Grounding/shielding questions

[dfro]

I am currently building the controller box for a router mill.

I have read some of the cnczone threads on grounding and I read through the pdf that Al suggested, but I have some more questions. I find explanations on grounding, shielding, and ground loops very hard to understand. I just cannot seem to grasp it. Al's explanation of a giant copper plate makes the most sense to me.

I have gotten this far with the controller box: The black and white wires from the 115V mains go to a terminal strip, which feeds the 30VAC and the 9VAC transformers. The mains power switch and the fuse are wired on the black/hot side. The green mains ground wire is bolted to the controller chassis. After I have rectified, and smoothed the AC off of both transformers, I have 45VDC and 8VDC wired to separate +/- terminal strips. (Each stepper driver board has a 7805 vreg, which takes the 8VDC down to 5VDC for the logic/signal circuitry.) I have placed the 45VDC and the 8VDC terminal strips far away from each other in the box to avoid capacitive coupling.

Can I now tie all of my signal and motor-power circuitry to their respective terminal strips and then connect the negative of each strip to the green chassis ground? By grouping it this way, I can keep my wiring more isolated and uncluttered. I would find it very difficult to connect all of my 45VDC and 8VDC grounds to one bolt.

I also don't understand if transformer isolation is necessary or if isolation is eliminated by tying all grounds to the green mains ground wire.

I understand that the white and green mains wiring are both tied together at the building's service panel and grounded to a pipe driven into the earth. So, by grounding all of my DC grounds to the green mains wire, I am eliminating any transformer isolation, correct? Both sides of the transformers are ultimately connected through the green chassis ground, correct? I don't know if I am making any sense.

Also, I am wondering how to shield my cabling. I bought 4 conductor, 18 gauge, shielded cable to use for the motors and limit switches. How should I attach the shielding? I am thinking I would group the motor cable's shield wires together at the cable connectors in the controller box and run a single wire to the 45VDC ground terminal strip, which then connects to the green chassis ground. I would also group the limit switches' wire shields together at the cable connectors in the controller box and run a single wire to the 8VDC ground terminal strip, which also has a wire connecting it to green chassis ground. Is this a good plan?

Should I have the shields on the stepper and switch cabling only connected at the controller box end and not connecting to the machine frame?

I am not sure how to ground the cnc machine. How should I transfer the green chassis ground to the cnc machine's metal frame?

I hope these questions make some sense.

Thanks for any ideas,
Dave

[dfro]

Could an administrator move this post to 'General Electronics Discussion' or the like. I realize this is the wrong section to be posting these questions. Sorry.

[Al_The_Man]

After the safety aspect is covered, There are different philosophies as to the methods of integrating all the different systems together, I have always grounded the common of any power supplies that I use back to a common ground buss point.
If you ground the DC common in this way, the AC secondary cannot be grounded.
See the Hardware engineering details here by AMC, they can apply to most systems, not just AMC.
http://www.a-m-c.com/content/support...reference.html
Personally I take all shields back to this common ground plate also.
The Machine ground you refer to is called Bonding, this entails taking a suitable ground wire from the machine frame and any motors or spindles etc, back to the common ground where the service ground is also connected.
Some may say this is encouraging ground loops, but the object is the contrary, ground loop occurs when there is a potential difference between various grounded objects in a system, if you ensure by suitable conductor that these points are definitely at the same potential by bonding, it makes it difficult for ground currents to occur.
Wire Shielding should always be grounded at one end.
When using PC based systems, I also make sure the mother board common is taken to this GND plate also.
If you decide to subscribe to the alternate method of keeping all systems completely isolated from each other, then it is not possible to ground all the power commons used in these systems.
It is usually a choice you make, and this decision may be swayed by manufacturers recommendations etc.
Usually the problems occur when either method is done haphazardly.
Al.

[Galanty00]

First i believe you should know how much ressonance from AC and DC transformers you
have, a few capacitors will do the job after the calculations, keep two ground bus one for AC another for DC

[dfro]

After the safety aspect is covered, There are different philosophies as to the methods of integrating all the different systems together, I have always grounded the common of any power supplies that I use back to a common ground buss point. If you ground the DC common in this way, the AC secondary cannot be grounded.

Is the common ground buss point that you mention located at the AC green chassis ground? Do you have all the commons connected to a buss that is then tied to the ac mains green chassis ground?
When you say the AC secondary cannot be grounded, do you mean that neither of the two ac secondary wires can be tied directly to the mains green wire? This would cause a short between the dc common and the rising and falling ac potential, correct? The grounding to chassis ground must happen after the rectifier bridge only, right? Sorry, for being so dense on this subject.

The Machine ground you refer to is called Bonding, this entails taking a suitable ground wire from the machine frame and any motors or spindles etc, back to the common ground where the service ground is also connected.

Should I have a connector and cable coming out of the control box that is there to connect the green chassis ground/safety ground/earth ground/service ground (do these terms all mean the same thing?) to the machine's metal frame?

In the AMC wiringpractices.pdf it states, "In many cases the signal ground and/or power ground can be referenced to earth ground. First decide if
this is both appropriate and safe. If this is the case, they can be grounded at the central grounding point."

This sounds like what you are describing, correct?


Thanks,
Dave

[m2ry51]

Hi, with reference to grounding and shielding:

1) as to your DC supplies try to have one common 0v / ground point from which all positive supplies (and negative supplies i.e. a -12v for your RS232 if used) are referenced to.

2) once a common 0v / ground point is in use this ought to be tied to any protective ground in use.

3) shield / screens should only be grounded at one end of the cable not both - connecting both ends of the screen can result in induced pickup within the screen making things worse than not having the screen in the first place.

I hope this proves usefull...

Regards, Si.

[Al_The_Man]

Originally Posted by dfro Is the common ground buss point that you mention located at the AC green chassis ground? Do you have all the commons connected to a buss that is then tied to the ac mains green chassis ground?

Yes, I usually tie all the grounds to a copper plate that has the service ground connected to it. They can mount up fast, so I usually drill and tap a bunch of holes in a piece of copper bus bar.

Originally Posted by dfro When you say the AC secondary cannot be grounded, do you mean that neither of the two ac secondary wires can be tied directly to the mains green wire? This would cause a short between the dc common and the rising and falling ac potential, correct? The grounding to chassis ground must happen after the rectifier bridge only, right? Sorry, for being so dense on this subject.

You can ground the AC secondary as is customary when a local 120vac control supply is set up as when the machine is only fed from higher voltages etc.

But if you do ground, say the AC secondary of a DC supply this prevents you from grounding the DC common, if this is want you wish to do.
IOW, it's one or the other.

Originally Posted by dfro Should I have a connector and cable coming out of the control box that is there to connect the green chassis ground/safety ground/earth ground/service ground (do these terms all mean the same thing?) to the machine's metal frame?

If you have a main case or enclosure where the machine supply enters, this is where you normally would set up the common point, or anywhere that is the most convenient.
You should have a ground or bonding conductor from the GND plate, out to all metal extremities of the machine, normally this is run along with the conductors for the servo's/spindle etc.

Originally Posted by dfro In the AMC wiringpractices.pdf it states, "In many cases the signal ground and/or power ground can be referenced to earth ground. First decide if this is both appropriate and safe. If this is the case, they can be grounded at the central grounding point." This sounds like what you are describing, correct?

Yes, this is essentially as I mentioned, this decision has to be made up front as to the method you wish to use.
By "appropriate & safe" means is it OK with the equipment you are hooking into to do this?
You either have to have certain knowledge to be able to assess this, or if in absolute doubt, you may leave it to the manuf. discretion, or recommended method.
Usually when I use my method that goes against the manufacturers recommended, it is up to me to assess the ramifications, i.e. how exactly is the equipment interfaced to other parts of the system.
Sorry I could not give you anything more concrete to go on.
This is all part of the fun of control electronics.
Al.

[dfro]

Thanks for all of the responses. This is so helpful.

You can ground the AC secondary as is customary when a local 120vac control supply is set up as when the machine is only fed from higher voltages etc.

Al, sorry, I do not understand what you mean here. Could you rephrase this?

Also, in my posts the quotes do not have the "Originally posted by xxx" part. Could someone tell me how I do this?

Al, in response to my question about how to connect the service ground to the machine's metal frame you wrote:

You should have a ground or bonding conductor from the GND plate, out to all metal extremities of the machine, normally this is run along with the conductors for the servo's/spindle etc.

I have bought 5 pin connectors to separately connect each driver board to its stepper motor on the machine - A+,A-,B+,B-, and shield/ground (which will be connected at the control box only and tied to service ground). I am not running all of the motor supply wires down a single snake cable. So, in this case, I should have a separate cable that connects the machine's chassis ground point to the service ground point in the control box, correct? What gauge wire would you suggest, also?

Again I apologize for being dense on this. I want to be 100% sure that I do this right and avoid frying my electronics with a wiring mistake.

Thanks,
Dave

[Al_The_Man]

Most hobbiest machines are run off of 120v outlet, this has 120v hot & a Neutral grounded at the panel.
In the case of a machine that, is fed by a 3phase or higher voltage source, in order to obtain 120vac control voltage, a step-down isolation transformer is often used.
In order to reproduce the type of 120v supply you get from the outlet, and to satisfy safety conditions, it is permissible to ground one side of the 120v secondary immediately at the transformer terminal, and take a conductor as well as ground from this point and use this as a neutral.
This then replicates the ordinary 120vac outlet circuit.
Normal minimum conductor size for ground bonding is 14g, you could get away with 16g in a pinch.
BTW, Any conductors that carry appreciable current like servo's, steppers and spindles etc should be twisted conductors, if separate conductors are used it is best to manually twist them if possible.
Do not include the ground wire in the twisting if it can be avoided.
Al..

[dfro]

Thanks Al. That makes more sense to me now.

The 4C, 18 gauge, shielded cabling I bought has all four conductors twisted together and surrounded by an aluminum foil shield. Inside the control box I will make sure to twist any +/- wire pairs together. In audio tube amplifier builds, I have seen many examples of people using shielded cabling to route signals inside the chassis, and they are careful to only ground the shielding at one end.

Thanks,
Dave

[dfro]

I would like to rephrase a description/question I posed in the first post.

The star pattern is the best for grounding and shielding, because it ensures a ground reference to one single point and it avoids ground loops, correct? But, can one use more of a tree pattern?

The controller box chassis bolt to the green service ground is the trunk, which branches out to a 45VDC terminal strip, a 8VDC terminal strip, a cable shielding terminal strip, and a ground cable which connects to a bolt on the machine's metal frame. Further, the bolt on the machine now branches to the various machine components that need grounding, like the shielding that is on all of the cabling running around the machine. As long as the branches do not connect back to previous branches and create a ground loop, I am okay, right?

Thanks,
Dave

[Al_The_Man]

Take a look at the fig4.2 it may help understand one of the common methods.
http://www.divshare.com/download/5002681-38f
Al.