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DC Power

Published : 30th July 2007

class="style2">> How does the DC vs AC help this issue?

In terms of colour temperature, most powerhouses on the major lots have precise control over voltage and current so chances are the DC your getting on stage is exactly 120v DC and, if you were to put an Oscilloscope on the line would show a beautiful flat DC line.

AC dimmer systems don't necessarily provide true line voltage at 100% so lamps may be a touch warmer when running on large dimmer systems, especially if your talking about socapex runs of 300' Plus. I've seen AC dimmers showing voltage as low as 105v under load.

These days we may be a little dimmer happy. I'm certainly guilty of it, but in the last two years I've been forced to rig stages with a significant amount of DC because we have simply run out of available AC power--I've fallen in love it. Rigging power hungry sky-pans and cyc strips DC has saved me time and time again when I'm fighting for valuable dim-able AC power for more critical lamps.

DC power has some significant cost advantages over AC on long term projects as well. It's much less expensive to generate DC then it is to purchase AC from the city. The DC generators in Paramount's powerhouses for example require only 400A ac to provide 4000A of available DC service.

Also, when we rig a stage for AC, power is generally routed out of the stage, into an air-conditioned trailer or "dimmer shack" full of dimmers and then back into the stage as dim-able AC power. DC power, with the absence of dimmers and fans can stay inside the stage, cutting the required cable at least in half.

There are also significant cost savings when you substitute good old fashioned deuce boards for dimmer packs and ac switchboards.

class="style2">>>Some down side issues are the mechanical (and thus HUGE) >>individual dimmers

DC dimmers are essentially impossible to get these days in Hollywood, and while some lots still have a few lying around, most don't tie in easily with existing DMX dimmer boards. We had few on a show I did last year at paramount and they were great, but they were rigged up as a favour for me from the 40 shop there.
I would love to see rectified DC dimmer packs for 10k and 20ks, I think its a great idea.

Erik Messerschmidt
CLT, LA


In the "old days" we use to have DC on stage for lighting. AC service was available for props such as television sets.

I remember the beginning of the end for DC service on Hollywood sound stages. At MGM the stage was being rented to a commercial company. A production assistant plugged the producer's computer into the DC Service. Over the years, and with the advent of HMI's use on stage, DC service got fewer calls. The confusion over what was AC and what was DC required support from on-lot crew.

Roy H. Wagner ASC
director of photography


Hi,

class="style2">>>The DC generators in Paramount's powerhouses for example require >>only 400A ac to provide 4000A of available DC service.

Someone want to explain that one to me, because it sounds like you've just solved the world energy crisis.

Phil Rhodes
Video camera/edit
London


Phil ,

You must know the difference between , Direct Current and Alternating Current, wish you were correct about solving worlds energy probs. Hope you have good new year, regards.

John Holland , London.


class="style2">>>The DC generators in Paramount's powerhouses for example require >>only 400A ac to provide 4000A of available DC service.

"Someone want to explain that one to me, because it sounds like you've just solved the world energy crisis."

I think that must be... "plus a few hundred gallons of diesel".

Tim Sassoon
SFD vfx & creative post
Santa Monica, CA


Current is not power. If you have an M-G set and you put 1200V in and get 120V out, you can get ten times the current out and power is still conserved.

Actually, in real life, if you are making DC from AC, you get only seven times the current, since with DC the peak and average powers are the same and with AC it is not. Plus whatever loss in heat and noise come from the M-G sets.

Scott Dorsey
Kludge Audio
Williamsburg, VA


class="style3">>>DC dimmers are essentially impossible to get these days in >>Hollywood, and while some lots still have a few lying around, most >>don't tie in easily with existing DMX dimmer boards...I would love to >>see rectified DC dimmer packs for 10k and 20ks, I think its a great >>idea.

I'm up to date on variable DC power supplies to 35kW and would like to investigate developing a big DMX DC lighting pack.

One question: Are LA sound stage three phase tie-ins 208/120 volt wye circuitry like generator outputs? Frankly I suspect developing such a pack would generate the classic Engineer's retort "It can't be done, and if it could be done, it would cost too much money", but everyone needs a windmill or two in their life to tilt at.

Hal Smith
Engineer and Somewhat DP
Edmond, OK


Phil Rhodes wrote:

class="style3">> Someone want to explain that one to me,

I'm with Phil, can anyone point me to a good book or website explaining how it works?

Never known much about it's use in film.

Though I do know why we don't use DC in general overall, it doesn't transmit very far without loss, and we'd have to have generators all over the place.

Which makes it great for a studio lot, but that's about it?

Steven Bradford
Dir. School of Film
Collins College
Tempe Arizona


In film use its main use was for Carbon Arcs , Brutes etc , still a lot of Brutes around , problem is finding Generators to power them.

I am talking about here in UK.

John Holland , London.


class="style3">>>Though I do know why we don't use DC in general overall, it doesn't >>transmit very far without loss, and we'd have to have generators all >>over the place.

Actually watt for watt at a given voltage the loss is no different. It's the ability to easily generate, transform, and distribute AC at high voltages that make the difference. Ohmic loss is equal to amps squared times resistance, for the same power level transmitting higher voltages will have less loss. However at the extreme high voltages that long distance transmission lines operate at, AC peak voltage (as opposed to RMS average voltage) corona loss becomes an issue (the electricity literally leaks off the line) and there are quite a few modern EHV transmission lines operating with DC!

Hal Smith
Engineer and Somewhat DP
Edmond, OK


Erik Messerschmidt wrote :

class="style3">>>In terms of colour temperature...AC dimmer systems don't >>necessarily provide true line voltage at 100% so lamps may be a >>touch warmer when running on large dimmer systems

Erik is always a wealth of information. thanks.

I kept wondering if there was something strictly AC vs flat-line that had an effect on the colour temp - some exotic concept in filament harmonics (don't worry, I just made that up) - that's what I couldn't wrap my head around. But couldn't see forest for the trees, its the simple line-loss and lower voltage running the lamps warmer. Makes perfect sense of course.

Those stages you've rigged partly DC up in the perms - away from those pesky laptops getting plugged in, and powering the coops and skypans...same thing you did for us on 'Everybody Hates Chris', DC dimmers and all, right?

I think Paramount Lighting loved that and the DC room had one of its "submarine motors" happily spinning along for our stage.

Thanks for the info.

Mark Doering-Powell
LA based DP


Hi,

Allow me to restate.

Yes I'm fully aware of the situation with AC and DC which is why I was interested to find out why someone thought they were getting 4000A of 120V DC from, presumably, 400A of 120V AC (given that it wasn't specified.)

One presumes the motor-generators run off something other than 120VAC which makes the comparison rather meaningless unless it -is- specified, yes?

Phil Rhodes
Video camera/edit
London


class="style3">>>One presumes the motor-generators run off something other than >>120VAC which makes the comparison rather meaningless unless it >>-is- specified, yes?

Yes

Mark Weingartner
LA


Your right Phil, I wasn't clear. The motors which drive the dc generators on most major lots take 480v 3Ø AC @ 400A, and output 120VDC with a capacity of 4000A. It is a misleading number without the supplemental info. Regardless, its still an efficient use of power. Not all the DC you find on the lot is necessarily motor generated however.

Unless you require a serious load most likely the dc your using on stage is rectified in the power house.

Rectifiers are also the preferred method for carbon arcs these days as they allow us to use existing location AC power distribution.

Erik Messerschmidt
CLT, LA


class="style3">>>Those stages you've rigged partly DC up in the perms - away from >>those pesky laptops getting plugged in, and powering the coops and >>skypans...same thing you did for us on 'Everybody Hates Chris', DC >>dimmers and all, right?

Yes, on 'Everybody Hates Chris' our sky-pans and a number of 10ks and Coops were rigged DC with dimmers provided by Paramount's 40 shop. We were popular for it. On 'Bones' we have a similar rig, only sans dimmers (fox doesn't have any). In both instances there was no other way to rig it because we were out of available AC on the stage. On bones we use close to 3000A dc on a daily basis.

Erik Messerschmidt
CLT, LA


Hi,

class="style3">>> Regardless, its still an efficient use of power.

Much as I hate to be awkward, I'm still not entirely clear on how this can be efficient - the converters, of whatever type, have by definition a sub-ideal efficiency. Why is this such an attractive way to distribute power, from an efficiency standpoint?

I'd be interested to see how clean the rectified DC is, too.

Phil Rhodes
Video camera/edit
London


class="style3">>>Why is this such an attractive way to distribute power, from an >>efficiency standpoint?

In most situations on the major lots relatively unlimited DC power is available free of charge. Additional AC power on the other hand is very expensive. As I mentioned before the simplified distro in a DC system also makes it attractive. The "efficiency" differences between transforming the voltage down to usable 120vac and generating DC are irrelevant.

Erik Messerschmidt
CLT, LA


class="style3">>>"The motors which drive the dc generators on most major lots take >>480v 3Ø AC @ 400A, and output 120VDC with a capacity of 4000A"

Sorry for the previous idiotic comments. Someday I'll just shut up and learn something...

class="style3">>>"Rectifiers are also the preferred method for carbon arcs these days >>as they allow us to use existing location AC power distribution."

Like thusly. Intriguing; despite probably being old enough to, I've never used them. Would they be a useful alternative to a SoftSun for tighter beam spreads, or perhaps just better colour rendition than a big HMI? Hopefully not too stupid a question.

Tim Sassoon
SFD vfx & creative post
Santa Monica, CA


Phil, the rectified DC that's available on stages here in Hollywood doesn't need to be that clean - lights aren't precision instruments. I don't think anyone's really tested it but I could be wrong.

Ted, the reason that DC is 'free' on the lot is because the infrastructure is already there, and the company is already paying for the AC that it takes to generate the DC power. The powerhouse already has the DC generators in place, which use the existing AC transformers. There is circuitry run from the powerhouse to the stage. Adding AC to the stage means adding supplemental infrastructure, i.e. either a generator outside the stage (not good for a long term project as it takes up a precious parking space and creates exhaust and noise), or a power drop from the city (expensive, as it requires the lot to pay for permits, cable, transformers, etc.).

On a short term project, it's possible to pull power from an adjacent stage, but that's not very practical long term either as the company that rents the adjacent stage may want to use their power....

Ted Hayash
CLT


Ted Hayash wrote (edited for CML length):

class="style3">>>Ted... DC is 'free'... is because the infrastructure is already there... has >>paid for the AC it takes to generate the DC power.

Thanks for the explanation, Ted. Making use of the existing (DC) infrastructure makes sense.

Sounds like it makes shooting on the lot(s) economically attractive for productions that can use DC, or that are going to be using a lot of power for lighting, as Erik described.

Having to jump thorough hoops as you outlined to get AC into a stage is surprising and I'd bet a BFPIA?

Is that just due to DC being tied to cabling that has been there a long time, and a studio choice not to build a parallel AC-for- lighting infrastructure?

What powers the HVAC, non-production lighting and such?

Interesting and educational thread. Thanks!
Ted Langdell
Ted Langdell Creative Broadcast Services
Marysville, CA


Ted,

Shooting on the lots is economical for lots of reasons, not just the DC power. To be perfectly clear, nearly all stages in Hollywood have AC power, and lots of it. It's just that with bigger and bigger sets (don't quote me on the size, but the big set on Bones takes up an entire 30,000 square foot stage), and the need for control of every light or set of lights individually, means that sometimes the 10,000+ amps of AC power already permanently installed on the stage just isn't enough. That's where the DC circuits come in handy - it's another 4000 amps available by a telephone call. That's one reason to be shooting on the lot - it's only the old established soundstages that have that luxury available.

At Raleigh Manhattan Beach, which is a much newer facility where quite a few television shows are filmed, there is about 10,000amps of AC or more on each stage, but no DC and no powerhouse like on the older lots. It's still a nice place to work - but the other lots have really great set lighting departments, 'canvas rooms' where there are people sewing materials full time, a full time grip department that manages the grip equipment and hangs supplementary catwalks or green beds, and a '40 shop' which is the where the members of IBEW Local 40 maintain, repair, and sometimes build anything electrical that's used on the lot. This may seem like overkill in some sense, but it's really handy when you need some coloured Kino Flo tubes RIGHT NOW - just ask the lot best boy, he'll get them from the lamp dock if they have them (and production approves the cost, of course....)

Enough rambling....

Ted "this post is making me late for dinner" Hayash
CLT
Los Angeles, CA



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