I just checked the ground to housing on the VFD filter, VFD, computer, and the smaller filter and they are all internally connected to ground. The VFD is controlled via modbus so I don't know how to check that cable (which is just a phone cable with a phone jack on one end and a DB9 on the other end to connect to the computers serial port, it's a Automation Direct cable not home made.
Is injection to the mains coming back to the machine somehow? JT On 12/25/2015 6:43 PM, Bertho Stultiens wrote: > On 12/26/2015 12:42 AM, John Thornton wrote: >>> [240V mains]--+--[vfd filter]--[VFD controller]--[spindle] >>> | >>> +--[mains filter]--[ctl transformer]--[control] >>> The interesting part in now how earth is connected in the "VFD >>> controller" and the "control". If the GND of both are connected to earth >>> (internally), then you will create a loop and all hell may break loose. >> The filter is 115/250v so it could go on the upstream side of the >> control transformer. I'm not understanding the ground part. I thought >> all grounds go to a common point? Drawing tomorrow... > The filter, as I mentioned, is to prevent injection into the mains line, > so it should be on that side. Please note that those filters have an > orientation (input/output). > > The grounding is actually a bit more complicated. > > There is a difference between ground (GND) and earth. Ground is the 0V > (reference) potential used in the electronics, whereas earth is the > protective shield potential we are walking on. > > Background on earth: The mains has a central point connected to earth, > either at or near 0V (AC) potential. It is used as a protection for when > your equipment fails. When the L/N lines are unbalanced (differential > current), then the RCD/RCCD(*) will trip and cut the power. The second > way (without RCD) for earth to protect is when it is shorted to L, which > causes a current very much greater than the fuse can handle and is blown > by the short. The third reason for ground is that it can absorb any and > all noise because the earth is big (and therefore a great filtering > capacitor). > > > Now if the VFD controller has an earth connection that also happens to > be connected to GND. And, at the same time a controller with an earth > and a GND connection to earth, then you will create a loop: > > [ ]-----------+ > +--[VFD ctl] | > | [ ]--------+ | > | | | > | | | > | [ ]-signal-+ | > +--[ ctlr ] | > | [ ]--GND------+ > | > --- > - > earth > > If both devices have GND (internally) connected to earth, then there is > no way of knowing which way the current will flow and you have a > ground-loop. Ground-loops are notorious because both capacitive and > inductive coupling can induce high currents that can destroy your > equipment at worst. Or they can interfere with proper operation by > introducing transients into the control electronics. > > If this is the case, then you must break the loop. You can often measure > if there is a earth/GND connection with a multimeter for each device > separately (while not connected). Sometimes you have a capacitive > coupling, which can be a pain too (another story). > > For all digitally controlled systems, you often break the loop by using > opto-coupling or RF-coupling on the signals, such that you no longer > require the common GND connection. > > > (*) https://en.wikipedia.org/wiki/Residual-current_device > ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
