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.
