I would not ground the DC power to the cabinet. It's just asking for problems. I keep all my DC circuitry isolated from the high voltage.
Finally getting around to doing this. Question on dc power distribution. should I just ground all my dc power supplies to the panel, as well as ground all components needing dc power to panel, and then just wire the positive to each component? Or is it better to actually wire the positive and negative from component to power supply? Electrically I don't think there's a difference, but not sure if one way or the other is proper. Seems like a lot less wiring if just grounding everything straight to panel.
I would not ground the DC power to the cabinet. It's just asking for problems. I keep all my DC circuitry isolated from the high voltage.
That would mean I would need to isolate every component from the panel. The gecko g540, Ethernet smoothstepper, dmm bob, etc. Should these really not be grounded to the cabinet? Also, doesn't the a.c. ground on the power supply input already pass through to the dc ground output?
A unwritten law is that the enclosure you order always ends up being at least 5% too small!
I have often got deals on used enclosures off ebay, some came with disconnect, terminals, P.B.'s etc, I even got one that was stainless!
I have always earth grounded my DC supplies where possible to the Star ground point, the service ground should also terminate there.
I have even put two VFD's in the same enclosure as the rest of the control.
For hook up wire, of course always use stranded conductor and my favorite is TR64 as the insulation is much smaller and avoids filling up the wire trough.
Another general rule is to place heat producing items at the top of the enclosure.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
Sounds like I need to stop and get a better understanding of proper grounding. There seems to be some mixed opinions on whether or not to tie earth ground to dc common. I always assumed everything should be grounded to the same ground. Currently my dc components are grounded to the small enclosure I have, as well as the a.c. ground and it hasn't caused any issues, but maybe not the proper way to do it? Just wamt to make sure i have this right before spending time on the new cabinet
Hi, i connect the mains earth to anywhere where it is required/where there is an indicated earth connection. But i do not tie/connect the neg of any SMPSU to earth
There are two schools of thought out there, some that say you should never ground any of the DC supply commons, the other is to earth reference all supplies, I have always followed the latter practice, with so far no problems, there was a post here a while ago where a poster was experiencing all kinds of intermittent stoppages of his system where he had followed the isolation method, I suggested that he use the ground all method, where possible, the problem went away, there have been others where it has fixed the problem.
A star ground termination point is also very important.
There used to be a paper on the Siemens site on Equi-potential bonding in enclosure installations, but after they changed their web site, it dissapeared, I may have a copy somewhere.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
STAR grounding: everything to a single solid ground point.
Essential to prevent 'ground loops', which can be really disruptive.
Cheers
Roger
I use star grounding for all DC electronics, then I tie that star ground point to the the incoming mains ground. I do not gain or lose axis Step pulses.
I suspect that workplace safety laws will require full connection to the mains ground in all cases unless the entire machine is double isolated - which would be pretty much impossible for a CNC machine.
Cheers
Roger
Currently I have my machine wired the same way. Dc grounded to earth. I have hundreds of hours of run time without a single lost step or any hiccup at all. I think I'll just stick to how it's been. Right now it's just kind of messy and needs to be better organised with a few things added. Servo driven mill turn spindle, multiple solenoids for brake, coller closer, and coolant lines.
Some components require the DC supply to be isolated. Connecting the DC- to ground loses the isolation.
Gerry
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And often component suppliers will stress not to earth ground the common of their equipment, from what I have personally gleaned it is often not because of a technical reason, but to avoid any blame if you do ground it and if something inadvertently goes wrong with the installation you can blame their equipment.
If there is no technical reason otherwise, I have always ignored it and grounded anyway, with no ill results.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
A little late to the discussion, but I have been running the DC return to earth ground for years, works for me. One instance of where you would not want to do this is in the case of direct rectified line power without an isolation transformer, Anilam built a lot of systems like this. Grounding that would cause a dead short. As Gerry noted there are some other exceptions, but normally it is a good practice IMHO
Jim Dawson
Sandy, Oregon, USA
Also one publication aimed at machine/enclosure wiring is NFPA79, you most likely can find a copy out there in PDF.
I would add that just about anything used in a PC desktop/tower environment is going to be earth grounded simply because it is automatically done in the PC, e.g. P.ports, USB ports. Kmotion etc as well as any card that plugs into a PC slot, Galil, Acroloop, etc. are all connected to the PC ground plane that is earth grounded via the service supply.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
Hey Guys,
I realize this thread has been on vacation for a month, but I have a couple of related questions.
I've been running desktop CNC's for 5 years or so and have always tried to keep my wiring neat and isolated, but I'm getting ready to upgrade my machine from a Gecko G540 and open loop steppers to LeadShine controllers and closed loop steppers, so I need to design and build a new control cabinet. I'd like to use DIN rails and components.
My question(s) are:
Where is a good reference for general (or more specific) design standards? I appreciate Al_the_Man's link to the NFPA79 document and will search that out. Without going back to school and getting an electronic engineer's degree, are there others?
Where can I find direction on which components to use for specific functions? I know that's kind of vague, but for instance, it looks to me like a Contactor and a Relay both perform the same general function - turning an electrical component on or off - but when do you use one over the other? Or do I have that functionality wrong?
I also have questions about when and where to use circuit breakers - only on 120V supply circuits or others in addition?
Thanks in advance for any guidance,
Gary
Gary, you ask hard questions. This arcane knowledge is normally only passed down from the master to the apprentice, over a period of years. I don't know of any books on the subject other than NFPA, NEMA and UL codes.
In general you want supplementary protection (fuses or breakers) on both sides of a power supply or transformer.
Motor loads and drives should be individually protected on the power input side. Normally follow the device manufacturers guidelines for proper protection sizing and type, wire size will be determined by the protection device rating.
A contactor and a relay are functionally the same. But a contactor is normally directly switching the load (motor or other higher energy device), and a relay is switching the control side of things.
I hope this makes sense. There is a lot more, but it's pretty hard to try to condense 50 years of experience into a post.
Jim Dawson
Sandy, Oregon, USA
Jim,
Thanks for the quick reply.
Your advice helps me make a better start.
Gary
What Jim said: it is a matter of experience and judgement, not of 'rules' (apart from things like the UL guidelines, which are mainly for safety rather than 'good practice').
Contactors are usually 'bigger' than relays - in voltage and current.
I have over-current cutouts on each motor driver. You should NOT turn them off that way under normal circumstances, but they do handle cases where the axis has run into a hard limit and the motor current tries to go skywards.
Neat and tidy, with adequate protection, is always good. GOOD documentation is a pain to create, but utterly vital later on.
Cheers
Roger