I do not know the whole story, but I do remember seeing subway
incandescent lamps flickering. This was when I was studying EE at CCNY.
We students were disabused by the profs that it was from the use of 25Hz
to distribute power to the rotary converters that provided the traction
dc. The flicker was attributed to commutator ripple.
To tell the truth, commutator ripple never weighed heavily upon my mind.
But if someone can fill us in on the history of why 25Hz was used in the
New York subway, I would certainly be interested. I would also like to
know if subway rotary converters have been replaced by more modern
equipment. I would think that polyphase ac using solid state rectifiers
would be the way to go.
25 Hz was used in some locations to drive tractor motors, and the lower
frequency eliminate the need for a gearbox,, or pulleys & drive belts
for speed reduction at the required torque. It was also common in
mining towns. Depending on your persistence of vision, the flicker can
be visible at lower frequencies, but if it wasn't a constant flicker, it
was probably due to the train motors starting & stopping throughout the
system. I used to be able to see a single frame of 16 MM film when I
worked at a TV station. There were frames marked "Insert Commercial
Here" that I could see every time. No one else at the station could see
them, so we ran several experiments and I found them every time. On the
down side, I see a Fluorescent lamp start flickering before anyone else.
aioe.org, Goggle Groups, and Web TV users must request to be white
Commutator ripple? On a 25 Hz 3-phase rotary converter, that would be
150 Hz. That's not visible in an incandesant light or to the human eye. I
would think it's more likely due to varying voltage as trains stop and start
or enter and leave that power district.
http://www.nycsubway.org/ has several articles and many pictures on the
history of the New York subway system, including the original power plants
and the converter stations. Follow the link to Behind the Scenes -> Power
Generation. When these articles were last updated in 1999, only two
stations still had 60 Hz rotary converters, and these only ran during peak
demand periods. All of the 25 Hz machines are long retired. Over the years,
the rotary converters have been replaced first by mercury arc and then by
solid state rectifiers. When you get to the 1904 IRT book, note that the
original 11,000 volt power generators were driven by four-cylinder double-
expansion steam engines turning just 75 RPM.
I also found _Theoretical Elements of Electrical Engineering_, Third
by Charles Proteus Steinmetz, copyright 1909, on Google Books. In the
chapter on synchronous converters, on page 332, he explains why the
60-cycle converter is inferior to the 25-cycle converter.
BTW, one of my job interviews after college in 1984 was at an aluminum
refinery where they had converted from mercury arc to solid state rectifiers
just the year before. Except for the busbars, there was no indication of the
power capacity, something like 90000 amps at 800 volts as I recall.
I think you are referring to rectification or commutaTION ripple.
CommutaTOR ripple comes from segment to segment voltage ripple from a dc
commutator. I am sure that various brush phenomena get into the act.
Thank you, I think, for the subway link. I will probably spend too much
time browsing through it. As part of an ME course, the class went to the
subway maintenance shop. I still remember being impressed by chips about
an inch in cross-section comi9ng off the wheels.
I visited a Scottish aluminum smelter about a dozen years ago They had
dc generators run by hydropower. IIRC they were using about 500V with
the appropriate number of pots in series. When I was walking on a
platform above the bus bars, I took out my magnetic compass. Its
orientation changed as I walked along.
I've been reading up on 25 Hz ( see Thomas Hughes book "Networks of
Power"). Niagara was 25 Hz because they ordered the turbines before
they ordered the generators! When they finally decided to go AC,
Westinghouse wanted 30 Hz for running rotary converters, but this was
incompatible with the chosen speed of 250 RPM (120 * 30 Hz/250 RPM 14.4 poles, not realizable) - the other option was 16 2/3 Hz, but this
would have made for unusably flickery lamps. 25 was a compromise...I
don't know why they couldn't have run the turbines a little faster,
probably some deep hydraulic reason.
Sue- for some reason I thought you were in OZ where the sun is not allowed,
by law, to flicker, not in Dartmoor where it is impossible to know if it
flickers. Here on Vancouver Island, flicker is not allowed in summer and in
winter, it is frowned upon - it is only supposed to rain at night- just like
Camelot (which is supposed to be near Dartmoor -at least in the way I think
Don Kelly firstname.lastname@example.org
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