Reply To: Recharge Magnets on Fly Wheel

[wmohat]

foot_doctor:

The “Stevens Magnetizers” that the outboard guys use run off of 120VAC, full-wave rectified. They can generate maybe 8,000 gauss or so, which is OK for Steel and Alnico magnets……but are not strong enough to magnetize Samarium Cobalt or Neodymium magnets……for those, you have to generate magnetic field strengths on the order of 20,000 gauss or more.

The kind of magnetizer you are talking about (a capacitor bank discharger) will charge a huge bank of capacitors with up to 3,000 or 4,000 volts, and then dump that through the charging coil in just a few milliseconds…….creating 30,000+ peak gauss. Do NOT wear a watch in the room where one of those units is running! Keep your smartphone a BIG distance away as well…..or you’ll have to buy a new one.

See this video: (and note that they are charging a HUGE bank of capacitors up to 3,500 volts before doing the ‘charging’.
Also note that the actual charging time is just the duration of that “thump” sound that you hear. )

See: https://www.youtube.com/watch?v=PVIAZmrQJDU&index=16&list=PLG3u0gcV6DRNgwaPAt-UzL86h6qOawmf7

HOWEVER……… it doesn’t matter how much power you use to “charge” a magnet; the material it’s made of determines how much of a magnetic field it can hold. Steel, maybe 2000 gauss; Alnico up to 6,000 gauss…..and that’s it. For magnets made of these two materials, “charging them” with a huge capacitor-discharge unit will cause them to have NO GREATER STRENGTH than charging them with that old Stevens unit…………..the material itself can’t hold any greater “charge”, no matter what the “charging strength” it was exposed to. Unless you re-machined the flywheels and installed Cobalt or Neodymium magnets into the flywheels, the expensive capacitive-discharge magnetizer you were using was just a waste of money and time. (If you had a gaussmeter and measured your flywheels when charged with a Stevens unit, compared to that monster capacitor-discharge unit, you would be able to prove this for yourself. Charge it with either magnetizer, and you’ll get the same field strength when you are done…….the magnetic characteristics of the raw materials creates this physical limit.)

So far in my testing, I haven’t found ANY older motors with Cobalt or Neodymium magnets; all that I’ve seen so far have been either raw steel (1920s and 1930s) or Alnico (1940s and later). Now, I suspect some of the VERY NEW outboards (in particular, the smaller 4-stroke fuel-injected units that do NOT NEED A BATTERY TO RUN……..I’ll bet that those units are using Cobalt or Neodymium
magnets, as they have to generate a LOT of energy on each revolution of the crankshaft……….but honestly, I haven’t been able to verify this yet. Maybe some other reader knows for sure about this?)

As for the “charging time”……it takes microseconds for the magnetic material to take a “charge”, when exposed to a high enough external field. This is true not just of that expensive capacitor-discharge unit, but of the Stevens unit also. HOWEVER……unlike the small magnets shown in the video (referenced above), the old antique outboards had HUGE magnets, and sometimes VERY LARGE pole pieces. Rotating the flywheel a few degrees either way, and repeating the “charging” cycle several times, allows you to expose much more of the magnet material to the charging field, and DOES create a larger, final “charge” on the magnet structure when you are done. (Remember the old magnetic tape audio recorders? The tape accepted the audio information and recorded it as magnetic charges in THOUSANDTHS of a second………..the same is true for the magnets in your flywheels. Physics is physics, after all.)

As for how we are measuring the fields; my gaussmeter has a small, hand-held probe the size of a small pencil, with a hall-effect sensor embedded inside it. We are actually making contact with the magnets when we take measurements. Using a business card or some other “spacer” doesn’t really change the field any, over that short of a distance. We did, however, discover that the CENTER of the pole pieces typically has 1/4 to 1/10 of the field strength that you will measure at the EDGES of the pole pieces……and it’s VERY HARD to measure in the exact same location, short of using a “magic marker”, and marking on the poles exactly where your measurements were taken.

We really are not interested in laboratory-grade measurements; what we are interested in is the RELATIVE CHANGE in field strength before, and after, we “charge” the magnets…..and what effect it has. Did the magnetic strength increase by 25%? 50%? Did it DOUBLE? In each of these cases, could the magneto create spark at 25% LOWER R.P.M.? 50% lower? 1/2 as much? Is there any correlation between change in magnetic field strength, and the RPM at which spark can be generated? (So far, there’s a strong correlation between the two, but it’s NOT LINEAR…..and depends greatly on the physical design of the pole pieces, etc.)

About half of the outboards we tested so far showed either no, or very little change. About 30% lowered their starting RPM by 25%, and about 20% of the outboards lowered their starting RPM by HALF, making them MUCH easier to start than before.

There does seem to be a pattern emerging from our data, as to which generations of outboards are improved the most.
The 1920s vintage units typically had MASSIVE magnet structures……FAR larger than they needed. Even though they used low-grade steel magnets, most still have 75% of their charge, and most can generate spark at way under 100 RPM, even without being “recharged”. However, in the 1930s and 1940s, manufacturers started to use much smaller magnets and spark coils……and starting RPMs were around 200 to 300 RPM or so. For outboards in those time periods, it didn’t take much fading of the magnets to get you up to 300 to 500 RPM minimum (or worse) to generate a spark, making some of these outboards very difficult to start, and they just would not idle down unless they magnets were “re-charged” up to full strength.

AGAIN (disclaimer here)…..we have only tested 10 outboards so far. I’d like at least another 10 to test before I’ll stand by those tentative conclusions, and actually publish them as “fact”. But, I’m having troubles locating volunteers to haul motors up for us to test. Actual testing only takes about 15 to 30 minutes. Anyone interested in participating?

–wmohat

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Bill Mohat MS/CIS, BSEET, CCNA

"You can avoid reality, but you cannot avoid the consequences of avoiding reality." --Ayn Rand **********************************************