Rotor grounding

[thinker]

This is a well discussed subject. The VFD driven dual-ended induction motor (tandem application) has insulated bearings on both ends. Do we also need to specify a rotor grounding brush on both ends of the motor, or one side istalled brush would be sufficient to minimize the bearings current effect?

 

[aolalde]

Hello Thinker:

For very high frequency the inductive reactance and skin effect could develop very high impedance in a few inches of conductor, this could leave the farther bearing unprotected. I recommend two grounding brushes installed close to each bearing for taking care of induced capacitive discharges. This problem is more serious on applications involving large size machines.
See the following site:

http://www.est-aegis.com

 

[thinker]

Aolalde, thanks for reference. We have a 600 HP motor, and AEGIS also recommends to istall ground brushes on both ends for rotor diameter greater than 2".

 

[ScottyUK]

Have a look at

http://www.sohreturbo.com/.

Mainly big brushes for big machines, but some good information and useful mounting ideas.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[spargher]

I think you should always have only 1 grounding point on a shaft train. Otherwise, whit 2 ore more points you will have groud currents flowing trough the shaft and grounding brushes

 

[Skogsgurra]

With insulated bearings on both ends there should be no need for grounding brushes. At least not when it comes to bearing currents. But if the insulation is a thin ceramic layer on the outer ring, then the HF parts of the induced rotor voltage are known to leak through the resulting capacitor (it is around 10 nanofarads in a four-pole 500 kW machine). So you need to create a by-pass for the HF current using a grounding brush at each end of the shaft.

The truth is actually more complex than that. Most electric codes require that parts sticking out of an electric environment or having electric things mounted on them shall have a protective ground.

The rotor is just such a part sticking out of an electric environment. Hence it shall be grounded. Not only from a bearing current point of view, but also from a NEC (or other code) point of view.

I know that this is news to many of you and that your first reaction is that it is utterly nonsense. You just need to get acquinted to the thought and I am looking forward to your esteemed comments.

Gunnar Englund

http://www.gke.org

 

[thinker]

Gunnar,
You predicted the first reaction quite well. Could you please elaborate on how the Code (e.g. NEC)requires grounding of the "part sticking out of electrical environment"? (BTW, very unusual definition).