Magnets & Magnet Chargers

By Staff
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Figure 2: Schematic of Magnet Charger
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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)

The amp turns are simply the number of wraps or turns of wire around the poles of the electromagnet times the current through the wire. The permeability is a measured constant for most magnetic materials which says if I pass so many amp-turns through a wire wrapped around so-and-so materials, I will get such-and-such a magnetic field strength up to a point. Figure 1 shows the magnetization curves for four ferrous alloys. The x-axis is determined by and is proportional to the number of amp turns, while the y-axis is the actual amount of magnetic field generated. As you can see, cast iron is a pretty lousy magnetic material when compared to cast steel. Armco iron is the best but probably unavailable to most of you. Silicon steel is used mostly in transformers because it can be reversibly magnetized by AC coils with little energy absorption (heat, mostly). Steel is plentiful and will be adequate to use as a magnetic core material to recharge the quench-hardened steels used in most pre-1930 magnets.

Most magnetic materials (iron and steels included) have a saturation point. This means that no matter how many amp-turns you put around a pole, you cannot exceed the saturation magnetism. (There are only so many atoms in the metal which can be aligned creating magnetism.) The people who design starter solenoids and the like know this and figure out how many amp-turns at say 12 volts they need to saturate the core which draws in the starter gear. More turns on the coils would simply be wasted copper wire! So if you go the starter solenoid route, most of the amp-turns guesswork is already figured out for you. The bigger the solenoid, the larger the pole pieces you can fit and the better job of remagnetizing you can do.

I made my second charger from the two starter solenoids, some 1.25″ diameter rod stock and a piece of 2 x 3″ steel “U” channel. A piece of 0.5″ x 3″ x 6″ flat bar could also be used in place of the channel. This is shown schematically in Figure 2. I have tried both base styles and found no difference in performance. The important thing to remember is to select low carbon steel (mild steel is okay) for the pole pieces and base. Carbon, as in cast iron or tool steel, reduces the saturation point of iron, which means less magnetic field strength is able to get to your magneto. Again, prior to about 1930, most magnets were made using a quench-hardened steel. Generally, these hardening steels have the quality of being easily magnetized and then maintaining that magnetism for a long time. Ever wonder why those screwdrivers stay magnetic? These magnets will, over time, lose some or all of the magnetism. When I got my ’23 Fordson, the residual magnetism was barely detectable. Excessive heat, hammer blows or vibration can all lessen the field of a hardened steel magnet over time.

Low carbon steel makes a good pole piece, is easy to machine and supports a good level of magnetism before it saturates just as long as enough amp-turns are available. If you use a large starter solenoid, you can be assured a good strong field for charging your magnets.

How you assemble your charger depends on the type and shape of your magnets you care to recharge. If you think of magnetism in the same way you think of an electric circuit, assembling the pole pieces should be easy. Magnets like to form “magnetic circuits” much the same way you hook up batteries in series in electric circuits; that is plus-to-minus-to-plus-to-minus-etc. for batteries and north-to-south-to-north-to south-etc. for magnets. Figure 2 illustrates how this occurs in your magnet charger. The center leg or base which the poles bolt to is essential to completing the magnetic circuit through your magneto magnet. The field in the base plate is “induced” by the action of the two solenoids. The complete magnetic loop or circuit is necessary to getting the most possible magnetism through the magneto magnet thus charging it to its original level. If you are recharging plain old garden variety “U”-shaped magnets as shown in Figure 2, you’ll want the solenoid spacing such that the magnet is centered on the pole piece. If you make the base long enough and drill several holes in it, your charger will then be able to charge several sizes of magnet. For Fordsons and Ts, laying the magnet on it side (flip magnet in Figure 1 90 degrees towards you) works in lieu of making special angled pole ends. In magnetic circuits, poor contact (as in electrical circuits too!) area between the magnet and pole pieces limits the amount of magnetism transferred to the magnet to be recharged.

Let’s say I have a charger built as shown in Figure 2. First question is how do I hook up the wires to get the right field directions (norths and souths correct) to do some magnetizing? I go buy a cheap dime store compass and a flashlight battery. On the first solenoid, arbitrarily mark one wire as the plus and one as the minus. Hook up the battery plus wire to plus side of battery and minus to minus. Move the compass close to the top of the pole piece and observe which end is attracted. For example let us say it is north; mark the pole with an “N” with a permanent marker. On the other pole, simply hook up the battery whichever way gives you a south reading from the compass near this pole piece; mark it with an “S.” Mark the wires to the second solenoid accordingly with plus and minus. Now you know how to hook up the coil to your power supply.

If you are using a battery charger to energize the solenoids, measure the resistance of the solenoids. Once you know the resistance of each solenoid you can figure out whether or not to hook them up in series or parallel. If you are using a car battery, you can probably hook them up in parallel okay. If you are using a battery charger, they are sometimes current-limited and you may want to hook them up in series to prevent overloading the charger or power supply. Before you try out your magnet charger on a magnet, hook it up to your intended current source and check for a north and south pole on the magnet charger. Always double check your work with a compass to avoid mistakes. Reversing the direction of the magnetic field using this magnet charger will greatly reduce the charged strength of the magnet. The magnet may also lose strength quickly over time if the original field direction is reversed. Next, hold the compass to the magnet (preferably) removed from the magneto and look for the “N” and “S” poles. Mark them with a grease pencil or the like. Place the magneto magnet on top of the poles with the “N” pole of the magnet on the “S” pole of the charger. This will complete the magnetic circuit. Now simply “flash” the wire to the charger to the source (observe correct polarity) for a second or so. Try turning the magnet on its side and flash for another second. Repeat with the magnet in the vertical position on the last flash and your magnet should be good as new. Do not be afraid to flash the charger several times.

The charger shown in Figure 2 also makes a good demagnetizer for tools and such. Simply connect the coils to a 6 volt AC transformer and pass the magnetized tool over the poles. It should then be demagnetized. If you’re using heavy gauge wire and a welder, switch to low AC amp range, energize the coils and pass the objects to be demagnetized over the poles.

Reference: Circuits, Devices and Systems, Third Edition, by Ralph J. Smith, John Wiley and Sons, 1976 (see Figure 2).

Contact Paul Gray at Graystone Ltd., 3437 Blue Ball Road, North East, Maryland 21901

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