Richard Scott
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I thought these engines ran at a constant speed so there was always 750V for the DC bus?
There are a few electronic components between the generator and the DC bus of the unit. For a start, an engine-driven generator will produce AC that has to be rectified into DC first. As part of that process the voltage will also be normalised to the 750V required.I thought these engines ran at a constant speed so there was always 750V for the DC bus?
That's interesting that the engines are running that fast on pulling in, especially when there's absolutely no traction demand, and the ER braking should be in use. However those engines do sound as if they aren't under load, so maybe it is an issue with the demand management between the engines.Use of higher than normal (in some case full) engine speeds when decelerating or when stopped. There aren't a lot of regular runs of 769s on youtube and of course I haven't ventured down to Wales during lockdown to see for myself, but there's definitely been something odd about the way they behave up until now.
If the engines aren't under load, it shouldn't cause them to fail, - just increase their run 'times' a bit. It sounds like the tandem control system might still be a problem.Yeah exactly - they couldn't be under load as where would the energy be going? There's just something odd about how they're being managed in respect of the traction power demand. Since there aren't many videos of them other than test runs, that may well be an issue that's been fixed by now, but it does highlight that there were definitely some quirks and/or bugs with the engine control process that could explain some of the reliability issues. On pulling away, they sound much more like I'd expect from a DEMU, i.e. as you say without being thrashed at full speed from the get go, but considering how glacially slow they accelerate, one does wonder whether they are actually getting all the power from the engines they should. I wasn't expecting them to be fast, but I also wasn't expecting them to be as slow as they are.
This makes sense. It seems to me, then, that the reliability issues being discussed here are analogous to teething troubles when new units first enter service, which means I don't totally understand why people are really laying into the 769s when this sort of to be expected.Happens a lot. Reliability of 321s went through the floor when they moved to Northampton from Bletchley, for instance, though it picked back up again when they got used to them.
I wasn't suggesting them being overworked would cause the engines themselves to fail, rather that this odd behaviour may be symptomatic of a control system that has a few issues yet to be ironed out and thus, could potentially be unreliable. I may be completely wrong about that, it's just a guess.If the engines aren't under load, it shouldn't cause them to fail, - just increase their run 'times' a bit. It sounds like the tandem control system might still be a problem.
On the acceleration front, I wouldn't be surprised if the drivers are still holding back a bit, especially as units are failing more than they might expect.
Are there any on-board clips of the 769s around Hengoed. The down line gradients vary either side of 1:100 in that area, so their acceleration shouldn't be as slow as the 1:50 departures from Heath.
The 769s (and the 319s before them) have been subject to "London cast-off" abuse here since 2015, and I wouldn't be surprised if some of the criticism still relates to that.This makes sense. It seems to me, then, that the reliability issues being discussed here are analogous to teething troubles when new units first enter service, which means I don't totally understand why people are really laying into the 769s when this sort of to be expected.
It's a shame that there isn't any member of RUK who can share local knowledge of their problems. I think that we agree that there are continuing problems with the engine control system as southern442 says.I wasn't suggesting them being overworked would cause the engines themselves to fail, rather that this odd behaviour may be symptomatic of a control system that has a few issues yet to be ironed out and thus, could potentially be unreliable. I may be completely wrong about that, it's just a guess.
Yes, appreciate it's not a DC generator but point I was trying to make is that the engines run at a constant speed to ensure via the various pieces of equipment to ensure that the 750V DC bus is maintained at 750V DC.There are a few electronic components between the generator and the DC bus of the unit. For a start, an engine-driven generator will produce AC that has to be rectified into DC first. As part of that process the voltage will also be normalised to the 750V required.
Not necessary though, engine speed to a generator strictly speaking determines the frequency, not the voltage - which becomes irrelevant as it's then rectified to DC. The voltage requirements can be met as long as the engine has the power to counter the resistance it encounters when load is drawn from the generator, and of course that the generator and rectification components are capable of producing the current being drawn. It's electromagnetism and it's complicated, but a constant engine speed is only actually required when producing AC power. To avoid wasting fuel, DEMUs are capable of running at lower engine speeds when not at full power, within the engine's acceptable parameters of course.Yes, appreciate it's not a DC generator but point I was trying to make is that the engines run at a constant speed to ensure via the various pieces of equipment to ensure that the 750V DC bus is maintained at 750V DC.
I'm well aware of all of that but don't forget as soon as you take power you may end up with a significant voltage drop well below that of the 750V, fairly sure the engines on these run at constant speed whether electrical demand is low or high.Not necessary though, engine speed to a generator strictly speaking determines the frequency, not the voltage - which becomes irrelevant as it's then rectified to DC. The voltage requirements can be met as long as the engine has the power to counter the resistance it encounters when load is drawn from the generator, and of course that the generator and rectification components are capable of producing the current being drawn. It's electromagnetism and it's complicated, but a constant engine speed is only actually required when producing AC power. To avoid wasting fuel, DEMUs are capable of running at lower engine speeds when not at full power, within the engine's acceptable parameters of course.
Ok, I've not watched the video but 1050rpm sounds like a very fast idle. Other DEMUs don't have the issue of maintaining a constant voltage at the bus, was going on what I read and hoping someone had first hand experience of these units and was able to confirm or disprove what I was saying. Sounds like engines don't run at constant speed then but maybe still idle faster than most.I've no idea, it's not obvious to me based on what I've seen so far. I expect voltage drop will occur to a certain degree, after all it certainly does from the transformer of regular EMUs, but that isn't necessarily engine speed related. Engines running at constant speed based on demand is definitely not the case with these, or with many other DEMUs. Take a look at the video I linked on the previous page and jump to 6m40. You'll notice three discernable engine speeds - about 1050rpm at idle, about 1500rpm as it pulls away, then increasing to 1800rpm around the 7m00 mark.
it's not all that unusual for DEMUs, to be fair. Voyagers run around 1100 when in a 'ready' state but not under power, dropping down to 950-1000 ish when at stations.Ok, I've not watched the video but 1050rpm sounds like a very fast idle. Other DEMUs don't have the issue of maintaining a constant voltage at the bus, was going on what I read and hoping someone had first hand experience of these units and was able to confirm or disprove what I was saying. Sounds like engines don't run at constant speed then but maybe still idle faster than most.
Would the high engine speed whilst pulling up be some sort of “engine braking” whereby regenerated electricity is dissipated by spinning the engines rather than feeding into the more traditional resistor array?
I saw one fail at Pengam station. It looked like the brakes refused to come off as the train started. That said I am blissfully ignorant of the engineering on these units.It's a shame that there isn't any member of RUK who can share local knowledge of their problems. I think that we agree that there are continuing problems with the engine control system as southern442 says.
Anglia 170s MTIN MAA was 11,049 in January 2020 and 16,068 in January 2019 so already heading in the wrong direction.Possibly. But I wonder how well those 170s compared to the others when they were with their previous operator?
The Régiolis is a bi-mode train from new, and I presume has each diesel genset in the same car as the motors that it supplies. The difference with the 769 is that the train has a DC bus that is powered from transformer, 3rd rail or diesel drivern generator. As the motors are in a separate car to either of the gensets, it was necessary to feed the bus from both generaors at the same time. During development, there were problems of the two gensets hunting, i.e each one sensing that the other was providing enough power and taking a rest - and then both trying to meet the demand. This instability would be at its worst when the train was cruising as presumably a single engine alone could probably provide enough power for the traction system. The control system I believe is automatic in that it maintains the bus voltage - the driver's control acts on the traction electronics which expects the bus voltage to be mantained. It maybe that the load sharing between the two gensets needs further tweaking to prevent instability or unnecessary high tickover speeds.Possible, but wouldn't think it's a desirable thing to do particularly. I can't help but wonder if the engines are left high-revving in order to allow quicker power delivery in the event of the driver shutting off power and reapplying fairly rapidly (coasting or being signal checked for example), but this implies both incredibly sloppy control systems that don't have any visibility of speed and terrible response from the engines powering up from idle, neither of which I'm convinced would be true
It'd be interesting to compare these to the Alstom Régiolis which use the same engine, though some quick youtubing has failed to turn up any useful videos
The Régiolis is a bi-mode train from new, and I presume has each diesel genset in the same car as the motors that it supplies. The difference with the 769 is that the train has a DC bus that is powered from transformer, 3rd rail or diesel drivern generator. As the motors are in a separate car to either of the gensets, it was necessary to feed the bus from both generaors at the same time. During development, there were problems of the two gensets hunting, i.e each one sensing that the other was providing enough power and taking a rest - and then both trying to meet the demand. This instability would be at its worst when the train was cruising as presumably a single engine alone could probably provide enough power for the traction system. The control system I believe is automatic in that it maintains the bus voltage - the driver's control acts on the traction electronics which expects the bus voltage to be mantained. It maybe that the load sharing between the two gensets needs further tweaking to prevent instability or unnecessary high tickover speeds.
The overall control system is roughly equivalent to Voyagers /Meridians i.e. without clever integration of engine and TCMS seen on the 80x (what TCMS on 769s!).It's not exactly without any basis. We don't profess to be experts, but it's definitely abnormal behaviour. Should 'traction idle' really be almost full speed?
So comparing the two, a Régiolis 'unit' seems to have two or more cars with a number of gensets each supplying a a traction system*, rather than the 769 case which has two generators feeding a DC bus from which the traction system gets its power.It's a bimode from new, but it uses the same engines (and both are BM-DV although the second voltage on the régiolis is 1500v DC rather than 750) which is why it'd be interesting to see how they both compared, particularly with regards to things like idle speeds. Whilst they've got one engine per motor set, as far as I can tell these are mounted remotely to the traction equipment, so it may not be the case that they are one engine/motor (or accurately, axle!) but are bussed together á la 769s
Thinking back to the class 205 DEMUs, there was no observable change in engine speed if the compressor cut in when the train was stationary. However, when the heaters were switched on, the engine did idle at a higher RPM.It may be relevant that the air compressor on the 769 is inherited from the 319 and fed from the DC bus. During deceleration the compressor is likely to start up to recharge the main reservoir, as the driver modulates the brakes.
Therefore the engines need to be running fast enough to maintain 750V on the bus while the compressor is running.
No increase in speed when the compressor kicks in or indeed out, is because it operates at what ever the output of the engine is. Whereas the train heat requires minimum voltage to operate efficiently thus the revs being higher when in train heat only mode. When in traction and train heat mode the drivers power handle dictates the engine revs with power output being shared between traction and heating needs in an unregulated way. As its around 40 years since I trained on thumpers I cannot remember if train heat remains cut out below a certain power notch level but I don't think it does?Thinking back to the class 205 DEMUs, there was no observable change in engine speed if the compressor cut in when the train was stationary. However, when the heaters were switched on, the engine did idle at a higher RPM.
How much load does the compressor present? I have no idea but I would be surprised if it was more than 10kW, which would probably be deliverable from the alternator when the engines are running not much above tickover speed. Similarly, there is the saloon heating (20kW per 4 cars), intenal lighting (3kW per 4 cars), various electrical/electronic equipment (5kW) - all in all the total of all non-traction power I would expect to be no more than 50kW which is a small proportion of the 500+kW total capability, and probably deliverable from a relatively low input speed.It may be relevant that the air compressor on the 769 is inherited from the 319 and fed from the DC bus. During deceleration the compressor is likely to start up to recharge the main reservoir, as the driver modulates the brakes.
Therefore the engines need to be running fast enough to maintain 750V on the bus while the compressor is running.
I think that the Thumpers had DC generators and a 600V bus but apart from the lower low-speed performance of a DC generator, the principle is the same. However their engines did hunt when they were on tickover.Thinking back to the class 205 DEMUs, there was no observable change in engine speed if the compressor cut in when the train was stationary. However, when the heaters were switched on, the engine did idle at a higher RPM.