92TS refurb/replacement

The weld repairs are mainly for corroded sections of floor which have now become thin or have holed areas these need to be cut away and new sections or plates welded in, structurally there is no apparent problems with the bodyshell itself, the corrosion is mainly down to water/salting damage under the old vynil.

as the bodyshells are aluminium these need specialist welders to do the work.

Will we be able to see this train being tested out and about on the line? Or will it be tested at Old Dalby as before?

Text of post was embedded in quote - repositioned. (Tom)

Would the track infrastructure need to be changed to pump out AC voltage if it is currently pumping out DC

This is a valid question.

Although it is "pumping out current" at issue, not voltage.

It is true the train does the DC to AC conversion, yes, but there are still traction current supply effects; the trainborne kit does not address what Amps are drawn from the supply over the whole acceleration phase from standing start.

The main issue is I^2R heating effect (square of the current x resistance). This heating effect issue is in the lineside cables and distribution equipment rather than an issue for the conductor rails or the trains.

WITHOUT going too deeply into it, in v-e-r-y simplyfied terms, a DC motor is a constant voltage variable current machine - this puts out very high, albeit very brief, high current peaks that rapidly drop off. An AC motor is a constant current variable voltage machine - this puts a constant current on the traction supply.

With DC motors, you get a very high current peak, hence a (high)^2 heating effect peak, but it drops off very very quickly. With AC motors you get a constant current, so a constant I^2R heating effect, which, while much lower than the high DC motor peak, is significantly higher than the DC motor troughs.

If the new AC motor constant current effect is greater than the peak to trough DC motor effect integrated over time, then there may be an issue needing addressing in the supply system. The integration concerns sucessive trains over the same section, not trains in isolation; indeed the whole issue is about the section, not just the train(s).

Main line NR had to do a lot of this in the Southern electric third rail zone before the likes of Cigs Veps and Cigs etc could be replaced by 375 377 450 etc. There was a massive power reinforcement project put in hand. Even more recently, a new substation was Farringdon for Thameslink was only partly for the increased train service, it was also partly that AC motor 700s took over from DC motor 319s - there was enough resiliencne there for the 319\377\387 mix, but not enough for all 700. And even now, another underground, there is ongoing work under the Mersey for 777s to take over from 507\508s.

4LM did have some power reinforcement done; partly for migration to 750 V and partly because there are more trains, but also partly because AC motor S-stock replaced DC motor A|C|D stock. Hence the new sub-station at Edgware Road, for example, and others.

Setting AC motor traction packs on board trains to emulate DC motors emulates their performance, not their current draw, the motors have to draw the right current through the pack. It is possible to have the AC traction pack limited in such a way it won't exceed what a DC draws, but, the result is not good, suboptimal performance, worse than keeping DC motors.

But what I write is the general principle, if the traction supply is already robust enough, and I am pretty certain the Central is robust, then it might be nothing need be done.

Also, traction power kit is not fixed in stone for ever, there is a constant overhaul and replacement to ever higher standards. It may already have been picked up in the past couple of decades of routine renewals. In fact, likely, since retractioning 92TS has been on the cards for eons; perhaps the delay in not doing the trains to date has been down to supply works coming first.

One other thing; I assume, and this is an assumption, the retractioned 92TS will be capable of 750 V operation. If so, that step alone might address the issue, since more volts available = less amps need to be drawn. No idea if 750 V is being done as part of this; if it is, 630 to 750 DC supply have to wait for all 92TS to be retractioned first; probably with a software cap on the retractioned units before it gets increased.

This will all have been thought of by CLIP \ CLOP, no-one would set off on a project like this without having evaluated what needed to be done, which might be nothing. Effects there will be, but it might not anything doing now.

Sorry it's got very tekkie, there is no alternative on something like this.

It is an exceedingly complex issue. I am not a traction engineer; I did do electrical machines in my degree, the man doing my first year lecture was ex-MetroVick traction, and even covered Metadynes, so I think I was taught by someone who knew his stuff. What I have posted really is simplified, and numerous details omitted.

The point is, the original question is valid, and, yes, the subsequent response thT trains do the conversion is also valid, but the trains do not fully do conversion of the elements that impact on the trackside infrastructure. Whether that infrastructure is already up to it, or needs something further doing, is outside my theatre of operations.


HTH.


EDIT : typos and other errors sorted out; after my original post, my ISP played up and could not immediately correct things. Most of you read typoese anyway.

I thought we weren't meant to use such abbreviations any more.

Does train 2 have the AC motors? Not mentioned! Train 1 tested AC motors. Perhaps they have done these trains in parallel as prototypes, before fleet is treated with all modifications done at same time?

I think that’s how it’s been done, each prototype train has different quantity of modifications done, then the production trains will encompass all the mods that have been successfully tested elsewhere.

This seems like a good time to clarify the context in which information (especially media) on this project currently stands for the benefit of all members.

The vast majority of the media relating to Central Line Improvement Project (CLIP) comes with the clear understanding that they are for internal use only. Those on the Forum who've worked in and around transport will be acutely aware of the consequences when these warnings are not adhered to. This is one of the many reasons why we have Rule 7f among others:


District Dave is in the public domain and is viewed by an extremely diverse audience. With that comes added scrutiny about the information that is posted. Bearing all of the above in mind, written updates currently provide the most appropriate medium to share information about this project. 

The enthusiasm for Central Line Improvement Project (CLIP) internally aswell as externally is self-evident. Should media become available that is suitable for the public domain, members can rest assured it will be shared as and when it is appropriate to do so.


goldenarrow

I mean, theoretically you could have a 6-rail system to deliver three phase AC to the train, but I suspect it would rather fail the cost/benefit analysis compared to having switch mode supplies on board the train!

The few three phase AC railways generally use two overhead conductors and the track to supply the three phases.

The thing about modern AC traction drives is that the supply has a fixed frequency (DC = 0 Hz) but the asynchronous motors don't respond well to a fixed frequency. The inverters are know as variable voltage, variable frequency systems, delivering gradually increasing frequency and voltage as speed increases.

This isn't the place to go into the technical aspects, but Googling Variable voltage, variable frequency motor drives offers articles such as this: electricalbaba.com/vvvf-speed-control-of-induction-motor/

At least two of the three-phase AC systems, both in Switzerland - both rack and pinion mountain railways - did indeed feed AC more or less directly to AC synchronous motors whose speed is locked to the frequency. This was in the days before there were any electronics! The Jungfrau railway train had a two speed gearbox with a higher torque/lower speed ratio for the steeper sections of the line.

<<rincew1nd: quote attribution fixed>>

Depending on what you are trying to prove, some theory requires considering DC is infinite frequency rather than zero.

I'm not going to elaborate on that, it being somewhat off subject, will bore the xxxx off readers here, and doubt I could do it succinctly anyway.

Yes, all three were DC choppers.

eeeek, yes, I skipped a whole section there, it was meant to go through the steps from clockwork controlled DC motors, through electronic power controlled DC motors [choppers], and then AC motors control; you had to have choppers first, as the variable voltage variable frequency current applied to asynchronous motors does have a form of chopping to get there. Mian line BR SR Romeo and Juliet were DC choopers, as were 86TS.