On Sunday 21 October 2007, [EMAIL PROTECTED] wrote: >John Kasunich wrote: >> Even better, if you can get some, is a braid that can be expanded enough >> to run the three motor leads through it, then stretched lengthwise so it >> snugs down around the motor wires. Again, connect one end directly to >> the motor frame, and the other directly to the VFD ground. >> >> The idea here is to have the return path for stray currents as close as >> possible to the outgoing path. > >There are cables produced just for this purpose with a braid and outer > plastic sheath. You can also get them with 2 wires extra for temperature > sensor if the motor have such. > >It will contain noise carried by the cables provided it is connected to a > good ground at least at both ends. Additional grounding along the path > improves efficiency. Ground by using clamps around the braid and against > the ground rail. Pigtails act as inductors and render it less efficient at > higher frequencies. And of course a spiral conductor around the leads is > just a mechanical protection. It is a coil, and thus not effective at AC > which is what worries us here. > >Any ground cable that might carry noise (current spikes) should have as > large surface as possible. That means many thin strands. The grounding > strap from engine to chassis of modern car is a good example. Welding cable > is also fine. It should go in a straight line, never coil it up. Use > mentioned straps to ground your cabinets to the machine frame and each > other. Use the grounding bolts built into the cabinet for this purpose, or > if there is none, make sure there is no paint or corrosion hindering a good > connection when you install it yourself. Don't forget to ground the cabinet > door using 1 or 2 straps. > >Also make sure your machine have a good connection to the grounding system > of your building. If you run sensor cables or data cables to the machine > cabinet, those should also be shielded and connected at both ends, and the > device at the other end must be considered just another machine cabinet > (grounding). This last point is less strict if using galvanic isolations, > and no worry if using plastic/glass fibre. > >Then with the appropriate filters installed, you should be able to run your > machine while your wife watches the football game. ;-) > >Einar > Regarding motor cables: I am using a Belden microphone cable called Star-Quad, which has 4 conductors in a good shield that I use with my xylotex kit, one cable per motor, with the shield well grounded at the xylotex board, and not connected to anything at the motor end.
This cable is available in gauges as large as 20, but what I found was 24 gauge and its working quite well. The wire layup in this cable is 1 2 3 4 and intended to be used 2 wires in parallel for low impedance mics, where 1-4 is used as one wire, and 2-3 is used as the other so the net pickup effect of external noise is that all wires pickup as if they were perfectly centered & balanced. I use 1-4 for the A coil and 2-3 for the B coil. I can run an AM radio 3 feet away without hearing it. I didn't trim the length of the motor leads, so theres about a foot of wire from the plug to the motor exposed. That wasn't one of my more brilliant moves, but it worked so I haven't fixed it. My point is that this might be a good technique to use with the encoders too, by using it exactly as Belden intended from the encoder to the diff amp. Sure, that's twice the wires, but if it works, well... Another thing to consider is that the diff amps are often not very immune to 'longitudinal' noise, noise where the spikes are alike on both inputs, but the amplitude of the spikes exceeds the power supply rails. So your 'termination' resistors might have to be adjusted in value so that you can tie one end to ground, one end to the supply rail, and use the junctions /2 voltage as a bias thereby giving equal noise margins going both directions. What you are doing there is setting the input bias of the diff amps so that equal noise margins are available for both polarities of spikes. Because of unbalance in actual R values, it might be best to set that up as one pair of r's across the supply, in which case they can be a higher value, with the junction bypassed to local ground if that local ground is quiet enough otherwise don't bypass it, and the other two term r's, say 56 ohms hooked from the junction to the inputs. This would allow the use of common 10% resistors rather than fawncy and ex$pen$ive .01%ers. These bias setting r's can be higher values, but should be low enough to absorb the noise and constrain it to within the supply rails. It really should stay at least a volt away from the rails if possible. Food for thought guys. From an old fart C.E.T. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Over the years, I've developed my sense of deja vu so acutely that now I can remember things that *have* happened before ... ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
