A bit OT - but a good puzzle!

On a motorcycle newsgroup to which I contribute, there was the following posting. Not entirely our thing, but I know we do love a problem to gnaw at.
This is what he said
"Dear All, I have a Lucas 12 volt generator with electronic regulator on my bike that has stopped working. That is, no charge showing on the ammeter and full discharge when the lights are turned on. However, just before I removed it from the bike to peer inside it, something made me decide to test it in situ.
So I removed the generator drive components and did a motoring test whilst applying rotational load to the armature shaft. All was well with no jerkiness, hesitation or stopping. I then plugged it back in and ran it up with an air drill and it works just fine with the correct charge showing on the ammeter and a multimeter across the battery showing 12.3 volts.
Put the drive components back on and start the engine and no charge. Spin it with the air drill and all correct again. So why does it work with the air drill and not with the engine running?"
See what I mean? Answers eagerly awaited ;o))
Regards,
Kim Siddorn
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Three thoughts. The air drill may be running at far higher speed than the bike can rotate the generator therefore a weak unit may be persuaded to give an output. There is a loss of drive to the generator due to a sheared key or similar. This would mean that the unit seems to spin OK but isn't reaching normal speed, hence poor output. This can be found by checking for hot components in the drive system due to friction. Finally, there may be a short between the armature windings and the shaft which is going to earth via the bikes drive mechanism. Remove that electric pathway to operate it with an air drill and away it goes. Then of course I could be wrong with all three.
John

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I knew you'd like it!
A later post said the air drill was running loaded at 4,500 rpm, a close approximation of the bike doing 80mph.
He'd thought of an armature winding earthling through the core and thence to the frame, but replicating it with a wire or isolating it made no difference.
However, the sheared key has not come up and I'll pass it on.
Interesting idea Peter, you're quite right I'd not thought of that.
Regards,
Kim Siddorn
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On Mon, 8 Dec 2003 13:16:02 -0000, "J K Siddorn"

12.3 volts ain't going to charge no battery no way, that's the standing voltage of the cells without charge.
Sounds like he has a short to ground somewhere as John mentioned.
With electronics in the regulator he should be seeing 13.60V to 14.20V with a reasonably fully charged battery.
Suspect a funny in the regulator/generator earthing, again as John mentioned.
Kind regards,
Peter
Peter Forbes Prepair Ltd Luton, UK email: snipped-for-privacy@easynet.co.uk home: snipped-for-privacy@easynet.co.uk
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Peter, I agree that the voltage is low but assumed that the bike generator was one of those low output jobs which was struggling to charge a battery which was already well flattened. Knowing the age (V. old) of the motorcycles which excite Kim, I thought this a reasonable assumption. The fact that the ammeter showed a charge would support that.
John
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Wouldn't matter, the cells are still a nominal 2V each so 12.3 really is almost flat.
Looks like there may be a real funny in the system, like negative earth regulator and positive earth battery...
Peter
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mmm, not too sure I'm afraid. If a battery is flat it will take a lot of charging current. If the charger cannot deliver that amount, the battery will not reach 14 volts. If the charger is capable of supplying the current, yes the battery holds at 14 volts. I have seen very flat batteries connected to (old, small) battery chargers where it takes quite a while for the battery voltage to reach 14 volts. Plenty of current flowing but still below 14 volts as the charger cannot supply that much. Of course, there is also the question of how accurate is the meter. Personally I suspect it is reading low.
John
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On Fri, 12 Dec 2003 09:06:35 -0000, "John Manders"

Unless you have a battery that performs like no other, the cells will respond immediately to a reasonable charge current, as long as there is no other loading on the battery at the time.
A 10AH battery which is flat, will show around 12.4 to 12.5 volts, and a 2A charge will raise that terminal voltage almost immediately to near 13V, a 5A charge would raise it to 13.4 or so and so on. Nicads operate on a more extreme voltage rise, a flat Nicad cell will go from 1.3 to 1.6 volts with almost any charge, but unlike lead-acid cells, you cannot use the cell voltage to determine state of charge of the cell.
Kind regards,
Peter
Peter Forbes Prepair Ltd Luton, UK email: snipped-for-privacy@easynet.co.uk home: snipped-for-privacy@easynet.co.uk
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is he turning it the same direction with the drill as when it's in the engine? sammm

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To conclude this thread, he's fixed it - but like many electrical problems I've encountered in my time, he knoweth not what he did!. Here's his posting.
"Dear All, Well, every thing inside the generator checked out serviceable so with a dab of grease on the bearings, reassembly, a motoring test on the bench and back on the bike. Ran it with the air drill on the bike and it all checked out. The ride home produced a charge and balanced with the lights on. What was the problem, who knows, one of those mysterious electrical problems I suspect. A few years ago a fault on another bike's generator turned out to be lack of electrical contact between the generator body and the engine. I thought that this may have been the cause this time but thinking back, this problem always passed the motoring test on the bike with the earth going back through the body to the engine so that proved that. Thanks Kim. Yes I know that there should have been 13.8 volts across the battery under full generator charge and I just knew that someone would comment on that. Considering that the battery voltage at rest with the lights on was approx. 11.9 volts, then when the generator ran up it stepped up to 12.3 volts and with the air drill not quite getting it up to speed, then I figured that it was pretty close to what I wanted. Thanks for all the comments, hints and tips."
- and rest ;o))
Regards,
Kim Siddorn
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