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Old 7th Dec 2016, 5:00 pm   #101
threeseven
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Default Re: Mercury Arc Rectifier? Where to start

Yes, easy to set up an initial test rig, just to see it in operation.
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Old 7th Dec 2016, 6:29 pm   #102
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Default Re: Mercury Arc Rectifier? Where to start

Plus some inductance to force continuous current, too... A resistive ballast won't do this (on 3-phase there wouldn't be a problem due to phase overlap).
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Old 7th Dec 2016, 7:37 pm   #103
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Default Re: Mercury Arc Rectifier? Where to start

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Originally Posted by merlinmaxwell View Post
a 50-0-50 transformer
A 55-0-55v power tool transformer will likely be more widely/cheaply available, just be aware the secondary centre tap is usually earthed on these.
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Old 7th Dec 2016, 10:32 pm   #104
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Default Re: Mercury Arc Rectifier? Where to start

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It seems to me that there are three things needed to get this going, an ignition source/solenoid, enough volts to strike and enough current at a sufficient voltage to maintain an arc. I have a thought for the latter two a 50-0-50 transformer with it's primary in series with some ballast (say 20A at 20V for the rectifier, that is 400W so a small heating element would suffice). I will bow down to Lucien's advice on this one (these three!).
Doesn't the load go between the cathode and the secondary centre tap (in series with a choke for smoothing / to keep the current flowing during the zero crossings)? 400w would indeed be a bowl fire element - my wife dislikes my old Pifco bowl fire beaming at my head from 18" at the best of times; attached to a MAR she'd really get the hump
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Old 8th Dec 2016, 12:28 am   #105
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Default Re: Mercury Arc Rectifier? Where to start

Hi all,
The MARS without exciters & just a dipper arm is wired as attached. Missing from the drawing is the big choke & controlers. On the nevilectors the primary of the transformer was tapped off at lots of points and the mains fed in through a heavy duty (100 Amp) fine & corse stud switch. This gave good control over the arc current & you could go up or down in steps of about 5 Amps at a time with the fine switch or about 20 Amps with the corse.
They were a joy to use & gave a good steady light with no flicker.
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Old 8th Dec 2016, 7:06 am   #106
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Default Re: Mercury Arc Rectifier? Where to start

"They think it is an antique glass what?" Urinal?

Andy.
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Old 8th Dec 2016, 9:00 am   #107
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Default Re: Mercury Arc Rectifier? Where to start

Quote:
Originally Posted by kalee99 View Post
Hi all,
The MARS without exciters & just a dipper arm is wired as attached. Missing from the drawing is the big choke & controlers. On the nevilectors the primary of the transformer was tapped off at lots of points and the mains fed in through a heavy duty (100 Amp) fine & corse stud switch. This gave good control over the arc current & you could go up or down in steps of about 5 Amps at a time with the fine switch or about 20 Amps with the corse.
They were a joy to use & gave a good steady light with no flicker.
So it looks as though the mercury arc was struck into an open circuit before the projector carbon arc was struck?

Martin
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Old 8th Dec 2016, 11:21 am   #108
kalee99
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Hi Martin,
Some had a push to start button but on mine you just struck the arc & the dipper arm was activated by the solenoid which went out of curcuit once the arc struck up. I think they had a current relay? Its a few years ago now. If anyone wants them they are still there in my old cinema. It was a nightmare getting them up the stairs

Paul.
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Old 8th Dec 2016, 11:28 am   #109
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Default Re: Mercury Arc Rectifier? Where to start

I think the carbons are shorted to start with, and then pulled apart along with tap control to allow arc voltage and current to both increase.

But that would be just one application, and possibly not the best application unless you wanted a UV laden suntan in double quick time.
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Old 8th Dec 2016, 11:33 am   #110
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Default Re: Mercury Arc Rectifier? Where to start

That sketch schematic isn't 100% representative - it omits the ignition ballast, load sensing contactor and choke for example - and the Nevelector equipments do not use an auxiliary supply, only the main secondary. Exactly as Paul describes, the rectifier responds to striking the lamp arc. The carbons have the AC secondary voltage between them through a few hundred ohms until struck, which completes the ignition circuit. This starts the bulb in a fraction of a second so that by the time you stop winding the carbons in and start winding out again the supply is on; the delay is imperceptible. The arc current immediately operates the load contactor to isolate both the dipper and the solenoid.

I will eventually get to do the video of this happening in the real unit, and then if time permits, an equivalent using home-brew control gear. Unfortunately working both day and night shifts at the moment...
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Old 8th Dec 2016, 2:34 pm   #111
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Hi all,
The circuit I posted was just the basic bones of a generic MARS. Striking the arc in the lamphouse was something any projectionist who did it will well remember. With the power off you lined up your carbons visualy in the lamphouse with the door open. leaving a gap of about 1/2 an inch between positive & negative. Then you closed the lamphouse door, applied the power and peering through the dense glass viewing window you operated the carbons feed untill they came together & struck up. A loud pop would be heard as the current rushed to the tips of the carbons & you would see a view of a miniture ball of plasma through the dense glass.
Then you set the gap using a periscope type focusing device which shon a projected picture of the arc onto a card marked up with the correct placement & gapping of the rods. Once happy you could then engage the auto feed for the rods & you were ready to start the show.
I will never forget the hiss the carbons made when burning correctly, or the distinct smell like the London underground.
Happy days,
Paul.
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Old 8th Dec 2016, 3:43 pm   #112
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Auto feed? What's that, Paul? (lol... Twin motor feed is super isn't it?)

There's a difference between power supplies for arc lamps and general DC power supplies. For arc lamps, a significant ballast is necessary due to the negative resistance of the arc. This can be achieved by resistors in the DC side, or more elegantly, chokes on the AC side, which won't waste power.

For the biphase MAR, there's got to be a DC choke on the output to force continuous conduction, but this is nothing to do with ballasting. A choke input DC filter, provided the choke etc is good, has very good regulation and hardly and sag of output voltage with increasing load. That's what most loads want, but not arc lamps, hence the additional ballast.
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Old 8th Dec 2016, 5:57 pm   #113
Lucien Nunes
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Quote:
For arc lamps, a significant ballast is necessary due to the negative resistance of the arc. This can be achieved by resistors in the DC side, or more elegantly, chokes on the AC side, which won't waste power.
And that's the main difference between unit rectifiers and a single DC bus. If the output of the rectifier serves only one lamp, its voltage can be allowed to follow the lamp voltage and hence AC control upstream of the rectifier can be used. The two selector switches that Paul mentions select taps at the two ends of a control choke in the transformer primary circuit. The disadvantages are that a separate rectifier is needed for each lamp and rectifier failure (albeit rare) puts the projector out of action.

Many cinemas had a shared DC supply with perhaps two rectifiers serving all projectors, slide lanterns and spots, usually with the ability for the show to continue with one unit out of action. However a common supply had to work at constant voltage so the ballasting needed to be done resistively on the DC side, significantly reducing the efficiency. The same was necessary with alternative DC sources such as motor-generators and engine-driven generators, indeed back in the days of DC mains, DC-DC M-G sets were necessary as there was nothing to transform, choke or rectify.

I wonder how many people today have practical experience, or an opinion on, of the difference in behaviour between a rectifier-supplied arc and an MG-supplied one?
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Old 8th Dec 2016, 6:16 pm   #114
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Auto feed? What's that, Paul? (lol... Twin motor feed is super isn't it?)
The auto feed was a shunt wound motor on the arc lamp & fed from the arc supply. It kept the carbons in trim when set up properly. Different makes of arc had different set ups. The BTH lamphouse had this clever "jig jig", solenoid device that pushed two toothed wheels around that fed the carbons. It was so clever and worked perfectly for me for a whole 20 minute reel of film. Keeping the gap and position of the carbons spot on. On the other hand Peerless Magnarc had a shunt wound feed motor that would burn out on a regular basis if not kept well lubricated.
Paul.
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Old 9th Dec 2016, 12:25 am   #115
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Default Re: Mercury Arc Rectifier? Where to start

It's OK, I do know but I've also had to hand feed with some really awful arc lamp houses which were just about worn out (Monarcs, by Frank Brockliss who later started to make Peerless under licence). And I've also used Autoarcs which used photocells to speed up or slow down feed motors according to the carbon tip position - OK in theory till the focussing lenses get covered in arc dust! Favourites were Kalee President with a separate motor for each carbon.

Reference separate or combined rectifiers, I've only worked on separate rectifiers (a single Westinghouse arc rectifier set dating from 1936 which I did most of my shows on as actually twin 3-phase transformers and twin bridge rectifiers - copper oxide I think - in a single cabinet). DC output was pretty smooth, there was no stroboscope effect on anything rotating. What MG sets would have been like, I don't know, though again if each arc had its own generator, it could be partially series wound and hence have a drooping characteristic.

I've never worked on MAR's though I know people who have... Lucky folks!
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Old 11th Dec 2016, 6:48 pm   #116
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Default Re: Mercury Arc Rectifier? Where to start

I have just been up to Derby and secured myself a 'thing'.
For nearly 3 hours on the way home I had to hear a 'ting' every time the dipper arm jostled up and down, but it was still attached when I got home. I am comforted by the fact that in use at high current the dipper seems to jostle around anyway, from the videos I have seen. Always wanted one. SWMBO is less than convinced, but you can't please everyone!

This latest cluster of history was discovered in a loft, I am advised, and would now appear to be all but sold.
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Old 11th Dec 2016, 8:15 pm   #117
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Welcome to the bulb collectors' club... Pictures please! Your other half will grow to love the bulb in time; they are a little daunting at first but their sheer elegance and undoubted utility win over even the most skeptical newcomer.

The dipper does tend to jiggle about when moved, there's nothing to damp its vibrations, not even air resistance, so I wouldn't worry. Provided you keep the mercury from surging up the arms while handling, you won't hurt it.

Depending on what time I finish work tonight I might have a go at making the video of Katie bulb in action. A 1.6R 20A resistance is standing by to serve as a load.
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Old 11th Dec 2016, 10:00 pm   #118
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Default Re: Mercury Arc Rectifier? Where to start

Excellent, thanks for welcome, do you upload your vids to youtube or elsewhere?
Will do photos, most likely next weekend, it is still boxed up from it's journey.

Single phase, shape is roughly of a GLS bulb. 2' high. All terminals are secure. I will certainly need guidance on the transformer/choke/load aspects in due course!

I am not sure what triggered this particular obsession, i think it may have been a documentary voiced over by John Peel, about the IOM tram/train systems. The maintenance man opened a cabinet and there it was. It made me question how i had reached the age of 40 without even knowing such surreal looking technology existed.
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Old 13th Dec 2016, 12:26 pm   #119
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Default Re: Mercury Arc Rectifier? Where to start

Similarly, I first saw them schematically in Newnes Electrical Technology, at about age 18.

First saw one in the flesh at tram museum, Crich. I was in my early 20s. They do exist, I thought.
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Old 13th Dec 2016, 4:37 pm   #120
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I've always assumed that the Tees Transporter Bridge uses these, although I've never seen them.
It was my earliest introduction to both DC traction, and the straight-cut gear sound that invariably goes with it.
I must try to find out. The bridge dates from 1911, but the trams ran until '35-ish, which would be about right for mercury-arc rectifiers rather than rotaries, I think?
It would be good to have a compendium of existing mercury arc installations throughout the British Isles.
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