light bulbs

I cannot think of any for which turning the switch on and of on a 10 second cycle will not result in intensity fluctuation, This comes pretty close to being a dumb question.

Bill

Burned out ones.

DC powered, or high frequency AC, like used in the sound circuit of old film projectors.

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 I would also like to know if subway rotary converters have been replaced by more modern equipment.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.

Bill

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.

snipped-for-privacy@gmail.com wrote on 9/19/2008 :

DC powered LEDs, incandescents or fluorescents. Halogens running on high frequency Power supplies are flicker free, at least to the human eye.

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.

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 Edition, 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.

Mike

Light bulbs are single phase (unless they have VERY strange bulbs on the subway) so at 25 Hz, you'd experience 50 Hz ripple, double the line frequency.

I am also CCNY graduate, no wunemployed, you know why? THIS GOVERNMENT SHIPPED NOW EVEN WHITE COLLAR JOBS TO CHINA, NOT BLUE COLLA,R NOT MANUFACTURING, NOW IT'S BEGINNING TO AFFECT US SHEETHEADS

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.

Bill

-------------------

25 Hz (polyphase) was commonly used in places like NYC and Toronto (and the original generators at Niagara Falls were 25Hz. The advantage was that the lower synchronous speeds of motors were closer to the speeds wanted at that time. (1500, 750, 500,.. vs 3600, 1800, 1200,... for 2,4,6 pole machines). Some of the 25Hz systems remained in use up to the 70's because, where they were used, they were adequate for the purpose. Railway use is one of these applications.

Note that typical European railway supplies are at 16-2/3 Hz with solid state converters/motor controllers on the trains. The lower frequency is advantageous for transmission in this case and individual drive wheel control allows optimization of tractive force by controlling torque to keep wheel slip in the optimum range (and outperforming equivalent diesel electric locomotives in terms of tractive force).

I can see 25Hz railway supplies being used for a long time-why fix it if it ain't broke?- and there are advantages to the lower frequency- reading comfort is low on the list of desirable features.

I doubt whether any subway station lamps, if these were the ones flickering, were supplied from the DC system but I can see them being supplied from a 25Hz railway feeder, and, in any case, commutator ripple would have been at a frequency and amplitude that would not cause visible lamp flicker.

Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer

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.

Bill

------------------------ Thanks for the added information. As I recall, turbine torque- speed curves are quite peaked and a 20% increase in speed would have reduced their performance appreciably.

Any bulb where the light is produced by a glowing element; as in, incandescent.

Even the sun flickers!

Bill

I wish!

-- Sue (Experiencing typical Dartmoor weather, grey skies, low cloud, slow drizzle, not even a hint where the sun may be)

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 of distances).

Cheers