Two-rail track, real wooden ties, wheels riding in sprung bearing trucks, miniature reversible motors that run on DC current, and not a piece of plastic to be found: are these trains the latest scale models available in HO? Nope. They are the first trains produced by the Lionel Manufacturing Company over 100 years ago that operated on track with a gauge of 2-7/8 inches. The 2-7/8 gauge line of trains lasted from 1901 until 1905. Standard gauge took over the following year. Yet they were cutting edge technology in their time. The motor used in the 2-7/8 inch gauge line was series wound, and designed to run on DC current. Lets look at the design and try to understand more about these motors.
Why DC? Well, most homes of the time had no electric service. The only power available to run these trains in most cases was some sort of battery. In its catalog Lionel offered dry cell batteries and a wet cell setup, for "...when utilizing Direct Electric Light Current" (Lionel catalog ca. 1901) with open glass jars of sulfuric acid. Thomas Edison was a proponent of DC power, so even if a home had electric power, most likely it was produced in an Edison plant, and therefore it was DC. Direct current flows in one direction, from positive to negative. Alternating current changes its direction rapidly. In 60 Hertz AC, the polarity reverses itself 60 times a second.
The field of Lionel's early DC motor is made of a solid mass of cast iron. When voltage is applied to the field coil, it becomes a huge electromagnet, with distinctive North and South poles. If energized with AC current, the same thing would happen, but in a cyclic manner. In layman's terms, the rapid (cyclic) changes in the magnetic flux, due to the alternating current, causes a heat build up in the solid cast iron field magnet which drastically lowers its magnetic capabilities, thus reducing the motor's efficiency. Since DC flows in one direction, there is no energy expended in the constant reversal of the magnetic polarity as would be the case if AC power was used. Therefore, little heat is created. Efficiency is much greater and therefore, the voltage requirements are much lower. At full throttle, these motors require less than 6 volts. This takes us back to the batteries. A simple battery, or more correctly, a simple cell, can only produce approximately 1.5 volts. This is because of the chemistry involved. When these cells are wired in series, they become a battery with the output equal to the sum of the cells. Therefore a 4-cell setup would produce around 6.0 volts.
Since the available voltage from batteries is relatively low, the slow speed performance of these motors is of great importance. The armature is a 3-pole-pairs type but is drum wound. There are 6 distinctive poles on the laminated stack, but the windings for each pole pair traverse the shaft and occupy spaces 180 degrees apart. This way, as each winding gets energized, there is a North Pole on one side of the armature and a South Pole directly opposite it. Since opposite poles attract, the armature rotates on its axis as opposing poles of the armature and field are attracted to each other, the North Pole of the field attracting the South Pole of the armature, and vice versa. As the armature rotates, the commutator is brought into a new position and the brushes energize the next winding, causing the process to repeat itself. This continues and the armature spins rapidly. The fact that two poles of the armature are attracted to the two opposing poles of the field at the same time lowers the voltage requirement.
The armature design would be retained when standard gauge trains were introduced in 1906, but the field would now be made of stacked steel plates. This design works well with AC since the thin steel plates reduce the magnetic impedance resulting in lower energy (heat) losses. Transformers were becoming available as homes were supplied with AC power and DC reducers were built for homes with DC. Standard gauge motors will run on either, although much more efficiently on DC.
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