VFD controlled wiring question.

[Greg M]

I've encountered a new situation regarding grounding of VFD controlled motors and need some opinions. I spec'd out the proper size VFD cable from the drive to the motor (125HP), but the customer is requiring a second cable (same size and type, with all conductors and shield connected at the motor and drive on the ground terminals. Can someone offer an explanation? Never seen this before. Thanks, Greg


[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1553804565/tips/VFD_grounding_sflxfn.pdf[/url]

 

[itsmoked]

It's lunacy.

However the high frequency currents that leak out of a VFD driven motor can cause a bunch of problems in a complex environment. They need a very low impedance back to the VFD. That is actually pretty hard to come up with using large round cable. If not using flat braid which has much lower high-frequency impedance than multiple big-round-cables multiple round ones will do a better job than a single one. As many as I see there, is overkill, but will certainly assure the highfreq current will go back to the VFD directly instead of thru every other wires in the area.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

It’s not lunacy. That is depicting a particular type of VFD shielded cable in which there are 3 ground wires nested in with the power wires for full symmetry of the conductors and ground paths, a recommendation on larger HP applications. They are also using one with both a shield and a drain wire for the shield, and it is recommended that shielded power cables be grounded at both ends. That is actually a best practice all around.

Example:

https://www.servicewire.com/Products/Service-Drive-Tray-2kV.htm

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LionelHutz]

Yup, it's a specific VFD cable with 3 ground conductors. The customer interpreted the drawing wrong.

A VFD cable with a braided shield using the proper gland connectors which fully connect to the shield is another good alternative.

 

[itsmoked]

Thanks Hutzy. I've never seen that special cable. Makes sense in that case.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Greg M]

Thanks, everyone. Makes sense now, I've just never seen all the ground conductors and shields connected on a diagram before.