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Old 31st Dec 2020, 4:01 am   #1
Top Cap
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Default How the Mercury Arc rectifier works

Came across this video of the Mercury Arc Rectifiers at Kempton Steam Museum. A fascinating visit if you have never been, the huge triple expansion steam engines are an amazing sight when they get started.
https://youtu.be/YhaQqgXrMMU
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Old 31st Dec 2020, 7:53 am   #2
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Default Re: How the Mercury Arc rectifier works

I used to drive past that building every day in the 80's when I worked at the BP research centre. Looks well worth a visit.
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Old 31st Dec 2020, 11:24 am   #3
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Default Re: How the Mercury Arc rectifier works

I really enjoyed that - thanks, very nicely shot and narrated.

Down in the comments someone helpfully provided a link to a page explaining how 3-to-6 phase transformation might work, for those like me whose intuition only extends to 'something involving centre taps'

http://www.vias.org/matsch_capmag/ma...ap6_12_07.html
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Old 31st Dec 2020, 11:35 am   #4
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Default Re: How the Mercury Arc rectifier works

I watched that YT clip yesterday, rather good. (The YT algorithm does work sometimes..)

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Old 31st Dec 2020, 12:18 pm   #5
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Default Re: How the Mercury Arc rectifier works

http://underground-history.co.uk/belsize.php

This was when I first heard of a mercury arc rectifier, at Belsize Park Deep Level Shelter (scroll almost half way down the page).
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Old 31st Dec 2020, 12:36 pm   #6
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Default Re: How the Mercury Arc rectifier works

Quote:
Originally Posted by The Philpott View Post
I watched that YT clip yesterday, rather good. (The YT algorithm does work sometimes..)

Dave
YT threw this one up for me to watch next. A guy makes the glassware for a MAR and evacuates it - although he has decided that he'll fill it with argon as he doesn't fancy the mercury getting out.

https://www.youtube.com/watch?v=LEE3e5Rxt0g

Half an hour in, he pans out and it becomes clear he is doing the glassware in his garage! How someone with an electronics knowledge also because a skilled glass blower is not made clear - quite a bloke.
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Old 31st Dec 2020, 12:52 pm   #7
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Default Re: How the Mercury Arc rectifier works

Quote:
Originally Posted by mark_in_manc View Post
Down in the comments someone helpfully provided a link to a page explaining how 3-to-6 phase transformation might work, for those like me whose intuition only extends to 'something involving centre taps'

http://www.vias.org/matsch_capmag/ma...ap6_12_07.html
Seems obvious when you see it! The only trouble is that it left me wondering how one achieves 12-phase.
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Old 31st Dec 2020, 12:57 pm   #8
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Default Re: How the Mercury Arc rectifier works

Quote:
Originally Posted by mark_in_manc View Post
YT threw this one up for me to watch next. A guy makes the glassware for a MAR and evacuates it - although he has decided that he'll fill it with argon as he doesn't fancy the mercury getting out.

https://www.youtube.com/watch?v=LEE3e5Rxt0g
Doesn't the rectification depend on the electrodes dipping into the liquid mercury and causing it to vaporise? I assume, therefore, that the argon device simply acts as a gas discharge tube, creating a purple glow but with no actual rectification.
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Old 31st Dec 2020, 1:05 pm   #9
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Default Re: How the Mercury Arc rectifier works

Yes, it's 'just' for display. Given he did all the difficult stuff - made glassware which could hold a vacuum! - it almost seems a shame he didn't take the next step and use mercury, but I guess I understand the caution.
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Old 31st Dec 2020, 3:07 pm   #10
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Default Re: How the Mercury Arc rectifier works

Thanks for this: at college in the 1960's we covered various transformer windng configurations for generating polyphase AC for producing DC with minimal ripple but never covered the hardware.
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Old 31st Dec 2020, 4:47 pm   #11
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Default Re: How the Mercury Arc rectifier works

Quote:
Originally Posted by Dave Moll View Post
Quote:
Originally Posted by mark_in_manc View Post
Down in the comments someone helpfully provided a link to a page explaining how 3-to-6 phase transformation might work, for those like me whose intuition only extends to 'something involving centre taps'

http://www.vias.org/matsch_capmag/ma...ap6_12_07.html
Seems obvious when you see it! The only trouble is that it left me wondering how one achieves 12-phase.
SIX phase is achieved from a standard 3 phase primary. Each secondary winding has a center tap. Each half of the winding produces voltages 180 degrees apart, and the six half windings produce six voltages each 60 degrees apart.
Twelve phase can be produced by two transformers with similar secondary windings, but different primaries to give a 30 degree phase angle.
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Old 1st Jan 2021, 11:53 am   #12
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Default Re: How the Mercury Arc rectifier works

That's one way to make six phase, but AFAIK the generally preferred system for 6-phase MARs was a double star secondary with an interphase transformer connected between their star points. This gives the advantage of better utilisation factor of transformer and rectifier by controlling the conduction overlap between successive anodes. With three centre-tapped secondaries, the overlap depends mainly on the leakage inductance which is higher between adjacent phases than opposite ones, resulting in ripple components at 3f. Each secondary section must carry the full load current at some point in the cycle.

The interphase transformer takes up the difference in instantaneous voltage between the conducting phase in one group and that in the other, so that both anodes can conduct at once and deliver a DC voltage equal to their mean. Ripple is then entirely at 6f and the peak winding current is reduced. Symmetry is maintained because the volt-seconds across the interphase transformer produced by the outgoing anode must equal those of the incoming one.

6-phase MARs were popular for the larger sizes but 3-phase were widely used too. Many applications in the 50-150A range used 3-phase, and especially grid-controlled ones (where grid control of a 6-phase bulb for a ripple-tolerant application such as a motor drive is unnecessary.) Naturally with a 3-phase 3-pulse rectifier (which is essentially three half-wave rectifiers) the transformer secondary carries DC, so a zig-zag (interstar) secondary is needed so that two sections carrying DC in opposite directions are wound on each limb.

Last edited by Lucien Nunes; 1st Jan 2021 at 11:58 am.
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Old 1st Jan 2021, 2:16 pm   #13
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Default Re: How the Mercury Arc rectifier works

MARs were considered robust enough to be used in the multiple unit trains for the Lea Valley services out of Liverpool Street in London, but were later replaced by silicon rectifiers.
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Old 1st Jan 2021, 3:32 pm   #14
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Default Re: How the Mercury Arc rectifier works

I was on a train to London from Cambridge many moons ago, the MAR went pop as I learnt from speaking to an engineer at London. We waited until the next scheduled service came along and we were pushed all the way to London. I think the East Coast Mainline was one of the first to use 25kV AC overhead lines. It was worth the wait, my girlfriend was still at Liverpool Street, now my wife of 30 odd years.
 
Old 2nd Jan 2021, 12:49 pm   #15
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Default Re: How the Mercury Arc rectifier works

The Kempton museum video was very interesting, thanks for posting. I live quite close to the site, I really must make the effort to pay them a visit when the situation permits. As an aside, I have always been under the impression (possibly incorrect) that you shouldn't look at mercury arc rectifiers for long due to the high UV content of the light.
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Old 2nd Jan 2021, 1:30 pm   #16
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Default Re: How the Mercury Arc rectifier works

Quote:
that you shouldn't look at mercury arc rectifiers for long due to the high UV content of the light.
The glass of the envelope stops nearly all of the UV just like a fluorescent lamp does. Even so the MARs at Kempton have Perspex covers, I found this out talking to one of the lovely volunteer staff a few years ago.
 
Old 2nd Jan 2021, 4:35 pm   #17
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Default Re: How the Mercury Arc rectifier works

Some of the early AC electric locos, Classes 83 (EE) & 84 (GEC) used MARs but were unreliable & were replaced by solid state ones.
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Old 5th Jan 2021, 12:25 pm   #18
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Default Re: How the Mercury Arc rectifier works

Having seen those mercury arc rectifiers at Kempton I can vouch they are indeed quite impressive objects, the first things I saw when entering that magnificent edifice!
Quite how they were made (professionally tortured glass?) or even conceived beggars belief.
The engines are equally impressive too!
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Old 5th Jan 2021, 1:13 pm   #19
Lucien Nunes
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Default Re: How the Mercury Arc rectifier works

There is a series of illustrations in one of my rectifier brochures purporting to show the process. I think the arms in later units were normally moulded in straight sections, then assembled manually. I have seen glimpses of rectifier bulbs being worked in films, but not the complete job.

The unique and distinctive shape of the glass-bulb rectifier follows from a few specific requirements.
* The large bulb is the easiest way to provide sufficient condensing surface for the vapour to keep the pressure low.
* Housing the anodes within separate inclined arms reduces the likelihood of both cross-fire and metallic mercury tracking.
* Forming elbows in the main anode arms on units working at 100V or more protects them from ionic bombardment from the cathode pool, while maintaining the shortest arc length to mimimise voltage drop.

In practice the key to a successful envelope is as much about the seals as the glass itself. A large grid-controlled bulb might have up to twenty seals, of which the main anode leadouts run at high temperature carrying 100A or more.

Later this week I will be testing a small single-phase bulb that I am about to use on a work project. Watch this space...
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