How Magnetos Operate on Cars and Motorcycles
Magnetos have 3 main components; a magnet, an armature and a condenser (capacitor).
There are a number of different magneto designs that have been used over the years. They all use the same basic principle to generate a spark. The most popular in vintage cars and motorcycles is the shuttle magneto. In this design a wound coil is rotated in a stationary magnetic field provided by a horseshoe magnet. The other is an inductor magneto, in which a permanent magnet is on the rotating armature and the coil is stationary.
Explaining the workings of a magneto
A magneto works by collapsing a magnetic field quickly causing a voltage spike. It follows a 2nd order transfer function.
There are two windings on the coil of the magneto, this goes for both types (shuttle magneto and inductor magneto) described here. The coil is wound like a transformer with a primary winding, consisting of a few turns of thick wire and a secondary winding, consisting of lots of turns of very thin wire. The winding of this coil is critical, because it is this that generates the voltage for the spark.
As the magneto spins, the wires in the coil pass through the magnetic field produced by the permanent magnet. Passing a wire through a magnetic field produces a voltage. If a lot of turns pass through the magnetic field then a voltage is induced into each turn of the coil, producing a voltage dependent on the number of turns. This can only allow a current to flow if there is a circuit from one end of the coil to the other.
This circuit is completed via a timing contact which interrupts the current flow at the point at which an ignition spark is required. When this contact opens, the current ceases to flow in the magneto primary winding so that the magnetic field collapses extremely quickly. As the voltage produced by the secondary winding, which has a much higher number of turns of wire, is directly proportional to the rate of change of the magnetic flux a high potential appears at the secondary winding terminals. This high pressure is held at a reasonable level because of the discharge across the points of the spark plug. In the absence of a spark plug the energy may be discharged across an internal safety gap at about 15kv to avoid damage to the secondary winding insulation. A capacitor is connected across the timing contacts to absorb the energy created in the primary winding due to back EMF (reverse electro-motive force) created by this magnetic activity thus avoiding excessive damage to the timing points.
One end of the secondary is connected to the top end of the primary winding, still close to ground as far as the secondary winding is concerned. The other end of the secondary is connected to a slip ring, which a carbon brush contacts. This carbon brush carries the potential to the HT lead connection.
If it is a single cylinder magneto then the connection to the HT lead is fixed, however if its a multi cylinder magneto, then the connection to the HT leads is done through a distributor on the back of the magneto. In this case, the carbon brush contacts the slip ring, then carries the potential down an insulated spindle with a wiper arm on the other end. There is another carbon brush on the end of the wiper arm which contacts the inside surface of the distributor. So as the wiper arm spins it contact each of the HT connection points in turn. A gear alters the rotational speed of the wiper arm, such that each rotation of the armature fires one HT lead.