The principal parts of the generating system of the magneto are the magnets, the armature, the armature winding, and the current collecting device, of which the armature and its windings are the rotating parts. The production of current in the magneto is the result of moving or rotating the armature coil in the magnetic field of force of the magnets. When any conductor is moved in a space that is under the influence of a magnet a current is generated in the conductor which flows in a direction perpendicular to the direction of motion. The value of the current thus generated depends on the strength of the magnetic field, the speed with which it is cut, and the number of conductors cutting it that are connected in series. Roughly, the voltage is doubled, with an increase of twice the former speed, and with all other things equal, the voltage is doubled by doubling the number of conductors connected in series.
By employing powerful magnets, and a large number of conductors (turns of wire) on the armature it is possible to obtain sufficient voltage for the ignition system at a comparatively low speed. The number of amperes delivered depends principally upon the internal resistance of the armature and the external circuit, and not on the number of conductors, nor directly upon the strength of the field. For this reason, low voltage machines that are intended to deliver a great amperage have only a few conductors of large cross section, while high tension machines have a great number of conductors of small size. In all cases the magneto, or ignition dynamo must be considered simply as a generator of current in the same way that a battery is a source of current since the current generated by them is utilized in precisely the same way.
The class of ignition system on which the magneto is used determines the class of the magneto. The low tension magneto is used principally for the make and break system, although it is sometimes used in connection with a high tension spark coil or transformed in the same way that a battery is used with a vibrator coil. The high tension magneto is used exclusively with the jump spark system and high tension spark plug.
These classes are again subdivided into the direct and alternating current divisions, depending on the character of the current furnished by the magneto. Briefly a continuous current is one that flows continually in one direction while an alternating current periodically reverses its direction of flow. As the alternating current magneto is the most commonly used type, we will confine our description to this class of magneto. The alternating current magneto is much the simplest form of machine as it has no commutator, complicated armature winding, nor field magnet coils, and in some types the brushes and revolving wire are eliminated.
As the magnetic flux of an alternating magneto is changed in value, that is increased and decreased, twice per revolution, it follows that the current changes its direction twice for every revolution of the armature. Each change in the direction of current flow is called an alternation.
The voltage developed in each alternation or period of flow is not uniform, the voltage being low at the start of the alternation, rapidly increasing in voltage until it is a maximum at the middle, and then rapidly decreasing to zero, from which point the current reverses in direction. As we have two such alternations, in a shuttle type magneto, per revolution we have two points at which the maximum voltage occurs; that is in the center of each alternation. These high voltage points are called the peak of the wave and consequently the sparking devices should operate at the peak of the wave or at the point of highest voltage. The spark therefore should occur when the shuttle or inductors are at a certain fixed point in the revolution at which point the peak of the wave occurs. The peak of the wave occurs when the shuttle is being pulled or turned away from the magnets.
In what is known as the “shuttle type” alternating current magneto, the generating coil is wound in the opening of an “H” type armature. This iron armature core is fastened rigidly to the driving shaft and revolves with it. As the armature revolves, it is necessary to collect the current that is generated by means of a brush that slides on a contact button B, the button being connected to one end of the winding.
(98) Low Tension Magneto.
The winding of the low tension magneto consists of a few turns of very heavy wire or copper strip, one end of which is grounded to the armature shaft and the other passing through the hollow shaft from which it is insulated. The end of the insulated wire is connected to the contact button (B) on which the current collecting brush presses. As one end of the winding is grounded, one brush, and one connecting wire is saved as the current returns to the magneto through the frame of the magneto. As the shuttle revolves between the magnet poles the magnetism is caused to alternate through the iron of the armature, thus causing the current to alternate in direction and fluctuate in value.
Since there are only two points at which the maximum current can be collected during a revolution with the alternating current magneto, it is necessary to drive it positively through gears, or a direct connection to the shaft so that this maximum point of voltage will always occur at the same point in regard to the piston position. If it is driven by belt without regard to the position of the piston, it is likely that there will be many times that the voltage is zero or too low in value when the spark is required in the cylinder. Alternating current magnetos must be positively driven, and the armature must be connected to the engine so that the peak of the wave occurs at, or a little before the end of the compression stroke.