Overhead DC electrification - a potential solution to the conundrum of EMUs?

Far too big a drop in voltage, unless you refitted new motors and other systems-which rather defeats the point. The EATM have a 110v wander lead for moving trams round their depot. It works perfectly for that application, but would be painfully slow to run on.

 

Far too big a current draw too. The low voltage would quickly become very dangerous when a train was drawing power.

 

I don't see this, the power requirement goes up as the cube of speed. At a heritage line speed of 25mph the train would take 7% of the power compared with say 60mph on the main line.
I agree with the volt drop issues and the feeder point would need to be a lot closer together, but the transformers would be much smaller than those on the big railway.

 

I’m not so sure - i was interested to read of the power draw of the GE 4/4 locomotives (900V DC) on the Swiss MOB being such that they always have to use two pantographs.

Edit. It is worth note that fundamentally identical locomotives in the Rhaetian Bahn, using Swiss standard AC electrification, do not need to take the same precautions.


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Not just the top speed, it’s the acceleration to get there. From 0 - 25mph the acceleration characteristics - and therefore power requirement - could be the same as the mainline, just without then carrying on. So the point about needing high current at low voltage to deliver the required power still stands.

(And in any case, the power relationship with speed is rather more complex than a simple cubic relationship, which is primarily about air resistance. At low speed, rolling resistance snd internal friction dominate, plus the effects of gradient. So needing 7% of the power to do 25mph relative to 60 is I would suggest a marked underestimate).

Tom

 

Are you an electrical engineer?

 

Yes.

Assuming we are talking about old-school DC traction motors with resistance starters, torque (tractive effort) is related to the current and voltage is related to the speed. At standstill the motors don't have any back EMF, and starting resistors are required basically to stop the motors burning out due to excess current.

Now on running on reduced voltage, (the resistance of the starter will need to be lower) the starting current (tractive effort) will be the same, but as the speed builds up you will need to sequentially remove the resistors more quickly, than for the high voltage situation.

Eventually as the motor back EMF builds up, acceleration will fade. In my view, as this is just a heritage line, you are only running the trains for demonstration purposes. you're not trying to run a high frequency commuter service. It also pre-supposes that the ORR would allow third rail electrification, and that any heritage line would actually want to do it

 

I come back to my previous observation, which is that there are railways that run on DC and have to contend with very high power demand at low speed, so much so that they have to take specific mitigations to avoid welding the locomotives to the overhead wire.

 

It may well be the case.
Any train actually uses more power at high speed than low speed, and it is just the inefficiency of the old-school resistance control systems that require a high current when starting, I would imagine this problem occurs more with heavy freight than with EMUs. It could be part of the reason why the 1500V DC coal trains on the Woodhead line had multiple locomotives.
Incidentally the reason for the starting resistors is to limit current to prevent this sort of damage. but they also have the effect of reducing the starting tractive effort.

 

I'm thinking of the Montreux Oberland Bernois, which has relatively light trains but very steep graidents

 

That's logical, the power is the product of voltage and current so, for the same power, the lower voltage locomotive requires more current and therefore two pantographs. My understanding of this thread is that we are talking about running EMUs at low speed on heritage lines. Power is also the product of tractive effort and speed, so at low speed the power requirements are low. The problem with resistance starters is that, when starting, a lot of the energy is wasted as heat.

 

We are - but my curiosity was borne of the disproportionate impact of relatively lightweight trains on overhead equipment, in the context of the OP

 

Thanks for the clarification, though could you please expand on "disproportionate impact"? From a technical point of view, I go to work every day on Manchester Metrolink, which has very good acceleration and reasonable top speed for two and four coach trains running from a 750V DC overhead line. Judging by the sound they make they have electronic power converters, rather that switched resistance starters.

 

The OP was about using very lightweight equipment, for heritage use. Thinking of SR DC EMUs (the original use case), my question is about how little can be got away with.

 

Thanks for that, I think it comes down to how much starting current a heritage EMU takes, and from the discussion above it could be quite high, if you're planning to retain the resistance starters. Using OLE, voltage won't be a concern, although you won't want to increase this as it will mean significant modifications to the traction motors etc. As I understand it the typical conductors on the 25kV AC OLE are rated about 200A. I hope this helps.

 

The OP's question was this:

My question was, and is, whether the installations required to allow preserved vehicles to work with "only" the addition of pantographs would be as "sympathetic and reversible" as the OP hopes, without requiring radical changes to the underlying control equipment.

 

I'll leave the question of volts and amps to those who might have more practical knowledge than I do, but it mustn't be forgotten that when it comes to EMUs and the like it is not just traction motors that we are talking about, it is also brake compressors, control systems, possibly motor generator sets and lighting systems, and probably other things as well. These are unlikely to do their jobs effectively, or even work, at a substantially reduced voltage.

I cannot imagine for one moment that any heritage line is going to countenance fitting overhead DC electrification along their preserved branchline, regardless of the practicalities. I'm not even sure a museum site with a demonstration line would, given the need to alter historic vehicles. Not something museums tend to do.
Steve B

Edited for spelling

 

Mulling over this question a bit further, if you get it wrong there's a risk of a lot of hot wire falling on a platform somewhere.
I don't really want to go there, but given some of the other sentiments on this thread I expect we probably won't.

 

To be clear, I'm curious as an intellectual exercise, not in any expectation that it could ever be real.

 

Likewise, in most photographs I've seen, the DC overhead conductors have been quite chunky.