The present task was to make up a usable magneto. He'd got the base sorted out, and was in the process of re-wrapping some rather dubious looking coils.
I've always been a bit skeptical of these coils insofar as it seems to me they're easily damaged, and there's no easy way of determining if they've got shorted turns. After all any insulation must be around 100 yrs old, give or take, and have lived most of that time in quite a demanding environment.
So I suggested a very accurate low-ohms meter, or maybe a Time-Domain Reflectometer might be usefully employed in testing said coils before they were offered up to the base and installed into the engine. Chris initially said he had three TDR's, but later amended that number to zero ... upon which I enquired if he had an oscilloscope.
Bear in mind Mr T is an electrician of some standing, so I didn't think I wasn't being completely unreasonable!
Apparently he didn't have a 'scope, or at least that was what I interpreted the response to mean, however I remembered that some time ago I'd purchased a small 5MHZ device to carry around in the car, which was with me; I had an idea we could make up a small jig to test his coils using this and whatever else was laying around the workshop.
A few short minutes later and we had just such a thing. Here's a photo of it; essentially it's a Model T magneto magnet clamped with a bolt at mid-mass, and that chucked up to a battery drill held in a vice. Adjacent to the outer swing of said magnet is three pieces of timber arranged in order to hold a magneto coil.
The theory was that with the magnet spinning in the drill and the coil connected up to the 'scope we should see a resultant waveform, and that it should be possible to measure the peak of that in order to determine the condition of each coil relative to its compatriots. The thought was that a damaged coil would probably not generate as greater a voltage and/or the waveform would be 'different'.
As it turned out the idea seemed to work quite well. We tested all the coils, and found that all but one produced between 23 to 30mV RMS, with 25mV (47mV pk-to-pk) being typical. It's should be noted this was a very weak magnet spun at low low speed, and the coil was some distance from the magnet swing, hence the low voltage. I suspect the variation was at least partly because we were experimenting, and the coil mounting was fairly loose, however one coil consistently showed under 21mV, which also happened to be the worst looking coil from a physical perspective. To me this was suggestive of an internal issue with that coil that required further investigation and repair, or replacement.
Here's a couple of images showing the jig in operation:
Just to confirm the system we also tried a scrap coil that had been somewhat unwound, and found it to produce a result different to the others. Additionally we attempted to simulate a shorted turn using a nail jammed into the coil inner, but this didn't give quite the expected result. This probably requires a little more thinking on, or at least a more thorough test.
Ultimately it's hardly a precision instrument, and as a test device it has various issues on could pick holes in, however as a rough 'n ready check I think it worked remarkably well. If one took the trouble to make a tidier unit and have a good 'standard' coil + magnet + fixed RPM I expect you could begin to reasonably measure coils against the standard and scale them appropriately.
Using the same apparatus but substituting the magnets instead of the coils it should also be possible measure the relative effective strength of each magnet's field and determine the best units to install (yes, I know there is a tried 'n true 'old school' method that works, but this feeds the inner nerd
That's it, I don't even have a name for this thing, and it's hardly worth coming up with an official design, but perhaps it will be of interest to some.
Luke.
