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.
usenet at lklyne dt co dt uk
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
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
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"
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 ?
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
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).
Tony Williams. Change "nospam" to "ledelec" to email.
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 ....
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.
Die Gotterspammerung - Junkmail of the Gods
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