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Single Coil Magneto Magnet Charger

Boris

Subscriber
Last Subscription Date
07/25/2019
I have designed and built a single coil magneto magnet charger that is compact and powerful. It uses a single horizontal coil consisting of 160 turns of 12 gauge wire. It is powered by a 12v car battery and draws about 100 amps during the charge pulse. It weighs 15 lbs and is 8” tall. It is controlled by an ignition key switch witch activates a starter solenoid for the current pulse. The top armature pads can be adjusted from 5/16” to 4” gap, as well as set at angles.
The horizontal single coil design makes it compact, easier to build, and reduces the magnetic flux path length compared to the more common two vertical coil design. It has worked very well. I can post performance measurements and construction details if people are interested. Comments and suggestions much appreciated. Thanks, Robert
 

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Joseph Cissell

Registered
Last Subscription Date
01/07/2017
Robert, I would like to see the measurements and the construction details so I can build me one of these chargers. Thank you and have a nice day. Joe A.
 

Peter Holmander

Subscriber
Age
71
Last Subscription Date
12/23/2019
Cool design Robert. Did you use magnet wire for the coil? I too would be very interested in the details of how you made it. I've always wanted to make one, but the cost of magnet wire for 2 coils was out of my reach. And when you do see a used charger for sale, they are very expensive. Thanks for sharing !!
 

JoeCB

Subscriber
Last Subscription Date
02/28/2019
Thank you Boris, very nice … I like it ! Looks like you used regular insulated 12 ga wire, like the THHN that has a thinner plastic insulation, yes? If so, do you think that this wire impedes the magnetic field build ? The THHN sure is easier to come by than the enameled wire... I have a big coil and had been considering using it for a charger.

Joe B
 

Jake Jacobs

Registered
I have designed and built a single coil magneto magnet charger that is compact and powerful. It uses a single horizontal coil consisting of 160 turns of 12 gauge wire. It is powered by a 12v car battery and draws about 100 amps during the charge pulse. It weighs 15 lbs and is 8” tall. It is controlled by an ignition key switch witch activates a starter solenoid for the current pulse. The top armature pads can be adjusted from 5/16” to 4” gap, as well as set at angles.
The horizontal single coil design makes it compact, easier to build, and reduces the magnetic flux path length compared to the more common two vertical coil design. It has worked very well. I can post performance measurements and construction details if people are interested. Comments and suggestions much appreciated. Thanks, Robert
construction details please. thanks
 

Boris

Subscriber
Last Subscription Date
07/25/2019
The coil core used in this charger is a 1” x 4” mild steel rod. The side and top armature pieces are 1 1/4 x 2 1/4 x 3” iron blocks. The 2 flux conductor blocks are also the same size. I ordered the 6 blocks precut from a local metal supply company. I flew cut the mating surfaces of the blocks with my mini-mill for a close fit but the blocks from the supplier came cut quite square. A good filling to smooth them out would be sufficient. Everything is bolted together using 10-32 flat head screws. This minimizes the removal of metal in the flux path. The armature assy is screwed to a 1/4” aluminum plate 6” x 8”. An aluminum 3 x 5 x 7” box encloses the circuit below. I plan to add a wood base.

The core is wound with about 60 ft of 12 gauge solid THHN building wire. A 100 ft spool can be purchased at your local hardware store for around $25; no need to use expensive hard to find magnet wire. I used my mini-lathe to hold the assembly while I hand wound it; took me about a 1/2 hr. A second person could be used instead of a lathe. The wire was stripped bare for the first 2-3 turns of the coil to provide the ground connection for the beginning of the winding. No insulation was needed around the core rod or sides; the wire insulation is rated at 600V. 5 layers were wound.

The circuit is built with a generic remote mount starter solenoid and garden tractor 3 position ignition key switch. A 4 guage jumper cable set provides the power connection to a standard car battery which almost everyone has (just pop the hood and connect it). The ground connection of the cable is bolted to the al plate underneath. The + cable is connected to one side of the solenoid; the other side of the solenoid is connected to the end of the coil winding wire from the top layer. The ignition switch is hooked up to the solenoid in the standard way. 2 LED lights were used for indicators. No diodes are used to suppress induced reverse currents and so far haven’t had any trouble with that.

Before use you need to determine the polarity of the top armature pads. Use a compass about 6” away and pulse the unit to see what the polarity is and mark it. Adjust the top pads to accommodate your magneto and arrange the flux blocks if needed. Search the web to see how your magneto is charged. Activate the charger with the key switch for no more then 1 second per pulse. I use three pulses. The coil will get warm but not hot. Never hold the key on for more then a few seconds. Its frequently stated that a keeper must be installed after charging a bare magnet. I have found that it is not necessary. You can’t do that anyway when you charge up a complete magneto.

Hope this helps. So far I’ve pulsed it over 100 time with good results and no problems. I have done some performance tests that I will post later.
 

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cobbadog

Registered
G'Day Boris, thank you for sharing this great design for a magneto charger. I now have to ask some silly questions, sorry!
1 - Is 'Iron' the same as "mild steel' ?
Just so when I go to the metal supplier I look as silly as I am.
2 - You mentioned that you stripped the insulation off the wire for the first 3 layers, but you pic of it in the lathe does not show that just insulated wire. Is the bare wire under a cover?
3 - I can almost figure out in my head your wiring but I'm not sure where the second end of the coil wire is connected. Do you have a schematic diagram please that also identifies the positions on the solenoid?

Sorry to annoy you with these questions but this looks like a project I need to do.
Cheers Cobba. (John).
 

Boris

Subscriber
Last Subscription Date
07/25/2019
I love silly questions!
My metal supplier had 1 1/4 " cast iron bar stock which is what I used. I have since found out that wrought iron is better for magnetic applications, and mild steel is better yet (and easier to work with). So try to find mild, low carbon steel for the armature structure around 1" thick. Thick armature structure conducts the magnetic flux better.

Look at the left hand side of the core rod in the lathe pic. You will see at least one bare winding. I drilled a small hole 1/4" deep on the inside of the armature just above the core rod. I then stuck the bare end of the wire into it, bent it over, and started the winding. The back side pic of the charger shows on the right the other end of the 12 guage wire passing through the side armature and passing down through the aluminum base plate which is then connected to the solenoid.

The starter solenoid you get should have two large high current connections and two smaller connections to activate the internal solenoid switch. If you look at the wiring pic on the left side you will see the red wire from the coil connected to the top large bolt on the solenoid. The bottom large bolt is connected to the red + of the battery jumper cable.
I will try to draw up a schematic of the charger and post it.
 

cobbadog

Registered
Thank you so much, I look forward to the diagram. !" round mild steel is readily available from our steel supplier.
 

Peter Holmander

Subscriber
Age
71
Last Subscription Date
12/23/2019
I look forward to a wiring diagram as well Boris. I'm pretty good at wiring, but am unclear on a couple of things. Was the start of the bare winding wire fastened to the 1 inch round bar, or the square block on the left end in your picture? Was any insulating material needed between the steel blocks and the aluminum base? Do you know why the two coil charger plans require such large steel stock for the armature? If I remember correctly, some use 3 inch diameter stock. Your design only requires 1 inch. I also think that some of the old mag chargers use a 6 volt battery too. Your design is so much cheaper to build. It's almost a no brainer. Thanks again for sharing.

---------- Post added at 09:54:01 AM ---------- Previous post was at 09:28:23 AM ----------

Boris, disregard the question about here you started the coil wire. I re-read your construction details again and it explains it very well.
 

Boris

Subscriber
Last Subscription Date
07/25/2019
For the connection at the start of the winding I drilled a small hole 1/4" deep on the inside of the armature just above the core rod. I then stuck the bare end of the wire into it, bent it over, and started the winding with a couple of bare uninsulated coils and the rest were insulated. Its a bit "iffy", but works, and the next one I build may have a better design without distorting the first few windings. No insulation or paint between the armature and baseplate was used; it would block the current flow.
Yes it only uses a 1" armature core which limits the flux capacity. Larger cross section is better; longer length is not. I have obtained a high purity iron 1" rod for the next charger I build. The drawback to a larger core is that design uses more wire per coil, which creates more resistance and less current for a given voltage.
When the old chargers were first built, there was no high current batteries available. So they had to use hundreds of windings of small guage wire for the low currents. The first high current batteries were 6V; usable but at half the current of a 12V battery for the same configuration. 100 amps is a piece of cake for today's automotive batteries.
Attached is a schematic for the circuit.
 

landreo

Registered
Just a few considerations when designing a magneto charger. You do not have to have a charger that weighs 200 pounds, a lighter version is possible but will get warm or hot. To charge a magnet every 10 seconds 8 hours per day is going to need a heavy charger, to charge a few magnets and then allowing the charger to cool is possible with a much lighter and less expensive charger.

The magnetic field has to be made strong enough by the electromagnet and that magnetic field has to be transferred or directed to the magneto magnets. Both part are critical. The fancier the magnet the stronger the field needed. An ALNICO magnet may need a field 3-4 times stronger than the field needed for the old horseshoe magnets.

I built my own years ago and did some research and calculations prior to building. Most seem to say that you need around 20,000 amp-turns for the electromagnet to charge the horseshoe style or Wico EK magnets. Looking into the science behind all of this and it appears that 20,000 amp turn recommendation is correct for the minimum number of amp-turns. That number assumes a close packing of the wire, i.e using magnet wire.

The size of the core depends on the type of iron or steel but for mild steel a cross section area of about 4-5 square inches is needed otherwise all you are doing is wasting the magnetic field from the electromagnet since it will not all get through the core.

The magnetic field is greatly reduced by even a small air gap from poor fitting of the steel core or from the thickness of the insulation. Magnet wire is used for electromagnets and motors since the effects of the thick insulation is reduced.

The charger I made was to charge some old telephone magnetos and some Wico EK magnetos. It will not do anything more than that level of magnet. It will not come close to fully charging an ALNICO magnet. I used somewhere around 24,000 amp turns for the electromagnet and used a 2 inch round mild steel core. That steel core will have a little over 3 square inches of area which is below what is really needed. Too small. If I was going to do it again I would use the same electromagnet but a 3 inch core. My charger will charge the magnets but not completely. It is useful but not as useful as it could have been.

100 amps using 160 turns of wire gives 16,000 amp-turns which is a bit low and the effectiveness of the magnet is significantly reduced by the thickness of the insulation on the wire. You cannot get a tight packing like you can with magnet wire. Cloth covered wire like what may be found on old magneto chargers was still thinner than the insulation on THHN wire. The effectiveness of the 16,000 turns is really a good bit less due to the thick insulation.

The 1 inch rod for the steel core gives a cross section of around 0.8 square inches, way less than a reasonable 4-5 square inches, and will result in more loss of the magnetic field as it is trying to work its way to the magneto magnet.

My statements are not a criticism of the design but just some things to think about for those that may be considering their own design. You do not need a 200 lbs charger for the old Wico and horseshoe magnets but there are some basic minimums to getting good results. You can charge a piece of steel and make it a magnet but to fully charge requires a minimum magnetic field strength.
 

Boris

Subscriber
Last Subscription Date
07/25/2019
Thanks landreo for your informative comments. Any info on magnet chargers is very helpful for guys considering building one.
I knew that a 1" core would be a limitation. Thats why for the next charger I obtained a high purity iron rod specifically formulated for magnetic applications. This should held avoid saturation.
Could you explain how insulation thickness can greatly reduce the magnetic field? Is it the proximity of the coils to the metal core thats important? With only 5 layers of windings I don't think using THHN wire would have much of an effect.
I will post some test results that seem to indicate that this design works well. I would appreciate your further comments and perhaps do the same tests on your charger.
 

Mitch Malcolm

Sponsor
Age
65
Last Subscription Date
01/14/2020
I agree with Landreo 16000 amp turns will do a very poor job of charging any alnico magnet and it certainly will do nothing on a AB33. Any decent charger needs 40000 amp turns to saturate a magnet. that's my 2 cents worth and 25 years of experience doing it daily .
 

landreo

Registered
Any air gap will decrease the effective magnetic field whether an air gap between the poles or an "air gap" from the thickness of the insulation. A tight pack is best. I should not have stated that the effectiveness of your coil is a good bit less because of the thickness of the insulation and its effects on the spacing of the coils but it will have some effect. Does it have a good bit of an effect? I think it would but I may be wrong on the amount, it may only have a small effect.

The core size does have a significant effect if it saturates at a too low of a level as does my 2 inch core. I had the 2 inch steel rod so I used it but soon realized that I should have just stopped and got a piece of 3 inch rod. The equivalent of what I have is using a 8 volt charger for a 12 volt battery, you can get some of the way to full charge but never all the way. My magneto charger will get some of the way but never will fully charge the magnet due to my poor decision when I used the 2 inch rod.

There are many ways to design a charger, I made a 2 pole charger with each layer of wire, I believe I used 20 gauge, connected in parallel with all the other layers. So electrically I had 5 or so individual coils or layers connected in parallel. Those coils were then connected in parallel with the other pole. That way I had high current flow and plenty of amp-turns.

If your charger does what you need it to do then I would consider it a success.
 

Boris

Subscriber
Last Subscription Date
07/25/2019
In order to test the performance of this charger I build a gauss meter to measure the before and after charging results. I ordered 4 horseshoe magnets from a well known Ford Model T parts supplier. They charged up 2 and left 2 uncharged before they sent them. The charged ones showed a slight improvement and the uncharged showed a 60% improvement.

A good test of a charger is if it can charge a magnet to saturation. And if it can it will not matter if you reverse charge it; it completely erases the polarity. So I reverse charged a magnet after charging it correctly and it showed about a 10% gain. I then reversed charged it again back to its original polarity and it showed an overall improvement of 22%

I charged up Wico XK, Bosch A-B 34, and a Dixie mag off of operating engines. The Wico showed a small improvement; the A-B 34 none, and the Dixie a 25% improvement. The Wico had a armature pull strength of 32lbs after charging.

I also hooked up the gauss meter to an oscilloscope to test how fast the field was generated. It reach full strength in 1/10 th sec and dropped to 0 in 1/8 sec. So a 1 sec pulse is more then enough.

The amp-turn rating of a charger is important but there are other factors as well that determine how much flux actually gets to the magnets. The armature dimensions and material used, total flux path length, air gaps and mounting arrangements can all affect its performance.

So this charger worked in most of the tests that I did, and I think it is "good enough" to charge many weak magnetos up to a level where they will work. There is always room for improvement, and my next charger will have better armature materials. And considering how compact, easy and cheap to build compared to most others, I think this charger has its advantages.
 

cobbadog

Registered
Now you have me curious about your next one and the materials to use. I will wait and see what you come up with as I only want to build one charger but to a good standard.
 
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