While reading Amtrak's documentation for the recovery act, I learned that the signal systems are fed by dedicated 100 Hz supplies and they want to upgrade them to run on 60 Hz through a converter that still runs them at 100 Hz. First I'm wondering why 100 Hz to begin with, and second, why not standardize onto 60 Hz during the upgrade?
I can see at least one advantage to staying with 100 hz (if that is the actual signal power, and not one of the code frequencies): very easy to prevent interference from stray 60 hz signals from power lines, which are all over the place.
I can also see one major cost savings, too, based on the same premise: if all the signals are designed to run on 100 hz power, you have a choice: change all the signals (many many many bucks) or provide converters (very very few bucks). I know which one I'd choose...
jchnhtfd I can see at least one advantage to staying with 100 hz (if that is the actual signal power, and not one of the code frequencies): very easy to prevent interference from stray 60 hz signals from power lines, which are all over the place.
One of the places such 60 Hz power lines are is directly right overhead of the signals, at 138,000 volts (I think) from Phila. Electric Co.'s transmission line "overbuild" that occurred sometime after the PRR's initial electrification construction.
jchnhtfd I can also see one major cost savings, too, based on the same premise: if all the signals are designed to run on 100 hz power, you have a choice: change all the signals (many many many bucks) or provide converters (very very few bucks). I know which one I'd choose...
This seems like the usual "backwards-compatibility" problem that is often encountered with upgrades - because the old stuff has to keep running as it is gradually replaced by the new, the new stuff has to be compatible with both the old technology during the installation, and then also with the new technology after the changeover occurs. But let's see what a signals person has to add to that . . .
- Paul North.
I can't say for sure if this is true or not. The story that I had always heard was that when the PRR was building what is now Amtrak's N.E.C., they specified odd voltages and frequencies to prevent the possibilty of pilferage of electricity by nearby homeowners. This would have been about 90 years ago when private homes were just beginning to be wired for electricity and quite costly.
WM7471I can't say for sure if this is true or not. The story that I had always heard was that when the PRR was building what is now Amtrak's N.E.C., they specified odd voltages and frequencies to prevent the possibilty of pilferage of electricity by nearby homeowners. This would have been about 90 years ago when private homes were just beginning to be wired for electricity and quite costly.
The reason for the choice of 25Hz. electricity has everything to do with electric propulsion motors, and nothing to do with worrying about theft. Low frequency AC power can be fed into a traction motor constructed exactly the same way as DC power, through a series wound motor and across a commutator. The limiting factor before arcing becomes a problem is the frequency of the AC supply. There is no hard and fast number about the useable frequency, just that as the frequency goes up brush life goes down. The Pennsy chose 25 Hz. as a good compromise between a frequency high enough to avoid flickering of lighting in passenger coaches, and low enough to give reasonable traction motor brush life. The Germans and the Swiss chose 16 2/3 Hz. as it is easily produced by a six pole AC motor generator running on their Commercial Standard AC frequency of 50 Hz.
Now as to why the choice of 100 Hz. for signal systems, you have to remember that you are sending a flow of current down one rail at a frequency of 100 Hz., you detection equipment is connected to the other rail looking for the return signal. What causes a problem is that the same rails also serve to return the current from the AC overhead equipment at either 25 Hz. or 60 Hz. depending on location. A further complication is you need still another frequency for the crossing gates. So you may have current flows through the rails at three different frequencies. If you think about things like harmonic interactions and other inferences from one of these flows with the others you see why the frequencies are carefully chosen and not abitrarily made.
Grade-crossing frequencies are 86, 156, 285, 348, 430, 525, 645, 790, 970 Hz. The reason for the different frequencies is the grade-crossing approach circuits overlap in just about any territory with speeds greater than 20 mph and crossings closer than a mile apart. By using different frequencies the Grade Crossing Predictor for crossing "3" in the middle of five crossings can be tuned for its frequency and ignore the frequencies for the approach circuits for adjacent crossings 1, 2, 4, and 5 which overlap its approach circuits. Frequencies more closely spaced bleed into each other. Note that these frequencies are not multiples of each other so the peaks and valleys are offset.
An approach circuit for a 70 mph crossing in double-track might have a 8,113' approach circuit -- 25 second warning time, 5 second equipment activation time, 1 second clearance time, plus any time for a traffic signal interlock.
Fortunately most of the NEC is grade-separated. Grade-crossing circuits in high-speed electrified territory are a difficult problem with very poor solutions.
RWM
Ahh, the harmonics would definitely caused huge problems.
Thanks, finally something I can understand!!
Sounds to me like Amtrak just wants to yank out the 100Hz cab signal carrier supply and use commercial power with local 100Hz power supplies. They likely already have commercial power everywhere they need it for the second carrier frequency of the Rube Goldberg ACES signal system.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
Unfortunally I do not know the source power for the PRR 100Hz signal power. If it comes from the 25Hz Generators at the PRR hydro generators then a simple motor generator (running at 1500 or 750 RPM) could provide the 100 HZ power. Otherwise other sources for the 100Hz power had to come from other persons. Now days the ability of commercial transformer/rectifiers - inverters (4 wave) allow easier use of any power and very cheaply. So now back up 100HZ power supplys can easily and relatively cheaply be purchased almost off the shelf allowing a commercial 60Hz 240V (or the high voltage line supply) hookup to supply the 100Hz needed for older signal equipment.
The second item is that the New Haven - Boston AMTRAK segment signals are powered by 60 Hz 240v. Originally as that route was upgraded either commercial power or a few solar sources were used to retire the pole lines. After the electrification of that route the commercial power signal connections were discontinued and a dual source system extracted by 25Kv - 240V transformers were connected to each CAT line to supply signal power (each track CAT is isolated from the other track with gaps at the crossovers). Unfortunally the commercial power source were disconnected and now AMTRAK is going to reconnect the local commercial connections as a furthur back - up of signal and crossing power. Since the upgraded signal system did not have a three (or four where there are three tracks) source switching capability some money will have to be spent to buy the necessary auto switches. These replacements are in the documentation of the recovery act.
Under the recovery act AMTRAK is rebuilding 3 Rotary Frequency converters at Lamokin Pa ( southwest of PHL about 12 miles). Assuming these are 60Hz - 25 Hz converters they would operate at 300 RPM and as such would be monsters. AMTRAK is also replacing 6 - 20megawatt transformers at that location.I have no idea how they are going to be wired but I suspect each transformer will supply commercial 60Hz electricity to one of the rotary converters and then each converter output will provide 25 Hz to a transformer. Various switch gear is also going to be installed to supply this to the overhead transmission lines. This is a $60 million project at Lamokin. Also seveal substations are getting new transformers at an additional cost of $25 Million. This appears to be a first step to eventually converting the NEC to 60Hz. Anyone have more information?
blue streak 1 Unfortunally I do not know the source power for the PRR 100Hz signal power. If it comes from the 25Hz Generators at the PRR hydro generators then a simple motor generator (running at 1500 or 750 RPM) could provide the 100 HZ power. Otherwise other sources for the 100Hz power had to come from other persons. Now days the ability of commercial transformer/rectifiers - inverters (4 wave) allow easier use of any power and very cheaply. So now back up 100HZ power supplys can easily and relatively cheaply be purchased almost off the shelf allowing a commercial 60Hz 240V (or the high voltage line supply) hookup to supply the 100Hz needed for older signal equipment. The second item is that the New Haven - Boston AMTRAK segment signals are powered by 60 Hz 240v. Originally as that route was upgraded either commercial power or a few solar sources were used to retire the pole lines. After the electrification of that route the commercial power signal connections were discontinued and a dual source system extracted by 25Kv - 240V transformers were connected to each CAT line to supply signal power (each track CAT is isolated from the other track with gaps at the crossovers). Unfortunally the commercial power source were disconnected and now AMTRAK is going to reconnect the local commercial connections as a furthur back - up of signal and crossing power. Since the upgraded signal system did not have a three (or four where there are three tracks) source switching capability some money will have to be spent to buy the necessary auto switches. These replacements are in the documentation of the recovery act.
[emphasis added - PDN.]
A couple of comments/ repsonses:
The PRR never owned any hydro generators, to the best of my knowledge, at least not for the 11,000 volt, 25 Hz electrification of the 1930s. I understand that most - but not all - of that power was purchased from Philadelphia Electric Co.'s Conowingo Dam (northeatern Maryland), and Baltimore Gas & Electric Co.'s / PECo's jointly-owned Safe Harbor Dam (Pennsylvania). Both dams are on the lower/ southern Susquehanna River - between them is the Holtwood Dam (Pennsylvania), formerly owned & operated by Pennsylvania Power & Light Co. That too may have served the PRR within PP&L's territory - Lancaster/ Harrisburg extension of the electrification (now Amtrak''s Keystone route service), but I don't recall ever having read that. I'll try to remember to look that up in either Middleton's book or one of my PRR references.
For Amtrak to discontinue the commercial power supply to the signals - even as a back-up - to run solely off the catenary power supply seems really short-sighted to me. What happens if the CAT power goes down, from either a failure of its peculiar power supply, a wire mishap or damage of some kind, or even for maintenance of same ? Then - it seems to me - the signal power and system would be totally lost as well. As a result, not even diesel-hauled trains could operate under anything but very slow-speed emergency conditions - not a very desirable condition. Although even diesel-powered trains would of course be slower and likely a make-shift operation there at best, with functioning signals it would be less a lot less disruptive.
Paul: you are right. When I rode the New Haven - Bos route after electrification I was able to question a road foreman about it when I observed the commercial 240V lines disconnected. His answer was the decision came from much higher up and he couldn't comment, When I said something along what you just posted he still replied "no comment". Well now the stimulus plan is correcting that short sighted decision.My guess is the higher up said since the two or more tracks are separated electrically that is enough back up. Don't you know they found out the hard way. I have no idea what their battery backup capacity is if any installed. Any bets per the above decision?
About the generating plants. Didn't know they were not AMTRAK owned and that really puts my e mail to you about the transmission line rebuildings to present more questions that need to be answered.
That might explain why the scenic train I took in Norway last summer ran on only 16 2/3 Hz instead of their regular 50 Hz (it ran from Myrdal, on the Oslo-Bergen line, down to Flåm (the Flåmsbanen) connecting with a fast ferry to Bergen, where I caught Norwegian Coastal Voyage to Kirkenes). I don't know what the voltage was, or what the train Oslo-Bergen ran on. Spectacular trip but v. crowded and no place for luggage.
--Roger Williams, longtime TRAINS subscriber, Boulder CO.
rogeracThat might explain why the scenic train I took in Norway last summer ran on only 16 2/3 Hz instead of their regular 50 Hz (it ran from Myrdal, on the Oslo-Bergen line, down to Flåm (the Flåmsbanen) connecting with a fast ferry to Bergen, where I caught Norwegian Coastal Voyage to Kirkenes). I don't know what the voltage was, or what the train Oslo-Bergen ran on. Spectacular trip but v. crowded and no place for luggage.--Roger Williams, longtime TRAINS subscriber, Boulder CO.
All electrified Norwegian rail lines are electrified at 15kV/ 16.7 Hz, not just the scenic line that you traveled on.
beaulieuThe reason for the choice of 25Hz. electricity has everything to do with electric propulsion motors, and nothing to do with worrying about theft. Low frequency AC power can be fed into a traction motor constructed exactly the same way as DC power, through a series wound motor and across a commutator. The limiting factor before arcing becomes a problem is the frequency of the AC supply. There is no hard and fast number about the useable frequency, just that as the frequency goes up brush life goes down. The Pennsy chose 25 Hz. as a good compromise between a frequency high enough to avoid flickering of lighting in passenger coaches, and low enough to give reasonable traction motor brush life. The Germans and the Swiss chose 16 2/3 Hz. as it is easily produced by a six pole AC motor generator running on their Commercial Standard AC frequency of 50 Hz.
Westinghouse started development of single phase commutator motors in the 1890's, and found that 10 HP was the practical upper limit for 60 Hz. The first use of AC commutator motors for traction was the Indianapolis & Cincinnati approx 1904 (and used 25 HZ at 3300 volts). The New Haven was the first mainline electrification and was the first to use 11,000V at 25 Hz. The Visalia Electric was an oddball using 15 Hz.
25Hz power does have some advantages, easier to make low speed motors with 25 Hz than 60 Hz (fewer poles), easier to send power over long distances, circuit breakers work better, etc. Perhaps most importantly, 25 Hz made for better performing synchronous converters (the ones used on the NYC's Grand Central Terminal electrification used 25 Hz). Between 1900 and 1920, there was about a third as much 25 Hz generation added as 60 Hz.
- Erik
Concerning possible conversion to 60 Hz, the recovery act project document only states that the existing equipment is old and reliability is not known--they do not address conversion to 60 Hz.
Furthermore, some speculation in this forum was that the existing 25 Hz equipment south of New York underwent a major overhaul in the middle of the 20th century (1950s?) and would have been a waste to scrap.
As for the generators at the three dams, they were not owned by the PRR but were commissioned to be built for the PRR by the local utility. They all have provisions to produce 60 Hz power via motor-generators at the cost of several kilowatts of efficiency.
Conowingo Dam on the Susquehanna has, or may have had in the past, a 25 Hz generator, but its web site states that it was only used for a long-abandoned rail line near the dam and POSSIBLY may have been used on another line on the Eastern Shore of MD which has not been electrified for many decades. I believe this information may be slightly incorrect, but I cannot find anything more on the internet that indicates the 25 Hz generator at Conowingo Dam is in use at all.
Anyone know how much of the once PRR electrification tracks that did not go to AMTRAK still has this same 100HZ signal power? ie the Trenton cutoff for one. Also what about the old RDG electrified lines?
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