[PRCo] Re: voltage and frequency

[robert netzlof]

--- Boris Cefer <westinghouse at iol.cz> wrote:

> I am not so far experienced, but 16 2/3 Hz (some time back it was
> changed a 
> little to 16.7 Hz to avoid reverb) allowed to use DC motors with
> alternating 
> current without rectifiers.

I've read some in old electrician's texts. Around the turn of the
20th century, "motors" were in effect DC motors. The rules of thumb
for designers of electrical systems were:

1) For motors, DC is best. If AC must be used, the lower the
frequency, the better.
2) For transformers, the higher the frequency, the better.
3) For lamps, the higher the frequency, the better.

This boiled down to:

If the major part of the load on your system is industrial (motors),
use DC or a low frequency. If the major part of the load is
residential and mercantile (lighting) use a high frequency. If the
load is mixed, it may be worth considering separate facilities for
the industrial and business/residential districts.

The advantages of being able to step AC voltage up and down were so
great that DC lost out (but note that it was within the past 2 or 3
years that ConEd finally told their DC customers in NYC that DC would
no longer be available).

In the US and Canada, things eventually settled onto 25 Hz for
industrial, 50 or 60 Hz for general use. Eventually 60 Hz became The
Standard. Railroads, of course, were mostly motor loads, so 25 Hz was
favored and once set up for 25 Hz, it would have cost a lot to
change. 

Europe (where, we are told, everything is done better) went to 16 2/3
Hz and 50 Hz although I vaguely recall reading of some other low
frequencies, perhaps as low as 11 Hz, used in some early railroad
applications.

It may be interesting to note that at least in its later years, the
Pennsylvania RR used 100 Hz AC to power signal apparatus. That was
apparently chosen to avoid inductive coupling from 60 Hz AC, as well
as the 2nd and 3rd (120 and 180 Hz) harmonics.

Bob Netzlof

Bob Netzlof a/k/a Sweet Old Bob



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