IEEE Power & Energy Magazine - September/October 2016 - 83

and its chief engineer Michael dolivodobrowolsky advocated and promoted
three-phase ac, the next polyphase
class. The partner company Maschinenfabrik Oerlikon (MfO) near Zürich,
Switzerland, shared this strategy and
manufactured the three-phase generators for lauffen on the River Neckar,
from where the legendary long-distance
power transmission took place to the
international Electrical Exhibition in
frankfurt am Main, Germany, in 1891.
But shortly afterwards in 1893, the municipal power station at frankfurt was
equipped simply with single-phase generators, which were acceptable for the
needs of electric lighting and other foreseeable minor purposes.
This was the situation when a current system had to be selected for
the Rheinfelden project. There was a
challenge: find the best polyphase ac
combination for a multipurpose system that is most suitable, all in one,
for lighting, power applications, transmission and distribution, and which,
as an additional requirement, needs
only a minimum of components and
conductor material. The solution was
the interlinked three-phase ac system,
as decided by AEG and proclaimed by
General Manager Emil Rathenau in
an 1896 survey: "The advantages of
three-phase ac, as outlined here, were
in their totality decisive for applying
this current type in the Rheinfelden
transmission system."
Of importance is the specification interlinked, which denotes a clever system
compression. Normally one would think
that three phases need a total of six connections for the input and return leads,
thus necessitating also six conducting
wires for power transmission. The artifice was to recognize that the current
of each phase, shifted by one third of a
period, finds its way back over the other
phases and needs no separate return conductor. The interlinked three-phase ac, in
fact, requires only three connections and
wires. The same is true for magnetically
interlinked three-phase transformers,
which get along with three magnetic legs
and no return paths. The current-system
decision for Rheinfelden was convincseptember/october 2016

ing and sent out a strong message that
it was the system of the future. interlinked three-phase ac was later adopted
throughout the world.

Pioneering Power Plant
Promoting 50-Hz Frequency
during the same period, the electrical
world was also affected by a "battle of the

frequencies." As, at that time, the frequency of ac had no primary significance, its
choice was free. Hence, the early-frequency values varied over a wide range
from about 140 to 25 Hz or lower, depending on manufacturer and equipment.
in North America, to begin with, the
1891 Telluride system was run with 133
1/3 Hz (resulting from 16,000 current

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83
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