I just looked at the control transformer and L1 and L2 (240v side are fused with type CC fuses and X1 only on the 120v side is fused with a slo-blow fuse. So I assume they intended X2 to be the "neutral".
JT On 1/1/2016 10:15 AM, Gene Heskett wrote: > 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 ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
