On Friday 01 January 2016 07:00:31 John Thornton wrote: > Hi Gene, > > The frame is most solidly grounded. I'll do the tests in a bit. Keep > in mind that I have a 240v to 120v step down transformer to supply the > 120v not a normal house circuit. Again an effort to kill the noise. > Which by the way I'm running 2.7 now after changing the wires on the > limit/home switches to twisted pair shielded wire. I've attached the > VFD side wiring diagram. I assume the connection is through the > neutral bonding screw at the panel because the step down transformer > is isolated from ground. > > Happy New Year to you too. > > JT
So this then was my 2nd guess, is the capacitative interwinding coupling in that transformer capable of supplying a very few microamps of actual current, probably less than 1 or 2. With no loads plugged in, you could connect either of the output phases of that transformer to machine frame "ground" without any fireworks which would bring the other up to 127 to "ground". I believe in that case I would synthesize a std 127 volt circuit out of the control transformers secondary by connecting the lower voltage wire to the machines frame, along with the static ground wire in a short bit of romex & using the std color code where the black wire is the high side of the transformers secondary, feed it to a duplex or 4 plex to power the computer, monitor, and any other 120 volt only loads on the machine as long as the total load is within the ratings of the transformer. That should be 100% safe for everything. And it should reduce the coupled noise just because its all bouncing in unison. Some of the noise coupling is coming into the 120 volt circuit from the "longitudinal" coupling of the windings un-avoidable capacitance, and grounding one side of it, and the loads static ground to the machines frame & ground should absorb a good share of it. In really obnoxious cases, a small, perhaps a .01 to .1 uF capacitor rated at least at 600 volts from the high side, black wire to the duplex, to the machines frame should gobble up the rest of it. And not just for S&G, I'd fuse the hot lead of that cap since a failure would take out the control transformer. Fused at less than the transformer is rated of course since you want to blow the fuse instead of the transformer. That was my theory when I hooked this room up, and I have not lost a piece of gear during an electrical storm in over 10 years since I did it. With the 50kw can that supplies 4 houses on the pole across the street, that pole has been nailed quite a few times, and I even got a grab the doorknob shock that jumped out of a wired keyboard once. So I know this whole room full of electronics has bounced at least 25Kv. Not even a computer crash when it happened. Electrical shock is a weirdly defined thing. Below 20 micro-amps directly thru the heart, a currant so low you may not even feel it, is generally harmless, but at 20 micro-amps up to about 20 milli-amps it can disturb the beat, causing fibrillation and eventual death if no one removes the power or you from the source and applies the defibber paddles. Above 20 milli-amps, the survival rate is better because the heart is frozen, and when the currant is removed, and it hasn't been frozen so long you are brain dead from lack of oxygen, then the heart will often start back as if nothing has happened. Your trivia factoid for the day. :) My ex had a cousin that I met once in the 1970's after he had stuck an alu ladder he was carrying into a low hanging 7200 volt line. Lost part of a foot, and the shoulder blade and arm the ladder was laying on. Never was the sharpest tack in the box, but I'd have guessed him at an IQ of 105 to 110 or so after the event. Some surgery, a specially built boot and a while to heal, which he was still doing when I met him at some sort of a family doin's, but by now he's probably taken over the painting business his father started 50+ years ago and run it, if not retired from it. That was a "few" thousand days ago. :) The ex left 31 years ago, ending any excuse I had to keep track of someone in Wisconsin I only met once. > On 12/31/2015 5:50 PM, Gene Heskett wrote: > > On Thursday 31 December 2015 16:20:40 John Thornton wrote: > >> On the 120v side if I measure from the hot to the ground I get 79 > >> volts, if I measure from hot to neutral I get the expected 128v... > >> what is that telling me? > >> > >> JT > > > > My first guess is that the Bridgeport itself, is not grounded. It > > really should be, just to keep it from becoming lethally hot when > > somethings insulation fails. Generally speaking, if I can measure > > more than a volt between neutral and the static ground, it concerns > > me UNLESS its a wild phase , which I don't believe you have since > > its not a 3 phase circuit. > > > > TBE if something is running that uses that neutral you might see > > more volts, but I'd re-measure after turning whatever it was off. > > > > As to the 79 volts, I would put a small light bulb (7.5 watt night > > lite) from the Bp frame to neutral to see if there is any real > > current, or its just capacitative coupling. If capacitative > > coupling, a 7.5 watt night light bulb will remain dark, and that > > voltage should drop to less than 1 or 2 volts. If it lights up at > > all, and the voltage doesn't drop drastically, there really is a > > fault someplace. I'd start disconnecting motors for starters. > > > > Happy New Year John. > > > > Cheers, Gene Heskett Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
