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Old 11th Sep 2018, 3:35 pm   #21
Scott37
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Default Re: Last public DC mains supply in the UK?

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Originally Posted by Richard_FM View Post
I've heard some DC radios were made for the cabins of cruise ships, I'm not sure what voltage they would have ran on.
I now remember my mum (I think it was) used to talk about a superstition that ship's electrical equipment would not work on land.
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Old 11th Sep 2018, 4:04 pm   #22
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Oxshot in surrey allegedly had DC mains until the 1970s.
Really. I've had relatives in Oxshot as long as I can remember and as far as I know they never had DC mains.

Perhaps it was only a small part.
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Old 11th Sep 2018, 4:13 pm   #23
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Default Re: Last public DC mains supply in the UK?

I seem to remember that a friend of mine who was a radio officer on merchant ships told me that several of the vessels he was on had 220-440 volts DC some had 115 volts but at 400 cycles and one which was a passenger liner had 115 at 400 cycles and some 240 at 50 cycles for passenger use
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Old 11th Sep 2018, 4:43 pm   #24
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Default Re: Last public DC mains supply in the UK?

There must have been a lot of DC mains in 1947.
A severe coal shortage was disrupting electricity supplies, and to relieve matters, the Royal Navy used moored diesel electric submarines to supply DC power to some coastal towns.

This was hated by the crews of the submarines as the continually running engines resulted in a freezing cold draught throughout the vessel.

Google "operation blackcurrant" for more details.
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Old 11th Sep 2018, 5:14 pm   #25
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Default Re: Last public DC mains supply in the UK?

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I've heard some DC radios were made for the cabins of cruise ships, I'm not sure what voltage they would have ran on.
I think this is the reason why so many Eddystone radios were designed as AC/DC sets to run on the 110 VDC marine power supply. They were typically installed in the first class and officers' cabins of the ocean liners of the 1950s. Here's an Eddystone 'cabin radio' from the Radiomuseum:

https://www.radiomuseum.org/r/eddyst..._670as670.html

I can imagine that a decent short-wave radio in the cabin might then have provided a greater variety of news and entertainment than the limited satellite TVs installed in today's cruise ships!

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Old 11th Sep 2018, 5:39 pm   #26
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Default Re: Last public DC mains supply in the UK?

The manual for the Technics SA-X900L which must be late 80's does mention not to use this product on ships as it might be DC power, or words to that effect.
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Old 11th Sep 2018, 5:41 pm   #27
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Default Re: Last public DC mains supply in the UK?

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I recall a period in the early 1960s when I worked at the then relatively new Doncaster 'B' power station that DC was still being generated at the old Doncaster 'A' station by means of substantial rotary converters. The main DC supply was 600V for the trolleybuses, but there was one machine dedicated to the local printing works whose supply contract still required that the Board must provide DC.
Interesting! Any idea why the DC was produced by rotary machine, rather than mercury-arc rectifier plus transformer? I would have thought that the MAR would be the cheaper solution - though I daresay that the rotary converter may have been easier to vary the output voltage of... I'm thinking that if a single consumer was being supplied, a pair of remote voltage sense wires could be installed to improve voltage accuracy at the point of load (just like many regulated power supplies have!)

Quote:
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I think this is the reason why so many Eddystone radios were designed as AC/DC sets to run on the 110 VDC marine power supply. They were typically installed in the first class and officers' cabins of the ocean liners of the 1950s. Here's an Eddystone 'cabin radio' from the Radiomuseum:

https://www.radiomuseum.org/r/eddyst..._670as670.html
I've got one of those, quite a decent little radio! No problem DC, 50Hz, or 60Hz though I have only used it on 50Hz so far.
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Old 11th Sep 2018, 6:14 pm   #28
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Default Re: Last public DC mains supply in the UK?

Quote:
Interesting! Any idea why the DC was produced by rotary machine, rather than mercury-arc rectifier plus transformer? I would have thought that the MAR would be the cheaper solution - though I daresay that the rotary converter may have been easier to vary the output voltage of... I'm thinking that if a single consumer was being supplied, a pair of remote voltage sense wires could be installed to improve voltage accuracy at the point of load (just like many regulated power supplies have!)
Just guessing here, but perhaps a rotary converter retains the ability of the DC supply to sink current as well as source it? As I understand it, DC mains, just like AC mains, could absorb power as well as delivering it. I can imagine this being useful in motor control applications especially. The power would go to the other customers, of course, just raising the line voltage slightly. With a dwindling number of customers, any equipment trying to push current back in to the supply would find that it might have nowhere to go (no trolleybuses to accelerate!). A mercury arc rectifier certainly wouldn't work 'in reverse', but perhaps a rotary converter would - push current into the commutated DC output and it would flow in the AC primary windings as long as the machine kept rotating?

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Old 11th Sep 2018, 6:28 pm   #29
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Default Re: Last public DC mains supply in the UK?

Why not use a dynamo, as I assume was used before the 'invention' of AC?
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Old 11th Sep 2018, 6:31 pm   #30
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Default Re: Last public DC mains supply in the UK?

I have been on a number of cruises in the last decade (my wife is very good at getting last-minute cut price offers), and AFAIR they have all had 60Hz AC supplies, even the ones built in European shipyards for European companies. In the staterooms, some have had 230V only, others both 230 and 120V sockets. Apart from an elderly Thomson vessel that only had the French sockets that will not accept a Schuko plug, most of the others have had UK 13A, Schuko, and US flat pin sockets. None had DC.
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Old 11th Sep 2018, 6:52 pm   #31
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Default Re: Last public DC mains supply in the UK?

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Interesting! Any idea why the DC was produced by rotary machine, rather than mercury-arc rectifier plus transformer? I would have thought that the MAR would be the cheaper solution - though I daresay that the rotary converter may have been easier to vary the output voltage of...
I think that the answer to this question would need to be supplied by the erstwhile station chief engineer sometime soon after the First World War. Probably went along the lines of:

"I can't be doing with any of those new fangled glass bottle jobs - I've got to supply the trams with my station, and those tram motors need proper dynamos to drive 'em, especially when they all start up together after the St Leger!"

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Old 11th Sep 2018, 7:13 pm   #32
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Default Re: Last public DC mains supply in the UK?

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Why not use a dynamo, as I assume was used before the 'invention' of AC?
In the early days of DC dynamos were used of course, but suffered from drawbacks.
Firstly a dynamo had to generate at the utilisation voltage which seldom exceeded 650 volts and was often 500 volts.
This limited the distance between dynamo and load to a few miles. Consumers too far away had to be served by a rotary converter from 3 phase high voltage AC.

As reciprocating steam engines gave way to steam turbines, Dynamos were not easy to build for the high speeds at which turbines are most economic.
A dynamo with a maximum speed of say 800RPM required either a special low speed steam turbine, or reduction gearing, either of which added to costs complication and losses.
By contrast an alternator could run at 3,000RPM. Since high voltage alternators were needed in any case to serve outlying districts via rotary converters, it became economic to standardise on generating AC only via modern large alternators driven by high speed steam turbines.
The limited DC load was served by converting plant, with the losses being cheaper than separate DC generation.
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Old 11th Sep 2018, 7:23 pm   #33
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Default Re: Last public DC mains supply in the UK?

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Originally Posted by cmjones01 View Post
With a dwindling number of customers, any equipment trying to push current back in to the supply would find that it might have nowhere to go (no trolleybuses to accelerate!). A mercury arc rectifier certainly wouldn't work 'in reverse', but perhaps a rotary converter would - push current into the commutated DC output and it would flow in the AC primary windings as long as the machine kept rotating?
You raise an interesting point Chris. How do Mercury Arc Rectifiers work on traction supplies where regenerative braking is a key part of the system?

For example, I know that, back in the day, trolleybus drivers in Doncaster tended to use the regenerative braking pedal. If MARs were to be used for the DC supply, presumably that reverse current would just have supplied the other buses on the route. A rotating machine supply, on the other hand, would have absorbed the power without difficulty.

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Old 11th Sep 2018, 8:21 pm   #34
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Default Re: Last public DC mains supply in the UK?

I think most regenerative braking was just dumping the power through a series of big air cooled wirewound resistors on the roof rather than attempting to push power back through the overhead line.

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Old 11th Sep 2018, 10:02 pm   #35
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Default Re: Last public DC mains supply in the UK?

I suspect that we’ve turned into an off-topic side-street. But briefly, the use of on-board resistors for electric braking is known as rheostatic or dynamic braking, not regenerative braking. The term regenerative braking is used solely where power is returned to the overhead line (or third rail in some railway cases). The MAR problem was usually dealt with by including resistor banks in the substations, switched in by voltage rise. There were some railway examples where substations had mercury arc inverters as well as rectifiers. For further reading, see:

The Regenerative Braking Story; Struan Jno T Robertson & John D Markham; Scottish Tramway & Transport Society & Venture Publications, 2006; ISBN 10 905304 161. The MAR problem is discussed on p.125.

Also highly recommended:

Electric Traction; A.T. Dover; Fourth Edition; Pitman, 1963.
Electric Traction Handbook (Control); R. Brooks; Pitman, 1954.

And this IEE paper:

Railway Electric Braking; F. Whyman; Paper #983, 1950.

Returning to the main theme:

Quote:
Originally Posted by Scott37 View Post
When was the last public DC electricity supply in the UK, and where? Given that AC/DC radios are discussed elsewhere, it cannot have been so very long ago.
As aspect here is that the sale of AC/DC radio receivers seems not to have been restricted to those consumers who had a specific need for them, i.e. they had a DC electricity supply. Rather they seem to have been sold more generally, and I suspect that many went to customers who had AC electricity supplies and so had no real need for them. Perhaps even more of the AC/DC receivers that were sold were used initially on AC supplies than were used on DC supplies. And I think not all models were offered in duplicate AC and AC/DC versions; some were AC/DC only. So a less desirable technique was propagated beyond its reasonable realm. On the safety side, there was an interesting article on the then BS415 rewrite in Wireless World 1953 January, p.02ff. One impression I have from that is that the standard perforce had to be written to accommodate AC/DC live chassis receivers, even if that involved some safety compromises. In hindsight, one wonders why the negative busbar technique, as used by some US makers, was not encouraged or even enforced, as this would have allowed earthing of the chassis itself.


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Old 11th Sep 2018, 10:25 pm   #36
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Default Re: Last public DC mains supply in the UK?

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I think most regenerative braking was just dumping the power through a series of big air cooled wirewound resistors on the roof rather than attempting to push power back through the overhead line.
Pedantically, dumping the energy as heat is rheostatic braking. Sending it back into the supply for use elsewhere is regenerative.
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Old 12th Sep 2018, 12:43 pm   #37
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Default Re: Last public DC mains supply in the UK?

I think the reason why a rotary converter was used instead of a mercury vapor rectifier has more to do with what was available at short notice when the station DC generators were no longer usable for whatever reason. I'm thinking that when they knew the generators were sick they just went with the first available options to keep customers on load. I imagine it wouldn't go down well if the supply was suddenly cut off for a week or more. Some lifts have big wire wound resistors to absorb the motors back EMF when the lift comes to its floor and the brakes are applied I guess the same principle as trains trams and trolleybuses just a bit smaller scale.
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Old 12th Sep 2018, 1:14 pm   #38
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Default Re: Last public DC mains supply in the UK?

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'I think the reason why a rotary converter was used instead of a mercury vapor rectifier...'
At the part of British Steel where I worked back in the 1970s (and present from much earlier, probably WWII) the 250V d.c. supply was generally provided by rectifiers: MAR then later, solid-state. But the standby arrangement was a big d.c. generator driven from a synchronous motor. This could be run at a leading power-factor and so would act as a power-factor compensation mechanism. When I say 'big' it was in a tile-lined pit about 25' x 12' in a room about 20' high.

Quote:
Some lifts have big wire wound resistors to absorb the motors back EMF when the lift comes to its floor and the brakes are applied I guess the same principle as trains trams and trolleybuses just a bit smaller scale.
Same as the lifting motors on overhead electric cranes. Series-wound using a 'Holmes' controller (copper segments on a cast-iron frame with blow-out coils). When the load descended, banks of resistors were connected across the motor instead of the supply to adjust the rate of descent.
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Old 13th Sep 2018, 7:35 pm   #39
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Default Re: Last public DC mains supply in the UK?

A rotary - coupled to a dirty great flywheel - had the benefit of damping-out short-term peaks and troughs of demand and so delivering a much better-regulated supply. "Industrial" loads are notorious for suddenly demanding massive current to get up to speed but then once everything's moving needing much less power.

Think something like mine winding-gear accelerating a few hundred tons of coal up a pit-shaft, or starting a half-mile-long conveyor-belt transporting those hundreds of tons of coal to the waiting rail-wagons.

One of my relatives was chief-engineer to the Nottingham coalfields in the 1960s: they ran their own DC generating-plant to deal with the heavy/peak loads because the National Grid wasn't up to it. The Grid provided the 'background' power for continuous-loads like pumping water from the mines [daily ventilating and shifting tens of millions of gallons of water from deep mines is energy-intensive] but they still had their own DC generating-plant so they could continue to operate when the Grid couldn't handle the load.
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Old 16th Sep 2018, 11:21 pm   #40
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Default Re: Last public DC mains supply in the UK?

Also, a rotary converter will run correctly on no load, or virtually no load.

On a summer bank holiday, the load on a DC main supplying say 200 households could easily drop to a amp or so, out of a normal load of hundreds of amps.
A rotary might be very inefficient at such low load, with perhaps 10KW or 20KW of losses whilst supplying 2KW, but it would work and reliably supply the consumer.


A mercury arc would probably go out or at least become unstable.
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