Cable from VSD to motor - screen earthing for multiple drives in one MCC section

[123MB]

Hi All,

Regarding the recommendations for earthing of variable speed drive motor cable, I am aware there are lots of recommendations out there and there is no shortage of rules on what to do and what not to do. However I have not been able to find a consensus on the following questions:

1) Lots of literature talks about the use of EMC glands or clamping of the screen (i.e. using a half saddle) in order to improve the surface area of the screen connections. However, in an MCC with multiple drives, is it really appropriate to do this? Such a gland would effectively earth the screen of all the drives together and connet to the cabinet gland plate which would not be sending the screen directly back to the drive PE and hence increasing the chance that the noise will affect other items in the cabinet?

2) It occurs to me that most people would pigtail the shield into a crimp connector, for example, and connect to the drive PE terminal. This would bring the noisy shield straight back to the drive, according to the rules, but at the cost of introducing pigtails. Some references say this is sacrilegious, but others (for example the Allen Bradley reference DRIVES-IN001) say that this is okay and even says your pigtail can be 1m long! This seems very long.

3) Considering the potential problem listed in item 1) above, being that terminating multiple motor's shields onto the cabinet gland plate/back plate may not be the best way to return the noise straight to the drive, and considering item 2) above that this may be resolved but at the cost of adding pigtails into the shield connections, do you think that the costs of introducing the pigtails would outweigh the benefits earned by not connecting all the shields together and earthing them to the cabinet frame?

Regards,

Michael.

 

[waross]

We typically use a gland connector that grounds the shield or armour to the glanding plate. The connector uses a bushing that has a terminal to connect a grounding pigtail. The grounding pigtails all connect to the ground bus. The PE terminals of the drives all connect to the ground bus.
The PE terminal should be and is connected to the ground bus.
Fault currents pass directly to the ground bus and do not rely on the drive mounting to complete the ground path.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[123MB]

Thanks for your reply mate.

So the scheme you are implementing is breaking the rule that the shields should be connected straight back to the VSD PE terminal. I assume this Hasnt caused you any issues?

 

[waross]

Our installations follow the rules laid down by the Canadian Electrical Code.
No issues nor concerns whatsoever.
Question. When you connect the shield directly and only to the PE terminal, what is the current path for a fault to the shield? Does the current try to find its way back through the mounting screws and through the paint on the mounting panel?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[123MB]

... just to be clear, what I am concerned with is the fact that in most cases if you ground the shield with an EMC gland, or with a saddle, you are probably not bringing it as close as you can to the drive. For example, if you use an EMC gland you are allowing the noise to travel through the gland plate, back plate and all the way back to the drive.

Has anyone seen any issues come about because of these things?

 

[waross]

Altough there is a connection through the gland plate, in addition, we use bushings with a wire terminal on the side. We use a copper wire to ground the gland connectors. We do not depend on the gland plate for grounding.
Our cables typically have an equipment grounding conductor inside the jacket.
The equipment grounding conductor is looped through the lug on the bushing and thence to the ground bus. This also grounds the screen or armour.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[123MB]

Thanks alot for your response Waross, I have learned something, especially those grounding bushings - I have not seen them used here in Australia.

But my question is less about grounding for electrical safety and is more about EMC considerations and the compromise that people find between terminating the screen as close as possible to the drive and the practicacility of installation

Sounds to me like you send the screen onto the cabinet frame and the noise going from there back to the drive doesn't cause you any problems, which is the kind of feedback I am looking for.

Anyone else use a similar arrangement?

Thanks, Michael.

 

[waross]

Eddy currents that may be induced in the ferrous cabinet by the high frequency noise and resulting current, The equipment grounding copper wire has a lower impedance and the majority of the noise current is conducted to the ground bus by the copper equipment grounding conductor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[controlsdude]

What waross is describing is what I typically do in my panels with vfds inside panels. Except the sheilds on the vfd cables are to the pe terminal and then the pe terminal goes to a central ground bus termination. I think waross is saying to not to let the metal structure act as a ground but the copper conductors to a central ground strip. Also the ground coming in from the mains power that ground is tied to the ground bus inside the panel.

I remember when the vfds are local at motors that the PE terminal on the vfd was terminated with a ground wire that went back to the main power ground bus as above. I thought that the motor case that had the ground terminal inside the motor termination box (peckerhead, hr bad term hehe) was daisy chained thru the vfd pe terminal then back to main power ground bus.