Lighting Phase Cancelations & Amps

[koncept]

Hello,

I got a questions about phase cancelations and amps... I have a 100 Amp three phase "company switch" and when doing shows we wire in a 8 channel nsi dimmer rack setup using single phase. When we threw an amp meter on there at hte last event leg1 (black) had 26ish amps on it (i was using an analog meter), leg 2 (red) had about 31 amps and the neutral had about 27 on it.

I thought that when you had a balanced load so say each leg in use had 30 amps on it, the neutral would have 0 amps on it. Am I wrong?

Thanks in advance

[soundman]

You didnt really have a balenced load, by not using the 3rd leg there was not full cancelation- I think

[koncept]

that is what i was worried about. we dont have the cable to make the dimmer three phase. we are looking to purchase it but have not done so yet. in the mean time we do a show once a week and need the rack (ideally we need double that). so until the cable gets ordered and everything else happends i wanted to get another opinion on the cancelation here.

also another question in a 110 single phase the neutral and the power leg should be ......
for us on a satelite pack they were the same

[STEVETERRY]

Originally Posted by koncept

Hello,
I got a questions about phase cancelations and amps... I have a 100 Amp three phase "company switch" and when doing shows we wire in a 8 channel nsi dimmer rack setup using single phase. When we threw an amp meter on there at hte last event leg1 (black) had 26ish amps on it (i was using an analog meter), leg 2 (red) had about 31 amps and the neutral had about 27 on it.
I thought that when you had a balanced load so say each leg in use had 30 amps on it, the neutral would have 0 amps on it. Am I wrong?
Thanks in advance

Actually, you did not have a balanced load because you connected to only two phases of a three phase service. Therefore, the C (blue) phase had nothing on it, so you did not get full cancellation.

Also, when using phase-control SCR dimmers like your NSI unit, you rarely get cancellation and in fact you may see neutral curents up to 1.3 times greater than the highest phase current, depending on dimmer settings. That's because an SCR dimmer is a non linear load, and does not draw current in pure sine wave, but in a chopped sine wave. This prevents the cancellation of current in the neutral. that's why the NEC requires a 130% neutral for portable dimming equipment using single conductor feeds.
Finally, you say you were using an analog meter. In order to get accurate readings of voltage on the output of an SCR dimmer, or current drawn by non-linear loads, you need to use a True-RMS-Responding meter,which would be very rare these days in an analog device.

Look for a digital meter that has True-RMS as a spec item. You can get one for well under $100. Otherwise, your readings are off by a large factor.

ST

[SHARYNF]

If you look inside the dimmer and see how it is wired up, typically you will see that there are 4 lug bars, and that the individual dimmer circuts are distrubuted across the three hots and the neutral IF it is wired up in three phase, and across two hots and the neutral in split phase. Again TYPICALLY so to convert to three phase, you might have a built in transfer taking the third phase load off the two phase legs, or need to move things around.
On the NSI dimmers inside you will see these bars, and you will see that the wires are moved from the third leg when you move them back for three phase, you will have 3 3 and 2 so you are never going to get a completely balanced load. You will also see that there are small fused connection on all the phases, these are for the lights to show what legs are active.

I have a bunch of the 6 channel NSI dimmers, since most places don't have three phase I usually leave them on single phase since it is still a bit of a pain to open them up and move the connections around and you are not looking at a very high load. IF you always do use three phase then it makes sense to change them over.
Also remember that when you use a typical meter you are measuring the leg values, not the phase values, So you need to multiply the values you get by the square root of three (1.732)

www.radioelectronicschool.net/ files/downloads/3phase.pdf

Three phase power and or just using two of the phases gets confusing, since we are typically used to hwome power which is different since there is a 180 degree shift on home power but a 120 degree shift on three phase power

Sharyn

[STEVETERRY]

Originally Posted by SHARYNF

Also remember that when you use a typical meter you are measuring the leg values, not the phase values, So you need to multiply the values you get by the square root of three (1.732)

Err....not exactly. A dimmer pack with 120V outputs connected to a 208Y/120 service is WYE connected, not DELTA connected. Your statement about multiplying the phase current by 1.732 to get the line current only applies to delta-connected equipment that does not use a neutral. In a wye-connected system, the phase current measured on each of the three phases equals the line current.

So where might we encounter delta-connected loads in the theatre? The most common is a power distribution feeding three-phase chain motors or automated lights that operate at 208V single-phase, typically arc lights with 1200W sources.

ST

[SteveB]

Originally Posted by STEVETERRY

Also, when using phase-control SCR dimmers like your NSI unit, you rarely get cancellation and in fact you may see neutral curents up to 1.3 times greater than the highest phase current, depending on dimmer settings. That's because an SCR dimmer is a non linear load, and does not draw current in pure sine wave, but in a chopped sine wave. This prevents the cancellation of current in the neutral. that's why the NEC requires a 130% neutral for portable dimming equipment using single conductor feeds.
ST

Steve

A question about harmonics. And a thread hijack warning.

I was under (the potentially incorrect) impression that harmonic distortions as you have described above - and in other articles over the years, do not become an issue if the demand load is under 70% of feeder size. As in, my total and typical demand load is around 500 amps per phase (architecture 'kinda limits us to anything more), with feeder sizes on my system being split across 2 - 3 phase/5 wire feeds ea. at 800 amps per phase (1600 amps total - 3 phase).

The neutrals were not upgraded (during renovation) to double conductors as it was deemed cost prohibitive to replace all the existing feeder cableing and conduits back to service entrance at the time of upgrade, as well as the existing service entrance 800 amp fused switches, AS WELL as the existing Con-Ed service entrance feeders back to the street vault transformers - which unless these feeds are upgraded as well, makes the whole neutral upgrade pointless.

According to what I was told (names of the potentially guilty parties withheld) I had to see over 560 amps per phase, on each of the feeder systems - or a total of about 1160 amps per phase total for the entire system, before I might see harmonic issues.

Any thoughts appreciated.

Steve Bailey
Brooklyn College

[STEVETERRY]

Originally Posted by SteveB

Steve
A question about harmonics. And a thread hijack warning.
I was under (the potentially incorrect) impression that harmonic distortions as you have described above - and in other articles over the years, do not become an issue if the demand load is under 70% of feeder size. As in, my total and typical demand load is around 500 amps per phase (architecture 'kinda limits us to anything more), with feeder sizes on my system being split across 2 - 3 phase/5 wire feeds ea. at 800 amps per phase (1600 amps total - 3 phase). The neutrals were not upgraded to double conductors as it was deemed cost prohibitive to replace all the existing feeder cableing and conduits back to service entrance at the time of upgrade.
According to what I was told (names of the potentially guilty parties withheld) I had about 560 amps per phase before I might see harmonic issues.
Any thoughts appreciated.
Steve Bailey
Brooklyn College

You are right. Harmonics and the resultant neutral overcurrents only become an issue where the total load in use is close to the total amount of service available.

Note that my previous comment about 130% neutrals only applies to single conductor portable feeders. That's because 4-wire feeders in conduit heat as a system and the only time we see neutral overcurrents in phase control dimming systems is when the phase currents are less than full. Single conductor portable feeders are rated at their free-air ampacity, so the "system heating" model does not work--hence the 130% requirement.

In past articles, I've often recommended the use of a 4-display, true RMS ammeter on the service feeding a dimmer system, especially systems with feeds derated below the system nameplate rating. This lets one know precisely what is going on with neutral overcurrent.

I also recommend Harmonic Mitigating Transformers for feeding dimmer systems (see upcoming article in the Fall ESTA Protocol).

ST

[JD]

Having a balanced load is more important then having low current in the neutral. The neutral is speced to be the same gauge as the hots because it is expected to handle current. Depending where you have your dimmers set, the neutral current will be all over the world during a live show anyhow. The only time you have near zero current on the neutral would be if you had three non-dimmed equal loads, one on each leg. At that point the current draw would be true delta on a wye feed. (very rare) Design balance into your show, but never expect it! SCR / Triac dimmers all work by chopping the waveform, so even with a balanced load there is going to be current on the neutral between the chop points on different channels. The trick is that it is only going to be flowing during fragments of the waveform, so I am not sure any meter is going to tell you the truth. If you wind a couple of turns of wire around the outside of the neutral line, and feed it to an oscilloscope, it looks pretty wild! (Done it, been there.)

[STEVETERRY]

Originally Posted by JD

Having a balanced load is more important then having low current in the neutral. The neutral is speced to be the same gauge as the hots because it is expected to handle current. Depending where you have your dimmers set, the neutral current will be all over the world during a live show anyhow. The only time you have near zero current on the neutral would be if you had three non-dimmed equal loads, one on each leg. At that point the current draw would be true delta on a wye feed. (very rare) Design balance into your show, but never expect it! SCR / Triac dimmers all work by chopping the waveform, so even with a balanced load there is going to be current on the neutral between the chop points on different channels. The trick is that it is only going to be flowing during fragments of the waveform, so I am not sure any meter is going to tell you the truth. If you wind a couple of turns of wire around the outside of the neutral line, and feed it to an oscilloscope, it looks pretty wild! (Done it, been there.)

As long as you use A True-RMS-Responding meter with a reasonable "crest factor" (the ability to measure a non-sinusoidal waveform), you can rely on the currents you read.

ST