The power supply cabinet I'm putting together for my Mill has a terminal strip with ground and 105VDC power (1500W) with one pair of terminals for each axis. From that strip I run one pair to each Servo Drive. From each Servo Drive a pair to the motor. The encoder cables enter the Servo Drives on a different side of the board and will be not be run in parallel with the motor cables. Stepper motor for the knee has its' own power supply and it too runs separately (power and ground) to the Gecko. These high power supplies are all run with 220VAC through a separate contactor controlled by the ESTOP.
The PMDX-126, the 24VDC and the PC/Monitor are also on a 220VAC circuit that is live from the Mill Master Switch. The 24VDC supply is what I'd refer to as the instrument bus and it's used for a variety of controls like the control side of the STMBL Servo drive and 12V supply for the DRO.. It will share the DC ground with the PMDX-126 breakout board, PC USB and ultimately a series of CAN bus based controls for tool changer etc. I wanted to also run the 24V into a 15V regulator to provide power for the control side of the HP-UHU servo drives but here is where that pesky ground sneaks in again. Connecting a 15V supply sourced from the instrument power that then runs the HP_UHU connects the Instrument power to the Servo Power ground. The step/dir/error from the HP_UHU is optically isolated so I don't have to worry there. The Servo Power Transformer has a 12VAC winding which through a bridge gives me roughly 16V which is the better source for the 15V that not only is regulated down to 5V for the HP_UHU but also supplies the gate bias voltage for the FETs. I'll only know if it works well enough once I put a scope on the running system. This is an example of where I'm not sure I see any value in ever connecting the two DC grounds together. As long as there isn't a return path for the high voltage DC through any part of the instrument bus DC circuit it doesn't matter if the high voltage DC ground became completely disconnected. But connect both of those DC grounds to Earth (metal cabinet etc) and now the potential for high voltage, through component failure running through the instrument circuit, to instrument ground to earth back up into high voltage ground is more likely. And touching that high voltage now ungrounded section and perhaps the metal frame puts the body into the circuit. And yet if you have, say an audio amplifier or radio transmitter with both 48V and 5V that share a common ground a circuit failure usually smokes things. But those are all usually in a sealed metal cabinet. A system where the various devices are placed around the equipment, not all in a box is different. John > -----Original Message----- > From: Jon Elson [mailto:el...@pico-systems.com] > Sent: July-14-18 8:57 AM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] Grounding > > On 07/14/2018 02:37 AM, Gene Heskett wrote: > > Not exactly 100% correct John. High frequencies are > > generally carried only by current flow at the skin of a > > conductor. And the large conductor, while having some rise > > in impedance, is still the better conductor because the > > smaller wire has far less surface skin area, raising both > > its impedance and its ohmic effects faster than the big wire. > There's also loops. Bundling the power supply and ground > return close together minimizes inductance. Having the > supply and return form a big loop increases inductance, and > can cause greater noise as well as radiation of EMI. So, in > general, you want the return wires to follow the supply > wires closely. > > Jon > > ---------------------------------------------------------------------------- -- > Check out the vibrant tech community on one of the world's most > engaging tech sites, Slashdot.org! http://sdm.link/slashdot > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
