I ran across a reference in section 19-161 of the 8th edition of Standard Handbook for Electrical Engineers where a railroad went for a carrier frequency of 91 2/3cps (Hz) for trackage where there was both the 11kV 25Hz “trolley” and DC third rail. The DC on the transformers and impedance bonds would cause the 4th harmonic of 25Hz to be generated which then interfered with the relays designed to pick up 100Hz. I recall seeing this in other sources as well.
The combination of 25Hz overhead and DC third rail sounds like Penn Station to me.
It’s quite possible that the passband of the on-board filters are wide enough to pass 91.67Hz as well as 100Hz. I’d suspect that the track relays are more selective - think many, if not all, use a reference signal derived from signal used on the track circuits. The track relays are the more critical as a continuous 100Hz signal would be interpreted as the most restricting aspect by the on-board equipment, while the track relay could give a false clear for the block.
If there was anyone on this forum who would know about the adverse effects of a DC current on a transformer or inductor, it would be you. [:)]
It was and is the onboard equipment that must have the filter charadteristics broad enough to accept 91.67Hz as well as 100Hz, but the track relay filters could be extremely narrow-band, since they don’t move to the 100Hz locations.
I designed a whole bunch of transformers for Mystic Transformers that were required to work with a dc primary offset. The trick was to determine the air-gap in the magnetic structure that would prevent oversaturation of the steel while still preserving useful efficiency (between 50% and 80% depending on the strenth of dc current, no. of coils, area of steel pole crosssection). I did not have formulas for this and developed my own rules of thum, based fortunateliy on trial calculations, not on prototype failures! With a dc componant, unless just a trickle, an air-gap is esential. (Of couse it isn’t air, it’s glass or other insulatng material.) With simple inductors, just adding more steel is usually the anwer, and this means also more coils. These were all successful designs with repeat orders. Mostly military.
What wonderful days those were! Engine pass on the B&M, commuting with my Raleigh bike on the front open platform of the wood combine behind the GP-7, riding the power unless I was outranked and had to ride the folding-bench in the baggage section or in a coach seat behind. Returning to my MIT dorm, usually via a very late dinner at Semeoni’s Italian Restaurant in Cambridge on the loop where the Type 4 Watertown cars turned off Central Square, always meant the question: Do I bike to the station and wait for the next train or bike the whole way downhill? The few times a steam subsituted, I could not enjoy a cab ride, because I had to be relatively clean at work at Mystic.
Thanks for the tale of the cab rides. A couple of classic cases for DC current in an audio frequency transformer would be a single ended output transformer (e.g. on an AC/DC set) and a plate modulation transformer.
In the process of looking up the section on ATC in the Handbook, I ran across to references to Railway Electrical Engineer magazine. Turns out that Google Books has a scanned copy of the 1922 issues with several reports of various automatic train stop and train control developments, though nothing on the coded cab signal system.
Mystic transformers was located in Winchester (may still be, ought to check), and the name came from the nearby Mystic River, not because there was any mystery about the products sold!