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Old 17th Sep 2018, 2:28 am   #41
Synchrodyne
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Default Re: Last public DC mains supply in the UK?

As previously noted in this older thread: https://vintage-radio.net/forum/showthread.php?t=132331, the legacy Edison DC system in New York City more-or-less went direct from rotary converter substations (with 25 Hz AC input) to solid-state rectifier substations (with 60 Hz AC input) in the 1960s, with minimal use of MARs. Perhaps ability to operate well on widely varying loads was a determining factor in that case.


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Old 17th Sep 2018, 6:31 am   #42
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Default Re: Last public DC mains supply in the UK?

This consideration makes the tramways and trolley-bus systems using MARs all the more interesting. The current demand of a stopped vehicle in daytime will be zero, and a moving one on the overrun down a hill will regenerate naturally so the current load can even try to go negative. With a number of vehicles in a section of track, the overall load ought to keep a MAR lit, but I assume they would have had to go out and to re-light on demand.

There was a trolleybus substation in our village, and I think there was a dead break in the lines down Somerset road, so I don't believe the network was all connected together. There certainly were tomes with no moving buses in the area, or just a single one descending the hill.

You've got me wondering how the power system worked. I always took it for granted. The buses were very visible, the power network invisible and unknown to the general public.

The buses were marvellous. Smooth, silent, quick accelerating. Don't we just wish we had them back, now?

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Old 17th Sep 2018, 10:28 am   #43
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Default Re: Last public DC mains supply in the UK?

Quote:
This consideration makes the tramways and trolley-bus systems using MARs all the more interesting. The current demand of a stopped vehicle in daytime will be zero, and a moving one on the overrun down a hill will regenerate naturally so the current load can even try to go negative.
Would it though? I confess to not knowing very much about tram and trolleybus motors, but I had understood that they generally used series-wound motors for the good torque / speed characteristic.

Such motors can never reverse the current, because if they did, the field would be reversed and the polarity then of the rotating armature voltage would also reverse, with the same direction of rotation. What happens is that there is actually no over-run, current just drops asymptotically to zero as speed approaches infinity.
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Old 17th Sep 2018, 10:41 am   #44
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Default Re: Last public DC mains supply in the UK?

A conventional MAR designed for general service does not extinguish at zero load. There is an excitation circuit with its own anodes, through which a few amps pass continuously, to keep the emission spot alive. The power lost to this circuit is negligible as the current is regulated by a magnetically shunted transformer; 100W will keep even a large bulb ready for load.

Induction regulators were used to control the DC output of both MARs and rotaries, but accurate regulation of a MAR can be trickier for low loads. A rotary converter ties the two sides together by a good approximation to an equivalent impedance. MAR Commutation occurs once per pulse (rather than once per armature coil in a rotary,) typically 6 times the supply frequency, using a 3-phase 2-group transformer to minimise ripple. The self-inductance of the transformer secondaries and interphase transformer governs the commutation process by causing overlap (two anodes conducting at once) which depresses the voltage somewhat. When currents become discontinuous, the regulation goes off rather rapidly.

The advantage of mechanical inertia mentioned in connection with rotaries is significant, and was employed specifically for winding duty e.g. in the Ward-Leonard-Ilgner drives. Another load situation that favours rotaries is the presence of transients. A voltage spike that causes a crossfire in a MAR practically shorts all the anodes together, so thorough overvoltage protection is needed if the output of a MAR is to venture far overhead on a highly insulated circuit.

Using grid control, it is theoretically possible for mercury arcs to invert power, but the complexity of the commutating circuitry made it uneconomical to provide for absorption of reverse power flow from a DC distribution system, as this tends to be a tiny fraction of the power delivered. A network with one vehicle descending a gradient is an edge-case better dealt with by dump resistors.
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Old 17th Sep 2018, 3:12 pm   #45
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Default Re: Last public DC mains supply in the UK?

I remember the Meadows area on Nottingham changing over from 200 volts DC to 240 volts AC in the early 1960's. The TV we rented never seemed to work very well on DC, it was always losing vertical & horizontal hold, Dad said "I'll be glad when we go on AC as we wont have this problem then." & the TV did work better on AC..

I started a DC electricity thread on 'Nottstalgia' a Nottingham local history site. One of the commenter's used to be a TV repair man & said "many TV's that had only ever worked on DC would expire when fed AC for the first time in their life's." Must have been a shock for the poor things, lol...
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Old 17th Sep 2018, 5:05 pm   #46
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Default Re: Last public DC mains supply in the UK?

Quote:
Originally Posted by kalee20 View Post
Would it though? I confess to not knowing very much about tram and trolleybus motors, but I had understood that they generally used series-wound motors for the good torque / speed characteristic.

Such motors can never reverse the current, because if they did, the field would be reversed and the polarity then of the rotating armature voltage would also reverse, with the same direction of rotation. What happens is that there is actually no over-run, current just drops asymptotically to zero as speed approaches infinity.
Interesting. So presumably no question of regeneration back to the line anyway. I guess the motors would still generate if switched onto a resistor though, as long as there was some residual magnetism in the field core to bootstrap things.
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Old 17th Sep 2018, 5:43 pm   #47
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Default Re: Last public DC mains supply in the UK?

I've just been thinking about series-wound machines. To work as a generator, the connections to the field (or to the armature brushes) would have to be swapped over, else the thing will never build-up from its initial magnetism. Or (in principle) you just drive the shaft the opposite way it turns when operated as a motor.

Regeneration into the line would work if the field was swapped, though the idea of breaking contacts at full current does not fill me with enthusiasm. I daresay you could have extra coils on the field instead, one for motoring, one for braking, series, shunt, or compound. It's the sort of think Lucien would know hopefully.
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Old 19th Sep 2018, 1:01 am   #48
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Default Re: Last public DC mains supply in the UK?

In brief, and looking at the pre-power electronics era, railway regenerative braking used separate excitation of series wound motors, railway rheostatic braking used either separate excitation of series wound motors, or self-excited series-wound motors, railway diesel-electric locomotive dynamic braking used separate excitation of series wound motors, trolleybuses with rheostatic or regenerative braking used compound-wound motors, tramcars with regenerative braking used compound-wound motors, late practice for tramcars (including the PCC type) with rheostatic braking was self-excited series. Some illustrative literature is attached. In connection with regen, there was some use of MAR inverter substations, e.g. the South African Railways 3 kV DC system, and rare use of MAR inverters onboard AC-DC locomotives, e.g. by SNCF, with excitron-type MARs.

Getting back to public general-purpose DC supplies in the UK, did any of these change over to MAR feeding, or did they stay with legacy rotary converters? Whereas changing to MARs would have made sense for traction and industrial applications, which would have been seen as having open-ended lifespans, even in the 1930s, public DC systems were likely to have been viewed as having limited life, so perhaps not worthy of much investment beyond status quo maintenance.


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Old 19th Sep 2018, 1:12 am   #49
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Default Re: Last public DC mains supply in the UK?

When public supply changeovers from DC to AC were made, was it a case of DC today, AC tomorrow? Or were DC and AC available side-by-side for a time, with a looming "drop dead" date for DC?

The Electrical Engineer’s Reference Book (EERB), ninth edition, 1958, shows quite a few UK locations still with DC distribution, although in the majority of cases additional to AC. But I guess that means that whilst a given town might have had AC and DC, both were not necessarily available to all consumers.


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Old 19th Sep 2018, 5:19 am   #50
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Default Re: Last public DC mains supply in the UK?

It depended on the physical layout of the mains network.

In areas served by a single company, it was usual to supply only DC up to a certain date and then AC thereafter.
With a simple mains network it was not viable to supply both AC and DC at the same time.
Exceptions occurred in places at the boundary between AC and DC undertakings.

In old congested urban areas, there would historically have been several competing supply companies. In time these often came under common ownership, but retained physically separated mains networks. This could include AC from one undertaking and DC from another, in the same road or even the same building.
I recall working in an old bank building in London in about 1990 that still had DC incoming cables and switchgear. No longer used, but it looked as though it had been used within living memory.
Some lighting circuits had changeover contactors from 240 VDC to 208 volts AC, again no longer used.
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Old 19th Sep 2018, 6:52 am   #51
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Default Re: Last public DC mains supply in the UK?

Fascinating! Thanks.

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Old 19th Sep 2018, 10:15 am   #52
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Default Re: Last public DC mains supply in the UK?

According to the Ian Allan "British Rail Handbook" for 1981, the Class 76 locos for the 1500VDC Manchester - Sheffield - Wath services had regenerative braking. It says " These locomotives were fitted from the first for regenerative braking, and the driver had to balance the regenerated voltage with the voltage in the overhead line before allowing regeneration to begin." Rheostatic braking was added later, integrated with the normal braking operation, and only required the operation of a switch to activate it. No other technical details given.

Electric locomotives of classes 81 to 84 initially had MARs, as did some of the early EMUs used on the suburban lines out of London Liverpool Street, all subsequently replaced by Germanium, and later Silicon, rectifiers. For classes 81 and 82, "The rectifiers were three multi-anode tanks of basically industrial design but modified to minimise splashing of the mercury pool." Single anode ignitron rectifiers were used for class 83, while class 84 had a special single anode design " . .. aimed at combining desirable features of ignitron and the exitron in a form specifically suitable for traction."

Getting back to the thread's subject, although Cardiff had both AC and DC public mains until 1970, the DC mains was really only aimed at industrial, not domestic, customers.

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Old 19th Sep 2018, 10:56 am   #53
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Default Re: Last public DC mains supply in the UK?

In the '50s part of my job as a TV engineer was to maintain the PA system at a fairly local paper mill. They had a quantity of large DC motors powered by the biggest rotary converter I have ever seen.

It was supplied with AC from a substation size oil filled transformer and was about 10' or more long and 4'+ wide and high, the brushes were about 4x2 " and it ran at a very leisurely pace, you could probably count the revolutions.

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Old 19th Sep 2018, 11:14 am   #54
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Default Re: Last public DC mains supply in the UK?

There are STILL areas in the London suburbs with mains intended for 3 wire DC.
The present day supply is of course AC, but non standard in various ways so as to utilise mains intended for 3 wire DC rather than 4 wire AC.

Single phase 3 wire is usually supplied to the former DC mains, obtained from 3 phase HV by various non standard transformers.
A small stock of 6 phase transformers and of 3 phase transformers with reversed polarity is still kept to replace failures on these rather odd systems.

The consumer receives 240 volts, single phase, 2 wire AC, But can't obtain 3 phase without a new cable from the substation.
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Old 19th Sep 2018, 11:50 pm   #55
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Default Re: Last public DC mains supply in the UK?

Here in Weymouth there are still one or two streets where the old DC cables are still in service. You can sometimes tell if an area has the old cables because the local substation has 2 transformers which I think are wound the opposite way round to each other. I did see a diagram of how it was done once maybe someone here has it and can repost it. My mate's shop is in one of the streets concerned and the wiring dates back to the 1930s he's got 1950's vintage flouro lamps fed by 1930's vintage twin twisted flex!!
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Old 20th Sep 2018, 10:39 am   #56
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Default Re: Last public DC mains supply in the UK?

Indeed.
Areas with mains intended for 3 wire DC but now served with AC need either a pair of substation transformers, one of which has reversed polarity, or alternatively a single transformer with a 6 phase output.

Considering the first example, one transformer is a standard type as widely used by the supply industry.
The second transformer is a "special" with a reversed output. This may be achieved either by winding the LV coils in the opposite direction to a standard type, or alternatively by connecting together the "top" ends of the 3 LV windings to form the outgoing neutral, rather than the normal practice of the "bottom" ends being the neutral.

The area to be served is divided into a red zone, a yellow zone, and a blue zone.
A new 4 core main is run around the area to be served with both ends going back to the substation.
The red core has one end connected to the red phase of one transformer and the other end connected to the red phase of the other transformer.
The yellow and blue cores similarly.
The black core is connected to the neutral terminals of both transformers, and earthed.

In the red zone, the red core of the new main is cut, and the old 3 wire mains are connected via fuses to the red cores. Former positive outers to clockwise red core of new main, former negative outers to the anti clockwise red core of the new main.
Centre wires of the old 3 wire mains jointed to the black core of the new main.
Similarly in the yellow and blue zones.

Each of the old 3 wire mains is thus supplied with 500 volts between outers, and 250 volts between centre wire and either outer.
Each consumer connected between one outer and centre wire receives 250 volts (dropping to 240 on load) single phase AC.

Legacy colours used for clarity.
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Old 20th Sep 2018, 11:09 am   #57
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Default Re: Last public DC mains supply in the UK?

In other cases, a single transformer with a 6 phase output is used, sometimes called a diametric transformer.

The HV windings are standard.

There are 3 LV windings each of 500 volts from end to end and equipped with a centre tap.
All 3 centre taps are connected together and form the outgoing neutral.
The 6 ends of the windings are each connected to terminals.
The transformer therefore has SEVEN LV connections, 6 phases and the neutral.

The terminals connected to the "start" of each of the 3 LV windings are identified as red, yellow, and blue in the usual way.
The other 3 terminals connected to the "finish" of the relevant windings are usually in written documents referred to as red* yellow* and blue*
In the spoken word they are often called "antiphase" red or blue or yellow.
Visual identification is by colour, with the addition of white for antiphase connections.
So fuses or single core cables connected to blue* or to "blue antiphase" should be marked blue and white.

Existing 3 wire mains are connected between phase and the corresponding antihase, with the centre wires connected to the transformer neutral and therefore receive 500 volts, with each consumer connected between one outer and the neutral of a 3 wire main getting 250 volts (reducing to about 240 on load) single phase AC.

Legacy colours used for clarity.
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Old 20th Sep 2018, 1:02 pm   #58
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Default Re: Last public DC mains supply in the UK?

Quote:
Originally Posted by Synchrodyne View Post
...railway rheostatic braking used either separate excitation of series wound motors, or self-excited series-wound motors... trolleybuses with rheostatic or regenerative braking used compound-wound motors, tramcars with regenerative braking used compound-wound motors, late practice for tramcars (including the PCC type) with rheostatic braking was self-excited series.
Thanks for informative post and supporting literature Steve! So basically with the series-wound DC motors, auxiliary coils were completely necessary for regenerative braking. Makes sense!
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Old 20th Sep 2018, 1:28 pm   #59
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Default Re: Last public DC mains supply in the UK?

It's always interesting to learn about non-standard electricity supplies.
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Old 20th Sep 2018, 2:36 pm   #60
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Quote:
Originally Posted by broadgage View Post
The terminals connected to the "start" of each of the 3 LV windings are identified as red, yellow, and blue in the usual way.
The other 3 terminals connected to the "finish" of the relevant windings are usually in written documents referred to as red* yellow* and blue*
Not "green", "violet" and "orange" then?
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