In message <ukkstc$3fmr0$1@dont-email.me>, at 15:57:33 on Mon, 4 Dec
2023, Muttley@dastardlyhq.com remarked:

>I was standing at farringdon today and watched a Thameslink train come into the
>platform. Under the footbridge the 25KV wire looked like it was mere inches
>above the roof of the train.

Air is an insulator, roughly 25kV per inch. TVs with CRTs commonly had
30kV inside them. Yes droopy wires can reduce the clearance, but often
there are measures in place to reduce that possibility.

Meanwhile, on some OHL routes locos aren't allowed because of the power
drain, rather than the clearance.
--
Roland Perry

<Muttley@dastardlyhq.com> wrote:
> On Tue, 5 Dec 2023 17:53:55 +0100
> Bob <bob@domain.com> wrote:
>> On 05.12.23 17:42, Muttley@dastardlyhq.com wrote:
>>> On Tue, 05 Dec 2023 16:24:36 GMT
>>> Recliner <recliner.usenet@gmail.com> wrote:
>>>> The 3-phase system would have been really hard to squeeze into a tunnel,
>>>> though, of course, the Jungfraubahn does exactly that:
>>>>
>>>>
>> <https://www.flickr.com/photos/recliner/7683489358/in/album-72157630773147320/l
>>
>>>> ightbox/>
>>>
>>> I wonder what the reasoning was that this was a better solution than a single
>>
>>> wire with DC or single phase AC.
>>
>> At the time of construction of the Jungraubahn (and other 3 phase
>> systems like that proposed for the Met), single phase AC was difficult
>> to work with in a railway environment due to the lack of rail compatible
>> rectifiers at that time. DC has its own limitations that were, and
>> remain, quite restrictive.
>
> Could they not just tap a single phase?
>
>

You need to do something with the other two, not easy in the early 20th
Century before grid systems became established. Its not that easy now to
provide for a heavy user of a single phase such as a railway without
causing balance problems in the supply hence substations for normal AC
electrification cannot just be placed and connected up at any point which
might look convenient.

GH

Recliner <recliner.usenet@gmail.com> wrote:

>
> The 3-phase system would have been really hard to squeeze into a tunnel,
> though, of course, the Jungfraubahn does exactly that:
>
> <https://www.flickr.com/photos/recliner/7683489358/in/album-72157630773147320/lightbox/>
>
>

Although it didn’t use 3phase the 1st line of the Budapest Metro was
already up and running with clearances and a loading gauge that has
similar restrictions to Londons bored tube lines,
they managed to make it overhead power supply using what was basically a
live rail fixed to the tunnel roof what we probably now call bar conductor.
This photo of one of the old cars that still runs shows the arrangement.
<https://commons.wikimedia.org/wiki/File:Museum_metro_line_1_Budapest_03.JPG>

I would think the MET would have used something similar as the Ganz
engineers would have been well aware of it.
Time and the cloak of Communism have obscured how Hungarians were at the
forefront of electrical development at the end of 19th -early 20th
centuries , the engineers who developed the 3 phase electrification soon
realised its limitations and went on to develop single phase supply
converting it to 3 phase on board the Loco.
This bloke

<https://en.wikipedia.org/wiki/Kálmán_Kandó>

is probably unheard of by most British Enthusiasts but his pioneering work
lead to the AC systems of today and arguably they will be around longer
than the era of steam was whose big names like Gresley , Churchyard ,
Stanier, Bulleid are regarded with reverence.

GH

On 05/12/2023 20:33, Marland wrote:
> This bloke
>
> <https://en.wikipedia.org/wiki/Kálmán_Kandó>
>
> is probably unheard of by most British Enthusiasts but his pioneering work
> lead to the AC systems of today and arguably they will be around longer
> than the era of steam was whose big names like Gresley , Churchyard,

Churchward

> Stanier, Bulleid are regarded with reverence.
>

--
Graeme Wall
This account not read.

On Tue, 5 Dec 2023 18:53:11 +0100
Bob <bob@domain.com> wrote:
>On 05.12.23 18:03, Muttley@dastardlyhq.com wrote:
>> On Tue, 5 Dec 2023 17:53:55 +0100
>> Bob <bob@domain.com> wrote:
>>> On 05.12.23 17:42, Muttley@dastardlyhq.com wrote:
>>>> On Tue, 05 Dec 2023 16:24:36 GMT
>>>> Recliner <recliner.usenet@gmail.com> wrote:
>>>>> The 3-phase system would have been really hard to squeeze into a tunnel,
>>>>> though, of course, the Jungfraubahn does exactly that:
>>>>>
>>>>>
>>>
><https://www.flickr.com/photos/recliner/7683489358/in/album-72157630773147320/l
>
>>>
>>>>> ightbox/>
>>>>
>>>> I wonder what the reasoning was that this was a better solution than a
>single
>>>
>>>> wire with DC or single phase AC.
>>>
>>> At the time of construction of the Jungraubahn (and other 3 phase
>>> systems like that proposed for the Met), single phase AC was difficult
>>> to work with in a railway environment due to the lack of rail compatible
>>> rectifiers at that time. DC has its own limitations that were, and
>>> remain, quite restrictive.
>>
>> Could they not just tap a single phase?
>
>The problem wasn't getting a single phase supply, the problem was
>driving a train with a single phase supply. Single phase AC is
>challenging to do useful things with when it comes to motors.

Millions of vacuum cleaners and lawn mowers might disagree!

On Tue, 5 Dec 2023 19:15:46 +0000
Roland Perry <roland@perry.uk> wrote:
>In message <ukkstc$3fmr0$1@dont-email.me>, at 15:57:33 on Mon, 4 Dec
>2023, Muttley@dastardlyhq.com remarked:
>
>>I was standing at farringdon today and watched a Thameslink train come into
>the
>>platform. Under the footbridge the 25KV wire looked like it was mere inches
>>above the roof of the train.
>
>Air is an insulator, roughly 25kV per inch. TVs with CRTs commonly had

Though I imagine that varies depending on humidity.

>30kV inside them. Yes droopy wires can reduce the clearance, but often

You could get a nice lightshow with some old CRTs in the dark putting a pin
close to the screen.

On 5 Dec 2023 19:16:43 GMT
Marland <gemehabal@btinternet.co.uk> wrote:
><Muttley@dastardlyhq.com> wrote:
>> Could they not just tap a single phase?
>>
>>
>
>You need to do something with the other two, not easy in the early 20th
>Century before grid systems became established. Its not that easy now to
>provide for a heavy user of a single phase such as a railway without
>causing balance problems in the supply hence substations for normal AC
>electrification cannot just be placed and connected up at any point which
>might look convenient.

Fair enough. But why then did most railway companies in the UK simply
rectify to DC and put that through a wire or conductor rail back in the 19th
century if it was such an issue? Seems to be this german (austrian?) railway
made a rod for its own back using 3 phase.

In message <ukpffr$mo8c$1@dont-email.me>, at 09:39:07 on Wed, 6 Dec
2023, Muttley@dastardlyhq.com remarked:
>On Tue, 5 Dec 2023 19:15:46 +0000
>Roland Perry <roland@perry.uk> wrote:
>>In message <ukkstc$3fmr0$1@dont-email.me>, at 15:57:33 on Mon, 4 Dec
>>2023, Muttley@dastardlyhq.com remarked:
>>
>>>I was standing at farringdon today and watched a Thameslink train
>>>come into the platform. Under the footbridge the 25KV wire looked
>>>like it was mere inches above the roof of the train.
>>
>>Air is an insulator, roughly 25kV per inch. TVs with CRTs commonly had
>
>Though I imagine that varies depending on humidity.

Not as much as you suppose. And to cause grief you need an actual arc,
not just the occasional crackle.

>>30kV inside them. Yes droopy wires can reduce the clearance, but often
>
>You could get a nice lightshow with some old CRTs in the dark putting a pin
>close to the screen.

--
Roland Perry

On Tue, 5 Dec 2023 19:15:46 +0000, Roland Perry <roland@perry.uk>
wrote:

>In message <ukkstc$3fmr0$1@dont-email.me>, at 15:57:33 on Mon, 4 Dec
>2023, Muttley@dastardlyhq.com remarked:
>
>>I was standing at farringdon today and watched a Thameslink train come into the
>>platform. Under the footbridge the 25KV wire looked like it was mere inches
>>above the roof of the train.
>
>Air is an insulator, roughly 25kV per inch. TVs with CRTs commonly had
>30kV inside them. Yes droopy wires can reduce the clearance, but often
>there are measures in place to reduce that possibility.
>
>Meanwhile, on some OHL routes locos aren't allowed because of the power
>drain, rather than the clearance.

Would they be allowed at night when passenger service (sleepers apart)
has ended?

<Muttley@dastardlyhq.com> wrote:
> On 5 Dec 2023 19:16:43 GMT
> Marland <gemehabal@btinternet.co.uk> wrote:
>> <Muttley@dastardlyhq.com> wrote:
>>> Could they not just tap a single phase?
>>>
>>>
>>
>> You need to do something with the other two, not easy in the early 20th
>> Century before grid systems became established. Its not that easy now to
>> provide for a heavy user of a single phase such as a railway without
>> causing balance problems in the supply hence substations for normal AC
>> electrification cannot just be placed and connected up at any point which
>> might look convenient.
>
> Fair enough. But why then did most railway companies in the UK simply
> rectify to DC and put that through a wire or conductor rail back in the 19th
> century if it was such an issue? Seems to be this german (austrian?) railway
> made a rod for its own back using 3 phase.
>
>

There were few significant railways using 3 phase. It made the motors
easier, and the load on the distribution network easier, but the OHLE was
very much more complicated.

Early systems tended to use DC, with the then hard bit, the rectification,
done lineside, or low frequency AC. Much of the German (and others) system
still runs at 16 2/3 Hz, with all the complications of providing a separate
traction supply network.

This posting from Quora is useful:

It’s a “historical accident”. That is, it was a good idea when it was
chosen but no longer. And there’s now too much existing investment in the
oddball low frequency AC systems to rip them all out and start over.

Variable speed motors are required for any form of electric traction, and
in the early days there was only the brush-type motor. Tesla’s AC induction
motor, though simpler and more rugged, could only run efficiently at a
fixed speed (or one of a small set of fixed speeds) determined by the AC
supply frequency.

Brush-type motors have separate field and armature windings. Although they
run best on DC, they can also run on AC. (They’re sometimes called univeral
motors for this reason.) The direction of rotation is determined by the
relative polarity of the voltage fed to the field and armature windings so
the motor will continue to turn in the same direction when the AC polarity
changes. To reverse the motor, you reverse the polarity of the field but
not the armature, or vice versa.

But there’s a complication. Motor windings (especially on the stator of a
parallel-wound motor) are essentially big inductors. Their impedance to the
flow of AC current increases with frequency. Small brush-type motors (e.g.,
in power saws or vacuum cleaners) can handle 50 or 60 Hz but not the large
motors in an electric locomotive. And that’s how we got unusually low
frequencies like 16 2/3 Hz in Scandinavia and the German-speaking parts of
Europe and 25 Hz in the southern half of the Amtrak Northeast Corridor in
the USA.

The early need for low AC frequencies didn’t last long. With the mercury
vapor rectifier and later the solid-state rectifier, a locomotive could
convert any AC frequency to DC for its motors. Today, with high power
transistor switches, locomotives can finally use AC induction motors
powered with variable frequency AC drives from DC and AC supplies of any
frequency, so new lines generally use either DC or the local grid frequency
(50 or 60 Hz).

On Wed, 6 Dec 2023 10:11:12 -0000 (UTC)
Tweed <usenet.tweed@gmail.com> wrote:
>Early systems tended to use DC, with the then hard bit, the rectification,
>done lineside, or low frequency AC. Much of the German (and others) system
>still runs at 16 2/3 Hz, with all the complications of providing a separate
>traction supply network.

Surely they could do as the french and belgies are doing converting their DC
OHLE to 25KV AC and buy dual voltage/freq trains and the gradually switch the
old 16Hz lines over once older stock has been scrapped?

<Muttley@dastardlyhq.com> wrote:
> On Wed, 6 Dec 2023 10:11:12 -0000 (UTC)
> Tweed <usenet.tweed@gmail.com> wrote:
>> Early systems tended to use DC, with the then hard bit, the rectification,
>> done lineside, or low frequency AC. Much of the German (and others) system
>> still runs at 16 2/3 Hz, with all the complications of providing a separate
>> traction supply network.
>
> Surely they could do as the french and belgies are doing converting their DC
> OHLE to 25KV AC and buy dual voltage/freq trains and the gradually switch the
> old 16Hz lines over once older stock has been scrapped?
>
>
>

It’s the same arguments about converting 3rd rail DC to overhead here -
cost. In the world of modern electronics it’s probably easier to use
converter stations from the public 50 Hz supply. Perhaps one day every item
of electric traction will be 50 Hz 25kV capable, but I imagine that’s a
long way in the future. Lots of NL is still running on 1500V DC overhead.

We are now starting to use fancy electronic substations between the public
supply and the railway feed, primarily to take a balanced load off all
three phases despite feeding an unbalanced single phase load.

On 06.12.23 10:30, Muttley@dastardlyhq.com wrote:
> On Tue, 5 Dec 2023 18:53:11 +0100
> Bob <bob@domain.com> wrote:
>> On 05.12.23 18:03, Muttley@dastardlyhq.com wrote:
>>> On Tue, 5 Dec 2023 17:53:55 +0100
>>> Bob <bob@domain.com> wrote:
>>>> On 05.12.23 17:42, Muttley@dastardlyhq.com wrote:
>>>>> On Tue, 05 Dec 2023 16:24:36 GMT
>>>>> Recliner <recliner.usenet@gmail.com> wrote:
>>>>>> The 3-phase system would have been really hard to squeeze into a tunnel,
>>>>>> though, of course, the Jungfraubahn does exactly that:
>>>>>>
>>>>>>
>>>>
>> <https://www.flickr.com/photos/recliner/7683489358/in/album-72157630773147320/l
>>
>>>>
>>>>>> ightbox/>
>>>>>
>>>>> I wonder what the reasoning was that this was a better solution than a
>> single
>>>>
>>>>> wire with DC or single phase AC.
>>>>
>>>> At the time of construction of the Jungraubahn (and other 3 phase
>>>> systems like that proposed for the Met), single phase AC was difficult
>>>> to work with in a railway environment due to the lack of rail compatible
>>>> rectifiers at that time. DC has its own limitations that were, and
>>>> remain, quite restrictive.
>>>
>>> Could they not just tap a single phase?
>>
>> The problem wasn't getting a single phase supply, the problem was
>> driving a train with a single phase supply. Single phase AC is
>> challenging to do useful things with when it comes to motors.
>
> Millions of vacuum cleaners and lawn mowers might disagree!

Low power, low starting torque demands, fixed speed. Those limitations
are not a problem for those applications, but are a huge problem for
railway applicatoins.

Robin

On 06.12.23 11:34, Muttley@dastardlyhq.com wrote:
> On Wed, 6 Dec 2023 10:11:12 -0000 (UTC)
> Tweed <usenet.tweed@gmail.com> wrote:
>> Early systems tended to use DC, with the then hard bit, the rectification,
>> done lineside, or low frequency AC. Much of the German (and others) system
>> still runs at 16 2/3 Hz, with all the complications of providing a separate
>> traction supply network.
>
> Surely they could do as the french and belgies are doing converting their DC
> OHLE to 25KV AC and buy dual voltage/freq trains and the gradually switch the
> old 16Hz lines over once older stock has been scrapped?

There isn't much active conversion going on, it's more a case that lines
being newly electrified are beind done to the 25 kV standard (at least
in the case of France). DC electrification is very expensive in terms of
infrastructure needed on and around the track, making DC electrification
schemes significantly more expensive to construct. The costs associated
with the 16.7 Hz system are largely already sunk: providing the
generation/conversion and transmission backbone to support the railways.
As a high voltage AC electrification system, the 15 kV system, on
direct-to-the-railway basis is much closer in cost to the 25 kV 50 Hz
system, and does not have the kinds of power limitations that make DC
systems unsuited to higher speed running or heavy freight haulage (300
km/h passenger lines and very heavy freight haulage under the 15 kV
system have been routine for many years). In essence, there is not a
significant cost saving to operate a 25 kV 50 Hz rather than 15 kV 16.7
Hz system, and a very significant cost involved in conversion.

Robin

On 6 Dec 2023 09:30, Muttley@dastardlyhq.com wrote:
> On Tue, 5 Dec 2023 18:53:11 +0100
> Bob <bob@domain.com> wrote:
>>On 05.12.23 18:03, Muttley@dastardlyhq.com wrote:
>>> On Tue, 5 Dec 2023 17:53:55 +0100
>>> Bob <bob@domain.com> wrote:
>>>> On 05.12.23 17:42, Muttley@dastardlyhq.com wrote:
>>>>> On Tue, 05 Dec 2023 16:24:36 GMT
>>>>> Recliner <recliner.usenet@gmail.com> wrote:
>>>>>> The 3-phase system would have been really hard to squeeze into a tunnel,
>>>>>> though, of course, the Jungfraubahn does exactly that:
>>>>>>
>>>>>>
>>>>
>><https://www.flickr.com/photos/recliner/7683489358/in/album-72157630773147320/l
>>
>>>>
>>>>>> ightbox/>
>>>>>
>>>>> I wonder what the reasoning was that this was a better solution than a
>>single
>>>>
>>>>> wire with DC or single phase AC.
>>>>
>>>> At the time of construction of the Jungraubahn (and other 3 phase
>>>> systems like that proposed for the Met), single phase AC was difficult
>>>> to work with in a railway environment due to the lack of rail compatible
>>>> rectifiers at that time. DC has its own limitations that were, and
>>>> remain, quite restrictive.
>>>
>>> Could they not just tap a single phase?
>>
>>The problem wasn't getting a single phase supply, the problem was
>>driving a train with a single phase supply. Single phase AC is
>>challenging to do useful things with when it comes to motors.
>
> Millions of vacuum cleaners and lawn mowers might disagree!

And, indeed, thousands of air conditioning compressors

<Muttley@dastardlyhq.com> wrote:
> On Tue, 5 Dec 2023 18:53:11 +0100
> Bob <bob@domain.com> wrote:
>> On 05.12.23 18:03, Muttley@dastardlyhq.com wrote:
>>> On Tue, 5 Dec 2023 17:53:55 +0100
>>> Bob <bob@domain.com> wrote:
>>>> On 05.12.23 17:42, Muttley@dastardlyhq.com wrote:
>>>>> On Tue, 05 Dec 2023 16:24:36 GMT
>>>>> Recliner <recliner.usenet@gmail.com> wrote:
>>>>>> The 3-phase system would have been really hard to squeeze into a tunnel,
>>>>>> though, of course, the Jungfraubahn does exactly that:
>>>>>>
>>>>>>
>>>>
>> <https://www.flickr.com/photos/recliner/7683489358/in/album-72157630773147320/l
>>
>>>>
>>>>>> ightbox/>
>>>>>
>>>>> I wonder what the reasoning was that this was a better solution than a
>> single
>>>>
>>>>> wire with DC or single phase AC.
>>>>
>>>> At the time of construction of the Jungraubahn (and other 3 phase
>>>> systems like that proposed for the Met), single phase AC was difficult
>>>> to work with in a railway environment due to the lack of rail compatible
>>>> rectifiers at that time. DC has its own limitations that were, and
>>>> remain, quite restrictive.
>>>
>>> Could they not just tap a single phase?
>>
>> The problem wasn't getting a single phase supply, the problem was
>> driving a train with a single phase supply. Single phase AC is
>> challenging to do useful things with when it comes to motors.
>
> Millions of vacuum cleaners and lawn mowers might disagree!
>
>

Until recent decades most of those ran at a fixed speed with no need to run
backwards,
not a mode that would suit a locomotive or other traction unit.
I’m old enough to remember that most vacuum cleaners like my gran had
also had to work on DC mains supplies so were equipped with what were
called universal motors which are basically a DC series motor with a few
more bits around the windings , they depend on the commutator and brushes
to work well but the sparking means the latter wear quickly.
Domestic equipment really has an easy life compared to industrial kit often
only running for an hour or less a day so such a vacuum cleaner could last
years before a set of brushes needed changing but
in industrial use servicing motors and cleaning commutators was a regular
service task.
There was hardly a town without a small workshop doing such work and
rewinds, like TV and Radio repair shops progress ( and associated labour
costs ) has seen them become rare.

GH

<Muttley@dastardlyhq.com> wrote:
> On 5 Dec 2023 19:16:43 GMT
> Marland <gemehabal@btinternet.co.uk> wrote:
>> <Muttley@dastardlyhq.com> wrote:
>>> Could they not just tap a single phase?
>>>
>>>
>>
>> You need to do something with the other two, not easy in the early 20th
>> Century before grid systems became established. Its not that easy now to
>> provide for a heavy user of a single phase such as a railway without
>> causing balance problems in the supply hence substations for normal AC
>> electrification cannot just be placed and connected up at any point which
>> might look convenient.
>
> Fair enough. But why then did most railway companies in the UK simply
> rectify to DC and put that through a wire or conductor rail back in the 19th
> century if it was such an issue?

The rotary converters ,later rectifiers in the substations were connected
across all 3 phases so the load was taken equally across them so no balance
issues.

Seems to be this german (austrian?) railway
> made a rod for its own back using 3 phase.
>
>
It was early days ,everyone was pioneering.

GH

Rupert Moss-Eccardt <nin@moss-eccardt.com> wrote:
> On 6 Dec 2023 09:30, Muttley@dastardlyhq.com wrote:
>> On Tue, 5 Dec 2023 18:53:11 +0100
>> Bob <bob@domain.com> wrote:
>>> On 05.12.23 18:03, Muttley@dastardlyhq.com wrote:
>>>> On Tue, 5 Dec 2023 17:53:55 +0100
>>>> Bob <bob@domain.com> wrote:
>>>>> On 05.12.23 17:42, Muttley@dastardlyhq.com wrote:
>>>>>> On Tue, 05 Dec 2023 16:24:36 GMT
>>>>>> Recliner <recliner.usenet@gmail.com> wrote:
>>>>>>> The 3-phase system would have been really hard to squeeze into a tunnel,
>>>>>>> though, of course, the Jungfraubahn does exactly that:
>>>>>>>
>>>>>>>
>>>>>
>>> <https://www.flickr.com/photos/recliner/7683489358/in/album-72157630773147320/l
>>>
>>>>>
>>>>>>> ightbox/>
>>>>>>
>>>>>> I wonder what the reasoning was that this was a better solution than a
>>> single
>>>>>
>>>>>> wire with DC or single phase AC.
>>>>>
>>>>> At the time of construction of the Jungraubahn (and other 3 phase
>>>>> systems like that proposed for the Met), single phase AC was difficult
>>>>> to work with in a railway environment due to the lack of rail compatible
>>>>> rectifiers at that time. DC has its own limitations that were, and
>>>>> remain, quite restrictive.
>>>>
>>>> Could they not just tap a single phase?
>>>
>>> The problem wasn't getting a single phase supply, the problem was
>>> driving a train with a single phase supply. Single phase AC is
>>> challenging to do useful things with when it comes to motors.
>>
>> Millions of vacuum cleaners and lawn mowers might disagree!
>
> And, indeed, thousands of air conditioning compressors
>
>

Though many modern versions of those and their near relatives Heat Pumps
and Freezers now have an Inverter inside so it is easier for their
software controlled operation to match running to demand
and throttle them back to get more efficient operation rather than Run -
Stop-Run -Stop with all those wasteful compressor start ups.

GH

Marland <gemehabal@btinternet.co.uk> wrote:
> <Muttley@dastardlyhq.com> wrote:
>> On Tue, 5 Dec 2023 18:53:11 +0100
>> Bob <bob@domain.com> wrote:
>>> On 05.12.23 18:03, Muttley@dastardlyhq.com wrote:
>>>> On Tue, 5 Dec 2023 17:53:55 +0100
>>>> Bob <bob@domain.com> wrote:
>>>>> On 05.12.23 17:42, Muttley@dastardlyhq.com wrote:
>>>>>> On Tue, 05 Dec 2023 16:24:36 GMT
>>>>>> Recliner <recliner.usenet@gmail.com> wrote:
>>>>>>> The 3-phase system would have been really hard to squeeze into a tunnel,
>>>>>>> though, of course, the Jungfraubahn does exactly that:
>>>>>>>
>>>>>>>
>>>>>
>>> <https://www.flickr.com/photos/recliner/7683489358/in/album-72157630773147320/l
>>>
>>>>>
>>>>>>> ightbox/>
>>>>>>
>>>>>> I wonder what the reasoning was that this was a better solution than a
>>> single
>>>>>
>>>>>> wire with DC or single phase AC.
>>>>>
>>>>> At the time of construction of the Jungraubahn (and other 3 phase
>>>>> systems like that proposed for the Met), single phase AC was difficult
>>>>> to work with in a railway environment due to the lack of rail compatible
>>>>> rectifiers at that time. DC has its own limitations that were, and
>>>>> remain, quite restrictive.
>>>>
>>>> Could they not just tap a single phase?
>>>
>>> The problem wasn't getting a single phase supply, the problem was
>>> driving a train with a single phase supply. Single phase AC is
>>> challenging to do useful things with when it comes to motors.
>>
>> Millions of vacuum cleaners and lawn mowers might disagree!
>>
>>
>
> Until recent decades most of those ran at a fixed speed with no need to run
> backwards,
> not a mode that would suit a locomotive or other traction unit.
> I’m old enough to remember that most vacuum cleaners like my gran had
> also had to work on DC mains supplies so were equipped with what were
> called universal motors which are basically a DC series motor with a few
> more bits around the windings , they depend on the commutator and brushes
> to work well but the sparking means the latter wear quickly.
> Domestic equipment really has an easy life compared to industrial kit often
> only running for an hour or less a day so such a vacuum cleaner could last
> years before a set of brushes needed changing but
> in industrial use servicing motors and cleaning commutators was a regular
> service task.
> There was hardly a town without a small workshop doing such work and
> rewinds, like TV and Radio repair shops progress ( and associated labour
> costs ) has seen them become rare.

Universal motors is how early single phase AC railways operated. The
problem is at the high power levels involved, the sparking at the
commutator becomes a huge problem. The sparking intensity is related to the
AC frequency being used due to inductance effects. The solution therefore
is low frequency AC. Hence the CH/De/At etc 16.7 Hz system. On board
rectifiers solved that problem and when they became viable in the late
1950s for on board usage on locomotives, 25 kV 50 Hz single phase become
possible.

Robin

On Wed, 6 Dec 2023 12:50:08 +0100
Bob <bob@domain.com> wrote:
>As a high voltage AC electrification system, the 15 kV system, on
>direct-to-the-railway basis is much closer in cost to the 25 kV 50 Hz
>system, and does not have the kinds of power limitations that make DC
>systems unsuited to higher speed running or heavy freight haulage (300

Actually DC is actually better for high voltage for transfering power as
there are lower losses. National Grid has built some DC interconnects to
other countries for example. These days if building a railway from scratch
you could probably do worse than 25KV *DC* given converting it down to lower
DC voltages is now a solved problem with modern electronics. Of course you
wouldn't get the nice hum for the catenary in the rain :)

<Muttley@dastardlyhq.com> wrote:
> On Wed, 6 Dec 2023 12:50:08 +0100
> Bob <bob@domain.com> wrote:
>> As a high voltage AC electrification system, the 15 kV system, on
>> direct-to-the-railway basis is much closer in cost to the 25 kV 50 Hz
>> system, and does not have the kinds of power limitations that make DC
>> systems unsuited to higher speed running or heavy freight haulage (300
>
> Actually DC is actually better for high voltage for transfering power as
> there are lower losses. National Grid has built some DC interconnects to
> other countries for example. These days if building a railway from scratch
> you could probably do worse than 25KV *DC* given converting it down to lower
> DC voltages is now a solved problem with modern electronics. Of course you
> wouldn't get the nice hum for the catenary in the rain :)
>
>

Swings and roundabouts for “better”. See here

https://www.cencepower.com/blog-posts/hvdc-transmission-systems#:~:text=Therefore%2C%20DC%20power%20is%20inherently,underwater%20it%27s%20about%2024%20%2D%2050km.

One of the main reasons for our undersea cables being DC is it removes the
need to synchronise our grid frequency to continental Europe.

On 06.12.23 17:19, Muttley@dastardlyhq.com wrote:
> On Wed, 6 Dec 2023 12:50:08 +0100
> Bob <bob@domain.com> wrote:
>> As a high voltage AC electrification system, the 15 kV system, on
>> direct-to-the-railway basis is much closer in cost to the 25 kV 50 Hz
>> system, and does not have the kinds of power limitations that make DC
>> systems unsuited to higher speed running or heavy freight haulage (300
>
> Actually DC is actually better for high voltage for transfering power as
> there are lower losses. National Grid has built some DC interconnects to
> other countries for example. These days if building a railway from scratch
> you could probably do worse than 25KV *DC* given converting it down to lower
> DC voltages is now a solved problem with modern electronics. Of course you
> wouldn't get the nice hum for the catenary in the rain :)

High voltage DC has some very specific use cases where it makes sense,
but also comes with some very specific drawbacks that makes it unsuited
for railway traction purposes.

DC has lower losses for long distances where inductance and capacitance
pose a challenge to transmission. Making use of high voltage DC comes
with the challenge of producing a high voltage DC supply, and of doing
something useful with a high voltage DC supply.

Getting mechanical work out of electricity relies on the interaction
between electric current and magnetic fields. The forces involved depend
on the current flowing. Creating coils of wire increases the magnitude
of the forces by the number of turns, but the lower the current, the
more turns in the coils are required. As electrical power is the current
multiplied by the voltage drop, if you start with a very high voltage,
for a given power there is a very low current.

Having a high voltage also creates a significant challenge on
insulators. If you attempt to transition from a bare conductor to an
insulated cable with an earth potential screen (needed to be able to
safely have it near conductive objects at near earth potential like the
frames of a railway vehicle) starts to become challenging at around 10
kV, and by the time you get to tens of kV, you have to design equipment
quite carefully in terms of switchgear, insulation terminations and joints.

These two factors combined means that to create a controlable electric
motor within a railway vehicle, the voltage that the motor itself
utilises can not readily exceed single digit kV. This is why railway DC
electrification at above about 3 kV is practically unheard of.

To make a high voltage DC railway system viable would involve on board
power electronics to convert high voltage DC to high voltage AC, then
step that down to low voltage AC in a power transformer, rectify that
back to DC, invert it to VVVF 3 phase AC and feed that to a traction motor.

Likewise a substation feeding a high voltage DC installation would
involve taking an AC supply from the grid, transforming it to the right
volage, then rectifying it to high voltage DC.

Each conversion step incurs losses. While the OHLE itself, if fed with
AC, will experience inductive and capacitative losses not present with
DC, for the distances and voltages that are relevant to a railway
traction environment, these are less than the losses of the additional
conversion steps.

In short, to use high voltage DC, there is extra complexity in the
supply and on the vehicles, and associated losses, compared with mains
frequency AC, and their efficiency penalties are greater than the very
small gains from DC transmission. Power transmission lines operate at
hundreds of kV and thousands of A. The electrical effects that are
important in that range are not important at the tens of kV and tens of
A that railway electrification involves.

Robin

On Wed, 6 Dec 2023 18:00:37 +0100
Bob <bob@domain.com> wrote:
>These two factors combined means that to create a controlable electric
>motor within a railway vehicle, the voltage that the motor itself
>utilises can not readily exceed single digit kV. This is why railway DC
>electrification at above about 3 kV is practically unheard of.
>
>To make a high voltage DC railway system viable would involve on board
>power electronics to convert high voltage DC to high voltage AC, then
>step that down to low voltage AC in a power transformer, rectify that
>back to DC, invert it to VVVF 3 phase AC and feed that to a traction motor.

Sure, but you're forgetting that you could put inverters in series. If
each motor on a loco (probably 4) has its own inverter then you can immediately
divide whatever the DC input is by 4. For multiple units it would obviously
be more complicated to do the same but probably not insurmountable.

On 06.12.23 18:16, Muttley@dastardlyhq.com wrote:
> On Wed, 6 Dec 2023 18:00:37 +0100
> Bob <bob@domain.com> wrote:
>> These two factors combined means that to create a controlable electric
>> motor within a railway vehicle, the voltage that the motor itself
>> utilises can not readily exceed single digit kV. This is why railway DC
>> electrification at above about 3 kV is practically unheard of.
>>
>> To make a high voltage DC railway system viable would involve on board
>> power electronics to convert high voltage DC to high voltage AC, then
>> step that down to low voltage AC in a power transformer, rectify that
>> back to DC, invert it to VVVF 3 phase AC and feed that to a traction motor.
>
> Sure, but you're forgetting that you could put inverters in series. If
> each motor on a loco (probably 4) has its own inverter then you can immediately
> divide whatever the DC input is by 4. For multiple units it would obviously
> be more complicated to do the same but probably not insurmountable.

That's already done, and how it is possible to viably use DC up to 3 kV
levels. It does not scale to 25 kV, as first, there would need to be too
many levels (more levels than there are sets of traction motors), and
second, the high voltage equipment still needs to sit in an earthed
metal box, so the demands on insulation make the construction of such
equipment impractical.

Robin

<Muttley@dastardlyhq.com> wrote:
> On Wed, 6 Dec 2023 12:50:08 +0100
> Bob <bob@domain.com> wrote:
>> As a high voltage AC electrification system, the 15 kV system, on
>> direct-to-the-railway basis is much closer in cost to the 25 kV 50 Hz
>> system, and does not have the kinds of power limitations that make DC
>> systems unsuited to higher speed running or heavy freight haulage (300
>
> Actually DC is actually better for high voltage for transfering power as
> there are lower losses. National Grid has built some DC interconnects to
> other countries for example. These days if building a railway from scratch
> you could probably do worse than 25KV *DC* given converting it down to lower
> DC voltages is now a solved problem with modern electronics. Of course you
> wouldn't get the nice hum for the catenary in the rain :)
>
>
Has HVDC switchgear evolved enough that it could economically be provided
in the numbers a railway would need with its need to isolate quite a few
sections? DC switches that have to deal with the Arc from breaking a DC
load are far more specialised than equivalent AC ones.
Where the advantages of DC has lead to long distance interconnection there
will only be switches at the ends which could be hundreds of miles apart, a
railway would not have sections that long and need many more of the
specialised DC switches which as well as being costly in the first place
will be costly to maintain due to their complexity.

GH