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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