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Transport for Wales 769's

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gka472l

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769424 was in Liverpool Lime Street a bit earlier this morning, not seen one in before......it was using electric power though
 
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Neen Sollars

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Thanks for posting Bob and Adlee. These are very smart looking units, the TfW livery is the smartest I think. I hope these four car units are a step up from what went before. The trouble is that it is taking too long all over the country for new, refurbed and cascaded trains to be introduced.
 

Adlee Turner

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Thanks for posting Bob and Adlee. These are very smart looking units, the TfW livery is the smartest I think. I hope these four car units are a step up from what went before. The trouble is that it is taking too long all over the country for new, refurbed and cascaded trains to be introduced.
It’s no problem, just glad the units are making progress towards entering service hopefully! However, I did notice that the second run only got to Heath High Level again then returned back to the depot again, another engine issue?
 

Brissle Girl

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So 2/3 failures so far. Not to worry, on the Rhymney line passengers are used to frequent failures by the 37s, so it’ll be business as usual once they enter service.
 

AMD

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I wonder what the reason is for pulling into the station with the engines almost at full speed?
It's the set up, the engines are designed to supply 750v constant supply, to mimic the supply from the third rail; so not like a standard engine which would rev up and down according to power demand.
On another note, I see the pantograph has been taken off.
 

Meerkat

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It's the set up, the engines are designed to supply 750v constant supply, to mimic the supply from the third rail; so not like a standard engine which would rev up and down according to power demand.
On another note, I see the pantograph has been taken off.
Can you explain that to an engineering simpleton please - where does all the power go if the motors aren’t drawing it?
 

samuelmorris

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It's the set up, the engines are designed to supply 750v constant supply, to mimic the supply from the third rail; so not like a standard engine which would rev up and down according to power demand.
On another note, I see the pantograph has been taken off.
But why is the power demand so high when the train is decelerating? The engine speed drops off once it comes to a stand. It's still lower when departing then when it arrived at the station. That's not how other DEMUs behave.
 

AMD

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Can you explain that to an engineering simpleton please - where does all the power go if the motors aren’t drawing it?
In a conventional diesel electric vehicle the engine generates power which is passed directly to the traction motors to convert into motion through the axles (keeping it simple before anyone says more), a higher moving speed (as an example) requires more power to the traction motors so a higher demand on the engine. On a 769 the diesel engines are generating a constant 750v DC to be passed down the bus line (the high voltage cable) to the transformer, at this point the unit works like a 319 electric unit in that the power is passed through the transformer to be used from there; which is why a 769 still generates the buzzing noise like a 319.
The voltage output is a constant 750v which is why it should rev at it's maximum speed most of the time, there will be something technical around electric demand on the supply which means it does sometimes rev down and up but that's electrical and I haven't seen the 769 manual ;)
 
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Meerkat

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In a conventional diesel electric vehicle the engine generates power which is passed directly to the traction motors to convert into motion through the axles (keeping it simple before anyone says more), a higher moving speed (as an example) requires more power to the traction motors so a higher demand on the engine. On a 769 the diesel engines are generating a constant 750v DC to be passed down the bus line (the high voltage cable) to the transformer, at this point the unit works like a 319 electric unit in that the power is passed through the transformer to be used from there; which is why a 769 still generates the buzzing noise like a 319.
The voltage output is a constant 750v which is why it should rev at it's maximum speed most of the time, there will be something technical around electric demand on the supply which means it does sometimes rev down and up but that's electrical and I haven't seen the 769 manual ;)
Thanks, but if the engine is running full whack it is creating power which the motors aren’t using - where does that energy go, heat?
Apologies to my Physics teacher if I am being a muppet....
 

samuelmorris

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Thanks, but if the engine is running full whack it is creating power which the motors aren’t using - where does that energy go, heat?
Apologies to my Physics teacher if I am being a muppet....
No you're right, I don't wish to sound unkind but AMD's response doesn't answer the question. I suspect it may either be a software defect with the unit, or perhaps some sort of debug mode to have the rear engine operate at full speed continuously while the unit is in motion.
 

RichT54

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Thanks, but if the engine is running full whack it is creating power which the motors aren’t using - where does that energy go, heat?
Apologies to my Physics teacher if I am being a muppet....

Doesn't it depend on whether the the generator is connected to a load at the time? If there is no load then current can't flow, so electrical power is not being generated?
 

krus_aragon

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Doesn't it depend on whether the the generator is connected to a load at the time? If there is no load then current can't flow, so electrical power is not being generated?
If the diesel engine and its generator aren't connected to a load, you're still busy releasing chemical energy from the diesel combustion. If it's not being converted to electrical energy, then it must be converted to other forms (e.g. heat and/or sound); it's not going to disappear anywhere.

(Power is, of course, the rate at which energy is being converted or transferred.)
 

gingertom

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diesel engines are governed. They can run at high (ish) revs with little load on them and generate very little power but once the load comes on the revs tend to drop, the governor compensates by moving the fuel rack (throttle to you and me) to allow more fuel to be injected into the cylinders. The whole system is a closed loop and keeps the engine running at a more or less constant speed over a wide load range. You can get a clue to how much power is being developed from the noise the turbo makes. The greater the load, the more fuel going in and the faster the turbo spins.
 

samuelmorris

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Right, but in the video example, doing some quick tonal analysis, the unit arrives (with minimal load as braking) with both engines running at about 1750rpm. Once stationary (with the same minimal load) they are idling at about 1100rpm. Once underway and when power is actually required, that only increases to 1500 at first (a lower power notch for initial acceleration I assume), rising to 1800 shortly thereafter (presumably maximum). The point I'm trying to make here is that arriving at the stations with the engines running at that speed doesn't make any sense.
 

sheepy1991

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Right, but in the video example, doing some quick tonal analysis, the unit arrives (with minimal load as braking) with both engines running at about 1750rpm. Once stationary (with the same minimal load) they are idling at about 1100rpm. Once underway and when power is actually required, that only increases to 1500 at first (a lower power notch for initial acceleration I assume), rising to 1800 shortly thereafter (presumably maximum). The point I'm trying to make here is that arriving at the stations with the engines running at that speed doesn't make any sense.

My best guess and this is only a guess, is that as this is a retrofit rather than clean sheet design, any ancilliary and other high load systems still need supplying when on gen set supply.

On a conventional EMU it is not just traction motors that are supplied, heating is usually 240v and cab air con systems are a high load system, the battery charger I would imagine would also need to be running at all times to supply the remaining 110v supply for the control circuits and lighting as well as charging the batteries. Which may account for the gen packs running under load when braking or at a standstill.

When on OHLE or shoe gear supply the transformer would still be supplying these systems at a standstill or under braking, so I would imagine the gen sets would be mimicking the same as if on OHLE or shoe gear supply.

I might be barking up the wrong tree but I am basing the above on similar Mk3 derived EMU’s that I work on and how their systems operate.

Units that are built as diesel units from new (be it DMU or bi-mode) will have systems set up completely different, and I cant see the 769’s having all of the original systems removed for new items as it would then have been cheaper to buy a new build unit.
 

samuelmorris

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My best guess and this is only a guess, is that as this is a retrofit rather than clean sheet design, any ancilliary and other high load systems still need supplying when on gen set supply.

On a conventional EMU it is not just traction motors that are supplied, heating is usually 240v and cab air con systems are a high load system, the battery charger I would imagine would also need to be running at all times to supply the remaining 110v supply for the control circuits and lighting as well as charging the batteries. Which may account for the gen packs running under load when braking or at a standstill.
But only when decelerating, and not when stationary?
All the reasons above are I imagine why the engines are "idling" at 1100rpm rather than the 700-800 mark used by Sprinters, that makes sense. It just doesn't make any sense to me under braking outside the context of mechanical transmission. It's not like you can use engine braking with electric traction...
 

gingertom

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Are we certain there's no regenerative braking? If there is then the motors need amps fed to the rotating windings in order to generate power in the field windings, that would have to come from the genset. Hope somebody on here could confirm that or tell me I'm talking rubbish.
 

Optom1

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Engine idle noise did vary a little stopping at Lisvane.My video of the restart(as it’s in a cutting) gives a good idea of sound levels.Happy to post it, but I dont want to get involved with “Photobucket”, any suggestions for another preferably free app?
 

samuelmorris

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Engine idle noise did vary a little stopping at Lisvane.My video of the restart(as it’s in a cutting) gives a good idea of sound levels.Happy to post it, but I dont want to get involved with “Photobucket”, any suggestions for another preferably free app?
Youtube? It's free and only requires a google account.
 

Cardiff123

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The huge irony with the 769 farce is that by the time they are finally in passenger service, 2 years late, due to the social distancing and working from home that the government is strongly encouraging, passenger numbers are going to be way down on what they normally are anyway. So the extra capacity they provide won't necessarily be needed.

On a different but not unrelated note, I can possibly see the 769s maybe staying longer with TfW now than originally intended, as due to the Covid-19 outbreak, the electrification of the Core Valley lines is now bound to be delayed.
 

Optom1

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Thanks Samual Morris,took a while to get there.769 At Lisvane on the up line (they still call it the up line as it leads to Cardiff and then London)
 
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