Electrical Trouble Shooting

How Magnetos Work

| August/September 1992

This article is reprinted from the May 1934 issue of Motor Service. It was sent to us by August Baron of PO Box 2901, Danbury, CT 06813.

While battery ignition systems are used almost universally on passenger cars, magnetos are used on certain trucks and tractors. Those who service such magnetos do so by replacing worn bearings, putting in new interrupter points or installing new armature winding when these are defective. Better work can usually be done, however, if the principles are thoroughly understood, and in many cases, special peculiar troubles can be solved by the understanding of such principles.

The modern high tension magneto carries two windings just as a battery ignition coil has two windings, and in the case of the magneto, the heavy winding is the primary and the fine winding is the secondary, just as it is in the case of an ignition coil.

In our first explanation, however, we will show the elements of a low tension magneto which has only one winding, for this magneto explains the principles by means of which magnetism is used to generate the current.

The fundamental idea back of any generating unit is that a change of magnetism is produced in some way through a winding or coil and in Fig. 1 we see that our magneto armature is in such a position that the maximum amount of magnetism is going through the center of it. When the same armature has turned one quarter of a revolution or ninety degrees as shown in Fig. 2, we find that the pole pieces of the armature are now carrying the magnetism from one side of the magneto to the other so that practically no magnetism goes through the winding. At least none of it goes directly through the center of the winding as it did in Fig. 1.

When we come to Fig. 3, we again find the magnetism going through the winding but now it goes in the reverse direction as far as the coil of wire is concerned, while in Fig. 4, we again have the case where the winding itself carries no magnetism through its center.