Wierd Flourescent lamp behaviour

[Zelandeth]

Well, this is more out of idle curiousity than anything else. That and the fact that I can't remember the answer to one of the questions.

I was recently in London, and while on the underground, noted some odd behaviour of the lighting on a couple of the trains (24" T12 Sylavania Daylight flourescents).

One of them had dimmer rings around the tube, which were seen to be moving back and forth along the tube depending on the speed of the train - so likely something to do with the trains motor interfering in some way with the lamps ballast (was an older train, so most likely using a magnetic ballast). The effect was sort of like the "Snaking" which I've seen when first powering up new tubes - but much less random. I have seen an explanation of what causes this before, but I sure can't remember what it was.

Second one was a little harder for me to figure out. One end of the tube was lit normally, while the other was dark - I can't see how that'd work, the tube would either be on or off...not HALF on and half off. What's happening there?

[gwbaltzell]

I know I heard the term too but couldn't remember. Looked it up. It seems the term of this swirling is ....... swirling. Sometimes normal with new bulbs. Also could be a sign of cold bulbs or low input voltage. I have also seen the banding form, but on a non-moving light. I know I've seen a detail explanation but I can't remember where.

The half on tube I think is fairly common. Never seen a good explanation, but mine is this. Every instance I've seen is an instant start ballast, which are higher voltage. If the bulb or wiring fails on one end, the voltage is high enough to still send some current though the other. Just like those plasma balls.

[Chris M.]

I always thought the lighting in Tube trains was fed via high frequency ballasts from a DC supply, similar to buses. If that is the case, that explains the half-dim tube, kind of. I have seen that happen on a 12 volt twin tube light intended for caravans. Two 8-watt tubes side by side, connected in series and fed from the usual inverter at one end. After a little while of being lit, the top one was still bright but the bottom one was very dim at one end. Swapping them over, it did the same thing after a while. It comes from the high frequency DC the tubes are run from - one end goes to 0v, with the other end on the high voltage side of the output transformer. I *think* the one-way current flow causes migration of the mercury, and in modern tubes with low mercury content, it leads to a lower than adequate concentration at one end, meaning the arc is not as bright there.

But I could be wrong. I don`t claim to be an expert.

As for the banding/swirling/snake caught in the tube trying to get out [img]/ubbthreads/images/graemlins/tongue.gif[/img] varying with the train`s speed, again if it were fed from a DC supply that wasn`t perfectly regulated, it is likely that high frequency harmonics resulting from the motor speed controller could have been affecting the discharge somehow. Perhaps when it came very close to a multiple of the frequency of the ballast it would cause "standing wave" type interference. See this page of Mike`s Electric Stuff: http://electricstuff.co.uk/geissler.html to see what happens to a Geissler tube fed from high frequency DC. Just a guess but that could be what was happening in that tube you saw.

Again, I could be wrong. I see the "snaking" effect quite often in circular tubes that are nearing the end of their lives, running from plain iron ballasts. But also saw it in some new linear T8s that were fed from an electronic dimmer/ballast pack as they were being dimmed.

[img]/ubbthreads/images/graemlins/confused.gif[/img]

[jtr1962]

[ QUOTE ]
Zelandeth said:
One of them had dimmer rings around the tube, which were seen to be moving back and forth along the tube depending on the speed of the train - so likely something to do with the trains motor interfering in some way with the lamps ballast (was an older train, so most likely using a magnetic ballast). The effect was sort of like the "Snaking" which I've seen when first powering up new tubes - but much less random. I have seen an explanation of what causes this before, but I sure can't remember what it was.


[/ QUOTE ]
If I had to guess on this one, I would say that the frequency of the tube voltage is related to the speed of the train. The reason for this is very simple-the power which drives subway trains is DC, not AC. Therefore, any devices on the train which require AC cannot be powered via a simple magnetic transformer or ballast. On newer trains this AC is undoubtly made electronically but on older trains it may be made by a mechanical inverter, which is simply a DC motor which powers an alternator. The speed of a DC motor varies with the supply voltage, and this in turn may be somewhat related to the speed of the train. At low speeds the traction motors which drive the train draw more current which lowers the supply voltage, and hence the lamps AC frequency. At higher speeds the supply voltage is higher, and so is the frequency. If anyone has a better explanation, I'm all ears.

[ QUOTE ]

Second one was a little harder for me to figure out. One end of the tube was lit normally, while the other was dark - I can't see how that'd work, the tube would either be on or off...not HALF on and half off. What's happening there?

[/ QUOTE ]
The AC going to the tube is evidently not pure, but has a DC voltage superimposed upon it. This phenomenon is also common with low-cost one-transistor fluorescent inverters which end up blackening one end of the tube.