Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(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.

With this type of magneto the only point that is likely to give trouble is the point at which the brush makes contact with the contact button. If the brush should stick or not make contact, or if the button is dirty or rusty, the current will not flow; this point should always be given attention. Outside of this the only attention necessary is to keep the bearings oiled.

Fig. 101. Sumter Magneto Advanced.

Fig. 102. Sumter Magneto Retarded.

Fig. 103. Sumter Magneto on Horizontal Engine.

Fig. 101 and Fig. 102 show the Sumter low tension magneto as arranged for make and break ignition. The armature and its connections are of exactly the same type as that shown in the previous diagram. The magnets and frame are arranged to tilt back and forth so that the peak of the wave will occur at the advanced and retarded positions of the igniter. This arrangement allows the full voltage of the magneto to be obtained at any point within the range of the ignitor, an important item when starting the engine or running at low speed. When mounted on the engine, as shown by Fig. 103, the magnets are provided with an operating rod that is marked “start” and “run.” When the pin on the engine bed is engaged under “start,” the magneto is retarded, when the pin is under “run” it is advanced. A number of intermediate points are provided at which the operating arm is held fast by tooth engagements as shown in the slotted handle. As shown in the illustration the magneto is fully advanced. The gears by which the magneto is driven are clearly shown in the cut, the ratio between the gear on the crank shaft and that on the magneto shaft being exactly 2 to 1. One lead is carried to the make and break igniter in the cylinder head, the current being returned through the bed of the engine. The same make of magneto is shown mounted on a vertical engine in Fig. 104. In this case the magneto is positively driven from the crank shaft of the engine by a chain. The single conductor running from the magneto to the cylinder heads is clearly shown. To start the engine, the igniter is set in the usual manner and the magneto tilted to starting position, as shown in the illustration. The engine is then started in the usual manner and, when running, the igniter is changed to running position, and the magneto is tilted outwardly. It is not important which is changed first, the magneto or the igniter. It is easy to remember the “starting” and running “position” of the magneto, the running position always being that in which the magnetos are tilted in the direction opposite to that in which the engine runs.