Maximum speed on 750V DC third rail routes

[rob w]

Some years ago on one of the Network Day events I travelled on a Weymouth fast ex Waterloo. I cant remember if I was expecting a REP or WES (need to check the dates) but it was a 12 CIG or VEP formation which disappointed me. I remember standing near the intermediate cabs and timing the speeds from the mile post and finding the speedo pretty accurate. in those days it was non stop to Southampton and down the bank near Winchester the ride was awful because we were well over 100mph. Any idea what the unofficial speed record is for a CIG/VEP?

 

[Deepgreen]

This?

"..

In the build up to the Eurostar build, Alsthom engineers and SNCF were
sceptical about the ability to achieve satisfactory current collection
from the third rail so we arranged an overhauled 4 REP with maximum
diameter whelsets to go flat out between Tonbridge and Ashford in a
night time possession. The result was entertaining to say the least.
The speedo was off the clock (100 mph +) and a radar gun at
Staplehurst clocked us at 117 MPH. Ride was bloody awful and I'm sure
that had the straight been longer we could have cracked 120MPH. This
sort of performance is why 4 REP's were never supposed to run alone,
always with one or more TC's.

Another point to bear in mind is that 4xx stock has a balancing speed
on the level of 80 mph or 86 mph for "greyhound" units with a second
field weakening stage, whilst the Electrostars and Desiros will
balance at 100MPH."

https://groups.google.com/d/msg/uk.railway/7uPYi1m4xY0/dGVlJPO4oyQJ

Fascinating stuff, but surely the absolute top speed would not be different for a singe REP or one with a TC or two behind, just that the accleleration to that top speed would be slower. If pressed and permitted, surely a 12car formation could have got to the same speeds on, say, the long gentle descent from milepost 31 towards Woking? On the 117mph run, was the gearing altered to permit a higher maximum?

 

[Taunton]

An REP on its own may have had such acceleration that it would compromise the calculations for level crossing AHB closing times, on the basis that it could pass the strike-in point while accelerating and get to the crossing in less than the stipulated time. I seem to recall AHB minimum closure times are calculated on the basis of a light locomotive accelerating at maximum rate.

 

[Deepgreen]

An REP on its own may have had such acceleration that it would compromise the calculations for level crossing AHB closing times, on the basis that it could pass the strike-in point while accelerating and get to the crossing in less than the stipulated time. I seem to recall AHB minimum closure times are calculated on the basis of a light locomotive accelerating at maximum rate.

With 3200hp for four coaches, against, for example, 2550hp for a light locomotive (class 71 for the 1970s), I would guess that the 71 would beat the REP up to around 90mph, depending on the gearing of course. The REP may have gone on to a higher maximum - I have no idea what the 71s could achieve.

 

[reddragon]

Only 12 REPs were needed to make 24 442s.

The remaining 3 REPs carried on running for quite a while in various forms so could have been used for testing.

 

[gimmea50anyday]

One of the 3 was destroyed in the Clapham disaster unfortunately. This scuppered reformation and refurbishment plans NSE had and so the remaining two were Initially reformed into 6 car units with one REP motor converted into a intermediate motor and using driving trailers from TC's but subsequently withdrawn as non standard

 

[edwin_m]

Fascinating stuff, but surely the absolute top speed would not be different for a singe REP or one with a TC or two behind, just that the accleleration to that top speed would be slower. If pressed and permitted, surely a 12car formation could have got to the same speeds on, say, the long gentle descent from milepost 31 towards Woking? On the 117mph run, was the gearing altered to permit a higher maximum?

As the train gets faster the amount of tractive effort available decreases, due to limitations on the total amount of power the train can supply and also because of back EMF effects in the motors. The rolling resistance and aerodynamic drag also increase with speed, and the balancing speed (on level track) is when the two forces are equal. On an infinitely long level track the train will approach, but never quite reach, the balancing speed.

If the train is longer there will be more rolling resistance and more aerodynamic drag for the same speed, but if the extra coaches are unpowered the tractive effort at that speed will remain the same. Therefore the balancing speed will be lower on level track, but the extra weight of train might possibly increase the balancing speed on a steep descending gradient.

New wheels were mentioned in the quoted post, and these are slightly larger than wheels that have been worn in service and turned on a lathe to restore profile. This may slightly increase the maximum speed available in some situations, although as it also reduces the tractive effort at a particular speed I think the balancing speed would actually reduce.

 

[Bevan Price]

In addition to this, was 90mph still the limit for REPs and double-headed 73s (+ TCs) when they were running the remaining fast services during the early years of the 442s?

Not sure of the "official" limit, but I once had 103 mph by a pair of Class 73s, and I suspect there might have been some higher speeds by others.

 

[SpacePhoenix]

Before the DRAs were brought it, would class 442s have been able to get much over 100mph on the SWML?

 

[HarleyDavidson]

Don't know the what the relevance of a DRA is on the top speed of a train, the biggest threat is the all seeing OTMR/OTDR.

 

[SpacePhoenix]

Don't know the what the relevance of a DRA is on the top speed of a train, the biggest threat is the all seeing OTMR/OTDR.

I thought that part of the DRA's functionality was to record things like what speed a train is travelling at

 

[bengley]

I thought that part of the DRA's functionality was to record things like what speed a train is travelling at

DRA is simply a switch the driver operates when stood at a red signal to remind him the signal ahead is red. It stops the unit from taking power but does nothing more.

 

[swt_passenger]

I thought that part of the DRA's functionality was to record things like what speed a train is travelling at

You are thinking of OTMR (On Train Monitoring and Recording) and presumably getting the devices totally mixed up. I thought it would be OK to duplicate the above reply because I've explained the initials...

 

[gimmea50anyday]

The 442's were built and geared accordingly for 100mph but only had limited stretches where they could attain it. Not so much a problem when they shared tracks with the 50's and HST's but 159's max 90mph had a slight effect on pathing so I understand the 100mph stretches weren't maintained and reduced to 90.

 

[SpacePhoenix]

You are thinking of OTMR (On Train Monitoring and Recording) and presumably getting the devices totally mixed up. I thought it would be OK to duplicate the above reply because I've explained the initials...

https://www.youtube.com/watch?v=5cs_ibmc1uw

From about 1:16 he mentions that the DRA disconnects the traction and does the data recording. Could it be different on a Voyager or has he got it wrong?

 

[Domh245]

He sounds a little hesitant when saying that. Remember that it stands for Driver Reminder Appliance and is commonly used at stations. The OTMR will always be recording, otherwise it'd miss things like door closing commands and the like that'd usually be done with a DRA on.

 

[edwin_m]

https://www.youtube.com/watch?v=5cs_ibmc1uw

From about 1:16 he mentions that the DRA disconnects the traction and does the data recording. Could it be different on a Voyager or has he got it wrong?

The DRA does "disconnect the traction", but only when the driver operates it, which should only happen when standing at a station facing a red signal (so the driver can't take power before de-activating the DRA, which should act as a reminder to check the signal).

It also "does the data recording" but only in the sense that its operation is recorded on one of the data channels for use in any investigation.

 

[pethadine82]

Was just wondering why cant trains run at 125mph on third rail and they can on overhead wire?

Second question is on third rail the current is 750V but how many amps
and on 25k wire how many amps, and what is the resistance of the wire?

From my physics days I recall that Power in watts is resistance * current^2, so I could work backwards!

 

[Bromley boy]

Only two sections where 100 is attained now. Sevenoaks to Tonbridge (down) and between Ashford and Westenhanger (up and down)

Linespeed along the long straight between tonbridge and ashford is 100 both up and down. Not achievable by services calling at all stations but can be and is done on empties.

 

[A0wen]

Was just wondering why cant trains run at 125mph on third rail and they can on overhead wire?

Second question is on third rail the current is 750V but how many amps
and on 25k wire how many amps, and what is the resistance of the wire?

From my physics days I recall that Power in watts is resistance * current^2, so I could work backwards!

The current isn't 750v or 25kv - that's the voltage.

The current is measured in Amps. So what you're asking is what the current (in amps) is for the two systems.

 

[Class 466]

Gradients and 90 mph limit on the up.
--- old post above --- --- new post below ---



Interesting that if you couple together two class 375/3s the total is about 266 tons. A 395 weighs about 10 tons more.

Of course it is well known the 395 is heavily restricted on the current it can draw at higher speeds on DC. This also affects longer Electrostar formations.

Also of note that there is a 50MPH TSR through Sevenoaks Tunnel for at least the next 18 Months whilst NR decide how to stop it from caving in.

 

[AM9]

Was just wondering why cant trains run at 125mph on third rail and they can on overhead wire?

Second question is on third rail the current is 750V but how many amps
and on 25k wire how many amps, and what is the resistance of the wire?

From my physics days I recall that Power in watts is resistance * current^2, so I could work backwards!

OK. The current required for a 4-car class 387 with traction rated at about 1.68MW is around 67A from a 25kV supply and 2240A from a 750VDC supply. Thus the current required is 33.3 times more on the lower voltage system for a given power output. Even if the current collecting task is shared between four or more shoes per side, contact cannot be relied on all the time, even at slow speed so each shoe would be taking say well over 1000A average current on full load. The current of under 100A per pantograph through carbon strips in contact with a copper composite wire is no great feat, even at speeds over 100mph. In the case of the latest EMUs (e.g. class 700 and 801) the whole train normally collects from a single pantograph so the current is more like 200-300A but that is no issue for the system which can deliver that at speeds up to 140mph.
For the class 700 at 100mph, the current could be 6000A which would prevent two trains in the same power section exceeding the maximum that could be supported via 3rd rail, (around 8000-10000A absolute maximum). As a consequence, most of the newer generation 750VDC EMUs have software controlled reduction of traction output in order to keep within the lower capability of the system.
Running DC EMUs at 100mph requires much higher levels of maintenance to both conductor rails and pick-up shoes. It become unsustainable at speeds much above 100mph, and as such is not suitable for modern EMU high-speed traffic services.

 

[MrB]

Also of note that there is a 50MPH TSR through Sevenoaks Tunnel for at least the next 18 Months whilst NR decide how to stop it from caving in.

Ah that's what it is, I wondered why my trains had been going through there slowly for a while.

 

[swt_passenger]

In the case of the latest EMUs (e.g. class 700 and 801) the whole train normally collects from a single pantograph so the current is more like 200-300A but that is no issue for the system which can deliver that at speeds up to 140mph.

I think someone posted a few months ago that 12 car 700s run with both pans up, but a quick search hasn't found it.

However it suddenly occurred to me that a search on youtube might be quicker and it found plenty of videos that show both pans up...

 

[apk55]

On AC systems you can not have two pantographs raised and connected together electrically as you would get a short at neutral sections. On trains such as the pendelino there is logic to prevent two being raised at once.

 

[edwin_m]

On AC systems you can not have two pantographs raised and connected together electrically as you would get a short at neutral sections. On trains such as the pendelino there is logic to prevent two being raised at once.

True, but the 700 may be (I don't know for sure) designed as two separate single-cabbed EMUs permanently coupled together. So all the traction motors in one half of the train are powered from one pantograph with no power feed between the halves.

 

[Bald Rick]

True, but the 700 may be (I don't know for sure) designed as two separate single-cabbed EMUs permanently coupled together. So all the traction motors in one half of the train are powered from one pantograph with no power feed between the halves.

Correct.

 

[AM9]

On AC systems you can not have two pantographs raised and connected together electrically as you would get a short at neutral sections. On trains such as the pendelino there is logic to prevent two being raised at once.

OK, well even if that is so, the current draw per 12-car train is unlikely to be above 300A which is not a problem for a standard BR/BW High Speed pantograph. Even if 8000hp locos are introduced, the resulting 6MW through a single pantograph would only draw about 250A.
A 750VDC line would need an exclusive feed per loco to provide the nearly 8000A required.

 

[notadriver]

Linespeed along the long straight between tonbridge and ashford is 100 both up and down. Not achievable by services calling at all stations but can be and is done on empties.

Yes but it's not something passengers get to experience hence my original post about the usual attainable speeds in service.

Having said that on Sunday's services omitting Pluckley will reach 100 mph in both directions. The up is better as Headcorn is signalled with flashing aspects for swift approaches.


Sent from my iPhone using Tapatalk

 

[WatcherZero]

To put it all more succinctly third rail cannot physically supply enough juice for two trains to operate much over 100mph at the same time.