Make Your Own Magnet Charger

You can charge it!

magnet charger1

Peter Rooke recharges a Webster magneto on his homemade magnet charger. Note the use of the blocks to get good connection with the magnets. 

Photo by Peter Rooke

Content Tools

Part 1 described the thought processes leading to the design of a charger and the preparation of the plans. Next the enjoyable part: machining metal.

My usual supplier of steel was able to source some “black steel,” which is similar to C1018 in that it has less than 0.2 percent carbon content. In reality this has properties similar to iron, although is not as good as pure magnet iron. Round black steel has a rough surface, so it needs to be ordered oversize so that it can be machined to a good finish.

I ordered a 12-inch length of 3.5-inch diameter steel together with a 12-inch length of 4-by-2.5-inch steel that would be used for the base. In addition an 18-inch length of 3.5-by-1.5-inch steel was ordered to cut up and machine to provide four pole pieces.

While 200 turns of 10 AWG wire per coil would in theory have generated near 45,000 ampere-turns, the current drawn would be high at over 100 amps. This could be reduced to 74 amps by increasing the number of turns to 300, still achieving the same number of amp-turns. In addition, a standard 4-kg coil from the supplier held just sufficient to wind 300 turns (if the starting diameter of the core is 3 inches). Furthermore, winding 3-inch diameter for 5 inches of the core results in the winding being nearly 6 layers, which meant it could start and finish near the bottom. The copper wire ordered was class H winding wire, with a dual polyester coating capable of withstanding high abrasion and temperatures.

Some hard plastic, acquired many years ago and since stored in a corner of the workshop, was used to form the end plates and the platform to rest the magneto on while it was charged.
To help reduce arcing when the power was switched on and off, a 12-volt car starter solenoid was purchased, along with a simple press switch, some connecting wire (6 and 8 AWG) and terminal screws. There will still be some arcing of these points so rectifier diodes were needed to suppress it.

For a more professional device an ammeter can be added, which is very useful since it shows when peak amperes are flowing. This allows for the power to be switched off one second later, by which time the cores of the coils would be fully saturated.

Machining core and base
The 12-inch length of iron was cut in half using a power hacksaw before mounting one piece on the lathe. Ideally, one end should be supported by the fixed steady, but the core was too big for my steady. To get around this the core was clamped tight in the chuck and then, at a slow speed, a hole was drilled to enable the tailstock center to be used when turning. This center hole would be drilled out later and threaded for the securing screws.

First, one end of the core was trued up as clean as possible using a combination of a fine cut and slow speed. Next, the body of the iron was turned smooth, holding just 0.500 inch of the core in the chuck, to remove the roughness before turning down to 3 inches. Next, the core was reversed and again held in the 4-jaw chuck, then accurately centered using a dial gauge before the other end could be turned square and the roughness taken off the top 0.500 inch, which was left at just under 3.5 inches outside/diameter to support the plastic insulating ring.

Finally, the 0.274-inch holes were drilled in each end, ready to be tapped for the 0.313-inch set screws to be used to clamp the cores to the base and secure the pole pieces.

Having prepared the two cores, attention then turned to the base. Not having access to a surface grinder to obtain a smooth surface, the first step was to set the block on the milling table and then flycut it using a sharp, high-speed steel cutter at an extremely slow feed rate. Once satisfied with the quality of the surface, the 0.375-inch holes for the two securing bolts for the core were marked out and drilled, countersinking the underside.

The next step was to coat the bottom of the two cores with a thin coat of engineer’s blue before fitting them in place to check the fit. As discussed in the previous article, an air gap severely reduces the amount of magnetic force, so it’s ideal to have none. If you do not have a perfect fit then scrapers should be used. The best way to do this is to thinly coat a surface plate in engineer’s blue (a piece of plate glass is a good substitute if you don’t have a surface plate).

Place the surface to be trued on the surface plate and if you have an uneven coverage of blue, scrape the high points and keep repeating the process until nearly 100 percent coverage is achieved.

Before starting to wind the cores, the insulation rings at each end and between the core and the wire needs to be completed.

First, the two insulating washers must be made from some form of insulating plastic. Take care to know the plastic and that it does not conduct electricity (as some do).

The top ring will rest against the shoulder left when machining the core, and can be held in place by epoxy cement. For the bottom ring, roughen the bottom 0.250 inch to help the epoxy cement bind completely.

To get a tidy winding, a groove can be cut in this ring the depth of the winding wire diameter so the beginning is not under pressure from the rest of the winding. If you decide to knurl this bit of the core for adhesive to bind to, ensure you do it before bluing and scraping as the knurling will raise an edge that might prevent an airtight fit.

Once the insulating rings are in place the core itself can be insulated. Something stronger than winding insulation tape is needed, and a good medium is a section of a cardboard file that can be glued to the core fitting between the insulating washers, with an overlap at its joint of at least 0.500 inch. To be safe, I used two thicknesses.

Other fittings
While machining the pole pieces it was decided to make four so that there would be some flexibility to use different combinations when charging magnetos.

Their construction was a simple case of cutting the four blocks from the piece of 18-inch long steel, marking out the steel to minimize the number of cuts and waste of material.

After sawing, the sides of the blocks were flycut to both square and clean up the faces. (The use of a surface grinder would have been a quicker and easier option!)\

The slots were then milled for the securing screws, cutting the slots on different sides of each pair to make them more versatile.

In a similar way to fitting the cores to the base, each block was scraped to ensure no air gaps and a 100-percent fit with each other and the cores.

The base of the charger would be bolted to a 1-inch-thick piece of timber to provide a fixing point for the box containing the wiring. Alternatively, some steel box section could be used to provide a larger base and some stabilization for the narrow iron base.

Winding wire
Before starting to wind the cores it is important to remember that the windings need to follow the correct direction in order for the two cores to work together and not cancel each other out. The right hand grip rule applies: let your fingers point in the direction of the current and the thumb points to the north pole, so one core needs winding in a clockwise direction, the other counterclockwise.

The winding wire was ordered on two 4-kg spools so that no measuring or counting was required when winding. If you are using 10-gauge wire, it needs a bit of tension in order to seat properly, and of course the easiest way to wind it is to use the lathe after setting the screw pitch to the thickness of the wire, in this case 10 TPI for wire 0.100 inch in diameter. The wire was tensioned by passing it through a nylon sleeve clamped in the lathe boring bar tool holder. For this operation it is better if there are two people; one controlling the lathe jogging switch, the other ensuring the wire is feeding correctly.

If you don’t have access to a lathe then it is possible to make a crank handle-driven winding machine.

Tape a 12-inch tail to the insulating washer and wind the first two or three turns. Then use some rapid cure epoxy to hold it in position before starting to wind the first layer. Once the first layer has been completed it is best to give it a coat of rapid fix epoxy and leave it to dry, thus stopping the wire from moving when the next layer is wound. Reverse the lathe and then wind the remainder of the spool, leaving another 12-inch tail. Fortunately, the calculations were correct and the sixth layer finished near the bottom of the core. The final tail was marked with tape to identify it before wrapping some cloth trip around the coils and covering with epoxy resin for protection, which was later sanded down and covered with plastic electrical tape.

After the first core had been completed the second core was wound, taking care to ensure that the direction of the wire was reversed.

Back Electro-Motive Force
Back EMF is a voltage that occurs when power to a coil is shut off and the magnetic field of the coil collapses. The collapsing magnetic field produces a Back EMF that will try to keep the current from dropping to zero. Even using a starter solenoid will not stop the contacts from arcing when this occurs and they could soon become damaged.

To prevent this it is necessary to use a rectifier diode linked across each coil. This diode will have to be of sufficient rating in terms of amps and volts to take the surge, which could be 1,000 volts or more. The low resistance of the diode will short circuit the Back EMF and protect the switch.

To cope with the potential Back EMF for the design chosen (two coils of 75 amps each), 1,400-volt diodes were acquired, which were more than sufficient to neutralize it.

Wiring up
To finish each of the coils, some 8 AWG wire was soldered to the end of the winding wire after first carefully scraping off its plastic insulation. The soldered joint was then covered using heat shrink insulation before mounting the coil on the lathe for the last time to wind a couple of layers of insulation tape.

Once the coils were completed, the temporary bolts centerdrilled for turning were removed, and the joining faces on the base were cleaned then lightly oiled before they were securely bolted in place.

Car jump cables were used to connect the battery to the charger, so to make it easy to use the large alligator clips, two pieces of 1-inch brass were turned with a groove before cross drilling with the 0.188-inch hole for the 6 AWG cable to be used to connect the power source for distribution to the coils. Finally, 2-by-0.156-inch threads were tapped in both the top and the bottom of the brass in order to mount it to insulating plastic and act as hold-tight screws for the cable. After assembly, the positive terminal was marked with red paint to help ensure that the connection to the battery was always correct.

The car solenoid was a bulky item so a plastic box big enough to house it with the wiring, diodes and meter (if fitted) was the next purchase. The solenoid was purchased on eBay and came without a wiring diagram, so a few minutes had to be spent to identify the correct wiring sequence so that it would operate.

Terminal strip can be used to joint the various components. Alternatively, some homemade terminal blocks can be used with 0.250-inch threaded rods, onto which crimp terminals fitted to the wires can be bolted.

To activate the started solenoid, a simple 12-volt press switch can be used, switching off as soon as pressure is released.

The wiring was completed in stages, first the lugs for the battery, the switch and solenoid. Once this appeared to work correctly the first coil was connected, along with a diode so the polarity of the coil could be checked using a magnetic compass. The press switch was then pressed for a couple of seconds while holding the compass near to it, but not too close in case the compass polarity was changed. To be cautious, this first coil was then disconnected and the second coil connected and checked. Power was only applied for a couple of seconds to stop the coils from overheating.

The two coils were then hooked up and the ammeter connected, following the wiring instructions that came with it.

Charging magnetos
After testing the connections, the best way to test the charger is to recharge a magneto.

It is easier to recharge the magnets of a magneto when they are assembled as this saves the need to place keepers across the poles to retain the magnetism after removing magnets from the charger.

The first and essential step in recharging a magneto’s magnet is to correctly identify the north/south polarity, then mark it on the magnet with a piece of chalk. Incorrectly placing a magneto on a charger can result in the loss of all magnetism and make it extremely difficult to recharge it again to its full potential.

To find the polarity of the magnets, a compass can be used. Alternatively, suspend the magneto above the charger, using some cord so that it is free to turn. Switch on the charger and the magneto should align itself, using the principle of opposites attract; the north pole of the magneto needs to be placed on the south pole of the charger.

When setting the magneto on the charger, use the best combination of the pole pieces in order to concentrate the magnetic field in the magneto magnets. In cases involving the charging of more modern high-tension magnetos with alloy cases, it might be necessary to make special shaped pole pieces in order to focus the magnetic field where it is needed.

It only takes a second or two for the charger to work and there is no need for any other black art practices such as tapping the magnets as suggested in some of the very old articles on charging magnetos. If you have an ammeter then you can soon tell when the peak amps are being drawn; wait a further second or two and then release the switch. The cores of the charger will have been saturated and passed the magnetism to the magneto. If you feel the need, the charging process can be repeated.

After recharging my magnetos the difference in the pull of the magnets was clearly noticeable when turning the armatures.

When operating the charger it was noticed that the small car battery being used only delivered the current at 9 volts at around 50 amps rather than the calculated figure of 12 volts at over 70 amps. This could be caused by poor contact through the alligator clips, and fitting bolts on battery cable lugs to the jump leads would probably improve the connection. At 50 amps the amp-turns were still over 30,000 – more than sufficient for the older-style magnetos. If more amp-turns were needed then a more powerful battery set-up could be used.

The magnet charger has proven to be a very useful addition to the workshop and I gained a number of friends as a result. Weighing in at more than 100 pounds, including the pole pieces but without the battery, this is not something that can be taken to engine shows without building a suitable trolley like the John Rex design (referred to in Part 1). 

Contact Peter Rooke at Hardigate House, Hardigate Rd., Cropwell Butler, Nottingham NG12 3AH, England •