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Author Topic: Nonlinear harmonics?
147Wiz
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Icon 1 posted May 31, 2002 20:03      Profile for 147Wiz     Send New Private Message       Edit/Delete Post 
Hi All,

I was wondering if any of the computer savvy Geeks out here can
explain something about what happens beyond the power supply in the direction of the receptacle into which the machine is plugged.

Part of what I do at work is run power lines in offices which will feed desktop computers. Normally, when we run two or three circuits in common with a shared neutral with balanced loads the neutral wire is sized the same as the hot wires.

However, we upgrade the neutral wire at least one size when the circuits are for desktop computers. Generally we use a #10 AWG
with two #12 AWG wires.

When I ask other people such as the foreman or straw bosses at work the answer given is usually "Nonlinear Harmonics and how come you ain't done yet?" and no further conversation ensues. The engineers are never on the site at the same time we are and so I hope somebody here knows the electrical universe that lies beyond the silicon.

Thanks


Do you have any idea how many union electricians it takes to screw in a light bulb?

Ed


Posts: 30 | From: NYC | Registered: Jan 2001  |  IP: Logged
Jessycat

Solid Nitrozanium SuperFan!
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Icon 5 posted May 31, 2002 20:35      Profile for Jessycat     Send New Private Message       Edit/Delete Post 
quote:
Originally posted by 147Wiz:

Do you have any idea how many union electricians it takes to screw in a light bulb?



House or Production?

Sorry, must edit: I am theatre-centric, and re-reading your post made me realize that my question made no sense!!! Please ignore it.


Posts: 491 | From: NYC | Registered: Mar 2002  |  IP: Logged
147Wiz
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Icon 3 posted June 01, 2002 09:18      Profile for 147Wiz     Send New Private Message       Edit/Delete Post 
Hi Jc,

The question actually does make sense because my union does have jurisdiction at the Javits Center and I've spent 20 hour days as a production electrician (we're called show extras) doing show lighting for the Auto Show and the Boat Show and things like that.

Great money, aching body.

Ed


Posts: 30 | From: NYC | Registered: Jan 2001  |  IP: Logged
quantumfluff
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Icon 1 posted June 01, 2002 15:10      Profile for quantumfluff     Send New Private Message       Edit/Delete Post 
<rant>
I've done a lot of trade shows at the Javitts Center. I will say, with strong conviction, that the whole union system is morally bankrupt. Let's go through a list of problems:

Example 1: Our booth was in the center of a 20'x20' carpet and needed lights.
I am not allowed to plug the lights in. A member of the eleictrical workers union must do that. It requires running an extension cord from the outlet at the edge of the booth, under the carpet and out throught the middle. The electricion said there was no hole in the middle and I said, "OK, I'll cut one". He stopped me and insisted I get a carpet layers union employee to cut the hole (In my damm carpet BTW). This means at least a half hour wait for the carpet worker and then another half hour to get the electrician back to plug the friggin light in.

Example 2: End of the show. We have a portable booth but we own the carptet rather than renting it.
We collapsed the booth and got it ready. My brother started to roll up the carpet so we could pack it for shipment. Within seconds a union steward showed up to say "You can't roll up that carpet". My brother said "Of course I can, it's really easy". The steward pushes him away and says "You're puttin' someone out of a job". Then he got security to watch us as we waited an hour for his toady to come over and roll up the carpet. When that was done, we still had the responsibility of bagging it for shipment, so all arguements about weight and liability over accidents are moot.

Example 3: We had a complex booth that required assembly and we didnt' arrange with our booth vender for assemblers.
We had to use the local people. I spent a 10 hour day with my colleague watching 2 guys assemble our booth. We had detailed instructions which any 10 year old could have followed, but they would not read them. I had to watch as they tried to fit parts together which obviously would not fit - even though the instructions laid out what went where. When they tried to assemble things and it didn't work, they would undo it all and start again. It took from 9am to 7pm to put together a booth that the two of us could have built in 3 hours. It was so inefficient I wanted to cry.
</rant>


Posts: 2902 | From: 5 to 15 meters above sea level | Registered: Jun 2000  |  IP: Logged
LifetimeTrekker
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Icon 1 posted June 01, 2002 18:16      Profile for LifetimeTrekker     Send New Private Message       Edit/Delete Post 
Being the research rat I am, I couldn't help but look up the term to try to put some meaning to it in the context of your question. The best clue I had on your scenario comes from:

http://www.oulu.fi/~spaceweb/textbook/plhr.html

Not being an electrical engineer or physicist, the only suggestion I have is the different materials may be used to avoid the formation of VLF signals coming into the desktop system.

If any of you folks more into material science than I can elucidate, I'm a little curious to see the correct answer.


Posts: 669 | From: Albuquerque, NM, US | Registered: Sep 2001  |  IP: Logged
MrJ
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Icon 1 posted June 03, 2002 01:38      Profile for MrJ     Send New Private Message       Edit/Delete Post 
The question was about the neutral/ground/sinking current, leaving the computer equipment. The bigger neutral wire is meant to handle larger current; wires don't condition the line. Computers have single-phase switching power supplies that use pulse loads. The supply changes impedence to grab the peaks of the sine wave voltage, which is the nonlinear part. All the switching causes the harmonics (multiples of the frequency). With balanced three phase power distribution, normally the current waves of each phase cancel on neutral. However the 3rd harmonics (and other odd multiples) caused by the switching don't cancel and can add up to significantly higher currents on the shared neutral (try a switched load on each phase).

The best explanation I found was here, along with many solutions:
http://powerquality.copper.org/issues.html
Except they skip the actual math.

Brief explanation and things that can go wrong:
http://www.mikeholt.com/studies/harmonic.htm

Brief explanation and a little math (max neutral current):
http://www.powersmiths.com/faq/index.shtml


Posts: 35 | From: near Grand Rapids, MI | Registered: Mar 2001  |  IP: Logged
147Wiz
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Icon 14 posted June 03, 2002 17:50      Profile for 147Wiz     Send New Private Message       Edit/Delete Post 
LT and MrJ-- Thanks to both for your help. The web sites were very interesting. I haven't visited Fourier Analysis since my networks courses in the early sixties and I seem to have forgotten everything. At last, some use for this stuff!


qf, I fully understand your feelings about life at the Javits Center. I've done shows in other venues and had friends exhibit at the Javits. It's a hell of a system for small business owners and employees to confront but it's what's here in the city. It costs time and money to make money and in NYC the numbers go through the roof. When I'm a buyer in this kind of situation I just bring a chair and watch them work (better his knees than my arthritis.)

I do hope your shows are successful and you make enough for the aggravation to be worth it.


Ed


Posts: 30 | From: NYC | Registered: Jan 2001  |  IP: Logged


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