High voltage vintage equipment, question for the physicists  - Page 1
Forum section: electronics / projects

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2017, 12:05:21 am
content: Several years ago, a friend of mine shows me a vintage ignition test system. There are three electrodes per load. One electrode shares a common ground with the other loads. One electrode goes to each of the high voltage wires. Let’s call this the HV electrode. There are 14 loads total. I suspect this was built in the 30s or 40s and may have been used for testing radial aircraft ignition systems.The electrodes would erode very quickly during testing. A third electrode was added to solve the erosion problem. This electrode is not electrically connected to anything and I will refer to it as the floating electrode. The floating electrode is closest to the HV electrode. It is gapped roughly 0.030” from the HV electrode. All three electrodes are made from the same material. We are not sure what material was used but it is magnetic. Adding the floating electrode stopped the erosion or greatly reduced it.The gap is settable during testing. The gap could be opened as far as 0.4” or so to induce a strain on the ignition system. There was no outside gas feed. All testing was done at ambient pressures. The technicians knew what a good system could handle and would open the gap until the system would breakdown or not spark. There is a dial that indicates the gap.My friend who has this system started with the company where it was designed in the 60’s. He asked several people who worked at the company about how it worked and no one was able to explain why adding the floating electrode stopped the erosion.I am interested in understanding why adding this floating electrode would have such an effect on the erosion. I have attached a close up showing the three electrodes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 23, 2017, 12:45:05 am
content: Something about corona spray, preignition (the configuration resembles an ignitron), leakage through the "floating" electrode into the surroundings (is it wood, or phenolic? do you have a megger?), or just good old superstition (who knows?).I suppose I'd have to see a summary of its operation: how fast does it actually wear, with and without? What does the discharge look like? Is it coincidence, or is it statistically significant, for the same number of strikes, gap length, and spark energy?Tim

author: cvanc
author_rank: Frequent Contributor
timestamp: August 23, 2017, 01:01:41 am
content: Interesting, any chance we can get some more pictures?

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2017, 01:09:46 am
content: The base is phenolic. I do not have the test jig. As I understand it, there was a significant reduction in erosion, so much so they changed the design. It may be difficult to find many details as that generation is long gone. I would assume it was running with mags.

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2017, 01:18:48 am
content: Decades ago when I was on the tools we used these Briggs and Stratton ignition testers:They appear to have been superseded by a new version that apparently has the same functionality.We used both terminals, the wide one for a static test and the narrow for run tests.For aeronautical ignition systems the energy was many times greater, they could kill and any tests would have been done with industry approved jigs.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2017, 01:21:08 am
content: Quote from: cvanc on August 23, 2017, 01:01:41 amThe first picture shows the outside front face with the viewing holes. The knob on the front adjusts all of the gaps. Of course, they would have had to gap them all the same and this is where there was this constant adjusting as the tips eroded.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2017, 01:32:31 am
content: tautech, that is the closest thing I have seen. Do you have the documentation for them both that you could post?I suspect this jig would have been used with something like this:https://en.wikipedia.org/wiki/Continental_R-670

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 23, 2017, 01:34:49 am
content: There is a real chance that nobody ever knew the physics behind this. Sometimes people try stuff and it works. Then other people copy them.

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2017, 02:22:33 am
content: Quote from: joeqsmith on August 23, 2017, 01:32:31 amSorry I don't but there might be some explanations of use in old Briggs service school notes if you can find any.A good buddy (now retired) did his apprenticeship on multi row radials and I've already sent him mail with a link to this thread......I'll wait and see if he can shed some light on your jig.

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2017, 02:44:36 am
content: Thebest explanation is thisfrom aircraft magneto testing:"The teaser electrode is used to solve the problem of time lag when using pointed electrodes (versus a spherical electrode). Time lag results in inconsistent voltage to produce an arc (voltage tends to be higher or overshoot without the teaser). This third electrode is universal used when measuring arc voltage across pointed electrodes.The teaser electrode is insulated from the other two electrodes and is typically set to 0.1 mm gap from the main electrode (5 mm gap). As voltage rises on the main electrode a tiny arc (invisible) travels to the teaser which produces ionization in the main spark plug gap which helps set off the main arc.Without the third electrode you have no method of comparing your results with book values since the spark voltage will be different and variable. I would add that I believe that the 0.1 mm gap distance has to be accurately set on all electrodes in order to get even sparking across leads.A properly designed magneto tester with a properly operating magneto should produce such even sparks that the sound of the sparks is as regular as music. You should be able to detect a bad magneto from across the room with your back turned to the tester just from the sound of the arching. "Here is the science paper:http://naca.central.cranfield.ac.uk/reports/1925/naca-report-202.pdfSomething about a 10usec ionization delay and a big deal knowing firing voltages vs testing.

author: Someone
author_rank: Super Contributor
timestamp: August 23, 2017, 03:38:29 am
content: Quote from: joeqsmith on August 23, 2017, 12:05:21 amGiven the above information from floobydust, did the extra electrode slow the erosion or was erosion a problem because it varied the triggering voltage? Then the additional sharp point was decoupling the spark initiation from the main electrode shape (which changed from erosion) but not actually slowing electrode erosion, which is usually done with cooling and/or material choices.

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2017, 06:05:57 am
content: Quote from: tautech on August 23, 2017, 02:22:33 amHe said:They wereonlyused on the mag test bench after magneto overhauls and the # of electrodes on the test jig had to be the same or more than the # of cylinders. His experience was with 2 row 14 cyl DC3's.Each mag (2)(dual ignitions) had leads for each cylinder that would in the correct sequence be connected to the test jig and the mag driven by a multi-speed electric motor. The sparks would travel along the jig as each lead (cylinder) was selected by the mag distributor and stress tests were applied by opening the test jig spark gap.floobydust's explanation of the teaser electrode setting pre-ionisation of the spark gap makes perfect sense to reduce erosion and my buddy agreed with the great sounds when the test was being run.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2017, 10:35:55 am
content: That paper is an excellent source. Looks like it was published Jan 01, 1925.https://ntrs.nasa.gov/search.jsp?R=19930091268

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2017, 08:08:45 pm
content: The paper's (1925) theory for how the third-electrode works is interesting.Spark voltage can be lowered by a little UV light, and the theory is the streamer arc to the third electrode makes UV. This causes the main spark-gap to breakdown more consistently. Apparently even a feeble spark several cm away (with a quartz shield) can trigger the main spark.Heinrich Hertzin 1887 wrote "First note on effect of ultra-violet light in facilitating spark." So they knew that UV lowered spark voltage.92 year old science that's a pretty good read. They used rotating mirrors, oscillographs to study the usec phenomena even to 100MHz.Lives depended on decent spark ignition for aircraft, so I can understand the massive amount of research; 71 papers referenced, back to the mid-1800's with many greats in science.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 23, 2017, 08:32:20 pm
content: Initiation is easily understood and anticipated; I hinted at this earlier--Quote from: T3sl4co1l on August 23, 2017, 12:45:05 amBut I don't see how it could do anything about wear, unless there's something about an initiated spark that produces less erosion. Or it's simply the regularization of it: instead of sparks being fairly random in their initiation voltage (and thus, the discharge energy usually being more than the expected amount), the energy and therefore wear is more stable. But that shouldn't account for more than a modest (10s of %?) improvement, not a massive (50%+?) improvement.So again, the original claim remains doubtful, and one would need to do a controlled study to at least reproduce the claim.Tim

author: HighVoltage
author_rank: Super Contributor
timestamp: August 23, 2017, 08:34:23 pm
content: All old ignition testers that I have ever seen in Germany had this third electrode.It was called the ionization electrode and always had a very small gap to the high voltage electrode.This electrode was required on inductive as well as magneto ignition systems because thehigh voltage build up was so slow that it was difficult to jump the larger gap at once.With this third electrode, the gap was ionized and then the spark jump was much more preciseand repeatable.You can easily try this these days with a high voltage transformer or an ignition coil and have like a 10 mm gap.Just hold an insulated screw driver in the gap, close to the high voltage electrode and you will notice thatthe spark will jump much easier, because the metal of the screw driver helps to ionize the gap.On modern ignition coils with voltage rise times of 2 kV/us or faster, it will not make a big difference any moreand the spark gap will jump each time almost perfectly.But old ignition coils with a secondary inductance of 60H or more were just too slow and the ionization gapmakes a huge difference.discharge

author: HighVoltage
author_rank: Super Contributor
timestamp: August 23, 2017, 08:45:00 pm
content: This is one of these typical ignition coil tester, made in Germany by Prüfrex.It has also the third electrode for ionization purpose.

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2017, 09:45:02 pm
content: The only way I see it could extend electrode life is the decreased spark voltage.Output capacitance of the test fixture/wiring/coil adds bite to the arc when it initiates, which pits the electrodes.Kind of a capacitive-discharge at the start with energy ~V^2.But once the arc is established though, I don't think the electrodes erode nearly as fast. I'm not sure, it probably goes into relay-contact life science, plasma and metal vapor.Originally I thought the third-electrode could be a magnetic blowout, so it pushes the arc out and it extinguishes faster, so shorter arc duration would give better electrode life.

author: HighVoltage
author_rank: Super Contributor
timestamp: August 23, 2017, 10:13:20 pm
content: Electrode erosion / life of the electrodes has nothing to do with this.The handheld tester show in the video was used for short time test, to see, if a coils is OK or not.Electrode erosion is proportional to the sustained current flow and the DC voltage across the spark and the time the spark is applied.

author: TerraHertz
author_rank: Super Contributor
timestamp: August 24, 2017, 08:28:42 am
content: Quote from: HighVoltage on August 23, 2017, 10:13:20 pmNot sure about 'proportional'. I'd expect a quite non-linear relationship between surface erosion rate, and energy of electrons/ions impacting the surface. Below the energy range needed to knock electrode atoms off, not much erosion. Above it, lots of erosion.With a plain two-electrode gap:The hot electrode voltage will rise until the E-field across the entire gap is enough to cause an ionization cascade across the entire gap. This can be thousands of volts.As the cascade begins, electrons and ions are being accelerated across the gap in the full breakdown voltage - thousands of volts. They strike the electrodes with this energy, easily knocking off metal atoms.There's a short period in which transmission line effects are in play - the wire that was charged up to the high voltage has a discharge wave propagating along it, back from the now formed arc. For a few nS there will be very high current, as the wire transmission line dumps its stored energy at several KV and very low impedance into the arc, maintaining a high voltage across the arc, ie high acceleration of the ions/electrons.Once the arc has discharged the HV wire back to what the magneto can sustain under load, gap voltage will drop to the arc-holding voltage (few tens of volts) and so the arc ions are not being accelerated much, and don't impact the electrodes with much energy. There's a high net energy due to plenty of current, but it's distributed across the electrode tip - lots of electrons and ions, but of low individual energy.With the teaser electrode and small gap:The hot (main) electrode voltage begins to rise as above, but at an early point a small, low energy arc forms across the very small teaser gap, to the floating capacitive load of the teaser electrode body. Because that's only a few pF, not much current flows, and neither the teaser or hot electrode erode significantly. But that small arc (maybe only just a glow discharge) does form some mobile ions/electrons, and a little UV.Meanwhile the main gap E field is still rising. The ions/electrons from the teaser min-arc get pulled into that field, and start heading towards the other electrode. More ions/electrons from the hot electrode follow, into this conductive feeler. Which is conductive, ie shorts out the E-field in the space it travels, so the overall E-field becomes concentrated between the end of the feeler, and the other electrode. The feeler goes faster as this field concentration increases the rate of air breakdown in the remaining gap.Very rapidly, the feeler reaches the other electrode. Much more current begins flowing across that conductive path. The arc forms.Overall, in this case the arc formed a lot sooner than it would have without the teaser electrode mini-arc. So overall, much lower voltages were present, and the electrodes didn't get subjected to ions/electrons accelerated by such high voltages.Also the arc-feeler propagating across the gap from one end (rather than forming all at once) would start discharging the voltage on the HV wire a little, slightly spreading out the current dump from that wire into the gap. Another effect lessening electrode erosion.(Edited: a bit)

author: HighVoltage
author_rank: Super Contributor
timestamp: August 24, 2017, 08:57:00 am
content: OK, lets go a little deeper in to this subject. In the 1980s/1990s we did some research in to the spark discharge and found that every spark discharge is going through 4 phases:Phase 1: Pre-BreakdownPhase 2: BreakdownPhase 3: Arc-DischargePhase 4: Glow-DischargePhase 1: Pre-BreakdownDischarge time: up to a few usThis is mainly the electric field buildup between the electrodesThere is NO electrode wear during this phasePhase 2: BreakdownBased mainly on the parallel capacitance to the spark gap, including distributed capacitance and every little pF is counting.Discharge time: 1 to 10 nsDischarge current: can reach 1000ADischarge power: easily a million WattDischarge temperature: 50.000KThere is NO electrode wear during this phasePhase 3: Arc-DischargeDischarge time: 1 to 10 usThis is the time the electrodes melt and cause the erosionDischarge temperature: 5.000KPhase 4: Glow-DischargeDischarge time: 0.5 to 3 msThis is the time were the cathode is cold and there is not much wear on the electrodesEvery spark discharge is going through these mandatory phases.It is possible that the phase 3 and 4 alternate, if the pressure and charge movement are high enough around the electrodes.I should have scope pictures from those days (Tektronix scope with camera) but may be difficult to find in old boxes.