How to ground mains equipment on a +48VDC chassis?

[abdullahseba]

Thanks for that picture. So we are preserving the history of the Victoria line? 

Note that the 115 V AC output from the motor-alternator set is just as isolated as the 50 V DC. The way you would emulate the 115 V AC in a museum setting would be by the use of a suitable 240 V to 115 V isolation transformer. Happily, such transformers should be relatively easy to procure. With such an isolation transformer you could also connect one pole to the car body without any worries of short circuits or trips.

From an authenticity perspective it would be nice to keep the original fluorescent lighting intact and not replace it with LEDs as suggested by some.

Yeah we are keeping the fluorescent tubes  . just replacing the original ballast with 240v ones. We use to run them off a 110V transformer but they are 20 years old now and a lot of them are dying. 110V ballast are hard to get hold of so. We hire it to a lot of filming companies too who have requested selective control repeatedly so we are adding that but its all behind the seens stuff that doesn't affect the look and feel of the car.

Heres an old pic of the full windings:



They use a lot of odd voltages probably to account for voltage drops.

I can take a hi res pic of the full thing if you like

[abdullahseba]

Yes, 240/120V mains transformers should be readily available in a variety of different forms  And then you wouldn't need to change out the ballasts.
However, we don't know what was the frequency?  Was it 50Hz? (probably not 60Hz like here in the Colonies)
400Hz was (is?) popular in avionics because it greatly reduces the size/weight of the iron core.
If the ballasts were designed for 400Hz (or something significantly above 50 or 60Hz, they might not like mains frequency.

Almost half the ballasts have died after 20 years of use so replacing them with 240 is a better option. Most of the train circuits where actually 850Hz including the ballasts which run fine off 50Hz until now. Quite possible that 50Hz has shorted the lifespan of them though.

[Richard Crowley]

Quite possible that 50Hz has shorted the lifespan of them though.

Have you analyzed the failure mode?  I would expect that 50Hz would significantly overheat a transformer designed for 850Hz.

[IanB]

However, we don't know what was the frequency?  Was it 50Hz? (probably not 60Hz like here in the Colonies)
400Hz was (is?) popular in avionics because it greatly reduces the size/weight of the iron core.

Good point. However I have an idea that the on-train electrics in the London Underground has traditionally been 115 V 60 Hz due to the American heritage of the system. A higher frequency would not have been a benefit in previous decades since the supply from the motor-alternator went directly to the lighting and no transformers were involved (except for the magnetic ballasts in fluorescent lamps). Also the practical frequency would depend on the speed and number of poles in the motor-alternator set.

The earlier trains like the 1938 stock used incandescent lamps (much more inviting than fluoros).

[abdullahseba]

Quite possible that 50Hz has shorted the lifespan of them though.

Have you analyzed the failure mode?  I would expect that 50Hz would significantly overheat a transformer designed for 850Hz.

Transformer we used was a 50Hz one. 50Hz in and out. https://docs-emea.rs-online.com/webdocs/1531/0900766b815319c1.pdf  Its the ballast that was rated for 850Hz

[abdullahseba]

However, we don't know what was the frequency?  Was it 50Hz? (probably not 60Hz like here in the Colonies)
400Hz was (is?) popular in avionics because it greatly reduces the size/weight of the iron core.

Good point. However I have an idea that the on-train electrics in the London Underground has traditionally been 115 V 60 Hz due to the American heritage of the system. A higher frequency would not have been a benefit in previous decades since the supply from the motor-alternator went directly to the lighting and no transformers were involved (except for the magnetic ballasts in fluorescent lamps). Also the practical frequency would depend on the speed and number of poles in the motor-alternator set.

The earlier trains like the 1938 stock used incandescent lamps (much more inviting than fluoros).

I believe the 1957 tube stock was the first to use 850Hz systems not sure if older ones where 60Hz. Even the 240v fan supply generated by 50VDC inverters are 850Hz for some reason.

[IanB]

I believe the 1957 tube stock was the first to use 850Hz systems not sure if older ones where 60Hz. Even the 240v fan supply generated by 50VDC inverters are 850Hz for some reason.

If it's truly 850 Hz AC then any magnetic equipment like ballasts or transformers will absolutely not be compatible with 50 Hz electricity. It would be so incompatible that instant failure would be predicted if attempting to connect 850 Hz equipment to a 50 Hz supply. So while you clearly have access to better information than I do, I still wonder how you managed to get things to work.

[abdullahseba]

I believe the 1957 tube stock was the first to use 850Hz systems not sure if older ones where 60Hz. Even the 240v fan supply generated by 50VDC inverters are 850Hz for some reason.

If it's truly 850 Hz AC then any magnetic equipment like ballasts or transformers will absolutely not be compatible with 50 Hz electricity. It would be so incompatible that instant failure would be predicted if attempting to connect 850 Hz equipment to a 50 Hz supply. So while you clearly have access to better information than I do, I still wonder how you managed to get things to work.

The ballast are electronic and we ran them off 50Hz for about 7 years. We are not using any 850Hz transformers... the one that came off the alternator is in the second trailer car which we dont have.

[abdullahseba]

I believe the 1957 tube stock was the first to use 850Hz systems not sure if older ones where 60Hz. Even the 240v fan supply generated by 50VDC inverters are 850Hz for some reason.

If it's truly 850 Hz AC then any magnetic equipment like ballasts or transformers will absolutely not be compatible with 50 Hz electricity. It would be so incompatible that instant failure would be predicted if attempting to connect 850 Hz equipment to a 50 Hz supply. So while you clearly have access to better information than I do, I still wonder how you managed to get things to work.

Heres an image of one:



The original original ones as in (pre refurbishment) where magnetic. Keeping in mind that these where refurbished in 1995.

[IanB]

Heres an image of one

I see. Cool. Yes, those are probably not very sensitive to supply frequency.

[IanB]

The original original ones as in (pre refurbishment) where magnetic. Keeping in mind that these where refurbished in 1995.

If we are talking about 1967 stock, it interesting to recall that these were brand new shiny trains when I first rode them in the early 1970's. At the time their speed and acceleration was impressive.

(And what happened to Thorn?    )

[abdullahseba]

Yep these are 19

The original original ones as in (pre refurbishment) where magnetic. Keeping in mind that these where refurbished in 1995.

If we are talking about 1967 stock, it interesting to recall that these were brand new shiny trains when I first rode them in the early 1970's. At the time their speed and acceleration was impressive.

(And what happened to Thorn?    )

Yep this is the 67 tube stock. We actually have half of one of the pre refurb ones that had an accident.



Thorn is still going http://www.thornlighting.co.uk/