a carbon brush that completes the primary circuit
that black cylinder (the earth brush) is part of the secondary
circuit, ... bypassing the armature bearings and their insulators.
Thanks for catching the miswording.
To make sure the material used for the aftermarket slip ring isn't conductive ... The resistance was over 11 GOhms with a 2500 volt potential applied ... This amount of leakage is completely negligible, so the material in the slip ring is good, as is the insulation between the coil and the armature.
I'm not so sure you should draw the conclusion from this test that the slip ring is good. 11 gigaohms at 2500 volts can certainly break down to a far lower resistance at the higher operating voltage of the slip ring. Breakdown of aftermarket slip rings is indeed a problem, but in my experience it often only rears its head at elevated temperature. A slip ring may test good at room temperature, but start breaking down at operating temperature.
Your experience certainly is different than mine. I've used both an IR thermometer and a thermocouple to measure the external and internal temperatures of a number of magnetos, and my measurements found the highest temperature a slip ring will see under the worst of conditions is well under 50oC. There is simply no physical mechanism I am aware of that could increase the electrical conductivity of whatever impurities are in a phenolic-like plastic by many orders of magnitude over that limited range. And, even if there were, the failure mechanism for a slip ring is completely different than that of a condenser that is warm. In the case of a slip ring it would be dielectric breakdown of the material, not increased electrical conductivity of the material itself.
I don't know what measurements you made that found slip rings that are good at room temperature break down 20-30oC higher. All I can guess is that either the slip rings you measured started out with very much lower resistances than 10 GOhm, or you misinterpreted whatever observations you made that led you to this conclusion. Perhaps arcing through a track of carbon that you hadn't noticed. This is why I test them at 2.5 kV, to reveal any such issue.
I am also concerned that, with this test, you are applying the 2500 volts not only to the slip ring conductor and the HT tail of the HT winding, but also to the whole of the HT winding and more worryingly the LT winding. In operation, the insulation around the LT winding sees voltages of perhaps 250 V, and it might be fair enough to test it to three times that voltage, but testing it to ten times that voltage is, IMHO, asking for trouble. Even if it passes the test, the test may itself trigger the onset of a later breakdown. If you were a professional restorer and if you were to do that, I think you ought, as you have said in the past, to give your customers their money back.
You have a fundamental misunderstanding of the electrical circuit of a magneto. When the 2.5kV was applied to the coil in that test all of the wires of the primary and secondary were elevated to that potential. There was no voltage whatever between one layer of windings and the next, so the insulation on the LT (and HT) wires is irrelevant.
The only places where there was a large electric field during this test was between the operating surface of the slip ring (where I had wound the stainless wire) and the nearest piece of metal on the armature, and between the bottom winding of the primary and the core of the armature over which it was wound. Had the person who made this coil wound the first layer of the primary directly on the armature, rather than properly having a layer of insulation there first, the primary certainly would have broken down since whatever insulation is on the LT wire itself would have failed at less than 1 kV maximum. Had the 2.5 kV induced arcing of the primary to the armature, it would have been a good thing, since it would have revealed the fact the coil had been defectively wound. This is why this is an important test to conduct and, rather than giving someone's money back for having conducted this test, quite the opposite.
I use an Eisemann tester for extended tests of coils ... I ran the coil for an hour on this tester with the gap set to 5 mm (6 kV). The coil works fine so I will use it instead of winding one of my own.
The Eisemann tester looks like a nice piece of kit. But does it get the armature up to operating temperature, or do you heat the armature elsewhere and then test it on the Eisemann while it's still warm?
Good question. I should have written in that section that the only way to be sure the coil would not fail in operation would be to test it while at elevated temperature. This is what I would have done at this point in process of rebuilding one of my own magnetos. However, in the case of this Bosch, I knew I would later test the entire assembled magneto for an extended period at elevated temperature (to be described in a future post), so I bypassed testing the coil alone at elevated temperature. In several places in my posts I explicitly point out where I took shortcuts because of time constraints forced by the deadline, and I should have mentioned that here as well.
Currently another supplier is advertising replacement condensers that are of a type not rated for pulsed high current applications, and whose dielectric layers are made of a porous oxide. Even if they survive the current pulses of a stressed magneto, no extended environmental testing has been done of them, so there is no way to even guess how long they might last in the ozone-rich atmosphere near the points before delamination or breakdown of the oxide might set in.
If you are going to have yet another tedious
go about the Brightspark EasyCap, it might help people if you named names instead of leaving them wondering. The fact is EasyCaps have undergone extended environmental testing; they carry on working; they are guaranteed; if one were to fail, it is a ten minute job to replace it
without the need to pull the armature apart and mill out the old one
as you have described in detail above; and last but not least the customers and their bikes are happy.
I had written all that I planned to about condensers, so if you consider the following text tedious, you're responsible for prompting it:
Your website is not the easiest to navigate, but the only information I could find on "extended environmental testing" is a page in your FAQ section where you describe finding no degradation after one of your capacitors spent a day in a pan of hot water. That might be a fine test for sugar cubes, but says nearly nothing about the long term survival of your capacitors in a magneto. I can only infer that the fact you devote a page by itself to this test, as if it were meaningful, that it indicates you actually think it is meaningful, and that it somehow counts as "extended environmental testing." It does not. Further, your capacitors are located in the points housing, where there is an elevated level of highly reactive ozone created by the sparks from the points. I could find nothing on your site about testing whether the porous oxide of your capacitors degrades in the presence of ozone.
Given the long and checkered history of inappropriate capacitors being sold for use in magnetos, the onus falls on the supplier to conduct appropriate tests to indicate yet another new capacitor isn't going to fail. As far as I could find on your website, you have not conducted such tests, which are needed independent of whether or not you offer a free replacement if/when they fail. There is also the issue of the degraded performance of the magneto itself as a result of removing the armature to install any condenser, including yours, which I will deal with in a quantitative fashion when I come to the point of magnetizing the Bosch in a future post.
If you are going to have yet another tedious
go about the Brightspark EasyCap, it might help people if you named names instead of leaving them wondering.
I searched the archives and can't find that I've ever made a single negative comment about your EasyCap. The sidebar about capacitors in my previous post is the first time I've said anything about your capacitors, and even then not by name. Unless I've missed something, if you say people are having a "tedious go" about the product you are selling, it means people in addition to me have issues with these capacitors.
Yes, that was a very interesting article. One thing I did find puzzling about it was Dr Falco's reasoning that a breakdown of the waxed paper dielectric caused an increase in the equivalent series resistance of the capacitor, when schoolboy physics would suggest it causes a decrease in the equivalent parallel resistance of the capacitor. Did you understand that?
Schoolboy physics doesn't have much to say about the breakdown of capacitors. But, if it did, it would say that the use of an equivalent series resistance (ESR) is a very widely used approximate model found to be quite useful in describing the electrical behavior of capacitors when used in a circuit. If you Google "equivalent series resistance" (in quotes) you will get over 800,000 hits, indicating how widely used this model is. On just the first page of hits you will find good explanations for why it is so useful to lump together all the complex internal physical phenomena into a single ESR
... if you use any capacitor other than these Panasonics it is at your peril.
Crikey, that's not much help then, since they're out of production.?
The same criticism certainly could leveled about the Lucas
HT brushes I suggest using, since they've probably been out of production for 40 years. And yet, it's possible to find them.
installed it in the end cap using the minimum amount of 3000 psi epoxy necessary to insure it would stay in place.
We have seen some where the resin totally fills the end cap, bridges the gap between it and the windings, and encapsulates the tails of the winding. Removing a dud capacitor without also damaging the winding insulation or the tails is then, ummm, difficult to say the least.
Yes, this is one of the reasons I cringe when I discover a magneto has been "professionally restored," since this is one of the atrocities I sometimes find that has been committed. However, although it is a headache, I've found it to be not all that much worse than the one I had to deal with on this Bosch. It's easy to tell where the coil ends, so the epoxy holding the coil to the endcap can be milled away by mounting the armature tilted slightly from horizontal (or by carefully using a Dremel).
But, the broader issue is whether there is an easy way out. One that avoids having to deal with whatever condenser is currently in the magneto, but that still restores the performance to the as-new level. I would love it if there were. If there were, I wouldn't have had to waste an hour of my life milling away all the epoxy the guy used who restored the Bosch. However, the fact I went to all that trouble isn't because I enjoy cleaning up wads of epoxy chips. It's because my experience tells me there is no proven-reliable capacitor I could use other than the Panasonic. I wish there was an easier way to restore proper performance, but I am unaware of any such easy way out.
Some people use a blob of bathroom sealant, which seems to work quite well.
The use of bathroom RTV sealant is definitely the sign of someone who doesn't know what he is doing. If the RTV is still in place, it only means the electrical leads on the inappropriate replacement capacitor were strong enough to hold it in place for the short time the magneto was used before the capacitor failed, because the shear strength of RTV is too low to have done any good. Do not
use bathroom sealant.
After reassembling the armature with the new capacitors I checked the output using one of my Merc-o-Tronic 9800 testers. The blue arc in the window of the tester shows the operation of the coil is reliable at a test current of just over 1 Amp supplied to the primary. This is as it should be, so the completed armature passes this test.
Again, do you recommend a room temperature test or a hot test?
To briefly repeat the answer I gave earlier, about the test on the Eisemann tester, I "always" (except for this Bosch) run this test at elevated temperature. A future post will describe the extended test I ran of the assembled magneto at elevated temperature, which it passed.