Control/Dimming Self Balancing Dimmer Rack

[TheME]

I work at a local theme park that has a variety of theaters. In our main theater we have 2 ETC sensor dimmer racks with dimmers 1 & 2 on phase 1, 3 & 4 on phase 2, 5 & 6 on phase 3, 7 & 8 on phase 1, etc. aka "self balancing dimmer rack". Our patch bay is set up next to our dimmer rack and the circuits have male stage-pin connectors on them so you just plug them in to the corresponding dimmer like your plugging it into a wall socket (if wall sockets were stag-pin that is). My boss wants the patch bay to stay hooked up 1 to 1 but if i do that on rack A phase 1 will pull 263 amps and phase 2 will pull 198 amps, also on rack B phase 1 will pull 181 amps and phase 3 will pull 86 amps. I have called ETC support and they have confirmed that no a "self balancing dimmer rack" does not actually balance itself and if you have varying loads you must move things around to balance the load. Problem is my boss believes a "self balancing dimmer rack" actually balances its self and you don't have to check it, also he has a degree and worked in Las Vegas for a while so he knows better than me (and ETC apparently) so I'm wrong.
I've been researching for hours and cant find anything that i can physically show him that backs me up on this. If anyone knows where or what web site i can find that clearly lays out the capability, and incapability, of this rack it would be a huge help. I have searched the manual and ETC's website and they don't have anything directly talking about this subject.

 

[derekleffew]

First, if a dimmer rack has dimmers 1,2,7,8,13,14... on ØA; 3,4,9,10,15,16... on ØB; 5,6,11,12,17,18... on ØC; it's called a "balanced rack," not a "self-balancing rack." The intent is to somewhat randomize the loads among the three phases. There is not, to my knowledge, a dimmer rack that can automatically shift loads around on the three phases.
(The terms "balanced vs. straight (sequenced)" don't seem to exist outside the language of the CEM. Whoever at ETC invented the term did a disservice, but I can't think of a more descriptive term. Do other dimmer manufacturers offer both a "Numeric Edition" and "Designer's Edition" of the same rack?)

... I have called ETC support and they have confirmed that no a "self balancing dimmer rack" does not actually balance itself and if you have varying loads you must move things around to balance the load. ...

IF an ETC service tech told you this, you were given mis-information. As long as your currents are within the OCPD, the phases do not need to be balanced. STEVETERRY said so, but I can't find the exact quote. Will continue searching.
EDIT: Found it!

Guys--please just relax.

A properly designed three-phase dimming system should be able to tolerate whatever phase imbalance you can throw at it.

100% on A, zero on B & C , why not?

There is no reason to consider phase balance as part of the production check list.

Y'all have bigger things to worry about!

ST

See also this post by Mr. Terry, as well as the entire thread.

-----

... and worked in Las Vegas for a while so he knows better than me ...

Trust me when I say that working in Las Vegas should bear very little weight on whether someone knows anything or not. I'm proof of that!

-----

TheME, nice play on words in your screen name, Themepark Master Electrician. Stop by the New Member Board to give yourself a proper introduction.

 

[JD]

Currently, there is no product on the market that can be truly considered a "Self Balancing Dimmer Rack."

However-
Those day are not too far away!

Technically, using IGBT sign wave dimmers, the design is not far from reach! It would be a simple matter to rectify the incoming phases and supply a master +/- buss system that all of the dimmers would work off of. A 1kw load placed on channel #1 would draw 333.3 watts off of each phase A, B, and C.

The only thing that may prevent this from coming to market is the cost / need analysis. Do you pay 400% more for your dimmers, or do you have someone employed who knows how to balance a load? This becomes even less of a factor if the theater has been designed properly. As ST would say, a 100%, 0%, 0% load imbalance should not present a problem if the system is designed properly.

 

[xander]

Both JD and ST inserted a little prerequisite into their statements: a properly designed system. Perhaps you could expand on what a properly designed system entails so that the OP can tell whether they have one and can stop worrying?

-Tim

 

[derekleffew]

... so that the OP can tell whether they have one and can stop worrying?

The OP has no cause to worry. ETC would not allow an installed rack to be energized if it were not connected to an improperly designed system. Just one of the many reasons a factory-authorized tech is required for turn-on.

 

[cdcarter]

The OP has no cause to worry. ETC would not allow an installed rack to be energized if it were not connected to an improperly designed system. Just one of the many reasons a factory-authorized tech is required for turn-on.

I'm still interested in hearing the short version of the answer!

 

[JD]

I'm still interested in hearing the short version of the answer!

Short? As Derek said above, "As long as your currents are within the OCPD, the phases do not need to be balanced."

Dimmers, by their nature, are single phase devices. Enough current should be available on each leg to feed the dimmers attached to that leg. (Check NEC for overage allowances in theater.) In practice, systems are laid out with the intended hope of being balanced, thus racks with numbers of channels divisible by 3. The real world nature of a lighting plot and the need to run scenes assures that rarely occurs.

Balance suddenly becomes important in the touring world when the available supply falls short of what would be optimum. (Example- The venue that only has a 200a switch when a 400a switch was spec'ed in the rider.)

EDIT:
And then the long answer:
520-27. Stage Switchboard Feeders. (c) Supply Capacity. For the purposes of computing supply capacity to switchboards, it shall be permissible to consider the maximum load that the switchboard is intended to control in a given installation, provided that:
(1) All feeders supplying the switchboard shall be protected by an overcurrent device with a rating not greater than the ampacity of the feeder.
(2) The opening of the overcurrent device shall not affect the proper operation of the egress or emergency lighting systems.

The key word is "Intended." All systems are designed to have some capacity headroom. That word makes things a little gray. Perhaps ST could add some of the thought behind this.

 

[SteveB]

Remember as well that often times you have little control as the actual loading on the dimmers, which is usually determined by the equipment choices of the lighting designer as well as the location of the available circuiting.

Thus even if the rack has it's dimmers nicely distributed across the phases, that doesn't guarantee the loads will be distributed evenly across the dimmers.

 

[derekleffew]

...

520-27. Stage Switchboard Feeders. (c) Supply Capacity. For the purposes of computing supply capacity to switchboards, it shall be permissible to consider the maximum load that the switchboard is intended to control in a given installation, provided that: ...

The key word is "Intended." All systems are designed to have some capacity headroom. That word makes things a little gray. Perhaps ST could add some of the thought behind this.

He already has. See Dimmer feeds--How much power is enough? - ControlBooth .

 

[David Ashton]

presumably, a system with dimmers means that there are lighting changes happening all the time, each light state is a totally differently balanced load, so the concept of balancing dimmed loads is only a generalization in any case.

 

[STEVETERRY]

presumably, a system with dimmers means that there are lighting changes happening all the time, each light state is a totally differently balanced load, so the concept of balancing dimmed loads is only a generalization in any case.

David, that is precisely correct.

And, if the transformer, switch gear, and dimmer rack are Listed, by NRTL and installation is done to Code [in the US, the NEC], the system as a whole can readily tolerate up to 100% phase imbalance.

As to the concept of a "self balancing" dimming system, let's not waste a lot of time on that, since:

A. It's science fiction as of today.
B. It is totally unnecessary.

Beyond that, I don't have a lot more to say on this other than the articles and threads that Derek pointed out.

ST

 

[robartsd]

Technically, using IGBT sign wave dimmers, the design is not far from reach! It would be a simple matter to rectify the incoming phases and supply a master +/- buss system that all of the dimmers would work off of. A 1kw load placed on channel #1 would draw 333.3 watts off of each phase A, B, and C.

You could probably produce a sine wave that pulls 1/3 of the power from each phase, but you could not do it in a way that would pull a linear load on the supply side (the nature of 3 phase power is that the phases cancel each other out—simply taking 1/3 of each wave linearly would not change that), so the supply side advantages of the sine wave dimmer would be lost. Now if you wanted to provide some alternate waveform, IGBT equipment that can draw from all three phases might be worth investigating.

 

[JD]

You could probably produce a sine wave that pulls 1/3 of the power from each phase, but you could not do it in a way that would pull a linear load on the supply side (the nature of 3 phase power is that the phases cancel each other out—simply taking 1/3 of each wave linearly would not change that), so the supply side advantages of the sine wave dimmer would be lost. Now if you wanted to provide some alternate waveform, IGBT equipment that can draw from all three phases might be worth investigating.

Ahh! By using a star bridge rectifier (basically 6 diodes) you would produce a common DC buss for all the dimmers, thus distributing the load. (See 3 phase power supply for the Strong Gladiator.) It is actually exactly how a diesel locomotive engine works- Three phase generator output rectified to a common DC bus, then goes into an IGBT "chopper" which then generates it's own 3 phase sign wave output to drive each of the four motor "trucks" under the "engine." By the way, you should see the size of those IGBTs !!

As with any DC supply, the majority of the current draw occurs at the peak of the waveform, which gives it less than an ideal power factor.

But, as ST has said, why bother? (outside of the mental experiment.)

 

[shiben]

Ahh! By using a star bridge rectifier (basically 6 diodes) you would produce a common DC buss for all the dimmers, thus distributing the load. (See 3 phase power supply for the Strong Gladiator.) It is actually exactly how a diesel locomotive engine works- Three phase generator output rectified to a common DC bus, then goes into an IGBT "chopper" which then generates it's own 3 phase sign wave output to drive each of the four motor "trucks" under the "engine." By the way, you should see the size of those IGBTs !!

As with any DC supply, the majority of the current draw occurs at the peak of the waveform, which gives it less than an ideal power factor.

But, as ST has said, why bother? (outside of the mental experiment.)

So then an AC locomotive basically skips this process?

 

[robartsd]

Ahh! By using a star bridge rectifier (basically 6 diodes) you would produce a common DC buss for all the dimmers, thus distributing the load.

At which point you might as well just send DC through your circuits (unless you use dimmer doubling). It might even be cheaper to build DC dimmer modules; but unless the savings counters the costs of creating the DC buss, there wouldn't really be anything to gain.

 

[JD]

So then an AC locomotive basically skips this process?

They all start out as AC. The diesel engine drives a three phase alternator which produces AC, which is then rectified to DC. An AC alternator is much easier to build then a DC generator and a lot more maintenance free. The very old locomotives ran the AC output through an SCR chopper much like a light dimmer. No DC involved. Modern locomotives use DC and IGBTs to recreate three phase AC. The process is far less wasteful and it is easier to synchronize multiple engines pulling a long freight train. Efficiency = fuel = money.

I like the 2.9 megawatt spec on the generator! (full article- http://ankit09apllication.blogspot.com/2010/10/diesel-electric-locomotives-in-india.html )

At which point you might as well just send DC through your circuits (unless you use dimmer doubling). It might even be cheaper to build DC dimmer modules; but unless the savings counters the costs of creating the DC buss, there wouldn't really be anything to gain.

You could.... although I am told lamp life is reduced on DC as the redeposit of the tungsten in the halogen cycle is uneven. Also, DC is harder on connectors, (think DC Super-Trouper Arc) although the output of such a dimmer would actually be pulsed DC.

Again, cost puts all these ideas in dreamworld for now.

 

[SteveB]

For any students interested, there are any number of fine books on the subject of the development of electrical distribution systems. One of my favorites is "Edison, Tesla and Westinghouse and the Race to Electrify the World'. Has much to say about the differences and applications of DC versus AC current.

One of the fascinating things (in my mind) is how (at the time) AC won out over DC due it's ability, thru the easier application of step up/step down transformers, to allow electricity to be transmitted over long distances. All of which has been somewhat mitigated by the improvements in AC and DC rectifying circuits that now allow huge voltages and wattages of DC to be transmitted over distance and under water. One of the best uses is providing interconnections between large transmission systems where the DC connection eliminates the need to sync two systems of AC. Simply rectify to DC, transmit, then rectify back to AC. Widely used in Europe and Japan and seeing use here in the US as well.

 

[JD]

Guess this has kind of walked off topic! But.. I was reading about the "DC pipeline" project for connecting wind farms together at sea, and it is a fascinating change in the whole concept of high energy transmission. Things have indeed changed!

 

[dramatech]

They all start out as AC. The diesel engine drives a three phase alternator which produces AC, which is then rectified to DC. An AC alternator is much easier to build then a DC generator and a lot more maintenance free. The very old locomotives ran the AC output through an SCR chopper much like a light dimmer. No DC involved. Modern locomotives use DC and IGBTs to recreate three phase AC. The process is far less wasteful and it is easier to synchronize multiple engines pulling a long freight train. Efficiency = fuel = money..

All good stuff, but you don't have to go as far a locamotive. Take a look at the alternator in your car. It is a three phase AC generating device. There are 9 diodes. one for each of the three phases when the sine wave is plus, one for each of the negative going part of the sine wave on each phase and one each to send to the regulating circuitry. So it is essentially 6 half wave rectifiers, that all sum together and produce the slightly more than 12 volts DC. The three used in regulating, are used in a comparing circuitry so that the alternator doesn't over change the battery.
This is how the performance analyzing devices that you can plug into your cigarette lighter, and measure RPMs work. The DC used in charging the battery and running the car is never pure DC. It still has some of the components of AC generated by the alternator. As the speed of the car changes, and the alternator runs at different speeds, the peaks of the sine waves can be counted and give the RPMs.
Performance engine management computers used in race cars, will sometimes go nuts from power from an alternator, that has one or more diodes shorted, even though the rest of the diodes will work fine in charging the battery.
For those of us that are old enough to remember when cars had gererators instead of alternators, Your battery could go dead, if you idled the engine too long. The straight DC generator just isn't as efficient as an alternator running at the same speed.

Tom Johnson

 

[JD]

Yep! The reason I used the locomotive example is that once converted to DC, they go and change it back to AC with the same basic IGBT system used in the sign wave dimmer circuits. (And on such a grand scale!)