TELECOM Digest OnLine - Sorted: Re: 25 Hz Power

Re: 25 Hz Power

Eric Tappert (
Fri, 10 Mar 2006 21:30:26 GMT

On Fri, 10 Mar 2006 11:07:14 -0800, Al Gillis <> wrote:

> Hi Wes ...

> Sorry for the off-topic question but could you detail some of the
> advantages you mentioned?

> I know the Great Northern Railroad used electric locomotives through
> mountain passes in Washington State because electrical locos could out
> pull steam locomotives on the steep grades encountered there.

> Thanks!

> Al

> <> wrote in message

>> In a message dated Wed, 8 Mar 2006 12:45:38 CST, jsw
>> <> writes:

>>> In NYC up into the 70's at least, power for two of the subway
>>> divisions (BMT and IRT) was generated at 25Hz but converted to DC in
>>> the field.

>> Some electrified intercity railroads in the Northeast used 25 Hz
>> power. There are, or at least were in the past, some advantages to 25
>> Hz power for running locomotives. (Some railroads in other parts of
>> the world use or used 16-2/3 HZ power.

> (Many deletions from here down!)

Electric locomotives use series wound DC motors (lots of torque at low
speed...) on each driven wheel as the power source. DC motors will
operate fine on AC, but as the frequency increases you run into
commutation problems. As such, to gain the advantage of being able to
transform voltages yet still operate the DC traction motors without
intervening rectification, many railroads use low (lower than 50 or 60
Hz) frequency AC power.

I recall a tour of PECo's (now Excelon) Schuylkill Station in
Philadelphia, probably in the early 70's. This was PECo's first
central generation plant (it is still in service, mainly because the
steam is used to heat center city and the University of Pennsylvania)
and had a lot of old equipment retired in-place. Of particular
interest was a pair of 60 Hz to 25 Hz conversion sets that were used
to supply 25 Hz to the Broad Street Subway in Philadelphia. They had
been retired only a few years before, but about 2 years after that
there was a fire that destroyed the subway's converters, so they fired
them up again for a couple of months until the subway system could be
repaired. The really interesting part was that the stator on the 25
Hz side was mounted on a set of ring gears so that it could be rotated
to properly phase it to the 25 Hz bus. The nameplate said it was a GE
set, but when PECo tried to get the drawings for the archives, GE
claimed they had no records of ever making the machines. Legend has
it that Steinmetz designed it (they were old enough that it could have
been true!).

Eric Tappert

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