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From: wbrown@beva.bev.lbl.gov (Bill Brown)
Newsgroups: sci.electronics
Subject: Re: 50 Hz in Europe... Was the decision political?
Message-ID: <4450@helios.ee.lbl.gov>
Date: 13 Dec 89 16:01:04 GMT
References: <2332@ektools.UUCP> <787@larry.sal.wisc.edu> <8973@cbmvax.commodore.com>
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Reply-To: wbrown@beva.bev.lbl.gov (Bill Brown)
Organization: Lawrence Berkeley Laboratory, Berkeley
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X-Local-Date: 13 Dec 89 08:01:04 PST

In article <8973@cbmvax.commodore.com> grr@cbmvax.commodore.com (George Robbins) writes:
>Actually, If I remember all this sillyness, for power equipment including
>motors and transformers lower frequencies are still cheaper because AC core
>losses (eddy current, hysteresis, etc), are lower allowing less complicated
>construction (thicker/few lamination) and less cooling. 

This allowed the traction motors to be somewhat smaller, as less volume was
used up as insulation between the stator laminations.  For various reasons,
series-wound motors are generally more satisfactory as traction motors, altho
I seem to recall that both G.E. and E.M.D. are experimenting with using
synchronous motors (actually a form of stepping motor) driven by a solid
state frequency generator.  This will remove the maintenance problems
associated with commutators.
>
>Some of the major U.S. railroad electrification projects used 25 Hz for
>just this reason and were still using 25 Hz up to a few years ago, though
>the economies of custom power generation/converison had vanished with the
>advent of major power nets.

The use of higher frequency power supply was not practical until good
high-power rectifiers became available.  Several locomotives using
mecury ignitron rectifiers were built in the late 40's-early 50's, and
silicon rectifiers are now used.
>
>Don't ask me why they didn't use 30 Hz...

One railroad, the Visalia (sp?) Electric in California used 15 Hz
electrification; I believe that it was also used in Europe.

The frequency is a trade-off.  In distributing power to locomotives, it
is desirable to use the highest voltage possible in order to minimize
power losses.  There is (was?) no easy way to "step up/step down" D.C.
When A.C. is used, it was normally transmitted at high voltage (11 KV
being the most common).  However, as the frequency goes down, the ammount
iron required for the locomotive-mounted transformer goes up.

There are practical limits to the operating voltage for a series-wound
motor; it gets hard to build one to run on much more than a thousand volts
or so, especially given the cruddy environment in traction service.  The
early engineers had to make some interesting trade-offs to come up with the
system that eventually evolved.  Things are still changing.

						-bill
						wlbrown@lbl.gov

Disclaimer:  These opinions are my own and have nothing to do with the
    official policy or management of L.B.L, who probably couldn't care 
    less about employees who play with trains.