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AC line noise help.
Greg_Cameron:
--- Quote from: Barry Singleton on December 12, 2013, 05:28:19 PM ---Form a dollar and cents point of view since I am pretty much on my own with the power company and the manufacturer having done all that they are willing to do and a 7.5KW isolation transformer for the office looks more affordable that a transformer sized to run +180HP of lathes.
--- End quote ---
Either way, it does indeed sound like machines and the rest of the office needs to be on their own separate services. I vote for secondary transformer, both feed from the incoming utility service directly and not piggybacked.
Mike Sokol:
--- Quote from: Greg Cameron on December 12, 2013, 07:43:58 PM ---Either way, it does indeed sound like machines and the rest of the office needs to be on their own separate services. I vote for secondary transformer, both feed from the incoming utility service directly and not piggybacked.
--- End quote ---
Considering that you had sparks from the neutral, realize that a high-resistance neutral can inject all sorts of noise into a system. And there's even more problems if the safety ground and neutrals are double-bonded in the sub-panels which allows neutral currents to flow in the grounds. Time to check exactly where and how the incoming Neutral and Grounds are bonded.
Stephen Swaffer:
I agree with Mike check neutrals and bonding including the grounding electrode system at the service. I have worked on a lot of these machines and in similar situations. The machines will be 480 VAC delta -no neutral connection at all. The office panel coming from a 480/240VAC transformer is most likely fed 480 VAC delta-no neutral, secondary is 3 phase 4 wire Y creating its own neutral which should be bonded to ground. The ONLY common connection for a neutral (if it exists) on the 480 VAC system and a 240 or 208 system is the grounding bond. For the noise to travel over the neutral it must travel over/through the grounding system-which is what bonding and a grounding electrode is supposed to try and prevent. 360 HZ isn't really a high enough frequency to start misbehaving too badly in seeking a ground-if it has a good solid path somewhere.
Otherwise, a good electrical engineer should be able to build a low pass filter to supply the machines-allowing 60 HZ to pass and attenuating noise above that.
Mike Sokol:
--- Quote from: Stephen Swaffer on December 13, 2013, 01:58:21 AM --- The machines will be 480 VAC delta -no neutral connection at all. The office panel coming from a 480/240VAC transformer is most likely fed 480 VAC delta-no neutral, secondary is 3 phase 4 wire Y creating its own neutral which should be bonded to ground.
--- End quote ---
Another thing to check will be a high-leg delta transformer feeding the panel. That was commonly used in industrial buildings that need a lot of 240-volt 3-phase power for motors, and much less 120-volt single phase for office equipment. Of course a 480 high-leg delta would normally only split down to 240-volts single phase (if memory serves), but there were a lot of strange things done with 1970's power that we would never consider using modern code. Maybe there's a way they did high-leg delta 480v to single phase 120v. I'll think about this a bit, but a simple neutral to leg voltage measurement would confirm a high-leg delta.
Barry Singleton:
Hi All;
Tomorrow when I shut down and do the rest of the panel maintenance I will attempt to acertain where and how the ground and neautral are bonded and where they are not. I will have a licensed jounyman electrician with me for the day.
In answer to a couple of questions/suggestions:
The buiding was built in 1996 and there is no high leg.
The huge transformer that feed this block of buildings is about 200 feet across the street from the distribution/meter panel on the building. The power that feeds my building comes in underground from the dist/meter panel to a 480V panel that in turn powers some equipment like heaters , some air handling equipment ,some smaller machines and three step down transformers for the rest of my building. There was inadequate power in the building for the two new machines so, there was another 480V line of service pulled in from the distribution/meter panel straight down the wall to a new 480V panel just for these two machines. This service has it's own meter and nothing else is on it at all.
I don't recall for sure how many lines come from the main transformer across the street to the meter panel here on the building but I think there were six. How that is split up in the dist/meter panel I don't know.
What I think is most interesting is that I can also hear the sound of (sounds like arcing) in the main transformer across the street when the machines break and to me that's crazy!
Thanks for taking the time to reply with ideas and help.
All the best,
Barry.
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