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This topic comprises 2 pages: 1 2
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Topic: Portable digital projector that will work at a drive-in
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Stephen Furley
Film God
Posts: 3059
From: Coulsdon, Croydon, England
Registered: May 2002
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posted 10-25-2008 01:04 PM
quote: John Hawkinson
Just measure the voltage (hot-to-hot, i.e. small prong to small prong) on the two circuits.
If the voltage is ZERO, you have the same phase.
If the voltage is 208V, you have two differenct phases.
If the voltage is 120V, you have screwed up (phase-to-ground voltage).
If the voltage is 240V, something is whacko.
Or it's a domestic type centre-tapped single phase supply? I suppose you'd be unlikely to have that at a drive-in, but you might elsewhere.
quote: John Hawkinson
if the voltage is 277V or higher, seek professional help.
You really like to make your electrics complicated over there, don't you! ![[Roll Eyes]](rolleyes.gif)
quote: John Hawkinson
If you're running 100' extension cords, you might find the voltage sags noticable (perhaps from 208 down to 200 or 190) when you actually power up the projector. That could well be a serious problem.
--jhawk
Could you run two cables connected in parallel to reduce the resistance, or would your regulations not allow that? I'm not sure if it would be allowed here. The way you'd do that sort of thing here would be to simply run a cable of whatever size was required, with a pin and sleeve type connector on the end of it, 3-pin blue for single phase, 5-pin red for three-phase. No question about what Voltage you need. If you don't have a suitable supply available you hire a generator which in small sizes, up to 125A, use the same type of connectors.
120, 208, 240, 277, 347, 480, 600. Wye, delta, centre grounded, corner grounded, ungrounded, dozens of different NEMA sockets... We'd have electricians jumping out of windows if they had to cope with the number of different Voltages and systems that you have over there! ![[Confused]](confused.gif) ![[Smile]](smile.gif)
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John Hawkinson
Film God
Posts: 2273
From: Cambridge, MA, USA
Registered: Feb 2002
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posted 10-25-2008 07:30 PM
It's always dangerous when you try to be comprehensive, especially for the point of injecting humor. Yes, 240V shows up in the US for a single center-tapped phase (split-phase), which is typically only for residential service.
And if you have single-phase equipment, 240V might be acceptable to the equipment (check the nameplate!), or more scarily, the voltage drop (under load, only!) might bring it down to something acceptable, which is a dangerous way to go as well.
(We have some Irem single-phase rectifiers that are running through boost transformers at around 240V where they perform much better than at 208V. Their nameplate voltage is 220V, and they have trouble producing sufficient voltage to strike 2K lamps when fed 208V...)
Running two phases from different single-phase circuits is a temporary jerry-rigging that, if permitted at all by code, qualifies as a temporary installation. You can parallel two cables as temporary installation as well, yes. Depending on what kinds of currents we're talking about and what gauges of wire, it might or might not be reasonable. (But, of course, at that rate you're probably better off just renting the higher gauge cable from a local theatrical supply house.) Several of these answers are probably only things that should be done by people with more experience than I think Justin has...
For a temporary installation, to combat voltage drop, you could also use a boost transformer. Generally this is a no-no, because then you have voltage that significantly with the load, which is not a terribly safe condition. But if your load is constant and you're aware of it, and it is a temporary installation, it is not so terrible.
--jhawk
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Stephen Furley
Film God
Posts: 3059
From: Coulsdon, Croydon, England
Registered: May 2002
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posted 10-26-2008 05:32 AM
So, what sort of supply would you typically be asked to supply for a temporary installation for a large outdoor screen? Somebody suggested that a 6kW lamp would be appropriate; given less than unity power factor, losses and ancillary equipment I would guess that guess that you'd be looking at something like a 10-12 kVA supply. Would this generally be 208/120V three phase Wye? If so, then this is basically a half-Voltage, higher frequency version of what we have here. I would guess that you wouldn't try to send a supply very far at these Voltages, and that if you had an event any great distance from an existing supply, say in the middle of a park, then you would probably use generators.
Would the lamp rectifier be fed with 208V three phase, and everything else be 120V single phase? A 20A supply might do this, but it would be a bit tight, and I think the next size up is 30A, so I'm guessing that you'd put in 30A three phase 208/120V Wye.
How would you terminate the temporary supply at the remote end, would it be hard wired into a distribution panel, or plugged in?
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Barry Floyd
Phenomenal Film Handler
Posts: 1079
From: Lebanon, Tennessee, USA
Registered: Mar 2000
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posted 11-18-2008 12:16 PM
Justin,
We did a similar thing at my drive-in about 2 years ago. I had a guy call me who wanted to propose to his girlfriend and put a video on screen of their dating relationship, well wishes from friends and family, etc.. He was a film student at Watkins Film School in Nashville, and he told me he had a video projector that he could use as long as I had an extension cord.
When I met him out at the theatre one night after work, and he pulled out this little conference room table video projector, about the size of a small VCR. I was thinking to myself ... NO Way was this going to work as we were standing 275 feet away from the screen. He plugged it in and I must honestly say... I was AMAZED. It actually worked. Albeit, the mismatched lens made the picture about twice the size of the screen, and the picture was a little dark... but with about twice as much light and the right lens... it definetely had potential for something. (Advertising slides, private rentals, etc.) I anticipated the guy would leave the theatre with his "bubble burst" because his little dinky projector would be the equivalent of shining a flashlight on the screen from the back row.
He played part of his tape, and I walked up to the bottom of the screen expecting to see pixels the size of basketballs.... it was perfect.
This wasn't some fancy digital cinema thing, this was an ordinary business-style conference room table video projector.
I believe the name brand was "In-Focus", and the model number was the "Screenplay 5000"
We pulled 2 100 foot extension cords from the ticket booth and ran the projector from the roof of his car. Instead of the projector being back at the building, it was probably only 100 feet from the screen.
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