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Magnets & Magnet Chargers

Making your own magnet charger can save time and money on engine maintence

| April/May 1997

  • Magnet Charger
    Figure 2: Schematic of Magnet Charger
  • Figure 1: Typical magnetization curves for common magnetic materials.
    Figure 1: Typical magnetization curves for common magnetic materials.

  • Magnet Charger
  • Figure 1: Typical magnetization curves for common magnetic materials.

How many times have your restoration projects required a magnet charger? Maybe you have a magneto which seems to be all in order but might need a bit more magnet strength to produce a reliable spark. I have successfully recharged the magnetos on both a '23 Fordson and a '26 Model T using my home-built chargers. The '23 Fordson has started on three to four turns of the crank over the last three years. I have not tried to start the T yet but expect similar results. I like being able to run these off the flywheel magnetos instead of an external battery; very rewarding. Sometimes recharging a weak or dead magneto magnet will bring an otherwise fine engine to life, saving the owner considerable expense.

In recharging the Fordson and the T magnetos, which I think are the most challenging, I have built two magnet chargers and feel that I am one of many self-proclaimed, de facto experts on the subject. Until recently, I just built the chargers, they worked and that was the end of the story. A few weeks ago I thought I would try to explain to GEM readers what is important in building a charger and how to go about achieving it by several means. Not everyone has the same junk box as I, so some other options would surely help the budding builder of magnet chargers. Charging magnets is not difficult if you understand the rudimentary principles by which they function. It is like driving to Buffalo; several routes are possible, all get you there and some are more difficult than others. The following is written for both the "weekend warrior" who merely wants results and cares not about the gory details/theory, and for those of a scientific ilk who want to know how everything works.

The first thing which is important in building a magnet charger is to set up a magnetic field by wrapping wire around a cylindrical core and passing a direct current through it. The wraps need to be neat; starting from one end of the core to the other with several layers is usually required, all turns being in the same direction. How many turns you wrap depends on how you plan to energize the magnet. My first attempt took the high current road via a DC welder as the current source. (The welder has to be DC output or you can kiss any magnetism remaining in your magneto goodbye!) The welder had an output of about 140 amps possible so I used about number 10 insulated wire. This is large wire. On my core, which was about 7/8 inch diameter by three inches long, I was able to fit about 30 to 40 turns on each pole. What determines the strength of the magnetic field set up is in part the current times the number of turns of wire or simply the number of amp-turns. My first setup produced about 4000 to 5000 amp turnsnot too bad. This setup was cumbersome to use, as I had to use it in the garage where the welder was, the coils got hot really quickly and I had to avoid the arc from touching the stinger to a plate used to turn the current on and off. There was a lot of waiting time needed to allow the coils to cool. For a Fordson magneto where there are 16 magnets, you need a lot of on time for the charger so this may pose a problem. If you are zapping a hit-or-miss magneto to freshen it up, this may be okay if the welder is all you have.

On my second attempt, I thought to myself that if the number of amp-turns determines the field strength, then I could use a lot of turns of fine wire at a lower current. The current could come from a battery charger, battery or small DC power supply. Ah-ha, such a problem already solved in starter solenoids! On a recent trip to the junk yard I located two large 12 volt truck starter solenoids. All I had to do was carefully remove the spools of wire which constituted the electromagnets. That was easily accomplished with a hacksaw and some elbow grease. Now I could generate the same number of amp-turns with about 14 amps from a 12 volt battery charger. The solenoids have about 3-400 turns of 18 gauge wire (same 4500 amp-turns) on them.

Once you have a source of amp-turns, be it a few large gauge wraps of wire connected to a high current DC source, or many hundred of fine wire turns from a starter solenoid, you need some magnetic cores for the amp-turns to magnetize. Once the cores are magnetized, they can transfer the magnetic field to the magneto through a "magnetic circuit" and presto, you've recharged a magneto. In essence, you are making a big horse shoe electromagnet to mate with the magneto magnet in question.

In as basic as I can get math, the magnetic field strength is the product of: Magnetic Field Strength = (amp-turns) x (Magnetic permeability)


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