Paul Koning wrote:
> I don't actually know where 50 and 60 Hz came from, nor 100 and 115
> and 220 and 240 volts. There may be some ancient justification in the
> tinkerings of various 19th century engineers, but it seems to me it
> makes most sense simply to view them as random numbers.
Almost random, I concur, but there were definite technical factors
behind. It took a while for people to settle
on 60Hz or 50Hz. Some interesting tidbits mostly taken from
"Engineers and Electrons" by Ryder & Fink, IEEE Press:
First there was the AC vs. DC debate. Edison firmly believed
that DC was better, while Tesla-Westinghouse pushed things the
other way. Edison even tried (though friends) to push legislation that would
outlaw voltages greater than 300V, thus nullifying AC's advantage for
transformation to higher voltages (and therefore transmission over
longer distances). In 1888, a Harold P. Brown, now widely believed
to have been funded and helped by Edison, published letters stating
that "AC can be described by no adjective less forcible than damnable".
In Ohio, Brown even electrocuted several animals in front of witnesses
using AC in order to prove its perilousness.
Edison's interest was not safety: at the time, he held the patent to the
only available power usage meter, but it only worked with DC. Also, some
of the competing light companies used arc lamps, which require
high voltages. Finally, Westinghouse held Tesla's patent for the
induction motor.
Thus, when bids were called from Westinghouse
and General Electric to construct the generators for the Niagara Falls
project, Westinghouse won thanks to its AC experience.
What really tilted the balance in favor of AC was the 1891 demonstration
of AC 3-phase transmission at 30,000 volts from Lauffen to Frankfurt,
separated by 100 miles (Brown of "Brown-Boveri fame-another Brown- was behind
this). DC continued to be used only in transportation systems thanks to the
higher starting torque of the DC motors.
As for frequency, the factors that usually affected the choice were
suitability for use in different devices and the preference of
mechanical engineers to design steam engines operating at integral
RPM specs.
At roughly 25 Hz, flicker became "tolerable" for Edison's cotton filament
bulbs, but not so for arc lamps, which required higher frequencies.
However, higher frequencies made mechanical design more difficult
for AC motors. For example: for the Niagara Falls project,
both 16 2/3 and 33 1/3 Hz were proposed. 25Hz was ultimately
decided on. This was because the turbine speeds were set at 250 RPM
and you need an integral number of poles. A 1888 Westinghouse
alternator ran at 2000 RPM and had 8 poles for 133 1/3 Hz.
Transformers, filament and arc lighting worked fine at that frequency,
but it was too high for AC motors. Around 1890 a lower RPM figure
was used when steam engines first shared a single shaft with
alternators, and L.B. Stillwell proposed 3600 cycles per minute
or 60Hz, which was on the high end for AC motors but good for
all lighting and also for efficient and not-too-bulky transformers.
That way you could have 1800 rpm steam turbines or 225 rpm steam
engines.
Carlos.
--
Carlos E. Murillo-Sanchez
Dean of Engineering
Universidad Autonoma de Manizales, Manizales, Colombia
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Received on Mon Dec 13 2004 - 10:57:05 GMT