# vfd grounding



## EC2253 (Mar 7, 2008)

I am working on a bid and saw this in div 26.

"provide field installed shaft grounding"

"provide Aegis SGR or equal"

Has anybody installed these?


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## JRaef (Mar 23, 2009)

Yes. Fairly easy, they have a type that is like a split ring that can be retrofit onto an existing motor without having to remove the coupling.









If you have not installed the coupling though, you can use one that slips over the shaft.









Better yet if the motor has not been purchased yet, buy an inverter duty motor that has this built-in.


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## EC2253 (Mar 7, 2008)

Well the HVAC/mechanical contractor will provide the motors, I thought it was odd the engineer calls for the electrical contractor to provide and install these.

So it just grounds the shaft to the frame?


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## JRaef (Mar 23, 2009)

EC2253 said:


> Well the HVAC/mechanical contractor will provide the motors, I thought it was odd the engineer calls for the electrical contractor to provide and install these.
> 
> So it just grounds the shaft to the frame?


Yep.

The reason is that when you use a VFD, the high speed PWM DC pulses going to the motor make the motor stator and rotor act like a capacitor and a voltage potential builds up in the rotor. Without an easy path to ground, that voltage continues to increase until it exceeds the dielectric strength of the bearing grease and discharges to ground across the bearings. This etches the bearings and races, what is called "EDM damage" (EDM stands for Electric Discharge machining, because the damage is like that caused by a welder). The shaft grounding bushings have a carbon "fuzz" that is making contact with the shaft of the rotor, giving that voltage potential an easy path to ground so that it doesn't go through the bearings. But for that to be effective, the motor frame MUST be grounded too. That's likely why they assigned the task to you, the electrician.


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## EC2253 (Mar 7, 2008)

JRaef said:


> Yep.
> 
> The reason is that when you use a VFD, the high speed PWM DC pulses going to the motor make the motor stator and rotor act like a capacitor and a voltage potential builds up in the rotor. Without an easy path to ground, that voltage continues to increase until it exceeds the dielectric strength of the bearing grease and discharges to ground across the bearings. This etches the bearings and races, what is called "EDM damage" (EDM stands for Electric Discharge machining, because the damage is like that caused by a welder). The shaft grounding bushings have a carbon "fuzz" that is making contact with the shaft of the rotor, giving that voltage potential an easy path to ground so that it doesn't go through the bearings. But for that to be effective, the motor frame MUST be grounded too. That's likely why they assigned the task to you, the electrician.



Thanks for all the information..
I see these can be installed by the motor manufacturer inside the motor, that seems like a better application. I can just see the mill rights taking it off or something.


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## 360max (Jun 10, 2011)

EC2253 said:


> Thanks for all the information..
> I see these can be installed by the motor manufacturer inside the motor, that seems like a better application. I can just see the mill rights taking it off or something.


The shaft grounding bushings have a carbon "fuzz" that is making contact with the shaft of the rotor.

I'd imagine this would be on a maintenance checklist, maybe requiring replacement.


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## JRaef (Mar 23, 2009)

360max said:


> The shaft grounding bushings have a carbon "fuzz" that is making contact with the shaft of the rotor.
> 
> I'd imagine this would be on a maintenance checklist, maybe requiring replacement.


True, but what they say is that it lasts as long as the bearing seals, so change them out when you change the seals. Impro Seal makes a version of their bearing seals that has this built-in to them now.


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## John Valdes (May 17, 2007)

I have had some success with insulted bearings. I have never been asked to install this type of shaft grounding. But I have occasionally seen applications where something needed to be done as motor bearing failure/pitting became apparent.

Something to consider: What is their reasoning for this grounding? Are they experienced with this type of bearing issue/failure?
Seems a hard look at the installation and the problems associated should be considered long before you take this step.
I mean WHY are they having or expecting bearing failure and what can be done to prevent bearing failure/pitting in advance of a stop gap measure like this.


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## micromind (Aug 11, 2007)

One thing to look at when specifying shaft-grounded motors is cost. They're quite a bit more $$$

It's possible that the retrofit may be less $$$ even including labor.


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