Thanks for the e-mail Einar. There is a cabinet on each side of the lathe. The VFD is in the right cabinet and is grounded to the cabinet. A flex conduit with a plastic outer and a metal spiral inner carries three conductors to the motor connection box which is screwed to the motor housing. So, there should be metal surrounding the VFD to motor leads for the entire distance. The control electronics are in the left cabinet.
I did some more investigation today. The current encoder system starts with a Pico Systems Universal PWM Controller and a stand alone +12 Volt power supply. The UPC encoder inputs and +12 Volt supply are connected to a home built differential receiver, then a four-pair shielded cable and the receiver. The four-pair shield is grounded by a lead to cabinet. 6 inch unpaired unshielded leads connect to the spindle encoder. A +5 Volt regulator, input capacitor and output capacitor are on both the receiver and transmitter. I first by-passed as much as I could by running a CAT-5 unshielded four pair cable between the encoder and the UPC encoder inputs and on-board +5 supply. This worked properly, some of the pulses appeared fatter than others, but they where all in the right place and there were no extra pulses. I tried a thread and it came out as close to correct as I could tell. I added the differential boards to the CAT-5 and got the previous noise problem. Finally, I went directly from the encoder to the UPC and its +5 supply using the existing shielded cable which worked properly. Using an oscilloscope, when I probed the power supplies with the spindle running, I got about .5 Volts of ripple that had a three stair step up and down appearance. Probing the +5 Volt differential signals I got a very short 10 Volt spike on the rising edge of each pulse, but otherwise they looked well formed. Without the differential boards, the +5 Volt encoder pulses had a more drawn out spike on the rising edge and the tops varied about a Volt above +5. The whole idea of the differential boards where to reduce the susceptibility of the system to noise, but the boards themselves seem to make the effect of the noise worse. I think I need to place ferrite beads or other type of filter on the VFD inputs and outputs and then revisit the oscilloscope. I don't have much experience with tracking down noise with an oscilloscope, so if anyone has some words of wisdom, I would appreciate hearing them. I will try to get some scope pictures on my website too. If anyone has a good source for VFD appropriate filters, please let me know. I melted the original filters, so I know you can't use just anything. In the mean time, leaving the differential boards out, seems to work. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ On Fri, 2007-10-19 at 21:01 +0200, [EMAIL PROTECTED] wrote: > I may not have seen all the posts regarding this, but none that I saw > mentions using shielded > cable from the VFD to the motor? > > If not already done, change to shielded cable for the motor leads too! > The braid needs to be grounded at the VFD and the motor. I found another thread where someone suggested the same thing plus using an additional three conductors, grounded on each end, to make a three twisted pair motor connection. > Never ever use a pigtail to extend the braid to ground. This goes for both > encoder and power > cables. Clamp the braid itself to ground or use a grounding feedthrough! I have a central grounding post on the backplate of my cabinet. I have been trying to run all my grounds to this post, including leads from my cable shields. Are you suggesting tying the shields to the backplate where the the shield ends? > > If there is any potential difference between motor and VFD (cabinet), > equalise it with a > multistrand ground lead with a good cross section. Don't care if this might > look like creating a > ground loop through the braid. The current will go through the lead with the > least resistance (your > ground lead). Hence the solid cross section. > > Einar Sjaavik I have tried to avoid the dreaded ground loop (thats funny, pilots have a dreaded ground loop too) but the wiring has gotten so complex, that I probably wouldn't see it. -- Kirk Wallace (California, USA http://www.wallacecompany.com/machine_shop/ Hardinge HNC lathe Bridgeport mill conversion pending Zubal lathe conversion pending) ------------------------------------------------------------------------- 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
