25 Hz vs 60 Hz electrification (Was: Line-and-Shaft vs. Electric
nw-mailing-list at nwhs.org
nw-mailing-list at nwhs.org
Thu Apr 20 23:20:18 EDT 2006
What the gentleman told you isn't wrong, but it's a classic "truth but not
the whole truth," at least as it applies to railroad electrification. Yes,
there were industrial installations that started out with low frequency
electrification and kept it until economics, wear out, obsolescence and advancing
technology dictated a change to the 60 Hz that had become the utility standard.
By and large, the railroads who electrified with high voltage AC chose 25 Hz
for different reasons. These are well documented in readily-available
publications, so I'll try to be brief - railroad electrifications were stand
alone, not connected to the general utility system, so the lesser transmission
losses at the lower frequency became an important design and economic factor.
Where the wish was to utilize the superior operating characteristics of the
series commutator motor, in the sizes required for railway application such
motors simply would not commutate the 60 Hz without destructive sparking on the
commutators. At 16 2/3 Hz, as chosen in Europe, or 25 Hz, as chosen in the
US, they could be made to work.
Once rectifier technology advanced to the point that standard DC traction
motors essentially identical to those used under diesel-electric locomotives (or
under third rail EMU cars, as the case may be) could be used, once the
original 25 Hz equipment wore out, most systems that didn't simply dieselize
converted to 60 Hz and began to use utility feed rather than dedicated supplies.
Only in the former PRR electrification in the Northeast Corridor did the
economies of scale allow retention of the existing 25 Hz, and even there modern
solid state frequency converters replaced the original rotating sources.
In a message dated 4/20/2006 10:17:39 P.M. Eastern Daylight Time,
nw-mailing-list at nwhs.org writes:
The following comment may be of interest. It's from the chief electrical
engineer on one of the major Northeastern railroads which runs catenary:
The change from the old way resulted in the establishment of two
frequencies for AC power in this country. There was a lighting
frequency-which had to be relatively high, and an industrial frequency
that had to be relatively low. Using a low frequency for lighting
resulted in annoying flicker and stoboscopic effects, while using a high
frequency resulted in gear train installations to try to match the low
Early industrial frequencies were in the range of 15 to 16 2/3 hertz,
and when the Niagara Falls hydro plant was built, a frequency of 25
cycles was decided upon as one could build 300 rpm motors and that would
match the steam driven machinery.
That relic of the old steam driven plants resulted in many major
plants, shipyards, and railroads using single, two phase and three phase
25 Hertz until the rapid deindustrialization rendered this frequency
obsolete by the 1960's.
Oh, and by the way... I used the wrong term. The old machinery was not
called "line-and-shaft," but rather "lineshaft." I learn something new every
-- abram burnett
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