25 Hz vs 60 Hz electrification (Was: Line-and-Shaft vs. Electric Motors)

[nw-mailing-list at nwhs.org]

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.  
 
Dave Phelps
 
 
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
rpm machinery. 

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 
day.
 
-- abram  burnett


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