DC motor grounding conductor sizing 3

[thinker]

Please advise how to size grounding conductor for DC motor
fed by rectifier in adjustable speed DC drive. I could not
find a reference in NEC for this case.

 

[jbartos]

Suggestions:
1. Equipment ground is to be tied to surrounding conductive metals to keep all conductive metallic surfaces at the same equipotential level. The DC part from the speed drive may have a choice depending on the manufacturer instruction manual recommendations. If there is a ground fault detection/indication, then one fault to ground will be detected and annunciated. If there is one DC line grounded, then one shall follow the manufacturer instructions since it may need to be connected to the equipment ground, and a grounding conductor size shall be addressed.
2. To avoid electromagnetic interference, there are usually manufacturer recommendation covering shielding, grounding, circuit separation, conduit runs, etc.

 

[JBUDA54]

I agree with JBARTOS in that you should always apply the Drive manufacturer's specification into the design. If you don't, this allows them to say that their warranty will not be upheld. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jacobs Engineering Group Inc.
1041 East Butler Road
Post Office Box 5456
Greenville South Carolina 29606-5456
864·676·6458 Phone 864·676·4789 Fax
E-mail: jason.buda@jacobs.com

Jason W. Buda
Electrical and Control Systems Department
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 

I too, agree with previous comments. Lacking any advice from the manufacturer, I would size the ground the same as the primary conductors. The purpose is to allow an internal short circuit to case a path to ground so case is not at a dangerous potential. This will cause the protection device (fuse or CB) to fail-safe. An undersized conductor could cause overheating and start a fire, which is much worse than a blown circuit protection device.

 

[rhatcher]

I agree that the manufacturer's recommendations should be followed. However, I recommend that you understand exactly what assumptions their recommendations are based on as some older or special design power systems may differ from what they assume. As well, keep in mind that their recommendations will be "minimum" requirements.

In general, a ground conductor equal to the size of the armature conductors would be the best case, but economics may dictate that you try to select a smaller size.

Finally, a star for jbuda54...I would give one to dmkirk but he is not a member.

PS: wasn't 54 the high score on the final in our power class...?