Control/Dimming dimmer flicker - aluminum feeder?

[Plaza]

Have (6) Lite-Puter 12 channel x 20 amp units that are two years old. After an hour or so we'll start getting flickers. Ran new DMX, swapped out consoles, had elec company come in and verify supply. ALL checked out fine.

Then I find out old lighting dimmer rack had same flicker issue going back 8+ years. So drilling down the only thing that's common is supply and output cables to fixtures.

Brought electrician in and took supply apart. Each dimer is on own dedicated 3 phase / 208 v supply using #4 copper cable to all.

Then we discovered one of the supplies on one dimmer is #4 alum AND all 6 neutrals are #4 alum.

At this point our theory is that the #4 alum returns are causing back pressure since they can't handle the return loads when running a full concert rig.

Any ideas? Is it worth spending approx $800 to swap out all the alum for copper AND up size the neutrals to #2.

Thanks for any feedback.

 

[Les]

Maybe the aluminum wire expands after it heats up, causing resistance in the connection?

Is it worth spending approx $800 to swap out all the alum for copper AND up size the neutrals to #2

IMO, Yes. I'm no expert, but from what I heard, aluminum materials in dimmer racks nearly burned down Vegas >>[i]derekleffew[/i]

 

[derekleffew]

Plaza, upon reading your post, before you even got to the part about the aluminum wiring I was thinking "loose neutral," caused by expansion/contraction at the terminals due to heating.

I would find a company who can do a thermal analysis of the feeder terminals, perhaps using an instrument such as Fluke - Infrared Camera, Thermal Camera, Thermal Imaging Camera, Thermal Cameras. Try Googling "YourCity, electrical thermal imaging".

Unfortunately, it'll likely cost more than $800, and will diagnose, but not resolve, the issue.

Regarding the sizing of neutrals ...
NEC (2008) Article 520.53(O)(2):

Where single-conductor feeder cables, not installed in raceways, are used on multiphase circuits feeding portable switchboards containing solid-state phase-control dimmers, the neutral conductor shall have an ampacity of at least 130 percent of the ungrounded circuit conductors feeding the portable switchboard.

I don't know your installation, so don't know whether the above applies to you or not.

 

[church]

At this point our theory is that the #4 alum returns are causing back pressure since they can't handle the return loads when running a full concert rig.

Any ideas? Is it worth spending approx $800 to swap out all the alum for copper AND up size the neutrals to #2.

The conductivity of aluminium and copper are both very good which is why they are both used as conductors - aluminium being the prefered material for the high voltage lines because it offers better strength/weight properties than copper. It is not the material of the conductor that is your issue aluminium will not cause "back presuure" you will have to explain what you mean by that. However as someone has already said check that all the connections are tight and clean with no sign of arcing and make sure the conductors are not undersized. However flicker is usually a sign of a loose connection or a problem with dimmer firing circuit. is it alll the dimmers, one dimmer or a group of dimmers?

 

[epimetheus]

The fact that the conductors are aluminum is not a problem in itself. The problem would be if they are inappropriately sized, installed, etc. It should be noted the #4 copper and #4 aluminum DO NOT have the same ampacity.

The biggest concern to me with this installation would be a loose neutral, as derekleffew said. Aluminum has a higher coefficient of thermal expansion than copper does. This means that for a given temperature change, aluminum will expand more than copper. This leads to a greater potential for a loose terminal with aluminum cable, as opposed to copper cable.

Also, since phase-control dimmers are non-linear loads, I'd definitely be concerned that 5 of your 6 dimmers have a neutral with a lower ampacity than the phase conductors. I'm having trouble finding the code reference, other than what DL has already posted, that governs sizing the neutral conductor for dimmers. 310.15(A)(4)(c) specifies that the neutral is a current carrying conductor when supplying non-linear loads, and therefore the appropriate ampacity derating must be applied. Can somebody more knowledgible than me point out what NEC article would cover the conductor sizing for the supply of a permanently installed dimmer?

BTW, derekleffew's NEC reference above should be 520.53(O)(2) for NEC 2008.

 

[Plaza]

We had elec company come in and verify supply... all OK.

Electricians came in and ALL connections are tight w/ no arcing or other heat issues. Did thermal image a year ago and nothing in any of the wiring stood out but not sure lights were loaded and flickering when they did image (wasn't there at that time).

When dimmers are dumping surplus to neutrals at under 100% usage by lights that's when alum wire may be causing 'back pressure' to dimmers by not allowing all the surplus to be returned to elec grid. This could be screwing up 60hz wave and flicker is from drop to 55hz or so being caused by neutral. Trying to get a recording Fluke hz meter and see it that's happening.

This is 3 phase so neutral size and capacity of return is important to allow proper flow when dimming. #4 alum has about 20% less amp carrying capacity than #4 copper.

 

[epimetheus]

When dimmers are dumping surplus to neutrals at under 100% usage by lights that's when alum wire may be causing 'back pressure' to dimmers by not allowing all the surplus to be returned to elec grid. This could be screwing up 60hz wave and flicker is from drop to 55hz or so being caused by neutral. Trying to get a recording Fluke hz meter and see it that's happening.

This is 3 phase so neutral size and capacity of return is important to allow proper flow when dimming. #4 alum has about 20% less amp carrying capacity than #4 copper.

Not sure what you mean by "dumping surplus to neutrals at under %100". These are SCR dimmers, correct? I can't find a manual or data sheet on these dimmers - lite-puter's website doesn't seem to work for me. What's the required supply ampacity per the manufacturer? I'm not sure I understand this "back pressure" as you call it. When I've heard the term before it's been in reference to voltage drop, since in the water to electricity analogy, pressure is votlage. If this is the case, "back pressure" would be the higher resistance of your neutral, which would cause the neutral to heat up when loaded. This wouldn't cause a change in the supply frequency. It's very difficult for a load to cause the supply frequency to change when you're connected to a utility. The utility is essentially an infinite source since your load is so small compared to the available generation.

 

[Plaza]

my understanding is that while the dimmer pack is 12 @ 20 amp max or 240 amps at full load 3 phase it could at times be dumping 50, 60, or more amps to the grid on neutral leg. if the neutral can't handle the higher load is causes a disturbance the 3 phase wave form that manifests itself as a flicker in the lights.

"""It is important to note that NEC Section 520-27 (a) (3) requires that the neutral feeding SCR phase control dimmer racks be considered a current-carrying conductor. This is because of harmonics, since the neutral is not simply carrying the imbalance current of the three phases as it would in a linear load situation. The net effect is to cause derating of conductor ampacity to 80% of the values in table 310-16 when there are four wires (three phases plus neutral) in a conduit. This reduces the overall heating of the four-wire system. Note that this derating will not be required if the feeder conductors are installed in a raceway (not a conduit) with not more than 30 total conductors, and where the cross-sectional area of the conductors does not exceed 20% of the cross section of the raceway (NEC section 352-4). Even with the derating above, it may also be desirable to further increase the neutral size above the phase conductor size. The NEC does not specifically require this increase in size, but it creates a low impedance. This minimizes dimmer interaction and flicker, two common symptoms of undersized neutrals.""

 

[FMEng]

My advice is to get a different electrician. You are describing a problem that is a thermal intermittant. If the problem is indeed a power supply issue, you should be able to measure a difference in the supply voltage between when the system is cold, and when it heats up enough to act up. If a TRUE RMS voltmeter shows no significant change on PHASE to NEUTRAL voltages, then it isn't an AC supply problem. If you cannot measure a big change, then the problem is elsewhere, like the dimmer pack.

Did the electrician measure things before/after the system gets hot enough to fail? Did he make the measurements under load? If they were not made under load, then they are worthless.

He may have been concentrating on the wiring from the dimmer pack to the breaker panel. Did he look for a problem between that breaker panel and the main electrical service, or between the service and the meter shunt or the utility transformer?

How is the ambient temperature where the dimmers live? Does it get hot where they are after running hard for an hour. If so, there is your culprit.

We have discussed the wire material, and the wire size, but the other factor in voltage drop is the wire length. How long is the run to the breaker panel?

 

[Plaza]

Found out that problem predated current dimmer packs (installed in 2008) back to old lighting dimmer racks as far back as 1995. Only thing common to both is supply to racks and output to lights.

Transformer is 10 feet away from mains (900 amp each) on outside of building. Through wall and 6 feet to breaker box then individual circuits 10 feet to dimmer rack on stage left.

Both electric company engineers and our electrician agree all wiring, voltages, etc. are completely in spec up to the breaker panel. That's where the alum wire for neutrals came into play.

Finally got a hold of Lite-Puter engineer in Taiwan and awaiting his suggestions.

 

[sk8rsdad]

What has me puzzled is the OP has lite-puter dimmers and the discussion is around the wiring???? The first thing I would be checking is whether those dimmers have any dust build up, or are showing signs of component failure. Lite-puter does not have a reputation for supplying the highest quality gear.

 

[epimetheus]

my understanding is that while the dimmer pack is 12 @ 20 amp max or 240 amps at full load 3 phase it could at times be dumping 50, 60, or more amps to the grid on neutral leg. if the neutral can't handle the higher load is causes a disturbance the 3 phase wave form that manifests itself as a flicker in the lights.

First, full load on the pack would be 80A, 3 phase. When dealing with 3 phase systems, currents are called out as line currents, i.e. 80A per phase. If you want to use the 240A number, you need to clarify that as 240A, single phase.

Second, if all 3 phases were loaded equally with purely resistive loads, you would have no neutral current. That is the nataure of 3 phase systems. In the case of phase control dimmers, the dimmers introduce harmonics on the neutral. Even a channel at 100% will cause some harmonics content in the neutral, thus when all channels are fully loaded, you should have no fundamental current in the neutral, but you will have harmonic current. Though I doubt it will be 50A or 60A. That much harmonic current would lead me to think somehting is wrong with the dimmers. Of course, I've never acutally measured this, so I could be wrong on the anticipated amount of harmonic current.

Has anybody actually measured the harmonic current produced by phase control dimmers at 100%?

 

[derekleffew]

... Has anybody actually measured the harmonic current produced by phase control dimmers at 100%?

See this post, in particular the .pdf file attached. Although the experiment was conducted in 1981, the data is likely still valid.

See also this post and its attached files.

For more on harmonics, see So you think you know about power?. Just to tie that thread to this one, there's also mention of Lite-Puter!

EDIT: Sorry, epimetheus, I misread your question, thinking it hypothetical. Still, I'm leaving the above here for illustrative purposes. The answer to the OP's problem may very lie in Mr. Terry's Protocol article, except for the fact that the flickering doesn't begin until after an hour of use.

What has me puzzled is the OP has lite-puter dimmers and the discussion is around the wiring????

Since the same symptoms were experienced previously with a different brand of dimmers, yes. Although it would be interesting to bring in rented, quality dimmers for a test, that's likely cost-prohibitive.

 

[Les]

Don't you all know?? Lite-Puter pioneered DMX in the US back in 1999!

From Control/Dimming: "Facts" about DMX - ControlBooth:

1. "DMX" was created in 1986, but not widely used or known. At that time, the popular method for transmitting signal to dimmer packs was zero to ten volts.
2. "DMX" was revised in a major way and started gaining recognition in 1990. (But not in America).
3. The first DMX lighting controllers were displayed at the American LDI Lighting convention in 1999 by LitePuter.
4. "DMX" was introduced as the "Standard" digital signal for the World Wide lighting industry in 2000.

/sarcasm
/off topic

 

[STEVETERRY]

See this post, in particular the .pdf file attached. Although the experiment was conducted in 1981, the data is likely still valid.

See also this post and its attached files.

For more on harmonics, see So you think you know about power?. Just to tie that thread to this one, there's also mention of Lite-Puter!

EDIT: Sorry, epimetheus, I misread your question, thinking it hypothetical. Still, I'm leaving the above here for illustrative purposes. The answer to the OP's problem may very lie in Mr. Terry's Protocol article, except for the fact that the flickering doesn't begin until after an hour of use.


Since the same symptoms were experienced previously with a different brand of dimmers, yes. Although it would be interesting to bring in rented, quality dimmers for a test, that's likely cost-prohibitive.

This discussion has thrown off a lot of interesting topics. However, I'm prompted to repeat one of my favorite troubleshooting mantras:

"When you hear hoof beats, don't suspect zebras."

Take a look at the Liteputer pack before looking too hard at the power.

Only after doing that, get a Power Quality Analyzer on the feed and see what's really going on.

ST

 

[DimmerDotCom]

Have (6) Lite-Puter 12 channel x 20 amp units that are two years old. After an hour or so we'll start getting flickers.
Then I find out old lighting dimmer rack had same flicker issue going back 8+ years. So drilling down the only thing that's common is supply and output cables to fixtures.
Brought electrician in and took supply apart. Each dimer is on own dedicated 3 phase / 208 v supply using #4 copper cable to all.
Then we discovered one of the supplies on one dimmer is #4 alum AND all 6 neutrals are #4 alum.
Any ideas? Is it worth spending approx $800 to swap out all the alum for copper AND up size the neutrals to #2.

Hire an electrician with at least a scopemeter if not a power analyzer. You will want him to monitor the feeds and the neutral as you run the dimmers and take before and after flicker screen shots. You may be experiencing a distorted sinewave with a dip in the peaks. This could point to a loose or under size neutral or a transformer that is saturating.

As for mixing wire types...I would have all the feeds and the neutral in copper with the neutral upsized a min of 20% over the feeds. On studio rolling racks, when we get calls about flickering, we would tell them to double their neutrals and you would be surprised on the amount of problems that fixed.

 

[PeteEngel]

Also, has anyone looked at control as being the issue? wankie DMX signal or a repeater inline that isn't quite right?... after an hour of lights on and such, would the AC be kicking on? many potentials here...

 

[Chris15]

The conductivity of aluminium and copper are both very good which is why they are both used as conductors - aluminium being the prefered material for the high voltage lines because it offers better strength/weight properties than copper.

The main reason for aluminium being used for distribution is not to do with it's electrical properties, it conducts well enough (copper is still better by a way) but compare the pricing:

Metal Prices Per Ton
Copper - US$6,606.00
Aluminium- US$1,968.00
Last Updated: 21/6/2010

Source:Electra Cables Pty Ltd