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Re: Restoring a Rotating Armature Magneto
Ken Tee, R.I.P. #466315 12/05/12 12:34 pm
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Originally Posted by Ken Tee
Anyway, Magnetoman, what you said about Young and Griffiths is untrue (they were talking about a horseshoe magnet and keeper, not a magneto and its armature), and what you said about Spreadbury now also turns out to be untrue (he was talking about a generator, not a magneto). Very sad.
The physics of electrical generation by a generator is identical to that of a magneto. Maxwell's Equations apply to both in an identical fashion. Spreadbury describes how a generator/magneto that is to be disassembled in the field without the possibility of remagnetizing it has to be overdesigned if it is to still function, albeit at reduced output, to compensate for the loss of magnetism when it is disassembled. As I said more than once, a post-War magneto will still function after it has been disassembled, albeit at reduced output because of the loss of magnetism.

The quantitative loss of magnetism Spreadbury reports for the type of "generator" he addresses is consistent with what my measurements show for a magneto if it is disassembled and then reassembled. The power of an understanding of physics is that the same general principles (e.g. Maxwell's Equations) describe a wealth of phenomena, so understanding the commonality of a "generator" and a "magneto" lets data from one be correctly used to understand the other. What you wrote is as silly as saying measurements on a BTH magneto tell us nothing about a Lucas, because different names are stamped on their housings. For both books I correctly summarized for this thread the relevant conclusions that would have taken many pages to describe in complete detail. What I said about Spreadbury is completely true, as is what I said about Young and Griffiths.

For the reasons I explained earlier in this thread, the way your "internal magnetizer" functions leaves the magneto with significantly reduced output over that which it would have if properly magnetized. This is what an understanding of the content of these books shows, and this is what my measurements show. However, your lack of understanding of physics is no excuse whatever for the despicable way you have repeatedly called me a liar. Even if there weren't issues with your goods and services, I personally would never knowingly do business with someone who behaves like you.

p.s. I just noticed that Ken altered the text of mine that he quoted in his post, making it appear as if I had emphasized something that I had not, and that I had not emphasized something that I had. What I wrote was "F.G. Spreadbury shows that the output from a magneto with a Ticonol ("Alnico") magnet is reduced by 23% in actual operation if the armature is withdrawn and then replaced after magnetization." Taking the time to alter someone's text to change the emphasis and then presenting it as if it were a direct quote is unacceptable, and begs the question of whether he has altered anything else.

Last edited by Magnetoman; 12/05/12 2:39 pm. Reason: added p.s.
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Re: Restoring a Rotating Armature Magneto
Magnetoman #466398 12/05/12 9:56 pm
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Seeing that magnetoman has been outed by someone else I think people who are reading this tome should know who he is:

http://www.youtube.com/watch?v=cFxM6suZuw8

The good Dr. has one of the most extensive annotated motorcycle and related mechanical engineering libraries in the World. Not only has he read them all, but collected a huge database allowing him to research subjects quickly.

While his approach follows a strict academic model that some would consider overkill, he is not satisfied to have the skill to do something, but to also understand the underlying physics.
Now, if I could get Dr. Falco, Kevin Cameron and Rob Tuluie together with a tape recorder or better yet write for Vintage Bike...
John Healy

Re: Restoring a Rotating Armature Magneto
Richrd #466579 12/07/12 2:39 am
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Note: A version of the material in Appendix I and II was part of a series of two articles I wrote for the Fall and Winter 2011 issues of 'The Antique Motorcycle,' the journal of the Antique Motorcycle Club of America.

Appendix I: Post-WWII Magneto Condensers

The Condenser

For the reasons I explain below, if you have a post-WWII British bike, either your magneto's condenser already has failed, or it soon will fail. When the condenser does fail, there are at least a dozen different categories of replacements (tantalum, metalized polyester, ceramic,…) made by dozens of manufacturers that have the necessary capacitance and will fit in the available space in the armature. Unfortunately, despite anything you might have been told before, essentially all of these will fail in operation, because most lack the ability to handle high current pulses. However, as I will describe in a subsequent post, I have conducted a series of tests that identify the type of replacement condenser that will provide many years of service.

Symptoms of a Bad Condenser

Often people who have a malfunctioning magneto say "I need to send it in for a rewind." In my experience, once other potential sources of problems have been eliminated (most commonly, a fouled plug, cracked high tension lead, short in the cutout circuit, or a worn or seized high tension pickup), in nearly all cases the problem is a bad condenser, not a faulty coil (unless it is a replacement rewound coil, in which case the coil could have developed an internal short). If your engine runs fine when cold, but misses and backfires heavily as it warms up, odds are high it is due to a bad condenser. Obvious sparking at the contact breaker points is the "smoking gun" of a faulty condenser.

Why all post-WWII Lucas and BTH Condensers Have Failed, or Are About to Fail

In 1915 B. H. Davies wrote "Riders need not worry about the action of the condenser, which never gives any trouble." In fact, this was largely true at the time he wrote it. Unfortunately for us, pre-WWII mica condensers gave way to paper condensers that were developed in no small part because of the disruption in the supply of mica from Zimbabwe (Rhodesia) and India caused by WWII. Also, while mica condensers don't suffer the specific degradation problem described below, they can fail from mechanical delamination or corrosion of the leads.

It turns out condensers (capacitors) of identical materials and construction as the ones Lucas and BTH used in their post-WWII magnetos also were of much interest to the telecommunications industry. Because of this, the greatest solid state physics laboratory of the time, Bell Telephone Laboratories, researched these "impregnated paper capacitors" in great detail. A 1946 book by a Bell Laboratories scientist, along with eleven research papers on paper capacitors published between 1942 and 1961, contain information that fully explains the cause of the problems post-WWII British magnetos are facing today (the book and papers are listed in the References section at the end of this post).

The reason these particular capacitors merited so much research was they filled an important niche. Each of the millions of phones Bell Telephone supplied to its customers contained one of these impregnated paper capacitors as part of the circuit that made the phone ring, so cost and longevity of individual components was a major concern. Paper capacitors, impregnated with wax, were much cheaper to produce than any alternative, including mica, and they worked reasonably well for as long as they worked. They weren't needed to last forever, but only long enough to span the gap until the next generation of telecommunications equipment was deployed. Because of this, much effort went into finding chemicals to add to the wax to ensure that essentially none of the capacitors would fail for at least a decade. While it was essential that no capacitor fail for at least 10 years, achieving longevity beyond 20 years for a significant fraction of them was not of much concern for manufacturers of these paper/wax capacitors, since that was beyond the planned service life of the equipment that made use of them.

A perfect paper/wax capacitor would consist of thin sheets of metal foil separated by thin sheets of paper soaked with some appropriate wax whose dielectric constant is as large as possible and whose electrical resistance is infinite. When it is fresh, chlorinated naphthalene -- trade names included Halowax, Seekay wax, and Nibren -- works quite well as that wax. As an aside, this substance is a PCB, which is now internationally banned because it is carcinogenic. Although the electrical resistance of this wax is not infinite at room temperature, and decreases rapidly with increasing temperature, it still remains high enough when the wax is fresh not to result in unacceptable capacitor performance. However, even with the best chemical stabilizers, the wax still degrades with time, although accelerated tests showed it would be good enough for the required decade of service.

The following photograph and magnified inset shows the paper and wax layers in a Lucas condenser.

[Linked Image]

There are ~125 layer pairs of area ~1"x1-1/2", with a separation between metal foils of ~0.001". Waxes have dielectic constants in the range 2.1-3.1. Using a value of 3 for an estimate, capacitance = dielectric constant x permittivity of vacuum x Area/separation x 125 pairs = 0.11 uF. The actual capacitance from Lucas literature is 0.15-0.18 uF, agreeing very well with this estimate based on my measurements of the internal structure of the condenser.

Even with stabilizers, research showed it also was essential to hermetically seal the capacitors because the wax is somewhat hydroscopic, and moisture accelerates the breakdown. When the wax breaks down it releases hydrochloric acid which then attacks the aluminum sheets of the capacitor, releasing aluminum chloride. Unfortunately, aluminum chloride accelerates the breakdown of the wax further, in turn releasing even more HCl. While breakdown of the wax happens no matter what, the process rapidly accelerates in the presence of moisture. The slight statistical variation in the permeability to moisture of the plastic seals is why Lucas and BTH magneto condensers fail over a range of ages. However, as the Bell Labs aging tests showed, even if you found a perfectly sealed new old stock Lucas or BTH condenser from the 1960s. it now would be approaching its maximum lifetime due to the chemical breakdown of the chlorinated naphthalene wax that happens even without moisture, and even if the condenser has never been used.

A few years ago I was given a truly new old stock Lucas condenser. The condenser was sealed in thick wax paper, in a cardboard box, in turn sealed in thick wax paper, and finally wrapped in paper on which was printed the part number and manufacture date of September 1956. All of the layers, including the outermost paper, were in fine condition. However, when I measured the electrical properties after extracting it, the capacitance was 0.601 uF (~4x larger than when new), and the dissipation factor was 15x larger than when new. This condenser would not have functioned if installed in a magneto, so paying a lot of money to buy a new old stock one on eBay is a very bad idea.

For a magneto, the relevant electrical consequence of the breakdown of the wax is an increase in the Equivalent Series Resistance (ESR) of the condenser. The condenser is connected in parallel with the contact breaker points specifically to provide a low AC resistance bypass for the current, i.e. to suppress arcing. While a perfect condenser would have ESR = 0 Ohms, as the ESR increases due to the growing electrical losses in the wax, the condenser's effectiveness as a bypass decreases, and the arcing increases. Typically, the first sign of problems is a magneto that functions acceptably when cold, but fails when warm. The reason for this is the electrical conductivity of the deteriorated wax changes exponentially with temperature. As a result, at this point in the life of the condenser, when cool the ESR is still low enough for the magneto to function, but when warm it becomes too high to suppress arcing. Whether the condenser is used or not, the wax will continue to deteriorate with time, and the ESR soon will be too high for it to suppress arcing even on the chilliest night. Confirming this behavior, I have measured over 50 Lucas paper/wax condensers, and their ESR values neatly follow a smooth curve that allows me to calculate how much longer any particular still-functional condenser will continue to suppress sparks.

The results of accelerated testing were known in 1946, so it is quite likely that Lucas and BTH were aware at the time these paper/wax condensers would begin failing in significant numbers starting in a decade or two. But, they also knew only a fraction of vehicles would remain in use after that many years anyway. And, when the condensers did fail, they could be replaced with no more cost and effort than, say, fitting a worn engine with a new set of rings. Also, although mica was again readily available, the cost to make a condenser with it was 20x higher than that to make one of paper/wax. In an industry driven by customers who made their purchases based on "value" (i.e. cheapness), and where warrantees expired after one year, their choice to use paper/wax was quite reasonable.

References

B.H. Davies, The Modern Motorcycle: How to Run, Ride, and Repair It (C. Arthur Pearson, London, 1915).

M. Brotherton, Capacitors: Their Use in Electronic Circuits (D. van Nostrand, New York, 1946).

D.A. McLean, L. Egerton, G.T. Kohman, and M. Brotherton, Paper Dielectrics Containing Chlorinated Impregnant: Deterioration in D.C. Fields. Industrial and Engineering Chemistry vol. 34, p. 101 (1942).

D.A. McLean and L. Egerton, Paper Capacitors Containing Chlorinated Impregnants: Stabilization by Anthraquinone. Industrial and Engineering Chemistry vol. 37, p. 73 (1945).

L.J. Berberich, C.V. Fields, and R.E. Marbury, Characteristics of Chlorinated Impregnants in Direct-Current Paper Capacitors. Proceedings of the I.R.E., p. 389 (June 1945).

L. Egerton and D.A. McLean, Paper Capacitors Containing Chlorinated Impregnants: Mechanism of Stabilization. Industrial and Engineering Chemistry vol. 38, p. 512 (1946).

D.A. McLean, Paper Capacitors Containing Chlorinated Impregnants: Benefits of Controlled Oxidation of the Paper. Industrial and Engineering Chemistry vol. 39, p. 1457 (1947).

L.J. Berberich and Raymond Friedman, Stabilization of Chlorinated Diphenyl in Paper Capacitors. Industrial and Engineering Chemistry vol. 40, p. 117 (1948).

J.R. Weeks, Metallized Paper Capacitors. Proceedings of the I.R.E., p. 1015 (September 1950).

H.A. Sauer, D.A. McLean, and L. Egerton, Stabilization of Dielectrics Operating under Direct Current Potential. Industrial and Engineering Chemistry vol. 44, p. 135 (1952).

D.A. McLean, H.A. Birdsall, and C.J. Calbick, Microstructure of Capacitor Paper. Industrial and Engineering Chemistry 45, 1509 (1953).

L. Borsody, New Impregnation for Paper Capacitors. IRE Transactions on Component Parts, 15 (March 1960).

Paul D. Garn, Stabilization of Capacitors. Industrial and Engineering Chemistry 53, 311 (1961).

Re: Restoring a Rotating Armature Magneto
Magnetoman #467734 12/15/12 1:34 am
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Appendix II: Replacement Condensers for Post-WWII Magnetos
Note: A version of the material in Appendix I and II was part of a series of two articles I wrote for the Fall and Winter 2011 issues of 'The Antique Motorcycle,' the journal of the Antique Motorcycle Club of America.

Tests to Find a Suitable Replacement Condenser

The full range of parameters that affect a magneto's condenser are current, voltage, temperature, and frequency (i.e. for a condenser, it's not just the magnitude of the voltage across it, but how fast it is applied). Some of the less-common instruments that have allowed me to make my studies of condensers include a variable frequency impedance bridge for determining AC losses to within 1%, a resistance bridge with maximum range 50,000x higher than a standard "megger" insulation tester, and a 200 Watt pulse generator for high current pulses of rise time 0.01 us and repetition rate up to 1 MHz. I also have a four-channel 400 MHz oscilloscope, a 40 kV probe for directly measuring the time dependence of the output of the coil up to 75 MHz, a 2 kV probe for directly measuring the voltage across the condenser up to 100 MHz, and specialized current probes for directly measuring the flow in/out of the points, condenser, and high tension leads up to 60 MHz. Stated differently, this oscilloscope and probes have allowed me to simultaneously measure all electrical parameters of operating magnetos to within less than 0.02 us of the onset of ignition. Even on an engine at 6000 rpm, in 0.02 us the contact breaker points have opened by just 36 uinch (0.92 um), which is only twice the wavelength of light.

As for how I conducted my studies, after using manufacturers' specifications to select the most promising replacement condensers (described in the next section), I tested them when attached to an armature and contact breaker points, as well as with other appropriate laboratory tests made directly on the condensers. Using a modified distributor tester and two commercial magneto testers I was able to simulate actual field conditions in my laboratory, including with a condenser and coil in an oven at temperatures up to 150 oF (65 oC). Where appropriate, I then used those measurements to accurately simulate certain parameters under accelerated and/or overstressed conditions. For example, I used a 200 Watt high frequency pulse generator to subject the replacement condensers to current pulses 60x higher than the ones I had measured using a magneto tester, doing so at the equivalent of >100,000 rpm so that I was able to simulate "150,000 miles" of operation in minutes instead of months.

Some of my tests were a combination of simulations and "field conditions." For example, one long-term, test involved submerging condensers in 181 oF (83 oC) beakers of 30W Castrol and hot water for a number of months, taking them out periodically to measure using a low voltage capacitance meter (which measures them at only a few volts; much less than the several hundred volts they are subjected to during operation in a magneto) and a General Radio 500 V Teraohmmeter (which measures their resistance at an appropriately high voltage, but at DC rather than the ~100 Hz repetition rate of a magneto). Although neither of these electrical measurements tested performance under operating conditions, I designed this aggressive "environmental simulation" to test their ability to survive heat and solvents -- it would have been no good if a possible replacement condenser had the necessary electrical performance, but if it degraded in the presence of oil vapor or humidity.

The water test was particularly harsh since, even if a magneto fills with water, it will quickly dry once it is operating again, so seldom will the condenser of a functional motorcycle be in contact with liquid water for more than a short period. While the ones in oil had no detectable change in their capacitance or resistance, beginning at 1680 hours— the equivalent of them having spent 42,000 miles at an average speed of 25 mph submersed in hot oil and water — the resistances of the ones in water started dropping with time, from over 2 TOhm when new to ~100 MOhm at 3624 hours. However, 100 MOhm is still much higher than required to function in a magneto. Further, the ESR at room temperature of even the most degraded of them was still 3x lower than the lowest ESR I have measured of a Lucas condenser removed from a functioning magneto. Plus, the ESR remained unchanged at elevated temperatures, while by 120 oF (49 oC) that of the still-functional Lucas had increased a further 3x, to a value 10x worse than that of the most degraded replacement condenser.

Since my tests had established that prolonged immersion in hot water degrades the electrical properties of the condensers, albeit very slowly, at the 3364 hour point ("90,600 miles") I removed the water. After a further 120 hours ("3000 miles") in air at 181 oF (83 oC), the resistance of even the most degraded condenser had recovered to above 1 GOhm, and its ESR had improved by 58%. After an additional 336 hours ("11,400 miles" total since removing the water) the resistance was over 1 TOhm and the ESR more than 90% of its as-new value. The fact these properties were easily reversible indicates the measured degradation was due to the electrical conductivity of tap water that had slowly permeated the protective coating, rather than a permanent water-induced chemical breakdown of the dielectric. This means that in the actual operating environment of a magneto these replacement condensers would operate significantly longer than the at-least "90,600 miles" they survived immersed in hot water.

Periodically during my long-term "environmental" test of their resistance I also made a full set of measurements on these replacement condensers. After "90,600 miles" in hot oil and water, these replacement condensers then survived the equivalent of an additional 150,000 miles subjected to current pulses 60x higher in power than are generated by a magneto. I also connected the condensers and an armature coil to one of my magneto testers and measured all of their electrical properties at 150 oF (65 oC), comparing these measurements with ones I had done on them when they were new to see if I could detect any degradation. Judged from the lack of any apparent increased sparking at the contact breaker points, and unchanged oscilloscope patterns, all of these replacement condensers performed as well in a magneto after "90,600 miles" in hot oil and water as they had when fresh out of the box. Continuing on with hot oil only, my final complete set of measurements was at 7080 hours ("177,000 miles"), with the condensers again passing all the tests.

One typical accelerated lifetime test of electrical components is based on the observation that most chemical reactions approximately double in rate for every 10 oC increase in temperature. This "doubling rule" makes possible another kind of lifetime estimate. Assuming it applies to the chemical processes at work breaking down the dielectric material of the replacement condensers, surviving 7080 hours at 181 oF (83 oC) predicts they would survive at least 51 years parked in a storage shed at 73 oF (23 oC).

What Replacement Condenser Should Be Used?

The replacement condensers I recommend are a pair of Panasonic 0.082 uF polypropylene film capacitors (part no. ECQ-P4823JU). When soldered in parallel they produce a 0.16 uF condenser that fits into the available space in the end caps of Bosch, Lucas and BTH single and twin rotating armature magnetos. I hasten to add that it is possible to damage them by applying too much heat during soldering so, if you are not careful, they can be inadvertently made to fail before you even start. This capacitor has the published specifications that caused me to select it for my tests, the demonstrated electrical performance to survive the high voltage, high current pulses generated by a magneto, and the ability to survive the hostile environmental conditions of heat, oil, and humidity.

Although I believe the tests I've conducted are as thorough and comprehensive as they need to be, and although none of the capacitors failed, to extract a statistically meaningful minimum expected lifetime would require subjecting a much larger number of them to these tests. However, based on my measurements on a limited number of units, my conservative estimate is there is a very high probability these Panasonic capacitors will function without failure in a magneto for at least 140,000 miles or 40 years.

Unfortunately, these Panasonic capacitors are now out of production. However, certain other polypropylene film/foil capacitors made by other manufacturers are likely to function just as well as the ones from Panasonic, although I cannot recommend them until I have an opportunity to test them.

Importantly, no electrical measurements are even needed to know that any capacitor that easily fits into the cavity of an armature, with half the space left over, definitely is not up to the electrical rigors it will face. A half-century of developments in chemistry has resulted in significantly improved capacitor lifetimes, but no amount of development can overcome fundamental laws of physics. Surviving high current pulses requires relatively thick electrodes. Surviving high voltages requires relatively thick dielectric layers. From the known electrical properties of materials, what this means is any suitable condenser for this application necessarily must be quite substantial in size. Physics, not coincidence, is why the soldered Panasonic pair is remarkably similar in total length, width, and thickness to the magneto condensers it replaces.

Re: Restoring a Rotating Armature Magneto
Magnetoman #470880 01/05/13 9:29 pm
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Appendix III: Anatomy of a Post-WWII Lucas K2F Magneto

Although most students who took high school biology dissected a frog or fetal pig, I suspect few people who own a motorcycle have dissected a magneto. Much is to be learned in both cases (but without the aroma of formaldehyde for the magneto). Since the basic thread is about a Bosch ZEV, I've included several images to illustrate the similarities between it and the Lucas despite a half-century of evolution (just as the reason for dissecting a fetal pig is because of its evolutionary similarities to human anatomy).

The Armature

The next photograph shows the armature of a post-WII Lucas K2F twin magneto on the left, and a c1915-20 Bosch twin magneto on the right. Other than the asymmetry of the steel in the Bosch armature, needed because it is for a V-twin with offset firing angles, the similarity is remarkable. I won't go into the details in this Appendix, but the asymmetry of the armature and of the magnet pole pieces in the housing (shown later in this Appendix) advances and retards the firing each revolution by half the angle of the engine's V. That is, since this Bosch KEV is configured to work with the 45-degree V of a Harley-Davidson engine, one firing pulse each revolution of the armature comes 11.25-deg. (22.5-deg. engine) earlier than it would for a vertical twin like a BSA 650, and the second comes 11.25-deg (22.5-deg. engine) later, for a total difference of 22.5-deg. (45-deg. engine).

[Linked Image]

The next photo is repeated from an earlier post in this thread, and shows that the Bosch ZEV armature is made up of individual steel laminations. Early in the development of this type of magneto it was discovered that the eddy currents induced in solid-core armatures due to rotation through the magnetic field resulted in significant losses. Because of this, armatures are built up using laminated steel plates, each varnished to electrically isolate it from its neighbors to minimize eddy currents. The armatures of all Lucas (and Bosch, BTH, and all others) are similarly constructed using laminations.

[Linked Image]

The next photograph shows the armature of a Lucas K2F after I sectioned it through the middle. The six rivets holding the laminations together can be seen, along with the two holes through which long screws attach the brass end caps to the central armature/coil section. Also seen are the ~200 windings of the primary (closest to the core) and the ~10,000 of the secondary, separated from the primary by a thick layer of insulation.

[Linked Image]

Coil

The next photo shows the sectioned coil after removing it from the armature (it differs slightly from the previous one because it isn't the same coil, although they are nominally identical). Seen in this photo are that the coils of the primary are nearest the armature, and surrounded by the many turns of fine wire of the secondary. The horizontal wire exiting the coil carries the high voltage to the slip ring from the last turn of the secondary.

[Linked Image]

On the left of the next photograph is a magnified view of the left half of the coil of the previous photograph, and on the right is the same coil under additional magnification. The largest wire is 0.045" in diameter, and the small ones making up the secondary are 0.003" in diameter (i.e. roughly the diameter of a human hair). The relatively large wire of the primary (0.030") connects to the 0.003" wire of the secondary, which in turn connects to the 0.045" wire that leads to the slip ring.

[Linked Image]

The core of the armature is rectangular in cross section, as can be seen from the next photograph of coils cut from two armatures, one sectioned "horizontally" and one "vertically." As can be seen from the scale in the photograph, the core of the armature is ~1/2" x 3/4". However, the fact the core doesn't have a square cross section isn't particularly significant for operation.

[Linked Image]

Earth Brush

There are a few more electrical parts to examine before turning to the magnet. An earth brush is needed to complete the primary circuit, and the Lucas K2F holds this carbon brush in a hollow "screw" inserted at the drive end of the housing. The next photograph is of the earth brush taken through a window milled into the housing (red paint outlines areas where I've milled away sections of the housing). The carbon brush extends less than 0.06" from its holder, so it can be seen from this that the clearance between the housing and the armature is quite small.

[Linked Image]

This brush makes contact with the smooth brass endcap of the armature, completing the circuit (the brass on the armature in this photograph is rough, and would abrade the brush more rapidly than it should if used in this condition).

Pickups

The voltage needs to get from the coil in order to make its way to the spark plugs, and this is done using a slip ring connected to the coil along with a carbon brush held in a nonconductive pickup. The next photograph shows the slip ring and pickup through a window milled into the housing. The output wire from the coil is inserted in the slip ring where it makes electrical contact with a brass arc molded into the base of the slip ring. This brass arc in turn makes periodic contact with the carbon brushes of the pickups located 180-deg. apart in the housing.

[Linked Image]

The carbon brushes project ~0.08" from the end of the pickups, which is why the carbon itself has to be at least twice this long (or longer) to provide the necessary mechanical support for itself within the pickups.

Safety Gap

Under normal conditions the spark plugs fire at less than 5 kV, so the voltage experienced by the insulation in the coil doesn't exceed this. However, if the plug, plug wire, or pickup breaks the voltage would rise to a high enough value that it could break down the internal insulation. If that happens, permanent damage to the coil can occur. Because of this, a safety gap screw is located next to each pickup to keep the voltage within acceptable levels. The next photograph shows one of these screws viewed through the hole for one of the pickups (enlarged with an additional window milled into the housing).

[Linked Image]

The brass contact in the base of the slip ring is long enough that a safety screw is always over it when the voltage is high. Normally that voltage is routed to the spark plug, but if for some reason that doesn't happen it will jump the ~0.25" gap to the safety screw if the voltage ever reaches ~20 kV. Also, it should be clear from this photograph why if the safety gap screws are not removed before attempting to withdraw the armature, the relatively thin and brittle lip of the slip ring will be broken.

The Magnet

Having finished with the electrical elements of the magneto, it's time to dissect the housing to examine the magnet. The next photograph shows the inside of a Lucas K2F housing before and after I sectioned it. I've color coded the parts as follows: the Alnico magnet is blue, the laminated steel pole pieces are red, the remnants of the brass cap that held the pole pieces together with the aid of rivets is gold, and the remainder is cast aluminum.

[Linked Image]

The actual magnet is just the fairly small slug of Alnico at the top of the housing. However, the steel in direct contact with the Alnico is magnetized by it and "carries" the N and S poles to the right and left sides of the magneto, exactly as would be the case if everything shown in red were Alnico instead of steel. However, Alnico is such a strong (but expensive) magnetic material that inexpensive steel can be substituted for it in this way. It can be seen that the pole faces are symmetric since this is for a vertical twin engine with equal firing intervals, not for a V-twin.

From ZEV to K2F: A Half-Century of Evolution

On the left of the next photograph is the housing of the Lucas K2F and on the right is that of the Bosch ZEV (with just one of its two tungsten steel magnets in place). It can be seen that the magnets for both are in the shape of horseshoes (including the steel in the case of the Lucas), although a half-century of materials development allow that of the Lucas to be considerably smaller (the available energy from Alnico is ~9x greater than that of tungsten steel). The fact the Lucas directly evolved from the Bosch is readily apparent.

[Linked Image]

The asymmetric pole pieces of the Bosch show that it is for a V-twin engine. Together with the asymmetric armature shown above, this design fires one cylinder earlier and the other later than the even 180-deg. firing interval of the Lucas K2F. Although I haven't shown any examples here, the internals of single cylinder rotating armature magnetos (e.g. Bosch ZE1, Lucas KNC, BTH KD1, etc.) are essentially identical to their twin-cylinder siblings, differing only in some details.

From a rebuilder's point of view, it should be clear that the most time consuming aspect of a complete restoration of any magneto is winding a new coil. Removing the old coil is relatively easy if it is the original one, but can be quite troublesome if it is a replacement that has epoxy firmly bonding it to the armature (n.b. if the replacement had been properly wound it would not have failed). As can be seen from the cross sectional views, once the old coil has been removed, winding a new one requires ~10,000 turns of very fine wire in a close-packed arrangement, with insulation layered in appropriate places, connecting 0.003", 0.030" and 0.045" wires without breaking them, and then vacuum impregnating the final assembly. However, although time consuming to wind, a correctly rewound coil will outlast anyone reading this post by many decades, so it's worth doing right.

Re: Restoring a Rotating Armature Magneto
Magnetoman #507338 09/21/13 10:29 pm
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APPENDIX IV: DIAGNOSING A FAILED AFTERMARKET SLIP RING

Introduction

A few weeks ago when I was in Europe a friend gave me the aftermarket slip ring from the BTH magneto he has on his Velocette. The magneto had been professionally rebuilt in England but had failed after ~2000 miles due to shorting to earth of the slip ring and he asked me to do a post-mortem. He said these same slip rings have been supplied to other people he knows by at least four retailers in England so the information below may be of direct interest to others besides him. There are no manufacturer's markings on the slip ring so whether or not you have one of the same ones only can be judged by comparing its dimensions and appearance with the photographs below.

Aside from the issues I found with this specific slip ring the results of this autopsy again illustrate the broader problem of determining the suitability of aftermarket electrical components before using them. Unfortunately, as I've documented at several places earlier in this thread, with aftermarket magneto components one cannot take at face value unverified assertions like "Modern substitute, very high specification, zero failure" that turn out not to be true.

Preliminary Inspection

The following photograph shows a ~1 mm deep channel in this slip ring that starts at the trailing edge of the brass contact and runs for ~ 0.65". I can't know for sure, but it seems unlikely the rebuilder would have used this slip ring had this channel been visible when he was installing it, which means it almost certainly developed after the magneto was in operation.


[Linked Image]

As can be seen from the next two micrographs, this channel starts immediately at the edge of the brass contact, which means it cannot have been caused by wear from the pressure of the brush. Aside from the channel being irregular, if it were due to wear the depression would not start abruptly at the edge of the brass segment since the channel is much narrower than the brush and doesn't extend the full circumference of the surface. The features of this channel are consistent with "spark erosion" of an inhomogeneous material rather than wear having caused it. However, there is no way to know if the channel grew steadily over the 2000 miles, or if it developed rather quickly.


[Linked Image]


[Linked Image]

Electrical Resistance

One factor that could have contributed to the problem is if the black material used for the slip ring is electrically conductive. To check for this, I measured the resistance of the material itself by attaching the leads from a megohmmeter adjacent to each other on the flange:


[Linked Image]

The resistance measured this way at various places on both flanges was greater than 20 GOhm at 2.5 kV (i.e. the maximum R and V of this meter). To find the actual value of resistance I repeated this measurement with a more specialized meter (General Radio Megohm Bridge) and found a resistance of 10 TOhm at 1 kV (i.e. ten million, million ohms). This is a very high value, indicating there isn't an overall issue with the composition of the material used. However, possible inhomogeneities in the material (i.e. voids and/or conductive inclusions) are something that will have to be checked.

As can be seen from the above photograph I mounted the slip ring on an expanding mandrel to make good electrical contact with its inner surface as would be the case when mounted on an armature. I next measured the resistance between the brass segment and this "armature."


[Linked Image]

Although this resistance also should have been above the maximum of this meter (i.e. at least 20 GOhm at 2.5 kV) it was far lower as well as voltage dependent:

~1.5 MOhm at 250 Volts
~300 kOhm at 500 Volts
~150 kOhm at 1000 Volts (i.e. ~100,000x lower than 20 GOhm)

The meter couldn't go any higher than 1 kV due to the large current draw at this relatively low resistance. Also, although accuracy isn't especially important for this test, the 'overload' warning came on for the readings below 1 MOhm so those values may or may not be accurate. In any case, conductive path(s) to the armature of resistance ~150 kOhm or less is sufficient to cause failure of a magneto. For comparison, making the same measurement on a NOS Lucas slip ring using the General Radio instrument that goes to much higher resistances found 5 TOhm (5 million, million ohms) at 500 V.

The following figure from a book on magnetos shows that if there is a conduction path in parallel with that of the spark plug (i.e. as is the case here with the slip ring) of resistance lower than ~500 kOhm it begins to increase the minimum speed needed for the magneto to generate a spark. If the resistance drops below ~140 kOhm the magneto won't generate a spark at any speed.

[Linked Image]
Adapted from 'Automobile Electrical Equipment', 6th edition, A.P. Young and L. Griffiths (Iliffe, 1958)

What these measurements have revealed so far is that a conduction path (or paths) between the brass segment and the armature had developed during the 2000 miles of operation, and that the resistance of that path(s) became low enough to cause the magneto to fail. This is consistent with the formation of a carbon track(s) due to the breakdown of the material of the slip ring in the presence of an electrical discharge.

To locate the low electrical resistance path(s), I used a piece of paper to insulate all but a short section of the inner surface, as is shown in the next photograph.

[Linked Image]

By successively rotating the section in contact with the "armature" I found three low resistance regions: at the leading and trailing edges of the brass segment, and at the end of the channel. I used the 250 V setting on the megohmmeter for these measurements because, as above, the resistance of each of the three regions was low enough to overload the meter at higher voltage settings. In each of these three regions the resistance was below 1 MOhm. However, for all other regions of the armature the readings were above the 20 GOhm maximum of the meter.

Internal Structure of the Slip Ring

The above measurements show there is a relatively low resistance path(s) between the rubbing surface of the slip ring and the armature but it remains to be determined why this is the case. Next, I clamped the slip ring on my mill and used a 0.01" slitting saw to cut it in two at a place where I would be able to inspect the inner surface directly under the end of the channel, in the hopes of being able to see evidence of a carbon track. Note that the end of a carbon track large enough to have a resistance less than 1 MOhm would be quite small, so not being able to see where it emerged on the inner surface of this (black) armature would not mean it wasn't present. Although I was not able to see such tracks, the cross sectioning revealed the problem anyway.


[Linked Image]

As can be seen in the micrographs below there are quite a few voids in the material. The resin should be absolutely homogenous, so this shows there was a flaw in the manufacturing process. The location of the voids is consistent with a vacuum pump having been used during the molding. Although this should have eliminated all of the voids, if the resin set too fast, or was too viscous to begin with, air pockets would have been left behind. The fact the air pockets are concentrated toward the center of the thickest region of the slip ring is consistent with this explanation.


[Linked Image]
Cross-section of the slip ring. The highlighted region is magnified in the next photograph.


[Linked Image]
Magnified portion of the slip ring corresponding to the highlighted region of the previous photograph.


[Linked Image]


[Linked Image]

After inspecting the above cross sections, I then cut one of the sides in half in the other direction, revealing additional voids that are quite large (~2–3 mm).


[Linked Image]
Note: the discoloration and "smearing" of the brass is an artifact of the slitting saw that I didn't take the time to polish away with abrasive paper.

Reason for the Failure: Voids in the Resin

Note that I sectioned this slip ring only in a few places so there is no way to know the size, number, or distribution of additional voids at other locations within it. However, that they exist in the size and numbers we can see explains why the slip ring functioned at first but "died" after 2000 miles. On every revolution the voltage developed on the brass segment would have induced a corona discharge in any voids near the brass. The ozone produced by those discharges is quite reactive so it "burned" a conductive carbon layer into the surface of those voids, steadily extending the reach of the high electric field beyond that of the immediate vicinity of the brass. As the corona relentlessly extended its reach from one void to the next as a result of the conductive carbon, eventually the conductivity of the path or paths between the brass segment and the armature became so large (i.e. the resistance decreased to less than ~150 kOhm) that the magneto could no longer be turned over fast enough to generate a spark to start the motorcycle.

As an aside, as I noted earlier in this thread, the reason why the points cavity of a magneto needs to be vented is to reduce the concentration of ozone created by the residual sparking of the points that is present even when a proper condenser is in the circuit. The failure of this reproduction slip ring is graphic evidence of how destructive ozone is.

Other Issues with this Reproduction Slip Ring

While the above has identified voids left behind during the molding process as the reason for the failure, there are additional issues with this aftermarket slip ring worth mentioning. The next photograph shows that the slip ring is thinner than a genuine BTH item. Although the restorer should have used a shim on the inside end in order to center the track under the brush, the offset wear pattern shows he did not do this.


[Linked Image]
Genuine BTH slip ring mounted on armature at left. Reproduction slip ring at right.

Also, since both slip rings are sitting on the same surface in the above photograph, it can be seen that the flanges of the aftermarket one are ~50% thinner than on the genuine BTH. The thickness of the genuine ring's flanges taper from 0.17" at the rubbing surface down to 0.08" at the outer edge, while on the reproduction they are nearly constant at a rather thin 0.09" at the rubbing surface and 0.06" at the outer edge of the flange. Dimensions of a slip ring are not arbitrary, since they are chosen to guard against corona discharge and direct electrical shorts, so the inferior specifications of this aftermarket part are of concern.

Adding to the issues is the reproduction slip ring is 0.01" too thin (0.628" vs. 0.638"). Since the slip ring also serves as a spacer for the bearings, this means the bearings would be 0.01" too close together unless an additional spacer were added to compensate. The fact that the spacer(s) used by the rebuilder were incorrect can be seen from the wear pattern in the next photograph.


[Linked Image]

The longer white marks are at the sides and center of the slip ring and the shorter one is 0.05" to the left, showing the wear pattern is offset ~0.03"~0.05" from the center line.

BTH and Lucas Slip Rings are Not Interchangeable

Although they may look the same on an autojumble or swap meet table, a BTH slip ring is thinner and has a smaller OD than a Lucas:


[img]http://i1151.photobucket.com/albums/o626/ClassicVehicleElectrics/SR_160_zpsba843798.jpg[/img]
The black slip ring is the reproduction BTH and the tan one is a genuine NOS Lucas.


[img]http://i1151.photobucket.com/albums/o626/ClassicVehicleElectrics/SR_170_zps24f32a62.jpg[/img]
BTH in front; Lucas in back.

As an aside, if the 2000 miles were covered at an average of 30 mph and 2000 rpm, this magneto made ~2 million revolutions before failing. During that time the carbon brush was dragged across the surface of the slip ring some 120 miles at an average speed of 2 mph. Although the rubbing surface of this reproduction slip ring is fairly rough, on a proper one the brush will travel ~600¬¬ miles before being abraded away. I suspect the roughness of this one would wear out a brush twice as fast.

Implications for Using Aftermarket Slip Rings

The failure of this slip ring after 2000 miles has wider implications for anyone rebuilding a magneto. Although I don't have a new slip ring from the same batch to test, the fact that the resin in the flanges (where there are no bubbles) of this failed one has a resistance of ~5 TOhm means the resin itself is intrinsically fine. Thus, this slip ring very likely would not have failed if the manufacturing process had not left (hidden) air bubbles in it.

It's worth mentioning that while 5 TOhm is more than adequate for this application, the same measurement on a NOS Lucas slip ring was off scale on the highest range at over 1000 TOhm at 1000 V. Aftermarket slip rings are for sale on eBay with the claim they are made with "better materials" and are of "better quality" than NOS Lucas. Although no data of any kind is provided to support these bold assertions, I note that it would be very difficult (and completely unnecessary) to exceed the materials and quality of NOS Lucas slip rings. To be clear, my concern isn't whether an aftermarket slip ring actually is better than NOS, it's whether it might be significantly worse despite unverified advertising claims, as was the case with the one dissected in this post.

Although the voids in this slip ring would have been revealed by x-raying it, that isn't a practical test for most people to apply (unless they know someone who works in a dentist's or radiologist's office…). Unfortunately, because the reason for the failure was slow degradation due to repeated application of high electric fields near the internal voids, it is unlikely that any usual electrical test would have detected a problem when this slip ring was new.

A rule commonly used for high potential testing ("Hipot testing") of electrical components is to apply twice the operating voltage plus 1000 V for a few seconds during which time one looks for current flow above some minimum value. In the case of a slip ring, this means a 15-20 kV Hipot tester would be needed (note: Lucas rated their slip rings for 35 kV, so a good aftermarket slip ring would have no problem passing a 20 kV test). Unfortunately, even applying 20 kV to this particular slip ring for some number of minutes quite likely wouldn't have revealed a problem since it takes some time for a conductive path of carbon to form. In any case, the failure mode of this slip ring has resulted in me now investigating other ways for stress-testing slip rings that could be used by people who don't have the specialized instruments that I have available to me.

Re: Restoring a Rotating Armature Magneto
Magnetoman #507385 09/22/13 8:42 am
Joined: Jan 2008
Posts: 310
Britbike forum member
Offline
Britbike forum member
Joined: Jan 2008
Posts: 310
A very true & sad situation on aftermarket parts in general. Some are made poorly, while others exceed original spec.s. How is a consumer to know which is good & which is bad.
I too am very familiar with the porous slip rings (and pickups). In fact, I am the one selling the "better than Lucas" slip rings & pick-ups on eBay.
I have found that several large British parts wholesalers are selling pickups & slip rings made in both India & Taiwan. I have found them to somewhat conductive & allowing loss of spark. after much searing & testing to find a better product for my customers magnetos & located 2 companies that actually make these items in the UK to a very high standard & these have tested well & I have been very happy with both companies products. Because material that they use is less brittle & less likely to absorb moisture that the original Lucas products,I consider them to be "better than Lucas" . The Lucas made Bakelite parts are becoming very hard to find NOS & if found are very expensive & have no advantage over the English made products that I have found, in fact the Lucas ones have disadvantages.
Rather than write a few thousand words on Bakelite (or some call it Barkerlite), here is a link on the material:
http://en.wikipedia.org/wiki/Bakelite
Even Lucas knew of the tenancy for Bakelite to absorb moisture, in fact at least 2 Bakelite items that I am aware of were coated with Glyptol ( http://www.glyptal.com/ ) to reduce this effect. Glyptol is an red-orange paint on coating that you will find on the pickups on a Lucas K2FC competition magneto, and you will also find this coating inside the distributor cap on a Lucas SR2.
This is a similar situation to the original Lucas capacitors, people will buy them NOS, pay a ridiculous inflated price for them, even though a much better product is available. An original Lucas NOS slip ring, or pickup is just fine, but there are better materials available.

I am sure as time went on Lucas would have changed over from Bakelite as most other industries did, but as is well documented, the English motorcycle industry at that time was too cheap & wouldn't pay for any R&D unless it was less money, but that's another story for another thread smile

A side note: I also sell an Ewarts replacement plungers that use 2 o-rings, in place of a cork seal. I consider it to be "better than Ewarts" & much safer, but some will always want the original Ewarts plunger with the cork, so they can say it is original, & thats fine too.

Last edited by 57nortonmodel77; 09/22/13 8:44 am.

Magneto & Dynamo restorations & supplies

My Bikes
1948 Norton 500T Trials bike
1950 Norton Model 7
1952 Norton ES2
1957 Norton Model 77
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Re: Restoring a Rotating Armature Magneto
57nortonmodel77 #508762 10/02/13 9:28 pm
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APPENDIX V: AN ALTERNATIVE REPLACEMENT CONDENSER

Introduction

Unfortunately, the Panasonic condensers I tested so extensively are now out of production and no longer easily available. Although there never was a question that other proper high-quality substitutes were available, it takes time identify and test those alternatives. There were two obstacles to overcome before this could happen. First, I have enough Panasonic capacitors on the shelf to last me into the 22nd Century so doing this work had a hard time making it to the top of my to-do list. The other obstacle -- related to the first one --is it requires mind numbing work to page through catalogs and spec sheets to identify suitable candidates.

The specifications for capacitors are in different formats in different locations, not all of which are easy to find, so it takes sitting down for quite a while to systematically comb through catalogs and manufacturers' literature. Earlier this summer someone else did this for me and sent me a short list of capacitors that seemed to be appropriate given the specs I had sent him. Thanks to his work, I only had to go through a small number of web pages to arrive at a few prime candidates. Once I did that I ordered ten of each for testing.

Basically, an appropriate capacitor has to satisfy three criteria: have the proper electrical specifications, be of a size that fits into the cavity in the armature, and not dissolve in oil. I selected several that met the first criterion, possibly met the second, and would have to be tested to see if they met the third. Although one was back ordered and wasn't shipped until early September, the others arrived in early August. The construction of magnetos, and the size of appropriate capacitors, are such that every fraction of a mm counts and one can't be completely sure whether or not a capacitor will fit from the published dimensions alone. Anyway, two of the capacitors I ordered were just a teensy bit too fat to fit in a Lucas armature so had to be rejected.

Unfortunately, a few weeks after I started my tests the hot plate lost control and heated to nearly 200 oC sometime between the temperature checks I did every few days. This forced me to throw out those capacitors and start over using a different hot plate. However, I'm happy this happened a few weeks into this month-long test instead of many months into the years-long test I did of the Panasonics. On the right of the following photograph is one of these overcooked capacitors, next to one that spend 31 days at 102 oC in the center, and a new on the left.

[Linked Image]
Vishay capacitors from left to right: new; after 1 month in 30W Castrol at 102 oC; after some unknown time up to a few days in 30W Castrol at 200 oC.

It is easy to see which capacitor in the above photography was accidentally subjected to 200 oC, but the only visual difference between the other two is the slight yellowing of the base material of the one that spent a month at 102 oC. This shows that oil does not attack the outer case of these capacitors.

Recommendation for a Replacement Capacitor

Since not everyone may want to read the details of the tests described below, I'll go straight to the conclusion. The capacitors I recommend as replacements for use in Lucas, BTH, and other rotating armature magnetos are a pair Vishay 0.082 uF "AC and Pulse Double Metallized Polypropylene Film Capacitors," manufacturer's number BFC238320823. These capacitors have pulsed current and voltage ratings of 1400 V/us and 630 VDC with a maximum operating temperature of 105 oC. These specifications comfortably exceed those needed to survive for years in the hostile electrical environment of a magneto. They are available from Digi-Key for $1.35 each ($2.70 for the pair required to be soldered in parallel) under part number BC1883-ND.

Relevant Properties of these Capacitors for use in Magnetos

At the left of the following photograph is a Lucas condenser from a K2F-type magneto, in the middle a pair of Panasonics I've already assembled into a package with bracket ready to install in a magneto, and on the right is a pair of Vishays.


[Linked Image]
Left to right: Lucas condenser; a pair of Panasonic 0.083 capacitors soldered in parallel and ready to install; two Vishay 0.083 capacitors.

For a better idea of the size, the next photograph shows a pair of the Vishay capacitors in the cavity of a Lucas armature.


[Linked Image]
Vishay capacitors in the end cap of a Lucas magneto.

Finally, as the arrows in the next photograph show, the Vishay capacitors are thin enough to fit below the lip in the end cap of the armature so they will not interfere with the coil of the armature when it is installed.


[Linked Image]
Vishay capacitor in the end cap of a Lucas magneto.

As an aside, remember that it was someone other than me who took on the task of narrowing down the list of all possible types of capacitors that are on the market. All he had to work with were the specifications I gave him based on the measurements I made on operating magnetos. It is no coincidence that the Vishays he found have the same metalized polypropylene construction as the Panasonics. Because surviving high pulsed currents for extended periods requires thick metal electrodes, which in turn makes the capacitors fat, it is also no coincidence they occupy approximately the same volume as the Lucas and BTH condensers they replace. Also, like the Panasonics, the 0.16 uF version of the capacitor from the same family is too thick to fit into the available space so a pair of the thinner 0.082 uF capacitors has to be soldered in parallel.

Why These Magneto Capacitors Could Not be Any Smaller

To illustrate an important point made in the previous paragraph, the next three photographs show the internal construction of a Vishay capacitor.


[Linked Image]
Cross section of a Vishay capacitor. The shiny flat electrodes are at the left and right sides, but the individual layers of the capacitor appear black in this micrograph.


[Linked Image]
Higher magnification view of the lower left corner of the above photograph showing the metal layers connecting to the flat electrode.


[Linked Image]
Micrograph at approximately 300x magnification showing the individual layers of the Vishay capacitor. The layers are 7 um thick, but the waviness was caused by the fairly crude cross sectioning process I used (a slitting saw on my mill).

The flat electrodes at the far left and right connect directly to the external leads, and have cross-sections of 0.0011 square inches. This is equivalent to the cross section of #19 AWG, wire which is approximately the same as the #20 wire used for the magneto's primary. This is no coincidence, since these capacitor electrodes have to survive the same high current pulses as does the primary coil. From there the current spreads to the metal layers that make up the capacitor itself (along with the polyethylene layers that separate them).

There are only two ways these magneto capacitors could be made smaller: reduce the thickness of the metals, which would reduce their ability to handle the high current pulses without burning out; or, reduce the thickness of the dielectrics, which would reduce their ability to handle the high voltage pulses without being destroyed by arcing between the layers. Actually, these particular ones could be made a tiny bit thinner without affecting their electrical properties. Vishay uses the same outer shell for a family of 630 VDC capacitors of values from 0.082 uF (i.e. this one) up to 0.11 uF. Looking again at the top micrograph it can be seen that there is a white plastic spacer between the actual capacitor layers and the outer shell. The thickness of this spacer is incrementally reduced as more layers are added to give higher capacitance values within this family until there is no more room left. At 0.12 uF the outer shell is replaced by one that is 1.5 mm thicker and the process starts over. If Vishay used a different shell for every capacitance value, this particular one could be made ~1 mm thinner.

Other Possibilities

There certainly are other capacitors with appropriate sizes and specifications. However, my goal here was to identify just one appropriate replacement capacitor that is in current production, not all possibilities. A potential replacement capacitor from a different manufacturer was out of stock when I placed my order in August and wasn't shipped to me until a few weeks ago so I won't finish testing it for a few more weeks. However, although its electrical specifications are even more robust than those of the Vishays, it is slightly thicker so would not fit in Lucas and BTH armatures. I ordered some of them anyway because if they pass my tests they will be fine for magnetos that happen to have deeper cavities. Also, sometime in the future (years? months? weeks?) these particular Vishays almost certainly will go out of production, so someone will have to go through this testing process again when that happens.

Tests on the Replacement Capacitors

For those interested in more details, the tests I ran on these capacitors were an abbreviated version of the ones I described on the Panasonics in Appendix II:

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=467734#Post467734

Although the tests were not as extensive as the ones I conducted on the Panasonics, taken in combination with the manufacturer's specifications they are enough for me to recommend them. If I did not already have a lifetime stock of the Panasonics of my own, and even if it were not possible to conduct any additional tests on these Vishay capacitors, I would use them myself.

As I wrote in Appendix II, one typical accelerated lifetime test of electrical components is based on the observation that most chemical reactions approximately double in rate for every 10 oC increase in temperature. This "doubling rule" makes it possible to derive a lifetime estimate without having to conduct an experiment that runs for decades. Assuming it applies to the chemical processes at work breaking down the dielectric material of the Vishay capacitors, if they still functioned after a month in 30W Castrol at 102 oC this "doubling rule" would predict they would function for at least 20 years on a motorcycle parked in a storage shed at 22 oC (72 oF).

Further, although I've never measured a temperature as high as 52 oC in an operating magneto, but if we take that as a worst-case upper limit and assume that during the next 20 years the motorcycle also covers 30,000 miles at an average speed of 30 mph, that would be 1000 hours (42 days) of operation at that temperature. That time on the road is equivalent to an additional 31 hours at 102 oC. Specifically, the test itself had two Vishay capacitors spend 745 hours immersed in 30W Castrol at 102 oC, which is equivalent to 30,000 miles at a very high operating temperature plus 20.8 years storage at 22 oC (72 oF).

One of the specifications that caused me to select these Vishay capacitors is their 630 VDC rating is significantly higher than the voltage they will experience in operation, and thus they will be understressed in a magneto. Although I could have assembled an external voltage divider to allow me to test their resistance at 630 V, I just used the nearest built-in setting of 500 V. Although lower than their rating, this is still significantly higher than they will experience in operation. Also, the Milspec requirement calls for testing magneto condensers at only 400 V, specifying s a resistance greater than 2 MOhm at that voltage.

Before starting the test my General Radio Megohm Bridge found 8 TOhm at 500 V one minute after applying the voltage (the polypropylene in the capacitors slowly polarizes so their resistance continues to slowly rise). This exceeds the Milspec resistance requirement by a factor of at least 4 million. During the month-long test it was convenient to use a less specialized megohmmeter limited to 20 MOhm to make quick checks. However, at the end of the test, after removing the capacitors from the hot oil and letting them cool to room temperature the resistance at 500 V was the same ~8 TOhm after 1 minute as it had been at the start. I found no degradation of the electrical properties whatever caused by the test, with the yellow discoloration of the white plastic base shown in a photograph above the only change I could determine.

Having passed the resistance test, I next used a Hewlett-Packard 200 Watt high frequency pulse generator to subject the capacitors to current pulses of 1000 V/usec, which is more than 10x higher than the pulses I had measured using a magneto tester. I did this test at the equivalent of >1 million rpm so that I was able to simulate "30,000 miles" of operation of a twin magneto in only a few minutes. Although I did this test at room temperature, Vishay does not derate the AC or DC voltage of these capacitors for temperatures below 90 oC so this test, in combination with their published specifications, indicates they would have been fine if I had tested them at an operating temperature of 40 or 50 oC.

After having been subjected to a month in 102 oC oil, followed by "30,000 miles" of 1000 V/us current pulses, I made one final measurement of the resistance. The resistance at 500 V was the same ~8 TOhm after 1 minute that the capacitors had when they were new. Since this exceeds the Milspec requirement for the resistance of a magneto condenser by a factor of at least 4 million, my conclusion is these Vishay capacitors, like the Panasonics, should be good for at least 30,000 miles of operation plus 20 years in storage .

Summary

Although these tests were not as extensive as the ones I conducted on the Panasonics, the Panasonics are by far the most extensively tested magneto condensers that I am aware of, which means these Visays are now the second-most extensively tested condensers available for use in a magneto. Taken in combination with the manufacturer's specifications, and the fact they have the same internal polypropylene structure as the Panasonics, I would have no hesitation using them myself if I did not already have a very large stock of the Panasonics.

If you want to install a new capacitor in your rotating armature magneto and not have to deal with it again for a very long time indeed, based on my tests I recommend either the Vishay BFC238320823 or the Panasonic ECQ-P4823JU. These metalized polypropylene capacitors have capacitance values of 0.082 uF so have to be soldered in parallel to give the 0.16 uF required total. The Vishay capacitors are currently available from Digi-Key for $1.35 each under their part number BC1883-ND. Note, however, that there are other polypropylene capacitors on the market with much lower V/us ratings because they use thinner metal electrodes so it is essential to check the specifications if you plan to use any other than these two specific ones that I have tested.

It is worth emphasizing that 30,000 miles plus 20 years is only a lower limit because, other than discoloration of the white plastic base, these Vishay capacitors showed no sign whatever of degradation when I ended the test after a month. There is no reason to expect they would not continue to last quite a bit longer.

Re: Restoring a Rotating Armature Magneto
Magnetoman #540361 04/28/14 9:39 pm
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APPENDIX VI: ADAPTER TO ATTACH POINTS PLATE TO CONDENSER

Depending which end of the armature holds the condenser in a given magneto, the central bolt through the points plate may bolt directly to it. If this is the case, you will have to manufacture an adapter in order to replace the condenser with a modern one. The original condenser itself could serve this purpose if its top were machined away, but it is filled with a now-banned carcinogenic PCB so I definitely do not recommend doing this.

At the top of the next photograph are Bosch (left) and Lucas (right) condensers with the threaded "nut" on the underside to which the bolt through the center of the points plate attaches.

[Linked Image]

At the bottom of this photograph are the end caps of two armatures. Note that the cavity of the one at the left (Bosch) is much deeper than the one at the right, making it easier to deal with when adding a modern condenser. Also note that the one at the right has a "square" opening for the condenser nut, so the Lucas condenser above it does not go with it.

My approach to making an adapter "nut" for the replacement condenser starts with a 1/4"-20 brass screw as shown at the left of the next photograph. This screw came from a local hardware store and there is nothing special about its diameter or pitch (other than the diameter of the head, as discussed below), or even that it is brass. Although brass is a nice material to machine and tap, a stainless M6 screw would function as well.

[Linked Image]

I use a lathe to reduce the thickness of the head, shorten the screw, then tap if for whatever screw is used to hold the points plate in the magneto I'm working on. Depending on the magneto this could be BA, metric, or some now-obsolete American thread so a pitch gage is needed to determine the required tap. The next photograph shows a jig I made to hold the screw in the lathe while doing all the modifications to it (although everything could be done without such a jig).

[Linked Image]

The points plate mounting bolt needs to be insulated from the magneto housing, and the next photograph shows a nylon insulator in the shape of a top hat that accomplishes this. Delrin, acetyl, phenolic or some other insulating materials would work as well. The OD of this insulator fits the ID of the hole in the end cap of the armature, and the thickness and OD of the "brim" of the cap fits in the larger diameter recess. I tap the insulator to match the brass screw (i.e 1/4"-20), but I haven't listed other dimensions here because they depend on the particular magneto. Also, as noted above, the necessary insulator might not even be cylindrical, and anyway anyone who has the tools to make this piece will have calipers to make the measurements.

As shown in the next photograph the brass piece threads into this insulator. One of the condenser leads is soldered to it to make the necessary electrical connection to the points via the central mounting bolt.

[Linked Image]

Although it isn't clear from the photographs, the OD of the head of the brass screw is slightly larger than the OD of the main section of the insulator (i.e. slightly larger than the ID of the hole in the end cap of the magneto). To minimize the height of the final condenser assembly the top of the brass screw can be reduced in one dimension in order to fit between two condensers, as seen on the right of the above image as well as on the left of the next one.

Although fabricating this "nut" might sound time consuming, it actually doesn't take much time at all. Also, since it doesn't take much longer to make a half-dozen of the individual pieces than it does to make just one I have a stockpile of the components ready for assembly whenever needed.

[Linked Image]

The above photograph shows the final condenser assembly with two 0.083 uF Panasonic capacitors (left) next to an original Lucas condenser (right). In this particular assembly the capacitors are being held a little above the insulator by the springiness of the lead, but they would be in direct contact when epoxied into place in the end cap (not that direct contact is important, other than keeping the overall height low enough that interference with the armature isn't an issue). The assembly would look essentially the same if made with two of the Vishay capacitors I recommended in Appendix V.

The next photograph shows what the assembly looks like from the bottom. Although when epoxied in place in the armature the leads shouldn't be able to touch any part of the housing, I like to insulate them anyway. The black substance I use for this is "corona dope."

[Linked Image]

Although I used a 1/4"-20 screw, and mentioned an M6 would be fine as well, it can be seen from this photograph that there is plenty of room for an even larger one if desired for some reason.

As for strength, because of the design my only concern would be if the "brim" of the insulator could break because of pressure from the points plate mounting bolt. Although nylon should be strong enough to avoid this, to be sure it isn't an issue I use brass screws whose heads have an OD slightly larger than the OD of the major section of the insulator (i.e. slightly larger than the ID of the hole in the end cap). Because of this, although at very worst the "brim" might compress slightly with time, slightly reducing the clamping force of the central bolt, it cannot pull through.

If brass screws with large enough heads are not available for a given magneto it would be easy enough to build up the diameter of smaller ones with silver solder, or just to make ones entirely from scratch. Finally, it wouldn't hurt to put a few drops of glue on the brass screw before tightening it into the insulator just to be absolutely sure there never will be an issue of the brass/nylon assembly trying to separate even if the mounting bolt is screwed in and out a number of times over the years.

It can from the above photograph that the new assembly has basically the same dimensions as the old Lucas condenser it replaces. As mentioned several places in this thread, the similar size is no coincidence. For a condenser to survive high pulsed currents without burning out requires thick capacitor plates, to survive high voltages requires thick dielectric spacers between each of those plates, and to have the proper capacitance despite the thick dielectric spacers requires many layers of large area. All of these factors add to the volume, which is why these capacitors that I have tested to survive the high pulsed currents of a magneto for many years intrinsically are large.

The first photograph in this post shows that the cavity in the Bosch housing is much deeper than in the other one. In such a case two, thin 0.083 uF capacitors don't have to be connected in parallel to create the necessary capacitance, but instead a single, thicker 0.16 uF or 0.18 uF can be used. However, no matter what, the larger capacitor must be from the same "family" as the 0.083 uF ones that are rated for high pulsed currents. The blue bars in the next photograph show that one of these higher capacitance units is quite a bit thicker than the 0.083 uF ones which is why it won't fit in the cavities of many magnetos.

[Linked Image]

Remagnetizing

Finally, as a reminder, removing the armature from a magneto for even a few seconds "permanently" reduces its magnetization making the motorcycle more difficult (or impossible) to start because a higher kickover speed is required to generate sufficient voltage for a spark. The only way to restore full performance after removing the armature for any reason is to remagnetize it after it has been reassembled. To do this requires an appropriate electromagnet ("magnet charger"), and one that was designed for pre-WWII magnetos does not have sufficient strength to remagnetize post-WWII alnico magnetos.

Re: Restoring a Rotating Armature Magneto
Magnetoman #566392 10/06/14 12:24 am
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The 2014 Cannonball Run is over and the Bosch ZEV magneto performed flawlessly for the 3000+ miles without having had any maintenance since I originally restored it for use in the 2012 Cannonball.

Bill Wood happened to mention at least four failed magnetos in his daily posts about this year's Cannonball. Although there certainly could have been others as well, given the 1936 cutoff for this year's event meant most bikes did not have magnetos, so even four is a very high percentage of failures. However, between the bench tester and actual road miles my friend's magneto has traveled over 5000 miles so far, which is consistent with my contention that there is no excuse for a properly rebuilt magneto to fail. And yet, "professionally rebuilt" ones continue to fail with remarkable regularity...

I've asked that this thread be unlocked so it is open again for questions related to magneto restoration.

Re: Restoring a Rotating Armature Magneto
Magnetoman #598449 05/08/15 7:47 am
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Sir,

You are a scholar and a gentleman :-)

Your article is now required reading amongst the Classic Bike Owners of Wellington New Zealand and has been crtically examined by most of my friends.

You have our thanks- (and if you are ever in Wellington an open invitaion to visit several workshops and inspect several interesting projects).

Regards

John

Re: Restoring a Rotating Armature Magneto
johnm #598778 05/10/15 11:19 am
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Originally Posted by johnm
(and if you are ever in Wellington an open invitaion to visit several workshops and inspect several interesting projects).
John,

Thank you very much for your nice comments. However, you should be careful with an invitation like this since there's a fair chance I could be on your doorstep any given year...

Apropos of maybe nothing in this thread, but your post reminded me that I received two requests in recent months to give hands-on magneto refreshing/restoring/rebuilding workshops at club meetings this year. Unfortunately, I already had time conflicts with both dates so had to decline.

More relevant to this thread, though, is before too much longer I hope to be posting another appendix, this one on rebuilding a Lucas KNC1. First, though, I have to find the time to rebuild it...

Re: Restoring a Rotating Armature Magneto
Magnetoman #598886 05/11/15 1:05 am
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Hi

Make the given year after October next year :-)

Im currently posted by my company to Eastern Europe but should be home late next year. The guy who found your posts most useful is a long time bike engineer machinist who orginally trained as a post office comunications tech. He restores a lot of mags and was able to use the information to augment his own experience.

Regards

John

Re: Restoring a Rotating Armature Magneto
johnm #608790 07/16/15 1:56 am
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APPENDIX VII: Rebuilding a Lucas KNC1 Competition Magneto

I just rebuilt a Lucas Competition magneto that I'm told had been professionally restored less than 100 miles ago. As can be seen from the next photograph it looks great on the outside.


[Linked Image]

Since the construction is pretty much the same as the Bosch ZEV, and since it wasn't such an internal disaster, I didn't take nearly as many photographs of the rebuild as I did of the Bosch. The next photograph shows it after disassembly:

[Linked Image]
The important point is the earth brush on this magneto is hidden under the "Competition" tag, and it has to be removed before the armature can be pulled out of the housing. Next I pulled the bearing off the race at the end of the armature with the slip ring and then used a tool shown much earlier in this thread to extract the race using it and a two-jaw puller as shown in the next photograph.


[Linked Image]
Next to the slip ring are the oil slinger, a few spacers, the race, and the bearing. A quick test with my Merc-O-Tronic tester already showed the coil was good so next the end cap has to be removed to gain access to the capacitor.


[Linked Image]
No identifying markings are on the capacitor but it appears to be a 60 Hz line filter capacitor that magneto restorers often use, and which quickly self-destruct because they can't handle the high pulsed currents. I tried to test its capacitance but it was too heavily shorted to measure. At least the "professional restorer" hadn't slathered it with an excess of epoxy to make removing it time consuming.

I replaced the blown capacitor with a pair of Panasonics as on the Bosch ZEV, although attached to an adapter like shown in Appendix VI because it is required for a single-cylinder magneto. However, when I reassembled the armature I decided to use two screws from my stock that were in better condition than the ones I had removed from it. That turned out to be a mistake. As the next composite photograph shows the replacement screws were a few threads longer than the original ones so they protruded by ~1 mm..


[Linked Image]
I didn't notice the problem until I tried to put the end cap on and there was a gap of ~1 mm. So, I switched back to the original screws.

After getting the right screws in it I reassembled the magneto and installed new points since the ones in it definitely had more than 100 miles on them. I then put it on my lathe using a bracket shown earlier in this thread. It sparked reliably down to 300 rpm (600 rpm engine) on the uncalibrated dial on the lathe. I then I magnetized it at 84,500 Amp-turns and redid the test. This time it was still sparking reliably down to the lowest speed on the dial of 250 rpm (500 rpm).

Since the speed dial on the lathe is only approximate, rather than putting the lathe in back gear to go slower I moved the magneto to my modified distributor tester. However, I haven't used it since moving to my new house ten months ago and I discovered that in the meantime the battery in the tachometer had died. So, once I get a new battery for it I'll see how low this properly remagnetized magneto will go and still continue to provide a reliable spark

Send questions or comments to [email protected]


Re: Restoring a Rotating Armature Magneto
Magnetoman #610526 07/28/15 12:08 pm
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I was busy getting ready for the visit of a daughter this past weekend and forgot I hadn't posted the results of the running tests I made after getting a replacement battery for the tachometer on my modified distributor tester.

After rebuilding and re-magnetizing the KNC1 it sparked reliably down to 160 rpm (320 rpm engine). I then stuck a piece of reflective foil to the chuck on my lathe and used a non-contact tachometer to check its speed to know how the magneto had performed immediately after rebuilding, but before re-magnetizing. The 300 rpm on the uncalibrated lathe dial I had found for my previous post turned out to be 315 rpm (630 rpm engine).

As I wrote previously, a Lucas manual says 300 rpm (engine) is at the low end of kick starting speeds, with 500 rpm normal. So, the magneto is now performing up to its full, um, potential.

The 320 rpm vs. 630 rpm is actually a significantly bigger difference than I usually see before and after magnetizing. This means this particular magneto had become significantly demagnetized prior to ending up in my hands. However, even had the "professional restorer" not used an inappropriate capacitor that failed, these results illustrate how much performance can be lost if a magneto of unknown previous history is not properly remagnetized after rebuilding. The fact it worked at 630 rpm (engine) means a healthy guy could have gotten an engine running with it, but probably would have incorrectly blamed its lousy performance on being intrinsic to magnetos. It had been "professionally restored," after all.

Send questions or comments to [email protected]

Re: Restoring a Rotating Armature Magneto
Magnetoman #614254 08/23/15 5:49 pm
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Ha ha, Doc.

For me, reading "...a healthy guy could have gotten an engine running with it.." is the equivalent of hearing fingernails running down a black(chalk)board, mate.

Why not, "...a healthy person could have caused an engine to run with it..."?

This ugly, American, bastardisation of the English language really gets on my tits, son.

Ditto "snugged up" versus, the less cosy, "nipped up".

That aside, and being 99.999% confident that I'll never own a motorcycle with a magneto, this thread has been fascinating to read and really sends out a message to those who dabble in the dark art of magneto or, for that matter, any other type of *restoration*.

For the uninitiated, the world of older motorcycles is a minefield to tiptoe through, check you still have all four limbs connected, and then place your trust in the hands of the "professional" to deliver the service you require.

Here's just one example of what can happen when you entrust part of your motorcycle to a "pro":

I had a later cylinder head parked on top of the barrels of a 67 T120. It was one of those with the alloy block manifolds, rather than the screw-in stubs. It needed the plug holes coiling; new valve guides blah....

So, I took it down to L*n at The Cylind*r H*ed Sh*p, who at that time was still operating out of W*mbled*n, SW London, and explained my requirements: One, possibly two, plug holes to be sorted; new valve guides and seats cut to accept whatever Fandango valves they were flogging back then; cosmetic vapour-blast of the whole head; oh, and could you tickle these manifolds so there's no discernible step where they meet the head- in other words, just make the intake flow smooth, yeah?

L*n: No problem. Road or race?

Figgley: Er, it's a road bike, so road.

Several weeks(months?)and £400 later, I take that bastard drive from NW London to effing Wimbled*n to pick up the head that is going to make the Bonneville breathe freely and to the maximum of it's abilities.

All I had required, in terms of flowing, was a smoothing of the joint between the manifolds and the head proper and a general clean-up.

Initial impression was that the head looked pretty....very clean. As new. L*n, or one of his slaves had, as promised, sealed the joints between manifolds and head with something other than paper and locked the studs, which was part of what I desired.

Turning the head upside down revealed, obviously, there had been a shitload of alloy removal that I hadn't asked for. He'd opened up everything back from valve seats to carb-side manifold faces.

Cheers for that, you hoon.

OK for 32mm Mikunis but not what I employed you to do, Len, you C*NT.

Your, slapdash, approach to customers' needs; your general lack of interest in anything other than yourself; your hiding behind that effing SERCO and pretending to be an *engineer* rendered my expensive head unfit for the other EXPENSIVE gadgets I had to feed it, mate.

Give me back that alloy you stole. I know where you hang out, sunshine.

L*n Paters*n is a cowboy trying to ride a thoroughbred. The guy's an arsehole

So, I have a nice '72 cast head that's flowed, courtesy of L*n, to breathe lots of vapour but is useless for 1 1/8 or 1 3/16 Monoblocs/ 30mm Concentrics [email protected]

I still have the original head, which is in very poor shape, but I have to treasure it and keep reading this blog, sunshine.

Where did I start? Oh, yeah- old motorcycles are the route to insanity, geezer.

P.S. Back in 81/82, before I knew the score, Ian at Roebuck's in Rayners Lane, Middx insisted I needed to turn over my Smith's Tach in order to procure a set of Jap-made TR5 clocks.

Hopefully he died of cancer.

Dave x








Last edited by figgley; 08/23/15 8:48 pm.
Re: Restoring a Rotating Armature Magneto
figgley #614677 08/27/15 4:11 am
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Originally Posted by figgley
For me, reading "...a healthy guy could have gotten an engine running with it.." is the equivalent of hearing fingernails running down a black(chalk)board, mate.

This ugly, American, bastardisation of the English language really gets on my tits, son.
Thank you for your comment. Although grammar and spelling errors bother me as much as the next person it is time consuming enough to create a 36,000-word technical document that is accurate. Unfortunately, to make it grammatically flawless without benefit of an independent proof reader would require far more time than I could hope to devote to it. So, sadly, I'm sure it wouldn't be hard to find examples in this thread of dangling participles, split infinitives and other grammatical atrocities. I envy anyone who does better under the circumstances.

Re: Restoring a Rotating Armature Magneto
Magnetoman #614707 08/27/15 12:46 pm
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"Gotten" was good enough for Shakespeare, but not for Wiggley off the internet.

Think I'll go to the pub and not come home until I've gotten nipped up.


Amateur Loctite enthusiast.
Re: Restoring a Rotating Armature Magneto
triton thrasher #630960 12/20/15 9:06 pm
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Although the figure below is for a Zener I'm posting it because it's interesting in its own right as well as being a preview of a magneto-related instrument I'm working on that I hope to have done in a few months.

[Linked Image]

I purchased a used Lucas "12 V" Zener on eBay to use in an instrument I'm designing and needed to test it to be sure it actually works. The curve is the current-voltage characteristic of this Zener with a greatly offset x-axis showing the important region where the Zener begins to function to regulate the charging voltage for the battery. The voltage at which the current begins to sharply increase is the "Zener voltage" and here it's seen to be ~15 Volts.

Although at the scale plotted here it looks like the current below ~14.8 Volts is zero a tiny current actually is flowing. However, at 14 Volts this Zener is passing only 16 microAmps (R = 0.88 MegOhms). Why a Zener is useful a voltage regulator is illustrated by the fact that at 15.25 Volts the differential resistance has dropped by over a million to only 1/4 of an Ohm. That is, the harder the stator tries to generate more voltage than the battery wants to see, the better the Zener works to clamp the voltage at ~15 Volts by conducting excess current straight to ground. In essence, it's a nearly perfect insulator for voltages below ~15 V (~ 1 MOhm), and switches to being nearly a dead short for voltages above that (~1/4 Ohm).

I limited the current from my power supply to a little over 1 Amp for this measurement so the maximum power dissipated in the Zener was limited to ~15 Watts (Current x Voltage = Power). However, extrapolating the curve to 2 Amps (30 Watts) shows the voltage would only increase to ~15.5 Volts at that point (~15.65 @ 45 Watts, etc.)

I should add that the Zener voltage is temperature dependent to some extent (increasing with increasing temperature) so if I wanted to thoroughly test its characteristics I would need to instrument the Zener with a thermocouple (because its internal temperature, not ambient, is what matters) and make the measurements after reaching thermal equilibrium at several current/power levels. Although I could do this, the results aren't important for how I will use this Zener.

A battery doesn't care what internal temperature the Zener has, it only cares about its own temperature. Since the optimum charging voltage for a lead-acid battery that's at 20 oC is 14.6-15.2 Volts this Zener seems like it would be an excellent voltage regulator. However, on a warm day (with the battery at, say, 30 oC) the required charging voltage required drops to 14.2-14.8 while an even warmer diode will have an higher Zener voltage.

Of course, if you're riding 120 mph with your lights off through Death Valley in summer the poor Zener will be dealing with nearly all the power your stator is generating so the voltage will rise and the battery will suffer. Still, even with its intrinsic limitations a Zener is not a bad solution at all for most riding conditions, especially when you compare what you get with it vs. an MCR1 electro-mechanical voltage regulator.

Anyway, the above figure shows this Zener passes the test and so it will take its place as one of the many components in the instrument I'm building. More details in due time.

Re: Restoring a Rotating Armature Magneto
Magnetoman #631110 12/21/15 10:48 pm
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I was going to post this in a separate thread, but when I saw your link in my thread, I thought it might be more appropriate here.

I was just reading in some literature supplied with a battery charger that the correct finish voltage for a 12V flooded battery is 14.84, while the correct finish voltage for a 12V gel or AGM battery is 14.4.

This may cast some dispersion on the use of AGM batteries in our old British machines, and may explain why my Scorpion AGM battery only lasted one year. (I was withholding this conclusion because, during that year, I had a faulty alternator lead for a couple of weeks. However, I noticed the charging problem right away, and prevented the battery from going completely flat through external charging.)

After that, I bought a flooded battery quite by coincidence - I needed a battery in a hurry and the only one I could find locally was a flooded battery. However, this battery has performed flawlessly over the last two years. Notwithstanding the dreaded acid leak potential, and having finally figured out how to make the dang hose stay on the breather spigot, I may just stick with flooded batteries (or "sealed lead acid", which has the same charging requirements as a regular lead acid battery, but without the potential for acid seepage).

But this is with an "OE zener diode". It would be interesting to see if the specs differ with a modern rectifier/regulator unit. I believe many modern bikes call for AGM batteries, so I have to believe they've taken this into account in their charging systems.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
Re: Restoring a Rotating Armature Magneto
Mark Z #631211 12/22/15 2:57 pm
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Originally Posted by Mark Z
But this is with an "OE zener diode".
Keep in mind that my test was of only one device so there no way to know from it alone how wide the tolerance in Zener Voltages was for the components Lucas supplied at the time. Although it would be easy enough to test a dozen original Lucas Zeners to determine the answer to this question about variation it would take a few hundred dollars to acquire them on eBay. Or, wait a minute, maybe my friend has a couple at his shop...

I dropped by my friend's shop on the way to work this morning and asked him if he had any old Zeners on the shelf. He thought he remembered having seen one somewhere in the back room. I left the shop a little while later carrying 9 of them so even if a few turn out to be bad there will be enough to get a reasonable idea of the variation.

For reference on what is possible 40+ years later, I went to the Digi-Key site and filtered only for voltage tolerance, leaving all other options open. They show 21,207 different Zener diodes are available from them having +/-5% tolerance, but that drops to only 1,680 if I specify +/-1%, and they have just 3 at +/-0.5%.

If Lucas's tolerance wasn't better than +/-5% (a range of 14.25-15.75 V for a nominal 15 V) it would mean some people will have batteries with reasonably long lives, but others will have ones that cook fairly rapidly. Based on what is available today, I suspect a tolerance of +/-1% (14.85-15.15 V) was beyond what Lucas achieved at the time.

To do a reasonable test I'll park my good Zener at 1.0 Amps and see how long it takes for the voltage to stabilize when internally heated by the 15 Watts. Then it should be a quick matter to repeat the measurements on the other diodes. I probably won't get to this until the weekend but will post the results as soon as I have them.

Although the most significant outcome of this will be to satisfy my own curiosity a side benefit will be to give me enough information to select a diode for my Trident having the "optimum" characteristics within the intrinsic limitations of a Zener. That is, should I ever go to the trouble of measuring the one currently in it and then switching the diodes if necessary...

Re: Restoring a Rotating Armature Magneto
Magnetoman #631612 12/25/15 11:36 am
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All but one of the Zeners I got from my friend's shop earlier this week turned out to be good so now I have a reasonable amount of data from which to draw conclusions.

The electrical setup needed for this is quite simple. I wired a precision 1.0 Ohm resistor in series with each diode and used a 3-1/2 digit Fluke voltmeter of 0.1% accuracy to measure the voltage drop across it to directly display the current to the nearest mA. This is more than adequate because the I-V curve of a Zener rises so steeply the accuracy of the current reading isn't the limiting factor. A second 4-1/2 digit Fluke voltmeter measured the voltage drop across the diode to the nearest millivolt. In this case the 0.1% accuracy means the absolute value could be off by as much as 14 mV, although relative values between Zeners are accurate to 1 mV.

The change of Zener voltage with temperature is a real issue so I'll start with the results on the three that came bolted to their original finned heat sinks.

For one of the three heat-sinked diodes I ran the current up to 1 Amp (~15 Watts), then up to 2 Amps (~30 Watts), then cycled back and forth a few times between there and 1 Amp. Each time I was at one of those current values I quickly recorded the Zener voltage. I determined from these measurements that even without air flow the heat sink was effective in keeping the Zener at constant temperature at these power levels, at least for the minute or so all the measurements took. That is, the Zener voltage I measured at 1 Amp after decreasing from 2 Amps was within a few millivolts of the value I measured when I initially had increased the current to 1 Amp when ramped up from 0. For this diode the Zener voltage at 1 Amp was 14.195+/-0.005 Volts. At 2 Amps it was 14.275+/-0.005 Volts. Extrapolating, at 4 Amps (60 Watts) it would be ~14.355 Volts, i.e. an increase of only ~1% from the 15 Watt value. Hence, to regulate a voltage at a fixed value one could do a lot worse than a Zener.

For the other two heat-sinked diodes I only measured the voltage after the initial ramp up to 1 Amp. For one of them it was 14.357 Volts and for the other it was 14.644 Volts (uncertainties in both are ~+/-0.005). Hence, the spread in values of these three was ~0.45 Volts.

For the other six diodes I didn't take the time to bolt them into a heat sink. Instead, I ran the current up to 1 Amp (15 Watts) and measured the Zener voltage as quickly as possible after I had stabilized the current (~5 seconds). Because of the internal heating caused by the current, the voltages continued to drift upwards but the range of values, if not the absolute values, are indicative of the spread of Zener voltages that would have been measured had they been in heat sinks. That spread was a full 1 Volt, or +/-3.5%.

This is a digression from my current interests so I didn't take the time attach thermocouples to the diodes, bolt them in heat sinks etc. However, I'm confident drawing conclusions from the results. My speculation is the measured spread of +/-3.5% I found for this limited number of diodes would increase to more like +/-5% with a larger sample size. This would be reasonable given the state of semiconductor production technology in the 1960s.

Assuming Lucas aimed for precisely the 14.84 Volts that Mark Z mentioned in his post, a +/-5% spread means at the low end some Zeners they supplied to the factories were 14.10 V and as a result undercharge batteries, while at the high end others were 15.58 V and boil their insides out. If someone got lucky and theirs was the perfect 14.84 Volts at fairly low speeds when it was only needed to dissipate 15 Watts, at high speeds when dissipating 60 W it would increase to 14.99 V, which by itself isn't too bad at all. However, because the heat sink can do only so much the temperature of the diode, and hence the Zener voltage, would increase further due to heating.

A typical 14 V Zener has a 0.08%/oC positive temperature coefficient while a battery needs a ~0.1% negative. For example, even absent internal heating from the current, a 14.84 Volt Zener that was optimum at 77 oF would increase to 15.02 V at an ambient of 105 oF whereas a battery needs 14.54 V at that temperature, which is nearly 0.5 V lower.

So, even a diode having a "perfect" Zener voltage at a given temperature isn't perfect when used as a voltage regulator on a motorcycle. That said, I haven't (yet) measured any of the modern replacement voltage regulators (Boyer, Mity Max, Podtronic, Tympanium, Wassell, others?) so I don't know if any of them do any better than a Zener for either the absolute voltage, or for the temperature compensation required for correctly charging batteries whether riding in snow or riding through the Sahara. However, any such measurements will have to wait until after I finish building the instrument mentioned in my post of December 20 and that's at least a few months away.

To digress for a paragraph, it turns out the temperature coefficient depends on the Zener voltage, and diodes smaller than ~4.5 V have coefficients with the opposite sign. This means four 3.71 V Zeners connected in series would have the perfect 14.84 V at room temperature, decreasing to 14.72 V at 105 oF, which is only 0.18 V too high. Similarly, cooled to 32 oF the Zener voltage would increase by 0.2 V whereas the ideal for a battery would be only 0.1 V higher than that. That is, four of these smaller Zeners in a good heat sink would regulate the charging voltage to within no worse than 0.2 V of ideal over the entire temperature range from freezing to above 105 oF.

To conclude this departure from magnetos, because of the apparent ~+/-5% variation in the ones originally supplied by Lucas the odds are the particular Zener that came in your bike is only doing a so-so job keeping the battery in good health. However, it only would take a few pieces of electronic gear to select a "perfect-ish" one that would do much better (within a Zener's intrinsic limitations due to the physics of semiconductors). Had I seriously thought I might ever want to "optimize" the one in my Trident I would have used this opportunity to select one from the batch from my friend's shop to do just that. Maybe someday I'll regret not having done so (and maybe I still will do it...). But, there are so many projects, and so little time.

Re: Restoring a Rotating Armature Magneto
Magnetoman #643396 03/08/16 1:27 pm
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I see this is and old thread so maybe the good Dr. won't respond. But I missed in these articles how the high tension wire is attached to the slip ring. If someone could please advise me I'd appreciate it. Regards, RL

Re: Restoring a Rotating Armature Magneto
panman #643417 03/08/16 3:48 pm
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Originally Posted by panman
I missed in these articles how the high tension wire is attached to the slip ring. If someone could please advise me I'd appreciate it.
The photograph below is of the relevant section of a slip ring I discussed in an earlier post:

[Linked Image]

As can be seen there's a hole in the brass segment. The output wire of the coil has a slightly smaller diameter so it slips into this hole from the right, at the center of the projecting section of insulator, resulting in either direct electrical contact with the brass or a very tiny gap for the voltage to bridge. The first few photos in the following post will remind you how the slip ring is oriented with respect to the output wire of the coil:

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=470880#Post470880

Re: Restoring a Rotating Armature Magneto
Magnetoman #643428 03/08/16 5:44 pm
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Is it glued in with a small drop of epoxy then or does the tensional stiffness of the wire hold it in place. Thanks much for the quick response, I'm new to this form. I'm working on a Goldstar flat tracker frame that I'm putting my BB34 alloy clipper engine into with some original flat tracker parts. I've read a number of your articles in AMCA newletter? I think and a number of other places and have always found them interesting and highly informative. Thanks, RL

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