancient now), a minicomputer PSU (5V, 50-100A) can be used too. It needs to be an old one (linear), because switched mode PSUs don't enjoy this kind of abuse.

Reply to
Andy Dingley

No, if you replace it with a similar-sized lump, then you saturate the pole pieces and the spare flux locks the armature solid.

Reply to
Andy Dingley

In message , robgraham writes

Yeah, buy a Niobium magnet

I just happen to have one for sale for £80

Reply to
geoff

robgraham formulated the question :

That does work, but you would need to work out which way or where the magnets poles were before you started and it MUST be DC - AC is used to demagnetise. Another way is to stroke a strong magnet along the other magnet, close up along the old magnet between the poles, then far away, to return to the starting point.

Best option really is to see if modern rare earth (Neodymium) magnets can be fitted instead, you can get them for a few pence each on ebay for the small ones and they are extremely powerful.

Reply to
Harry Bloomfield

If the buyer can move it....

Reply to
Bob Eager

Just for interest, I've been fitting a different windscreen wiper motor to my SD1 Rover. The original was never much good - it would stall on a dry screen with the engine off so only 12 rather than 14 volts. In contrast, those on my BMW are so powerful they shake the car on fast speed. A quick glance showed the motors to be of similar size so I obtained a secondhand BMW one - made by Valeo.

They are very similar. Roughly the same size armature, etc. But the Lucas one has 'steel' magnets, the Valeo ceramic. And a rough and ready bench test showed the Valeo produced 5 times the torque off the same current limited (8 amp) bench power supply.

Reply to
Dave Plowman (News)

I have a Volvo 960, the 2.5l straight-six. Starter failed a while back, quite the worst starter to remove I've ever worked on. The replacement though is one of the new permanent magnet starters, which allows it to be a couple of inches shorter. Piece of cake to get it back on. Works as well too.

Reply to
Andy Dingley

I have a permanent magnet replacement on the SD1 too. The actual motor is very much smaller, but it has reduction gearing added.

Reply to
Dave Plowman (News)

Not if you replace it with the correct modern magnet, as sold for the purpose.

MBQ

Reply to
Man at B&Q

The one on my pickup's almost the size of a loaf of bread and weighs many pounds. Two bolts to remove it (plus a third for power), and there's enough space in the engine bay to do it from above rather than crawling around underneath. Power to it comes via a big solenoid mounted up on the inner wheel arch.

Our Toyota has one with reduction gearing though - the thing's positively tiny, and I hope it never fails because there's absolutely masses of stuff that would get in the way of its removal :-)

cheers

Jules

Reply to
Jules Richardson

If the torque is greater, why would the max speed be less?

Reply to
GB

I have some magnets taken out of hard disk drives. When my screwdriver lost its magnetism, a couple of strokes with one of these rare earth magnets restored it (and more so!). Next time I'll just use one stroke. Would that work with a Triang motor?

Reply to
GB

DC motor speed is determined by the voltage, the armature design and the magnetic field, which combine to generate back EMF, which (simplified) is the voltage the motor would generate if it were being used as a generator. Increase the supply voltage, while keeping the same field and it goes faster for the same torque. Increase the field while keeping the voltage stable, and the speed decreases, as the back EMF rises relative to the speed. The torque available (simplified)is proportional to the current and magnetic field, and the rate of rise in current under load is governed by the rate of change in the back EMF, which increases as the field increases, all else being constant.

Electric traction such as trains and trams used to do this in reverse, by having a weak field setting on the motors to increase the speed and decrease the torque once they had reached a high enough speed. Kind of an electrical gearbox.

Reply to
John Williamson

No, because you can't stroke a cube. If you made a long stack of them, you could try it.

The problem is that of remanence, the magnet's ability to remain a magnet. Your screwdriver is intended as a screwdriver, not a magnet, so it's made of a hard steel with a not terribly high remanence (by the standards of deliberate magnets). However it's a hard steel metallurgically, so it's actually fairly high (about as good as magnets of 1900). It will act as a magnet once magnetised, but it's 'soft' enough to be re-magnetised by stroking with a more powerful and remanent magnet.

Alnico though has a high remanence, far better than anything else of its period. Stroking wouldn't work with any lesser magnet, however a modern Nd magnet might be in with a chance.

Reply to
Andy Dingley

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