Fig. 112. Longitudinal Section of “Mea” High Tension Magneto.
The result of these conditions are known to everybody familiar with automobiles. They are the difficulty of cranking a motor on one of the average high tension magnetos, if the spark is fully retarded, and of operating the motor on the magneto at very low speed, particularly when it is overloaded, as for example, in hill climbing. Attempts have therefore been made to obtain the spark, independent of the timing, always at the same favorable position of the armature.
The distinct innovation and improvement incorporated in the Mea magneto consists in bell shaped magnets (Fig. 111) placed horizontally and in the same axis with the armature, instead of the customary horse-shoe magnets placed at right angle to the armature.
This at once makes possible and practicable the simultaneous advance and retard of magnets and breaker instead of the advance and retard of the breaker alone as the magnets may be moved to and fro with the breaker housing. It will be seen that as a result of this new departure the relative position of armature and field at the moment of sparking is absolutely maintained, and the same quality of spark is therefore produced, no matter what the timing may be. Furthermore, the range of timing, which with the horse-shoe type of magneto is limited to 10° or 15° at low speeds (i. e. at speeds at which a retarded spark is of value) becomes limited only by the necessity of supplying a suitable support for the magnets. With the standard types of Mea magnetos described in the following, this range varies from about 45° to 70°, but if necessary this range can be increased to any amount desired.
The bell-shaped magnets are fixed to the casing which is mounted on a base supplied with the magneto. The timing is altered by turning the casing and magnets together on the base.
Fig. 112 shows a longitudinal section of a four cylinder Mea magneto. The armature F with the ball bearings 17–18 rotates in the bell-shaped magnets 100, the poles of the magnets being on a horizontal line opposite the armature 1. The armature is of the ordinary H type iron core wound with a double winding of heavy primary and fine secondary wire. On the armature are mounted the condenser 12, the high tension collector ring 4, and the low tension circuit breaker 26–39.
The circuit breaker consists of a disc 27 on which are mounted the short platinum 33, the other contact point 34 is movable and is supported by a spring 30 which is fastened to the insulated plate 28 mounted on disc 27. Fiber roller 31 in connection with cam disc 40 which is provided with two cams is located inside the breaker. Revolving with the armature the roller presses against the spring supported part of the breaker whenever it rolls over the two cams which of course is twice per revolution.
Magneto of Roberts Motor in Advanced Position.
