Classic Harley Tech: Magneto Rebuild, Part 4 Disassembly & Detailed Inspection (continued)

This is installment #4 of a multi-part series by an anonymous friend (not me) who gave us permission to repost here.

5) The magneto has a rewound coil in it, as well as a new condenser (although,  as I wrote in a previous post, the latter already had failed). The electrical properties of the primary and secondary with the armature in the magneto were:

Primary___: R= 0.58 Ohms; L= 3.40 mH
Secondary: R= 4.67 kOhms; L=16.38 H

The inductances would be somewhat lower if outside the housing. For comparison, the primary of one of my two spare ZEV armatures is still good, and it has essentially identical values of R=0.57 Ohms and L=3.52 mH (next photograph).

Although I don’t have a functioning Bosch ZEV secondary to measure, the resistance of the secondary is reasonable, although the inductance is about 50% higher than that of a comparable Lucas armature. My conclusion was that the person who rewound this coil neither made it with significantly too many windings of too fine wire, nor too few of too coarse.

———Sidebar (not in chronological sequence) ———
I am writing the following three paragraphs after the fact. I realized after having sent the magneto back that I should have measured the transformer ratio, which would have told me if the secondary has the correct number of turns. All that is required is to run a small AC voltage from a signal generator through the primary and measure the voltage developed in the secondary. This transformer ratio is ~35 for similar Lucas and BTH magneto armatures. The actual ratio of turns is 50, but the transformer loss due to generation of eddy currents is ~30%.

The reason the number of turns in the secondary concerns me is that, not having x-ray vision, I have no way of knowing if the rewinder used insulation between its layers. Not doing so would result in a less robust coil. The R and L values for the secondary are consistent with wire that is ~20% too large in diameter, but with ~50% too many turns. Combined, that would give the correct resistance, but an inductance that was 50% too high. The resistance and the inductance of the primary are as they should be, consistent with the primary being made up of the correct number of turns (which determines L) of the correct diameter wire (which determines R).

In order for a coil with too many turns of too-large diameter wire to fit in the volume available means interlayer insulation likely would have been omitted. Omitting this insulation makes it easier to wind coils, which is all the more reason to think it might not be there. The secondary winding would have about 30 layers, and in normal operation the coil output would be limited by the spark plug to no more than 6 kV. This means there would be ~200 V between adjacent layers, which the insulation would be able to handle without problem. But, if a plug wire fell off and the voltage rose to 18 kV, adjacent layers of the secondary would have to withstand ~600 V. That still is within the limit a good quality insulation can withstand. But, I don’t know the quality of the insulation, or if the coil was vacuum impregnated so the insulation can’t abrade. Unfortunately, the limited time I had to get this magneto back to the engine builder made me overlook this test that I otherwise would have thought to make, so this will be a lingering uncertainty.
——— end of sidebar ———

Unfortunately, the restorer got carried away with his use of epoxy on the coil. The coil itself is not only potted with epoxy, it is firmly attached to the armature by excess. However, what is not possible to tell is if only the outside of the coil had been slathered with epoxy, or if it had been vacuum impregnated. I made a note of this because it is a concern, but I will have to think about how I want to deal with the uncertainty.

The reason vacuum impregnation is an issue is that the motion of any wire through a magnetic field subjects it to a sideways force. If the armature coils are tightly wound, there is no place for any of them to move, so that force wouldn’t matter. However, if there is any room for movement at all, the oscillating motion due to the field changing direction twice every revolution can result in the insulation on a wire slowly abrading away, eventually shorting it to its neighbor. Such shorts do not necessarily cause the armature to fail completely, but they do degrade the output. Vacuum impregnation using an appropriate low viscosity epoxy or other potting compound fills the voids, making motion of the wires impossible.

Winding an armature coil is a tedious job, and I don’t begrudge the ~$150 that people charge for doing this. However, I do begrudge the quality of some of the coils that are produced. Proper coil winding is a craft, and not all magneto coils I have seen were wound by people who mastered it. I’m reminded of something I learned from an instrument maker 40 years ago: “There’s a difference between handmade, and homemade.” Too many coils I’ve seen over the past 15 years are of homemade quality (and, again, I’ve only seen the insides of magnetos that have failed, so this is not a representative sampling).

Magneto armatures were not my first exposure to coil winding, so my disappointment with the quality of rewound armatures is based on first-hand knowledge of what is possible to do oneself. In any case, I felt I had no choice but to purchase the equipment needed to do this in my own workshop. Having a coil winder meant I was no longer dependent on anyone but myself for any aspect of a restoration. Anyway, I have my own German-made coil winder as well as equipment for vacuum impregnation of potting compound. However, the short time to get this magneto restored, interrupted by the trips I have to take before it has to be shipped back, means I would much rather use the existing coil if it survives my stress tests (to be described later).

I haven’t used my coil winder for a few months so the area around it has accumulated some clutter. But, the photograph shows it with the left half of the pair of brackets I use to hold armature like the one in the Bosch (and Lucas and BTH). A mating piece is held by the tailstock whose dead center can be seen at the edge of the photograph. The very thin wire of the secondary is fed from the spool through a spring-loaded tensioner to help keep it from breaking (partially shown at the top, with a pulley at the end of a pink rod that is connected to the tensioning mechanism), and is automatically layered by an auto-reversing mechanism whose pitch is adjusted to match the diameter of the wire. The speed is controlled by a rheostat in the foot pedal to allow starts and stops to be made smoothly, and a counter keeps track of progress.

After winding a coil I pot it with an appropriate low viscosity compound and use a vacuum pump to impregnate the liquid into the voids between the windings. I use thick-walled plastic containers to hold the compound and the armature under vacuum, allowing me to see what is going on without the risk of implosion if a glass container cracked.

I set up the next photograph using a partially-disassembled armature in the plastic container to show the scale. I am not being coy about not naming the potting compound I use, but recently I’ve been experimenting with several new ones and don’t want to “endorse” any until my tests of those are completed.

The excess epoxy on the present coil would be OK (except, see below), except part of the coil is slightly too large and has rubbed on the inside surface of the body. I can’t tell at this point in the restoration if it is simply excess insulation that is rubbing, or if wire is involved. But, it’s possible to check the output of the coil with my Merc-o-tronic tester. The output of the coil can be seen sparking across a 5 mm gap in the window in the tester, so it passed this test (however, I’ll do a longer test of just the coil itself before declaring it to be good).

As an aside, I have at least a dozen magneto testers, but I almost always turn to a Merc-o-tronic 98 or 9800 first when checking a coil. Although these can make six types of tests, all but one of them — the current required to generate a spark across a 5 mm gap — can be better made with more modern instruments. However, if I only could have one instrument for testing magnetos, this is what I would choose.

6) As long as I can remove the end cap that contains the condenser, removing the epoxy that encases it, although a headache, and replacing it with a proper capacitor should be straightforward (note, the photograph above was taken during a later test I made after I had removed the end cap).

Send questions or comments to classic.vehicle.electrics@gmail.com.

[detailed inspection to be continued]

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