Magneto Recharging

Pages From the Past - Tips for Antique Engine Magnetos
John Rex
April/May 2011
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“How often do magnetos need recharging?” is a question frequently asked by antique engine enthusiasts. The answer has to do with the permanence of the magnet on the engine magneto.

It is a common fallacy that a permanent magnet expends internal energy to create electricity from an engine magneto or generator. This fallacy leads to the belief that after repeated use the magnetism will all be used up and hence the magnet must be recharged. The energy to create the spark comes not from the magnet, but from the mechanical energy required to drive the engine magneto. The magnet merely acts to convert this mechanical energy to electrical energy.

Owners of antique engines sometimes report that their magneto has mysteriously lost its magnetism. In most cases, the fact that the magnetism has been lost cannot be questioned; however, the “mysterious” aspect can be disputed. There is nothing mysterious about it. If a piece of material can be magnetized, it is likewise capable of being demagnetized.

Magneto recharging tips 

• Take the case of an engine enthusiast who wants to get the maximum performance out of his engine magneto. He removes the magnets, takes them to his local magneto service center for recharging, then puts them back on the mag. Now he gets a weaker spark than before. Why? Because removing the magnets subjects them to demagnetizing forces, the recharging restored the magnetism, but they again lost some of their magnetism upon removal from the charger. What’s the right way to recharge them? Charge them on the magneto as an assembly. Removing the magnets from a magneto causes them to lose some of their magnetism.

• Don’t pile several magnetos together in a heap or pile. Having magnets close together can cause them to partially demagnetize each other. Keep magnetos away from each other by at least 3 inches.

• Don’t connect a battery to the terminals of a low tension magneto. The current can demagnetize the magnets and might burn out the coils. If you want to run a mag equipped engine on a battery and coil, make sure the mag lead is disconnected so battery current will not demagnetize the magnets.

• Don’t remove the armature or rotor from a magneto because the magnets can be weakened. The rotor can be safely removed by first installing a “keeper” across the end of the magnet poles. Leave the keep in place until the rotor is replaced. The Wico EK mag is an exception to this rule and the armature may be safely removed and replaced without using a keeper.

• Magnets that have been exposed to a fire or excessive heat will frequently lose their heat treatment and hence their magnetic characteristics. Such magnets may never recover their magnetism even when recharged because the characteristics of the metal have been altered.

• Don’t attempt to recharge magnets by methods which produce inadequate energy to fully recharge them. Wrapping a few turns of wire around the magnets and “flashing” the wire with an automotive battery or arc welder frequently exposes the magnets to demagnetizing forces and may weaken them rather than improve them. If recharging is required, it’s best to use a magnet charger specifically designed for the purpose.

• It is rumored that shock and vibration may demagnetize magnets, however I have never been able to observe this effect, even on early hardened magnet materials. I have seen magnetos that were dropped and the shock of the impact cracked the magnets, yet the mag still functioned, provided the magnets remained intact. In some cases, after breakage the magnets will not stay in place. In cases like this, the magnets can be arc welded back together and still work adequately. Use a mild steel arc electrode with good penetration such as E6011 and place small tack welds at each end of the crack. Don’t weld across the entire crack as too much heat will anneal excessive amounts of material and reduce the effectiveness of the magnet. Recharging after welding is required. Torch welding produces too much heat and should not be used.

This article is excerpted from an article written by John Rex for the November/December 1986 issue of Gas Engine Magazine. To read the article in its entirety complete with photos, visit www.gasenginemagazine.com/magneto-recharging.


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