Make Your Own Magnet Charger

Enlarge Image Image Gallery 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

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.