As mentioned in the above, all systems using a spark plug are termed high-tension systems; however, a low-tension magneto may be used with a transformer coil. Reference was made to the instrument having a single winding on its armature and employing a step-up coil with a primary and secondary winding. This type, termed a high-tension system, although a low-tension current is obtained from the instrument itself. However, this low-tension current is transformed into a high-tension by the coil.
These coils are similar to those used in connection with battery systems, and as but few more wires are necessary to use a battery current, the system is generally of the dual type, making use of the magneto interrupter and distributor for both magneto and battery current. The low-tension magneto is sometimes defined as the primary armature type, as it incorporates but a single or primary winding in the magnetic field.
The construction of a low-tension magneto is similar to that of the high-tension type. It consists of permanent magnets of inverted U-shape, and the pole pieces bored out cylindrically, mounted upon a non-metallic base. The armature is also of H section, carries a primary winding and serves to form a bridge for the magnetic flux between the pole pieces. The armature core is wrapped with primary wire until the slot is almost filled. The insulating cloth is then put in place and the armature banded.
A low-voltage current is furnished by the magneto armature to the primary winding of the coil, while a secondary winding in the coil transforms this to a high voltage.
The interrupter and distributor are similar to the high-tension type and perform the same functions in the system.
The main constructive difference between the low and high-tension types is that the former has but one winding on the armature, using a transformer coil to raise current pressure value, whereas the high-tension type has a secondary winding incorporated in the armature.
The principle of a rotating armature and the method of generating current in the magnetic field was explained previously. We may now discuss the method of transforming the low-tension current to a high tension.
Fig. 52 is a wiring diagram of this type, with external transformer coil. It will be noted that the primary winding of the transformer is so connected with the magneto armature winding that it completes the metallic circuit through the latter. That is, the transformer primary is in series with the armature winding. The breaker points are separated at definite intervals, and are so connected into the armature and transformer primary circuit that a direct short circuit through the armature winding is caused when they are in contact. The breaker points are here said to be connected in parallel with the transformer primary.
As the induced electric pressure within the armature winding rises in value, due to the motion of the armature in the magnetic field, it flows through the circuit formed by the armature winding and the circuit breaker points until the instant at which it has attained its maximum value, when the contact points are separated and the direct short circuit broken. When this separation of the points occurs, the induced electric pressure is caused to enter the transformer primary with great suddenness and create lines of force through the transformer windings with extreme rapidity. This entry by the current and consequent creation of lines of force causes the lines to cut the secondary winding during the formation of the magnetic field about the transformer, and this cutting induces an electrical pressure within the secondary.
Of course with this system the armature of the magneto is positively driven from the engine by gears, so that the points of maximum pressure induction in the armature winding may coincide with the instants at which ignition sparks are desired within the engine cylinders. Such a single transformer with a single breaker is employed and all the current for ignition is generated therein; it is necessary that some means be fitted for the distribution and consecutive connections of the transformer secondary with the proper spark plugs in the cylinders. This distribution is accomplished by a distributor, also made an integral part of the magneto, and driven positively and in a definite relationship with the circuit breaker as in the high-tension type.
Again referring to Fig. 52, it is seen that the ends of the transformer secondary winding are connected, for the completion of its circuit through the spark plugs, one to engine frame, or in other words, one end is grounded and the other the central carbon brush holder of the distributor.
This distributor is of the same construction as in the high-tension system, so that the high-tension current induced in the secondary winding will be forced to follow the path selected for it, depending upon the position of the carbon brush of the distributor. In this illustration the heavy lines illustrate the primary circuit and the light lines the secondary circuit.
Most coils in use at the present writing are of the non-vibrating type, the trembler type of box coil having been discarded long ago, and the interrupter is now operated mechanically instead of electrically.
These non-vibrating or transformer coils, as they are termed, are made in various styles, and sometimes incorporate the switch and a push button for starting on the spark.