Motor grounding provisions

[thinker]

We received new AC motor (140 HP). The cast iron terminal box is bolted with 4 bolts (through rubber gasket) to the flange. One of these bolts is marked and designated as a motor ground terminal for connection of equipment grounding conductor. The flange itself is bolted (through another gasket) to the motor frame. It seems the the only path for ground current is through the threaded connection between the flange and the frame. Is this considered as sufficient grounding provision? Or there is a standard which would define better connectivity to the frame (directly)? BTW, this motor has UL label although its grounding method looks questionable. Any comments, gentlemen?

 

[Skogsgurra]

The grounding system that you describe is not used by serious manufacturers any more. The reason is that, although it may (or may not, I would say) be sufficient for safety - it is not a good HF ground. And a good HF ground is what you need if you are running the motor from a frequency inverter. A good VFD motor cable is next to meaningless if you cannot terminate the shield in a gland that contacts a terminal box wall that is solidly (around the perimeter) connected to the stator frame.

Gunnar Englund

http://www.gke.org

 

[powerjunx]

thinker,

As i recall, we provide grounding requirement to our installation to "wound rotor induction machines" less on squirell cage induction machines.

It's better to provide grounding to your machine.