Building Magnet Chargers
Harry Matthew's Weidenhoff magneto charger.
The following comes from a recent topic on SmokStak, which can be found on the Internet at: www.engineads.com/ smokstak.cgi. As ever, various individuals started, commented on and concluded the following bulletin board thread.
I would like to build a magnet charger like the one John Rex wrote up in the January 1989 GEM (see Gas Engine Magazine, January 1989, page 22, 'Building a Heavy Duty Magneto Charger'). I've wanted to do this for years, but never could find a source for the wire needed. It takes 40-lbs (preferably two 20-lb rolls) of 10-gauge, heavy Formvar magnet wire. Where can the average man find some of this? Also, a good, heavy, 40-amp knife switch? I like this style charger because, as Rex states, it will saturate any magnet without removing the magnet from the rest of the assembly. - Jim
I made a charger like the one you're building. I got the wire from a place that rebuilt industrial electric motors. Told them exactly what I needed and they came up with two spools at a very good price. As for the switch, poke about in old city electric buildings or factory electrical rooms. My switch came from the electric distribution room of a local manufacturing company. They were ripping all the stuff out and tossing it in a dumpster. Traded a dozen donuts to the wrecking crew for the switch. - Nick
I recently built a charger to Dave Gingery's plan. Simple and easy to build, 115-volt converted to DC with a $5.95 bridge rectifier, two spools of 6-lb, 18-gauge magnet wire from motor rewinders. It's safe, draws 8 amps, weighs approx 50-lbs finished, and with a bar across the poles when engaged lifts it right off the bench! It can charge most mags without dismantling. The total cost was $48, having had the two-inch steel cores in my scrap pile. - Fenwood
Hi Folks, this is not meant to be rain on anyone's parade, but I feel it is necessary to point out a small fact about magnet chargers. I have been a full-time magneto restorer/repairman for a number of years, and have owned a number of original magnet chargers built by some of the leading names in magnetos. These were built by the manufacturers and intended for use by their own 'Factory Authorized' service outlets. Without exception they have all been LARGE, HEAVY pieces of equipment. The American Bosch unit that sets on my bench weighs 240 pounds. I am of the opinion that if F-M, American Bosch and IHC thought a 25-lb charger would do an adequate job, that is what they would have built.
I am sure that if all you want to do is punch up the charge on your own mags a little the $50 to $100 units will at least not do any harm. But if you are truly looking to have a real magnet charger, take a long, hard look at the one that John Rex designed. Please don't just take my word for it. Ask any of the many mag guys that are in this hobby. - Ted Two things go into design of an electromagnet; the number of amp-turns (AT) and the magnetic permeability of the pole pieces. That will determine the maximum field you can generate to charge magnets. The AT is just the number of amps flowing through the coil of 'n' turns (have to divide by the area of the pole pieces.) Having a large pole to charge a wide variety of magnets means you'll need a 'Rex' type charger with many turns carrying high current.
The permeability is a characteristic number for the pole pieces used and is the ratio of the amp-turns to magnetic field strength. Some steels have higher permeability than others. But, almost all steels max out or saturate at a magnetic field strength of about 1.2 Tesla. If you want to make a better charger, use Alnico steel.
In some cases, bigger is not better! You may think a large charger is better, but if it is operated at a current above saturation all it will do is make a nice hand warmer in winter. It is very important that the magnets you are charging fit snugly against the poles without an air gap, and that you put a keeper on the magnet before pulling it off the poles and that you magnetize in the same direction as the magnet originally was. I have had good luck with my charger made from two 12-volt truck starter solenoids. - Paul
I have an Allen heavy-duty magnetizer that will really do the job. It is very heavy and is 110-volt. I also have made one from windings from Delco starter solenoids that will charge, but nothing like the Allen. - Ed
A $5.95 bridge rectifier and two spools of 6-lb, 18-gauge magnet wire at eight amps is going to work like a toy compared to the big chargers. You can pretend that you charged the magneto, or you can charge hell out of it to last forever. Note the size of the Wico EK compared to the coils (see photo). This one was at least 200-lbs and cost almost $2 per pound. It works! -Harry
My decision to build the Gingery charger was primarily based on the electrical engineering calculations outlined, i.e., 100 turns (heavy) at 60 amps, or 600 turns (18-gauge) at 7.5 amps, both give you 6,000 ampere turns and the same effect at the poles. The Gingery plan claims, 'that at 1,600 total turns at a max of 10 amps the charger will provide 12,000 ampere turns of magneto motive force, at least twice what is required to saturate an ordinary magneto magnet.' I felt that was adequate to my needs and it will accommodate a fully assembled Wico EK quite nicely. - Fenwood
True, ampere turns do the job, but I still wonder about the actual mass of the iron. I'm no magnet expert, so I'll forgo the fineries other than to be proud of my Weidenhoff. - Harry
I do envy your Weidenhoff, and yes, mass plays a substantial role. On the Gingery the cores are two-inch diameter by six-inch mild steel mounted on a four-inch by 10-inch, one-inch thick mild steel plate. The $5.95 DC rectifier is a state of the art, full wave bridge rectifier rated to 30 amps. The oldies (chargers) are scarce as hen's teeth, so I recommended the Gingery based on cost, standard, readily available components, step-by-step, clear instructions and drawings and the electrical calculations that support the unit's performance. A great project, and a confidence builder for the do-it-yourselfer. - Fenwood
I read somewhere that soft iron was best because it doesn't stay magnetized well and separating the magnet from the magnet charger is a little easier. With the mild steel, do you have any strong residual magnetism after turning off the power? I have always thought that you can have less turns and more current, or more turns and less current, in the magnet windings and still do the same amount of work. That is, as long as the volts times the amps comes out to the same value. Let's say 10-volts times 100-amps, versus 100-volts times 10-amps, which in both cases equals 1,000 watts of power, or energy. The number of turns in the coils and the wire size, or gauge, has to be proportionate. Any thoughts? - Russ
I guess I knew what I was thinking, but maybe I didn't express it too clearly. I am not a metallurgist, so my knowledge of these materials is limited. I have read that soft iron, whatever that is, makes a better electromagnet because when you turn off the power to it the magnetism decays rapidly. I understand that the laminated pole pieces in transformers, motor field windings and any other similar coils used with alternating current have to be soft iron so the magnetic flux can be reversed at whatever frequency it is operating at. This may not be as important on a magnet charger, as the polarity is constant in one direction.
On the other hand, you mentioned using mild steel for the pole and base pieces. What happens when you remove the power. Is the decay of magnetism satisfactory? If mild steel is satisfactory for the pole and base pieces, then I can probably get it here locally in the smaller quantities. The problem with many of these small projects is finding the right materials. Some people are fortunate to be living in places where these supplies are more common than here in the small city boondocks. - Russ
I am nearly finished with my own design (made from stuff I had laying around) to charge EK mags. It consists of one 1-inch x 2-inch x 5-1/2-inch mild steel strap with two 1-inch x 2-inch x 2-1/4-inch straps tapped and bolted together to form a wide, flat U-shaped core. I have taken the four field coils from a Ford auto starter and I am in the process of mounting them on the short legs of the 'U.' May work, may not. Should I bother finishing it to see? - Wayne
The 'REX' type charger is the way to go and I am building one now. I talked to a local motor rewind shop about winding the coils so I don't have to buy small spools of wire. The steel for the core is CR 1018, very common. Lots of on-line companies sell steel by the inch. Rex's charger is 20,000 ampere turns. - Doug
After following this thread, I just have to build a 200-lb 'REX' charger to replace my toy that weighs about 20-lbs. - Paul
SmokStak is an engine conversation bulletin board with over 15,000 messages on file and is part of the Old Engine series of web sites that started in 1995 as 'Harry's Old Engine.' Harry Matthews is a retired electronic engineer and gas engine collector from Oswego, N.Y., now residing in Sarasota, Fla.