There was a discussion at the end of 2014 concerning what is necessary to achieve a proper and lasting charge on a magneto magnet (http://www.mtfca.com/discus/messages/506218/505495.html?1420394855) in which I learned that the "secret" to achieving a lasting magnet charge is to generate a minimum of 9000 Amp-turns.
My goal was to build a simple, single magnet charger similar to the one Robert Anderson posted, which would achieve at least 9000 Amp-turns and stay cool while doing it while working off of a single 12 Volt battery. My first charger overheated very quickly and worked by striking the end of a wire over the battery post clamp, a real safety No-No since the discharge cycle produces hydrogen gas right next to where the sparking is taking place. I added many more turns of wire and a starter solenoid which keeps the sparks a safe distance from any hydrogen gas formation.
I ended up using 75 feet of 14 AWG (generously provided by Toon Boer) which was the maximum length that would physically fit within the confines of the "V" shaped magnet and resulted in 210 total turns of wire.
I gave 5 quick "zaps" to each magnet while a steel plate connected the N and S legs acting as a shunt. The first "zap" seems to be plenty to effectively charge the magnet, but I gave the additional ones "just in case".
I also tried using two 12 Volt batteries, but it didn't seem to charge any better or stronger, the charger only heated up faster.
The magnets charged with this setup will pick up a cast iron piston with its pin. Magnets pulled from a steel workbench vise release between 5.5 and 6.2 lbs. (2.5 to 2.8 kgs). The magnet ends are not polished, filed flat or in any way modified. It can be assumed the weight holding would improve if the magnet ends were flatter against the steel vise surface.
The 11.8 Volts from a single battery divided by 0.1875 Ohms resistance allows the charger to consume 63 Amps. Calculations based on information provided by other posters show this setup to provide 13,216 Amp-turns which generously exceeds the minimum 9000 Amp-turns necessary for saturation.
If we contrast this setup with the engine's magneto coil ring to charge the magnets, calculations come up as follows:
34 Volts divided by 1.2 Ohms gives 28 Amps draw. The coils have 25 turns each which would give 50 turns total per magnet (one coil per magnet leg) so the Amp-turns would be 1417, falling short of the 9000 Amp-turns objective for full magnet saturation.
If my calculations are anywhere near correct, this would explain why the in-car charge is not as effective as a single magnet charge.
Do these calculations seem on track?
Nice design Eric but please note that in car charge can be done as effectively as demonstrated by Tom in this thread: http://www.mtfca.com/discus/messages/506218/528397.html?1427399216
I charged my magnets in the car some years back and I remember the results were fantastic. I could start on mag and everything!
The issue I wanted to address concerns the long term effectiveness of the multiple magnet charge using the coil ring versus a single magnet charge. The saturation factor seems to be key here and according to my calculations, the coil ring does not come close to saturation. This is what I am wondering if I calculated incorrectly.
In Tom's thread, he mentions that the coil ring gives 5000 Amp-turns which is just a little more than half of the 9000 Amp-turns which is the calculated amount necessary for saturation.
This is how I understand it to be but I could be wrong.
Is that 210 turns for each coil or for both coils (i.e. 105 turns each) ?
Bud, that's 210 turns total (105 turns each).
FWIW, according to "Automotive Electricity", 1932, by Consoliver and Burling, the magic numbers are 12,000 amp-turns for ordinary magnets and 5,000 amp-turns for Ford flywheel magneto magnets.
Other experts may be giving different values.
There is more to it than amp-turns. How efficient the coupling between the electro-magnet and the permanent has a lot to do with it. Suppose, for instance, that you had an electro-magnet with 15,000 amp-turns that was 10 feet in diameter. You can see that it would not be good at imparting its flux to a permanent magnet stuck into the center of it.
I roughly ascertained the saturation point of the flywheel field coil system by slowly cranking up the amps on my welder and noting (with a gauss meter) when the flux stopped rising.
It is fairly easy with on oscilloscope to find the point of saturation in a magnet system. Below are traces from my 'scope that show a Model T ignition coil saturating when 12 and 18 volts are applied to it. When I get time maybe I'll do some tests and see how few amp-turns can saturate a single magnet.
Current and flux are proportionate - to the point of saturation. With six volts, the coil barely saturates. With 12, the current (amps) exceeds the flux. They diverge at the point of saturation. 18 volts show even more divergence.
Ok, Tuesday night is Model T night, so I took some time and did some tests. I took a magnet and wrapped it with 40 turns of 14 gauge house wire. I then zapped it with a 12 volt battery. It drew about 200 amps with the 12 volt car battery that I was using. I then did the same thing with 24 volts. Because of the jumper cables and connections and stuff it didn't double the amps (it went up to about 300), but the flux stayed the same so I new that I had the magnet saturated. So, it seems that a single magnet will saturate at 8000 amp turns.
Here is the 'scope trace. The blue is flux, the red is amps (measured by voltage drop on a resistor) the green is my hall effect amp probe that maxes out at about 180 amps, so is useless without a shunt for this job.
So it sounds as if the "in car" charging setup would have lower saturation because of the gap than charging with the magneto coils out of the car which would be touching the magnets, correct?
Tom, have you experimented with finding the saturation voltage required:
a - When using a magneto coil ring lying on the 16 magnets while they are mounted to the flywheel?
b - When doing "in car" charging?
Eric said: "So it sounds as if the "in car" charging setup would have lower saturation because of the gap than charging with the magneto coils out of the car which would be touching the magnets, correct?"
That doesn't sound right to me.
I will do some tests at some time and try to find the answers.
Tom said: "There is more to it than amp-turns. How efficient the coupling between the electro-magnet and the permanent has a lot to do with it. Suppose, for instance, that you had an electro-magnet with 15,000 amp-turns that was 10 feet in diameter. You can see that it would not be good at imparting its flux to a permanent magnet stuck into the center of it."
I misinterpreted your comment about imparting the flux. Your example refers to the size of the coil, not the proximity which is what I had understood (coil ring touching the magnets or not).
Thanks for your input, testing and posting results.