Design Your Own Magnet Charger

You can charge it!
By Peter Rooke
September 2009
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An early horseshoe magneto from an Amanco, with the retaining band removed.
Peter Rooke

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This is Part 1 of a 2 part series. This article describes the development of Peter Rooke's homemade charger design and the preparation of the plans. Part 2 describes the machining of the magnet charger.

While a friend of mine had a magnet charger, I always felt a little uneasy about putting him to the trouble of ensuring that the battery that powered it was fully charged and the fact he had to clear his work bench to make room to use it.

Part of the satisfaction I gain from restoring engines arises from the turning of various bits of metal into a working piece of machinery, so I felt that to make a charger would be a new challenge, and that I would also learn something in the process. I knew little about electricity, apart from being able to change a plug, and knew even less about magnetism. Fortunately, I still met my old school physics teacher for a glass of beer most weeks, as we were both members of a local rifle club, more social members now with failing eyesight! However my hopes of an immediate flow of information just elicited the initial comment “yes, magnetism – a tricky subject,” and he started to talk about something else!

Next, I searched and a few hours were well spent gathering numerous comments as well as print outs of articles detailing plans to build magnetizers, including a comprehensive one by John Rex printed in the January/February 1989 issue of Gas Engine Magazine, and a copy of an article in Dyke’s Automobile and Gasoline Engine Encyclopaedia in 1918. There was also reference to a Dave Gingery design.

This was fine but I was no further along as I now had three schemes, each one using different size cores, all of which were, according to numerous people, successful. The only common point that I was able to identify was that 20,000 ampere-turns of copper wire appeared to be the magic number to achieve the full charging of a magneto. Ampere-turns refer to the number of turns of wire around the core multiplied by the amperes that the length of wire draws.

The first decision was to identify the optimum core size before calculating the gauge and amount of wire to wind around this, along with the power source to achieve the desired result.

I was still in need of an “expert” so I contacted Martin Percy, the Help Desk contact for magnetos in Stationary Engine magazine here in the U.K. I hit gold dust, as Martin was only too willing to help. He knew the subject of magnetos inside and out, and had built his own charger. Furthermore, he had some information he would photocopy for me about a charger built by Warwick Bryce and featured in Stationary Engine in 1988. It soon became apparent that I needed to understand a little more about the difficult subject of magnetism before designing my own charger in order to be satisfied that it would meet my future needs. So again, I approached my old teacher, who subsequently proved most helpful.

While I could have simply described the charger I built and how I achieved it, this would not be much help to anyone else wanting to build a charger to a different specification and also understand a small element of the theory.

My initial knowledge of magnetism was based on my school days when, as kids, we played with them. This knowledge was limited to the fact that a magnet had a north and south pole, other ferrous metals could be magnetized with the magnet and that like poles on the magnet repelled while opposites attracted each other.

Magnet steel
Up to the 1920s, horseshoe magnets were made from the toughest steel then available, tungsten steel.

Since then, other strongly magnetic steels have been made, containing chromium and nickel up and until the 1930s when compressed metal powders such as alnico and Alcomax were used.

Alnico consists of aluminium, nickel and cobalt, hence the name. These special high-energy magnetic materials are much more expensive than tungsten steel but they hold more than 20 times the magnetic energy.

The tungsten magnets had to be long and thin to prevent self de-magnetism, hence the large horse shoe magnets seen on the early magnetos, more compact magnets and magnetos being made when the new materials were available.

The downside to the later, non-tungsten, magnets is that they require a greater magnetic force to charge, but once they are fully charged, they create a stronger magnet.

Metals that can be strongly magnetized such as iron, nickel and cobalt are known as ferromagnetic, exhibiting a large attraction to magnetic fields and have a high ability to retain magnetic properties once the external field has been removed. They all get their strong magnetic properties through the presence of what is known as magnetic domains.

Magnetic domains
Within ferromagnetic materials, the atoms tend to have their own magnetic field, created by electrons that circle it. Small groups of atoms, consisting of more than a trillion, tend to align themselves in the same direction and these groups are known as domains.

Each domain has its own north and south pole, and in an un-magnetized state, the poles of the domains point in different, random directions with the result being that little or no magnetic force is displayed as they cancel each other out.

Placing magnetic material in a strong external magnetic field or passing an electric current through it results in the domains starting to align themselves in the same direction so the material starts to exhibit a stronger magnetic capability. The greater the magnetic force applied, the more domains will be aligned and the stronger the magnetic force. Once all the domains are aligned the material is said to have reached saturation and its magnetic properties will be at their maximum. They cannot be improved with the application of any more magnetic force.

With soft irons the domains align easily whereas more magnetic power is required with hard materials. However, with soft irons the domains will be scrambled again when the external magnetic force is removed, whereas in harder metals, a greater proportion is retained, making it a stronger magnet. This is why soft iron is used for the core of coils, allowing the rapid build up and release of magnetism.

Magnetic force
The best way to describe a magnetic field is to imagine invisible lines of force parallel to each other entering the magnet at its south pole, travelling through it before exiting at the north pole, in a closed loop.

The magnetizing force of an electromagnet, which is in effect a magnet charger, is proportional to the number of turns of wire in its coil to the current flowing through this wire for 1 meter of coil; e.g. for a coil with a 1-meter-long core of 250 turns of wire drawing 50 amps, the magnetic force is 250 x 50 = 12,500 ampere-turns.

The overall strength is known as the flux density of the magnetic forces and relates to the number of these flux lines in a magnet within a given area. The greater the density of the magnetic force the stronger the magnet and this is measured by different units including the tesla, one tesla being about 30,000 times as powerful as the Earth’s magnetic field.

If the magnetizer will be used to charge modern rotating magnetos, which are best charged assembled, an allowance should be made for air gaps. An air gap can also mean a non-ferrous material like aluminium or other material used in the construction of the magnetos after the early horseshoe types.

For two air gaps as small as 1 mm each, the strength of the electromagnet (flux density) might fall by a factor of five or more, so the flux density will need to be five times higher to compensate.

hile the thought of 2 mm in air gaps is impossible when re-charging a horseshoe magnet, if you look closely at the magnet, the surface is generally rough and uneven and it is impossible to get a 100 percent metal to metal contact.

Having said all this, I must also repeat my comments earlier under magnetic domains that when charging a magnet there is a point when no matter what force is exerted on it, its strength will not increase once all the domains become saturated. Saturation occurs at around 1.6 tesla for a tungsten steel magnet and 2.0 tesla for composite materials.

Designing the charger
There are three aspects to the design of the magnet charger: the size of the core and frame, the length and thickness of the copper wire to be used as the windings, and the voltage to be used.
Before starting the design, a clear understanding is needed of the type of magneto to be charged. If purely small magnetos are to be charged with a modest cross-section to the magnets then a small core of 1-1/2 inches will be sufficient.

Lucas, a major high-tension magneto manufacturer here in the U.K., recommended the following design criteria in its workshop instructions in 1953, stating that this would be sufficient to saturate all commonly met magnetos:
• Core material – soft (preferably Swedish) iron.
• Core area – 9 square inches.
• Recommended core winding – 65,000 to 70,000 ampere-turns.
Another design consideration is how the magneto will fit between the coils, either by using various pole pieces on the top of the cores or by moving the coils themselves.

Core and frame
In deciding on the surface area of the coils, the magnets on the low-tension magnetos available were measured, as the prime intention was to continue in the restoration of low-tension ignition engines. However, the construction of the charger was to be a major project, not to be repeated, so to give added flexibility it had to be made to a specification that made it capable of charging a wide range of magnetos.
 Where there were two magnets joined and working together on a magneto, the combined measurement was taken.
To get the best results in recharging a magnet the source needs to have between two and two and a half times the surface area of the magnet so the magnetizing force from the charger is concentrated in the magnets. As can be seen from the table, the largest cross section was 2 square inches, giving rise to an optimum core area of 8 to 10 square inches. Therefore 3.00-inch cores were selected for the coils, and to maintain this surface area of 9.4 inches, the bottom of the frame would be made from 4-inch-by-2-1/2-inch iron.
The core of a charger should be able to reach magnetic saturation as easily as possible, so the best material to achieve this is Swedish magnet iron, which is extremely pure. This is not readily available, so the next best option is to find a steel that has a very low carbon content, such as C1010 which has a carbon content between 0.08 and 0.13 percent, in effect having the properties of iron. The more carbon there is in the steel, the more magnetic force is needed to reach saturation, and some of this magnetism will be retained when the magnetizing force is removed.
To get the best results the core needs to be as short as possible, but the design of an excessively short and fat core makes it difficult to fit a magneto between them so there has to be a compromise. The arms of the cores on this charger would be made from two pieces of iron 6 inches long, which, after allowing for the insulation rings at each end and the platform at the top, left around 5 inches of the core for the actual winding of the copper wire.

Power source
Calculations for this charger were based on using a 12-volt battery, which is an easy source of power without the need to use a transformer to change mains voltage and a rectifier to smooth it out.

If a higher voltage is used, the amperage increases proportionally, thus increasing the amp- turns, so if necessary two batteries could be joined in parallel to obtain 24 volts. Another point to bear in mind when using batteries is that the voltage might drop to 10 volts when the battery is delivering peak current.

Copper wire
Copper wire is sold by weight, on different sized spools. The resistance of the wire is measured in ohms, putting a value on how easily an electrical charge will travel down the wire, which for winding wire is generally expressed in ohms per 1,000 feet. With a thicker and low resistance wire, more charge will go down it, so the diameter of the wire chosen needs to be sufficient to allow the amperage necessary to obtain the desired level of magnetism.

As the resistance varies according to the length of wire used, the resistance of a coil in relation to the number of ampere-turns also depends on the diameter of the core. For a 1-inch diameter coil the length of wire for one turn of the first layer is 3.1 inches; for a 2-inch core, 6.3 inches; and for a 3-inch core, 9.4 inches. Therefore, in calculating the ampere-turns of these different sized coils at 12 volts DC, a pattern emerges that the optimum wire thickness is less for the smaller diameter coil. There is also a point where the efficiency of the ampere-turns reduces rather than increases.

The ampere-turns table at the top of this page sets out a summary of the calculations for one of the two cores of a charger, the wire being wound over a 5-inch length of the core. This shows that doubling the number of turns does not necessarily mean a doubling of the ampere-turns, because of the resistance of the wire. In the case of the 3-inch core and the particular resistance of the 10 gauge wire used, it was found that the length of wire used increases proportionally to the turns so there is no change in the ampere-turns.

It so happened that 4-kg spools of 10 gauge wire were available, each of which held 300 feet. This was convenient as one spool of wire could be used per coil and there would not have to be any measuring or counting of the number of turns.

Wiring circuit
Before going any further, an important decision is how to wire up the two coils and the battery, whether in series or parallel.

Wiring in series doubles the resistance of the coils, thereby reducing the flow of current or amps. In parallel, the total resistance is half the value of one coil.

The resistance of the wire used in one coil is 0.318 ohm (300 feet of wire with a resistance of 1.06 ohm per 1,000 feet). This means that if the two coils were wired in series, the total resistance would be double, 0.636 ohm giving a current draw of only 18.9 amps (by the formula amps = voltage/resistance, which is 12 volts/0.636 ohm), giving 11,340 amp-turns.

Wiring the same two coils in parallel, the combined resistance is 0.159 ohm. The current is therefore 12 volts/0.159 ohm which equals 75.5 amps, giving 45,280 ampere-turns. This assumes a 12-volt voltage from the battery. If it reduces to 9, the ampere-turns reduce to 33,962.

When a current is passed through a copper wire, the wire starts to generate heat, the amount being related to the current in amperes and the resistance of the length of the wire. The shorter the length of wire the lower the resistance and the greater the heat generated. The resistance of the copper wire eventually selected was 1.06 ohms per 1,000 feet, therefore being 0.318 ohm for the 300 feet to be used in each coil.
• Power (watts) = amperes2 x resistance of the wire.
• Power = 382 x 0.318 = 459 watts.

As can be seen, the coil will quickly heat up, and it is therefore imperative that as soon as peak magnetism is achieved it is switched off again or else there is the risk that it will overheat. If thinner wire is used then the resistance reduces, and so will the heat, but this will also reduce the number of ampere-turns and the effectiveness of the charger.

Final plans
Having decided on the basic dimensions of the cores and wire to be used, the next step was to draw up plans to get a clear idea of how the charger would look and order the materials.

In Part 2, in the December/January issue of GEM, Peter takes us step-by-step through the process of making a homemade magnet charger.
Contact Peter Rooke at Hardigate House, Hardigate Rd., Cropwell Butler, Nottingham NG12 3AH, England •

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