Hi John, You are correct I have a 220vac and a 120vac line coming into the machine. I didn't pull a 4 wire 220 line to those outlets in the shop so no neutral is there. If making a star ground doesn't fix the issue then I'll pull the 3 wire out and replace it with a 4 wire run. Not much fun but doable as I ran conduit up the wall to the attic.
Thanks for the great explanation, I'll save that and other things I've learned for future use. JT On 12/20/2015 2:25 PM, John Dammeyer wrote: > Hi John, > > It sounds as if you have two AC lines coming into the machine. One for the > controller and one for the VFD. The comments about a single star ground > point are standard industry practice. One breaker at the panel which > protects the wire to the machine. Another breaker inside the machine that > protects the equipment. > > The Earth (green) comes in and is connected to a welded stud with a ring > terminal, not a spade and double nutted so it can't possibly come loose. > >From that point your controller and VFD are run. Never ever rely on the > frame to carry the AC Ground (Earth) from one section to another. > > DC grounds are isolated from Earth Ground. You can, connect the DC power > supply minus to the Earth ground and it may even be a code requirement where > you are. Be aware, some PCs do this inside their power supply as do other > types of COTS hardware. > > And here is the problem you have to watch out for. Just because your DC > volt meter reads continuity (Zero Ohms) between all the ground points > doesn't mean that there is actually a path of lowest resistance following > what you think it should. > > The moment you get into the switching power world which includes the VFD, > Stepper Motors and Servos along with switching power supplies you get > something called impedance. That's based on the DC resistance (usually low) > and a combination of the wire inductance and capacitance and frequency. > > Depending on how things are wired and routed the impedance of the return > signal of the VFD may be lower through the RS232 (RS485) shield than it is > through the green wire or even the shield around the power cable. So just > imagine the very noisy VFD signal returns through the communication shield > because that has an impedance of 200 Ohms while the green wire has 1K. > That's problem #1. > > There's two types of electrical noise. Electrical coupling and magnetic > coupling. Electrical you shield against. Usually with the shield tied to a > common earth somewhere. > Magnetic coupling is the same thing that makes a transformer or a motor > function. A rapidly changing magnetic field caused by rapidly changing > current in a wire is coupled onto another wire that is in close proximity. > Shielding doesn't protect against that. Twisted pairs do to a certain > extent. > > The best protection is distance. That's why you don't run the VFD power or > Servo power tightly tie wrapped to the encoder signal. > > Therefore put the AC power side for the VFD on one side of the cabinet and > run the control signals as far away as possible. > > There are lots of books written on this subject and probably as many > opinions on what to do. Ground shield at both ends. Ground at one end. > Don't ground. Twist the wires. Don't twist the wires. > > Start with the Star Grounding. Make sure there aren't any DC paths that are > unexpected as explained earlier. Then start looking at how the noisy > signals might find their way back through alternate paths. Filters, > Ferrites are all useful to block the signals. Always best to block at the > source. > > John > > > > > > >> -----Original Message----- >> From: John Thornton [mailto:j...@gnipsel.com] >> Sent: December-20-15 10:27 AM >> To: Enhanced Machine Controller (EMC) >> Subject: Re: [Emc-users] Grounding Issues >> >> >> I have a line reactor for the VFD but I've never hooked it up. >> >> http://www.automationdirect.com/adc/Shopping/Catalog/Drives/AC_Drive >> _%28VFD%29_Spare_Parts_-a-_Accessories/AC_Line_Reactors/LR-23P0-1PH >> >> Speaking of VFD's mine is grounded to the breaker panel and not in the >> machine anywhere. When I check from the ground terminal to the panel I >> get 0 ohms so I assume that the back of the VFD is grounded to the >> ground lug on the VFD. >> >> JT > > ------------------------------------------------------------------------------ > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
