ukclique > railway

Graeme Wall (16.09.2017, 09:58)
On 15/09/2017 22:32, Clank wrote:
[..]
>  Regenerative is nice to have but neither necessary nor adequate, which
> makes me struggle to consider it 'primary', but I accept I didn't really
> define the term.


If you can't use regenerative you divert the power to resistive braking.
Clank (16.09.2017, 10:40)
On 16.09.2017 7:58 AM, Graeme Wall wrote:
>On 15/09/2017 22:32, Clank wrote:
>If you can't use regenerative you divert the power to resistive braking.


And if all the breakers on the train have tripped, can you still do this?
(Genuine question! Maybe the resistor banks are automatically switched in
line if not held off? Although I've no idea how much actual 'control' is
required to manage the rheostatic braking with modern reaction motors -
i.e. if just sticking some resistors across the motor terminals is enough
:-)).
Recliner (16.09.2017, 13:20)
Clank <clank75> wrote:
> On 16.09.2017 1:10 AM, Recliner wrote:
> Oh piffle. The maximum theoretical figure I've seen is 20% receptivity on
> DC, *maximum* practically achieved at peak 15%, and 5% or less typical at
> other times or circumstances.


"There is a very long list of ’ifs’ and this reduces the risks
significantly. This piece of work quantified the issue, and enabled
informed decisions to be taken. Lots of theoretical models of 3rd rail
receptivity say that on a line with only a few trains, such as Southern’s
South Coast service, there would be very little receptivity as compared
with an urban area. However in real life, there is ’tons of receptivity’,
perhaps because the system is so heavily interlinked that energy is
redistributed over a very large area.

Initial results with just a couple of units return economies easily in
double figures – perhaps even a 20% energy recovery. There need to be more
tests to reassure London Underground (LUL) in those areas where the
electrical supply is common.

The original plan was to physically separate LUL sections - but in the
light of practical experience, this may be revisited. The effect on voltage
may not be that high, as the network is such a big sink."



Yet again, you're forgetting that any surplus energy that can't be fed back
into the third rail will be burned off in the resistors. That's obviously
not included in the 20% energy saving (which is more than the 15% or 5% you
suggest). And, as I said, the friction brakes are still used to bring the
trains to a complete stop, as regenerative braking is not effective at low
speeds. In practice, therefore, all three types might be used in a typical
stop from speed.

And as for speeds, they're generally lower on the third rail network
anyway, so there's kinetic energy to absorb.

> Nonsense.
> I completely accept regen is great technology, and on a pure AC unit could
> indeed be used to 'derate' the traditional braking system. Also, that in
> any event it will reduce wear and tear on the breaking system and extend
> (for example) brake pad life. Quite probably very significantly indeed, I
> don't deny.
> But the notion you are proposing that the first Thameslink of the day out of
> Brighton on a warm day* requires a brake van to be attached because it
> can't stop on its own until there are enough other trains around is absurd.


Now you're being absurd.

> (* Rheostatic braking is limited by your ability to cool the resistor banks)


They're better able to handle heat than friction brakes.

> I'll accept that rheostatic/regenerative is the 'normal' source of braking
> power for the 700s, but also that the traditional friction brakes are
> entirely capable of managing the train in service at a cost of increased
> brake pad replacement.


Note that the 700s are based on the third-rail-only 450s, which also have
regenerative and rheostatic brakes, so there's a well-established precedent
for this use on DC lines.
Garden6089 (16.09.2017, 13:31)
On Saturday, 16 September 2017 11:20:15 UTC+1, Recliner wrote:
[..]
> light of practical experience, this may be revisited. The effect on voltage
> may not be that high, as the network is such a big sink."
>


AFAIAA, LUL and NR electrification systems (certainly the former Southern Region) are not connected at all. There are gaps on the Turnham Green - Gunnersbury spurs and north of East Putney to separate the two sets of tractionpower supply systems. I am not sure of the arrangements north of Queen's Park but I suspect the same applies.
Theo (16.09.2017, 13:34)
Recliner <recliner.ng> wrote:
> As an aside, I heard of an electric car that failed its MoT because the
> brake disks were so rusty. So, yes, regenerative brakes do work very well.


That's quite common. On the Prius, which now has a 20 year track record,
it's not unusual to be on the original set of brake pads after 100-200K
miles. They fail MOTs because garages see rusty discs and assume the
braking system isn't working, when it might be working but just not used
much recently (if you don't brake heavily, regen means they're only used
below 7mph).

The pads are used during emergency stops like any other car, when disc rust
isn't really a problem - doesn't matter if it causes a bit more pad wear as
long as you stop in the same distance.

Theo
R. Mark Clayton (16.09.2017, 14:28)
On Friday, 15 September 2017 17:01:51 UTC+1, Clank wrote:
> Another exciting day of Thameslink & I find myself at a Harpenden pondering
> the wheels of the train opposite me..
> I note that the motor bogies of the 700 seem to be fitted with old-school
> tread brake blocks. I can't believe these are actually a primary source of
> braking and presume all axles are fitted with disc brakes - have these been
> fitted to address the leaves-on-the-line issue long attributed in part to
> the removal of tread brakes and their side effect of cleaning shit off the
> wheels?


It is in the description somewhere. The tread brakes are to ensure the driven wheel rims are kept clear of debris, e.g. leaves, and don't slip.
R. Mark Clayton (16.09.2017, 14:33)
On Saturday, 16 September 2017 11:34:35 UTC+1, Theo wrote:
[..]
> isn't really a problem - doesn't matter if it causes a bit more pad wear as
> long as you stop in the same distance.
> Theo


Indeed if any car, even a brand new one [from first hand experience] is left standing for few days after a trip in the rain, then the discs get a surface coating of rust. First brake application after this results in stronger braking action than usual, due to the extra friction, but the rust is soon rubbed off and ABS deals with any tendency to skid.

If only one or two discs are rusty, this can indicate a seized caliper or hydraulic system problem.
R. Mark Clayton (16.09.2017, 14:36)
On Saturday, 16 September 2017 07:58:34 UTC+1, Graeme Wall wrote:
> On 15/09/2017 22:32, Clank wrote:
> If you can't use regenerative you divert the power to resistive braking.


Indeed, but you can still brake the train by throwing the energy away as heat.

Similarly many coaches and lorries have electric retarders for long descents: -
Clank (16.09.2017, 16:37)
On 16.09.2017 11:20 AM, Recliner wrote:
[..]
>Note that the 700s are based on the third-rail-only 450s, which also have
>regenerative and rheostatic brakes, so there's a well-established precedent
>for this use on DC lines.


Nobody is disputing the existence, viability or desirability of regenerative
or rheostatic braking.

You on the other hand are claiming that trains so equipped are also fitted
with "one use only, train will need to be taken or of service if used
because they'll melt into the four foot" friction brakes for emergency use
only, which is what I find highly doubtful.
Robert (16.09.2017, 17:09)
On 2017-09-16 13:37:37 +0000, Clank said:

> On 16.09.2017 11:20 AM, Recliner wrote:
> Nobody is disputing the existence, viability or desirability of regenerative
> or rheostatic braking.
> You on the other hand are claiming that trains so equipped are also fitted
> with "one use only, train will need to be taken or of service if used
> because they'll melt into the four foot" friction brakes for emergency use
> only, which is what I find highly doubtful.


All that anybody wants to know about the requirements for railway
braking is here
<https://www.rssb.co.uk/rgs/standards/GMRT2044%20Iss%204.pdf>

Basically - this is what the brakes must be able to do, how you do it
is up to you...
Basil Jet (16.09.2017, 17:15)
On 2017\09\16 11:31, Garden6089 wrote:
> AFAIAA, LUL and NR electrification systems (certainly the former Southern Region) are not connected at all. There are gaps on the Turnham Green - Gunnersbury spurs and north of East Putney to separate the two sets of traction power supply systems. I am not sure of the arrangements north of Queen's Park but I suspect the same applies.


The fourth rail actually stretches to Kilburn High Road.
Recliner (16.09.2017, 17:19)
On Sat, 16 Sep 2017 13:37:37 -0000 (UTC), Clank
<clank75> wrote:

>On 16.09.2017 11:20 AM, Recliner wrote:
>Nobody is disputing the existence, viability or desirability of regenerative
> or rheostatic braking.
>You on the other hand are claiming that trains so equipped are also fitted
> with "one use only, train will need to be taken or of service if used
> because they'll melt into the four foot" friction brakes for emergency use
> only, which is what I find highly doubtful.


So do I, which is why I never said anything of the sort. You seem to
have caught Roland's Disease; I hope you recover.
D A Stocks (16.09.2017, 17:26)
"Clank" <clank75> wrote in message
news:1p01
> Nobody is disputing the existence, viability or desirability of
> regenerative
> or rheostatic braking.
> You on the other hand are claiming that trains so equipped are also fitted
> with "one use only, train will need to be taken or of service if used
> because they'll melt into the four foot" friction brakes for emergency use
> only, which is what I find highly doubtful.


ISTR reading somewhere that an EC225 set (class 91 loco + Mk4s + DVT) is
specified such that it can do an emergency stop from full speed using the
friction brakes alone, and then continue its journey. However, it then has
to go into the depot for all the brake pads to be checked and/or changed.
Graeme Wall (16.09.2017, 17:36)
On 16/09/2017 15:09, Robert wrote:
> On 2017-09-16 13:37:37 +0000, Clank said:
> All that anybody wants to know about the requirements for railway
> braking is here
> <https://www.rssb.co.uk/rgs/standards/GMRT2044%20Iss%204.pdf>
> Basically - this is what the brakes must be able to do, how you do it is
> up to you...


Though I gather anchors on lengths of chain are frowned upon.
damduck-egg (16.09.2017, 18:11)
On Sat, 16 Sep 2017 15:36:51 +0100, Graeme Wall
<rail> wrote:

>> All that anybody wants to know about the requirements for railway
>> braking is here
>> <https://www.rssb.co.uk/rgs/standards/GMRT2044%20Iss%204.pdf>
>> Basically - this is what the brakes must be able to do, how you do it is
>> up to you...

>Though I gather anchors on lengths of chain are frowned upon.


We had a boat train thread recently, too soon for another one.

G.Harman

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