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Old 27th Jun 2019, 7:27 pm   #1
Kentode
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Default Mercury arc rectifier.

Below are a few screen grabs of a program on Talking Pictures called " Final Performance" by Wilf Watters, a former BBC sound man, taken at the Regent Cinema Amersham in 1962.

The projectionist is shown checking the rectifiers, and at one point he taps the glass and a rivulet of mercury runs down!

Harder to see is the fitting of a fresh rod to what I think is an arc light? Apologies but I'm unsure of the terminology.

More to follow.
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Old 27th Jun 2019, 7:35 pm   #2
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Default Re: Mercury arc rectifier.

A final photo. Is that kalee ?

One of my abiding memories of being taken to the'flics" by my father was that, if l was bored, I'd look up to the ceiling and watch the cigarette smoke making abstract patterns through the light of the projector.

I wonder why I had asthma?
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Old 27th Jun 2019, 8:02 pm   #3
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Default Re: Mercury arc rectifier.

Can't see the projector, but the arc lamp in photos 4 and 5 look like a Peerless Magnarc (which was sometimes used with Kalee projectors).

Yes, this is fitting of a new carbon rod to the arc. It's the positive one, because it is facing the large concave mirror (most of the light is emitted from a crater which forms in the positive electrode).

Carbon consumption is typically a bit more than 2" during the standard 20 minute reel, and about half that for the negative rod.

The mercury bulb is interesting because it appears to have just 3 arms. While 3-phase half-wave rectifier circuits do exist, for an extra 3 secondaries on the transformer, you can have 3-phase full-wave, which is much lower ripple, kinder on the transformer, better all round!

The photo in the second post is indeed Kalee - it's a cover, but I can't work out exactly where it might have featured!

Last edited by kalee20; 27th Jun 2019 at 8:04 pm. Reason: Last sentence
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Old 27th Jun 2019, 9:02 pm   #4
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Default Re: Mercury arc rectifier.

I recall one of those mercury rectifiers in the Science Museum many years ago. I guess they removed it due to health & safety...
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Old 27th Jun 2019, 9:37 pm   #5
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Default Re: Mercury arc rectifier.

Kalee cover, the cover looks very similar to the one in these links:

http://primolux.co.uk/projects/kalee...12_12_2010.htm

https://pixabay.com/photos/kalee-pro...tique-2319630/

Lawrence.

Last edited by ms660; 27th Jun 2019 at 9:45 pm. Reason: extra link added
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Old 28th Jun 2019, 7:43 am   #6
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Default Re: Mercury arc rectifier.

Quote:
Originally Posted by Keith956 View Post
I recall one of those mercury rectifiers in the Science Museum many years ago. I guess they removed it due to health & safety...
Yes, I saw it working there back in the mists of time, but it was no longer there last time I visited some years back.
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Old 28th Jun 2019, 8:52 am   #7
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Default Re: Mercury arc rectifier.

Quote:
Originally Posted by kalee20 View Post
The mercury bulb is interesting because it appears to have just 3 arms. While 3-phase half-wave rectifier circuits do exist, for an extra 3 secondaries on the transformer, you can have 3-phase full-wave, which is much lower ripple, kinder on the transformer, better all round!

The photo in the second post is indeed Kalee - it's a cover, but I can't work out exactly where it might have featured!
I posted the last photo just for you Kalee20!

What I didn't show is that there is two rectifiers being used so it probably is 3-phase full-wave. I wish it was possible to post a video here of the nonchalant way that the projectionist taps the glass of both of them
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Old 28th Jun 2019, 9:09 am   #8
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Default Re: Mercury arc rectifier.

Here's a link to an 'urban exploration' of the former Belsize Park Underground Station made by Subterranea Brittanica, it has some snapshots of the rectifier that supplies the lift.

https://www.subbrit.org.uk/sites/bel...ep-shelter-sb/
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Old 28th Jun 2019, 9:18 am   #9
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Default Re: Mercury arc rectifier.

I recall these supplying the power for the Bournemouth trolley buses. There was a blower fan beneath them and the stroboscopic effect from the rectifier gave the impression that the fans were only just turning! I once replaced a faulty rect. in a cinema, with silicon diodes. The manager thought I was some sort of magician - being able to replace such a complicated device with what appeared to be a few nuts & bolts!
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Old 28th Jun 2019, 10:53 am   #10
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Default Re: Mercury arc rectifier.

Because all MARs are inherently common-cathode (like full-wave rectifier valves) it was not normal to implement a bridge, thus traditional full-wave rectification was standard on single-phase supplies. On 3-phase, the same applies; although 3- or 6-pulse rectification can be used, there must be as many phases as pulses, and DC flows in the transformer secondary. Unidirectional flux from this DC is avoided using a zig-zag secondary winding, at the cost of doubling the amount of copper. Each transformer limb carries two sections of secondary, each being half of each of two phases, arranged so that the DC currents oppose.

For six-pulse rectification equivalent to a 3-phase full bridge, two zig-zag secondaries of opposite phase shift are needed, along with a separate interphase transformer between the two star points to force commutation alternately between the two 3-phase groups (they cannot simply be commoned.) This adds cost and complexity and was not always used on rectifiers of cinema-size (tens of kilowatts). Larger plants, hundreds of kilowatts or more, invariably used 6-phase transformers. Typically, if more than one bulb was needed for total capacity, each bulb would have six anodes. In each case, a very large choke in the DC output assisted with reducing ripple and preventing arc extinction.

There were two approaches to supplying a cinema's proector arc DC with mercury arcs. One was via a DC bus that fed both / all projectors and other requirements such as followspots in a cine-variety installation. Two bulbs provided redundancy. If one bulb or its control gear failed, a show could still go on provided that two full size arcs were not burned together for long. Alternatively, especially for smaller screens, one compact rectifier could be used per projector, connected directly to (and usually installed underneath) its lamphouse, a system typified by the Hewittic 'UnitArc'. This enabled the individual arc current controls to be implemented on the AC side upstream of the rectifier by means of a tapped choke, rather than on the DC side with large resistances. These were often single-phase full-wave, with a hefty DC choke; hence, 2-, 3- and 6-pulse rectification were all used for cinema arcs depending on the size and configuration of the installation.

Last edited by Lucien Nunes; 28th Jun 2019 at 11:03 am.
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Old 28th Jun 2019, 12:54 pm   #11
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Default Re: Mercury arc rectifier.

Quote:
Originally Posted by Keith956 View Post
I recall one of those mercury rectifiers in the Science Museum many years ago. I guess they removed it due to health & safety...
If my memory serves me correctly, there was one on display in Birmingham Museum, too. It was long enough ago for me to have forgotten much of the information shown with it, but I was young enough to be impressed by it. I bet that's gone now, too.

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Old 28th Jun 2019, 1:00 pm   #12
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Default Re: Mercury arc rectifier.

I think it was mentioned in a previous thread that mercury arc rectifiers gave off a lot of UV light, which is probably one of the reasons they are hard to see in use these days.
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Old 28th Jun 2019, 1:35 pm   #13
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Default Re: Mercury arc rectifier.

The Manx Electric Railway used these for many years. I managed to gain access to the power house a few years ago just after they had gone solid state. The chap in charge told me that they had performed faultlessly for over 100 years and the only item ever changed was a bearing in the cooling fan. They had spare units still in their crates unopened!
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Old 28th Jun 2019, 1:49 pm   #14
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Default Re: Mercury arc rectifier.

UV is often mentioned relative to mercury arc rectifiers, without any figures to indicate how much is emitted. Many of these mentions are guesses or heresay, sometimes referring to the hazards of mercury sun-ray lamps emitting UV-C etc. Such lamps used quartz tubes which are UV-transparent, whereas mercury lamps used for lighting are always of glass (or have a glass outer bulb to provide UV filtering) since most glasses are UV-opaque especially at UV-C.

I am not going to perpetuate the guesswork by making any definitive statements until I have a chance to do some measurements, which may not be soon. However, it's worth noting that while MARs were usually shielded from continuous direct view, the light from the bulb was allowed to escape through the open vents etc, and even latterly when risk assessments have been made of continuing to operate MARs, it has not been considered significant. I am familiar with arc-eye from other activities, but have never experienced it from operating exposed MARs!

Quote:
they had performed faultlessly for over 100 years
I'm sure they would have done had they been given the chance, however the units at Laxey dated from the 1930s.
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Old 28th Jun 2019, 3:40 pm   #15
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Default Re: Mercury arc rectifier.

Quote:
Originally Posted by Lucien Nunes View Post
Because all MARs are inherently common-cathode (like full-wave rectifier valves) it was not normal to implement a bridge, thus traditional full-wave rectification was standard on single-phase supplies. On 3-phase, the same applies; although 3- or 6-pulse rectification can be used, there must be as many phases as pulses, and DC flows in the transformer secondary.
But if each limb of the transformer has a centre-tapped secondary, you can connect all the centre-taps together to get your -ve output terminal. The six 'hot' leads from the transformer go to six limbs on your MAR, you have what you could call 6-pulse rectification or 3-phase full-wave (the transformer primary won't notice the difference), the DC components cancel out, just as they do in our familiar EZ80 / 5Z4 rectifier circuits, and the output ripple is 300Hz.

It is possible to create phase-shifts with zig-zag windings (or other windings connected in delta, except that's useless with MAR's as there is no neutral point), connect another rectifier, and common-up the outputs with an interphase reactor. Ripple is even lower then, and higher frequency, and the input current starts to look really like a stepped sinewave. But complexity as you say increases, it's uncommon except in demanding cases (I'm working on a 30-pulse system in the day job, sadly not using MAR's!)
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Old 28th Jun 2019, 5:56 pm   #16
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Default Re: Mercury arc rectifier.

Quote:
But if each limb of the transformer has a centre-tapped secondary, you can connect all the centre-taps together to get your -ve output terminal. The six 'hot' leads from the transformer go to six limbs on your MAR
What you say is true, however most of the installations I have encountered used two-group secondaries. As far as I know, the main reason was to minimise conduction overlap due to transformer inductance, and the corresponding DC voltage droop. Without the interphase reactor, the commutating volt-seconds occupy a much larger fraction of the half-cycle, during which both active anodes are pulled to their mean voltage.
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Old 28th Jun 2019, 6:28 pm   #17
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Default Re: Mercury arc rectifier.

The common or garden fluorescent tube is a mercury arc, they use ordinary soda glass to stop the UV getting out.
 
Old 29th Jun 2019, 1:43 am   #18
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Default Re: Mercury arc rectifier.

Indeed, and special germicidal fluorescent tubes are manufactured with special glass that DOES let the UV out.
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Old 29th Jun 2019, 9:21 am   #19
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Default Re: Mercury arc rectifier.

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Originally Posted by Lucien Nunes View Post
UV is often mentioned relative to mercury arc rectifiers, without any figures to indicate how much is emitted ... I am not going to perpetuate the guesswork by making any definitive statements until I have a chance to do some measurements ...
Certainly the right approach Lucien. The relative intensities of mercury's UV emission lines depend very strongly on the arc conditions (temperature, partial pressure of the mercury vapour, presence of other gases) and also on the envelope material. In the case of fluorescent tubes the internal phosphor coating is also explicitly designed to absorb as much of the UV as possible and then to re-emit it as visible fluorescence (hence the name, although the distinction between prompt fluorescence and delayed phosphorescence seems to have been lost). So guessing the answer would be next to useless.

It might be worth pointing out though that one of the lines, which can be among the strongest (or can be negligible) is at 254nm. This is pretty much at the peak of the absorption curve of DNA https://www.researchgate.net/profile...en-320-and.png. It's rather effective at damaging the DNA and in terms of cancer initiation that can be A Very Bad Thing.

Interestingly one of the commercial ways of monitoring integrated UV dose used to be by exposing bacterial spores over a period of time and then seeing how many of those spores were still viable at the end of the exposure. I used to work in an environment where there was a lot of scattered UV laser light at 249nm and we looked at using those monitors to ensure that we stayed below safe exposure limits.

Cheers,

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Old 29th Jun 2019, 10:06 pm   #20
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Default Re: Mercury arc rectifier.

I still remember seeing the MAR and the carbon arc projectors at the cinema in my then home town of Nuneaton. The film projectors had an autofeed device to maintain the arc gap on the film projectors but the slide projectors for the adverts were manual feed.
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