PRR Catenary power

I was reading on wikipedia about the PRR GG1's, and the article said that the PRR catenary system was 11,000 Volts at 25 hz. Is this true, this seems a bit unusual to me.

slcguy wrote:I was reading on wikipedia about the PRR GG1's, and the article said that the PRR catenary system was 11,000 Volts at 25 hz. Is this true, this seems a bit unusual to me.

Yes it is true. So why is that unusual?

amtrakhogger wrote:

slcguy wrote:I was reading on wikipedia about the PRR GG1's, and the article said that the PRR catenary system was 11,000 Volts at 25 hz. Is this true, this seems a bit unusual to me.

Yes it is true. So why is that unusual?

Because the standard is 25,000 Volts 60 Hz

The "standard" NOW is 25,000 Volts 60 Hz. The "standard" THEN was 11,000 Volts 25 Hz. Delaware, Lackawanna & Western used DC overhead (3,000 Volts?). The Milwaukee Road's overhead was 3,000 Volts DC, yet the Great Northern's overhead was 11,000 Volts 25Hz.

From a historic viewpoint, 25,000 Volts 60 Hz is unusual.

PRR (and New Haven) electrification were introduced before rectifier-equipped locomotives. (The New Haven had, I think, one experimental motor-generator locomotive, and there were a few on other railroads, but they weren't common: don't know why not.) Synchronous motors (as on modern AC diesels) were operationally problematic: the locomotive would have only a limited number of efficient speeds. (This was o.k. on small systems with fairly uniform train speeds, but would have been crippling for the PRR.) As a result the best available electric motor technology was a type of motor (closely related to DC motors) that is o.k. with low frequency AC but has problems with higher frequencies. Result: decision to use 25 hz for the overhead. Many German and Swiss railroad lines were electrified with 16 2/3 hz current (voltage somewhere between 10 and 20 thousand, but I forget exactly what).

With the development of rectifier technology after WW II it became feasible to electrify railroads with "commercial frequency" AC (= 50 hz in Europe, 60 hz in North America). I think the first major installation was a line (with fairly heavy freight traffic) in northern France in the 1950s; its success convinced British Rail to go with commercial frequency for the electrification of the West Coast Main Line a few years later.

Yes, Germany, Switzerland, and Austria (and I think Sweden) electrified at 15,000 volts, 16-2/3 hz.

Thanks, ExCon90!
I was being lazy and didn't look up the voltage.
My guess is that there isn't much to choose (from the point of view of efficient operation with early 20th C technology) between (25 hz, 11 kv) and (16.7 hz, 15 kv), and once one had been adopted for a big project (probably the New Haven's on North America) others on the same continent just followed suit: sort of like how the equally arbitrary 56.5" track gauge got adopted! (Or how 50 hz became "commercial frequency" in Europe and 60 hz in North America.)

One further complication (I'm testing my memory here, so somebody please correct me if I'm wrong): you don't want the power current to swamp the signalling track circuit, so choices have to be made. With a DC electrification, it's easy: use AC for the track circuit. With an AC electrification, choose a different frequency. I.i.r.c., the 25 hz electrificattions in the Northeastern U.S. used 60 hz track circuits, and when some were converted to commercial frequency, the signalling had to be redone! I think it is now 100 hz in trackage with 60 hz power electrification.

Can't comment on what the signal frequency currently is, but it was 60-cycle back when the overhead was 25 cycle. In a few of the towers, the 60-cycle stuff was available for use with electrical appliances, such as refrigerators. Since televisions at night were prohibited, that was a non-issue (wink, wink).

Some older radios (prohibited) didn't care what the frequency was.

Signals in this area have run on 100Hz since electrification. 200Hz for track circuits in some areas has also been in use at least since the completion of the CCCT, but I'm not sure if the PRR ever used it prior. Reading didn't. PRR, Reading, and Septa cab signaling equipment was/is strictly 100Hz. Amtrak and NJT use a combination of 100Hz and 250Hz for cabs. Newer Electro-Code style track circuits use time coded DC pulses which go through a special interface for electrified tracks, and ends up being a 156Hz signal. Audio-frequency overlays use a variety of frequencies from 430Hz to 4KHz.

Allen Hazen wrote:One further complication (I'm testing my memory here, so somebody please correct me if I'm wrong): you don't want the power current to swamp the signalling track circuit, so choices have to be made. With a DC electrification, it's easy: use AC for the track circuit. With an AC electrification, choose a different frequency. I.i.r.c., the 25 hz electrificattions in the Northeastern U.S. used 60 hz track circuits, and when some were converted to commercial frequency, the signalling had to be redone! I think it is now 100 hz in trackage with 60 hz power electrification.

In the early days of PRR electrification it was found the 100 Hz used for cab signals was a fourth-order harmonic of the 25 Hz catenary AC frequency (25 Hz x 4 = 100 Hz) and they subsequently changed the cab signal frequency from 100 Hz to 91 2/3 HZ to avoid the problems caused by these harmonics.

GE-RULES wrote:In the early days of PRR electrification it was found the 100 Hz used for cab signals was a fourth-order harmonic of the 25 Hz catenary AC frequency (25 Hz x 4 = 100 Hz) and they subsequently changed the cab signal frequency from 100 Hz to 91 2/3 HZ to avoid the problems caused by these harmonics.

That is correct. However, most of the off-the-shelf equipment designed for 100Hz operation works fine on 91 2/3 Hz.

limejuice wrote:

GE-RULES wrote:In the early days of PRR electrification it was found the 100 Hz used for cab signals was a fourth-order harmonic of the 25 Hz catenary AC frequency (25 Hz x 4 = 100 Hz) and they subsequently changed the cab signal frequency from 100 Hz to 91 2/3 HZ to avoid the problems caused by these harmonics.

That is correct. However, most of the off-the-shelf equipment designed for 100Hz operation works fine on 91 2/3 Hz.

Yeah I would think it would work without any issues. Probably the only modification made (if any) would be a change in the low pass filters to allow for a cleaner signal instead of operating on the edge of a 100 Hz LPF, but who knows the 100 Hz LPF may be broad enough to pass 91 2/3 Hz just fine without any degradation in signal.