Electrical Trouble Shooting

How Magnetos Work

Magnetism goes through the center of the coil.

Fig. 1. Magnetism is going through the center of the coil.

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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.

From these illustrations it will be seen that, as the coil rotates, the effect of changing magnetism is produced in the coil, although the total magnetism going from the North to the South pole of the magneto does not vary to any great extent.

Because the magnetism is reversing through the coil, we will have a reversing or alternating voltage generated in the coil.

If a mechanic would look at these illustrations and take a guess at the high spot of the voltage, he might say that in Fig. 1 or Fig. 3 the current or voltage would be greater because the magnetism seems to be strongest through the coil. A man saying this, however, would forget for the time being that magnetism does not produce a voltage, but that change in magnetism does, so that unless we know when the change is greatest, we will not know when the voltage is greatest.

In Fig. 5 we have in the upper part four little armatures representing the armature positions shown in the previous illustration, and below these armatures we have a curve showing the strength of the magnetism through the coil in these different positions.

In position No. 1, where the magnetism is all going through the coil, we have the maximum value for the lines of magnetic force, and accordingly plot a point up from the horizontal line or zero line such that the height of the line represents the number of lines of magnetic force going through the coil.

In the No. 2 position of the armature where it is in a vertical position, there are no lines of forces, so the point we plot is on the zero line. Then in the No. 3 position, we have again the maximum lines of force but they are in the reverse direction so that now we put our No. 3 point below the zero line while the No. 4 position gives us another point on the zero line. In this way we draw the magnetism curve, which represents for any position of the armature the number of lines going through the coil.

We can now look at this curve of magnetism carefully and see at what point it is changing fastest. At position 1, for example, the curve is for an instant horizontal so that it is not changing while the same thing is true at position 3. On the other hand in positions 2 and 4 the curve is the steepest which shows that the magnetism is changing fastest at position 2 and 4. Therefore, contrary to the way we might guess, we have the peak of the voltage produced at the positions in Fig. 2 and 4, so that when we get to the point of adding an interrupter to our magneto, we will want its contacts to separate when the armature is approximately in a vertical position.

We now come to the essential circuits of a high tension magneto which are shown in Fig. 6 in which the armature has two windings, the heavy one being the primary winding. This primary winding is connected to an interrupter and across the interrupter points is a condenser, just as would be the case in the battery ignition system.

As the armature rotates between the magnetic poles, there will be a shifting of magnetic lines back and forth through the windings, the peak of voltage being obtained at about the point shown in the illustration. This is when the pole piece of the armature is just about to leave the pole piece of the magneto. The position can be determined experimentally by turning a magneto by hand. At one point it will turn easily and at another point will turn with difficulty. Just as it turns very hard and then suddenly lets go and allows you to turn it easily is the point of maximum voltage or current. It is also the instant at which the interrupter points should separate for, to get maximum secondary sparks, we want to break the primary circuit when the current is at its peak.

Magneto timing therefore involves more than having the interrupter points open at a certain time with respect to the dead center position of the engine. It also involves having the interrupter set with respect to the armature position. On most magnetos, however, this is taken care of in the original design where a keyway on the shaft makes it impossible to get the interrupter on in the wrong position.

Instead of a single spark as shown in Fig. 6, there is usually a distributor which sends the high tension spark current out to four or more different spark plugs.