Regenerating permanent magnet

Here is one for theorists.
How can you regenerate a permanent magnet.
The magnet in question is part of the rotor of a honda generator. The permanent magnet no longer has any power and the alternator will not start up. It requires the permanent magnet to get power to feed the electro magnet on the rotor.
Lawrence
usenet at lklyne dt co dt uk
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Not much of a permanent magnet then. is it ? :-)) The generator is actually spinning the magnet and not the coils ? Is it the brushes on the rotor that have worn down ?
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It spins both a permanent magnet and an electro magnet. The stator coils consist of the main windings for 120V/240V, A winding to power the electronics feeding the rotor and a separate 12v winding.
The tech manual for a similar generaor says there should be 24V +- 4V on the rotor coil. I connected external 12v source and got 200Ma current. Started engine and output was 90V on the 240V output. Seems reasonable to me, 24V will give circa 180V and the permanent magnet another 60V. The rotor resistance is in the correct arena through the brushes, and I have had them out to inspect.
The stator resistances are Ok and if the magnet was Ok and electronics not working I would expect to see 60V on 240V output and something on the coil feeding the electronics but there is virtaully nothing (1v possibly).
The generator is probably 10 years old and probably run about an hour a year. Was used by a pub to run freezers in power cut. They are not that frequent around here, (Wiltshire). I suspect not run for a couple of years.
My source of reference was some manuals from http://www.honda-uk.com/support.htm where you can find some technical manuals and user manuals for honda equipment. I used the tech manual covering the em3000 and the dead one is an em1500.
Maybe I will extract the rotor and dismantle it. Do something. Not worth buying a new rotor.
On Tue, 14 Oct 2003 11:48:27 GMT, "BigWallop"

Lawrence
usenet at lklyne dt co dt uk
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Then it's possible that the windings have loosened up over the years through weathering, or that one or two have become corroded/oxidised. The whole thing might just need a good clean out.
I'll have a look at some of the technical gumph I have here to see if I find anything else that might throw some more light on possibilities.
Bugger it, eh ?
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On Tue, 14 Oct 2003 17:15:40 +0100, Lawrence

You sure it's not just damp windings? Aim a fan heater at the genny for a few hours and try again. Low heat, mind.
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Is the pilot generator a permanent-magnet AC generator (ie, is there also a bridge rectifier associated with it) or is it a shunt-wound DC generator (ie, a direct DC o/p comes off it via it's own brushes)?
In either case there are faults other than a complete loss of magnetism that could be the cause of no output from it.
For a PMG setup there could be an open/shorted stator winding, or a shorted diode in the bridge, or a short on the dc side.
For a shunt-wound DC generator it could be that an overload has put the remanent magnetism into the wrong polarity, or a fault in the winding, (open or short circuit).
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Umm .. I've never heard it expressed quite that way before, but I think I understand. In my view, there's a reasonable probability of there being some other fault on the unit - permanent magnets generally don't 'lose all their power' so as to require 'regenerating'. It only requires a relatively small residual magnetic field to enable a generator to "bootstrap" itself anyway, so I think I'd still be looking for what's *really* wrong with your unit. However ... is it possible that it has had a strong AC current accidentally passed through the field coils when the generator wasn't running? That certainly *could* destroy a permanent magnet if the current was strong enough for long enough ....
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On Tue, 14 Oct 2003 11:35:32 +0100, Lawrence
With a magnetising rig, which you can make yourself. DAGS on dynamo rebuilding - the stationary engine crowd do this regularly. Or ask again in uk.rec.engines.stationary
Basic technique is to place the magnet in a coil, then to charge up a capacitor bank and discharge it through the coil.
Problems are that the coil may need to be a funny shape, especially for alternator / magneto work. Also coils need a lot of turns to generate a large field from a small current. But more turns means more inductance, which reduces the current you can force through them in a pulse. So there's some juggling to be done for optimum size of coil, capacitors and voltage.
Alternatively, ignore the old magnet (maybe saw it up for pole pieces) and slip some powerful new rare earth magnets into the middle of it. Modern ceramic magnets are so powerful that you can replace a big old magnet with something tiny these days.
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