Grounding of motor nuetral for VFD application 3

[hydropowerengr]

We presently have a 200 Hp ,460 volt motor connected in wye which is provided power from a SquareD VFD controller.We typically always bring motor leads together and connect them to ground but Square D has some concerns that this will cause problems with their VFD drive. Is this reasonable to leave the motor nuetral leads ungrounded
"floating" in this application.

 

[peebee]

In what jurisdiction are you located? Seems to me that grounding your motor neutral would be a violoation of NEC -- if you want to "ground" your motor neutrals, the best you could do per code would be to run a neutral wire (not a ground wire) from your system source to the motor neutral.

I don't see any technical reason that letting it float would be a problem -- and I would generally defer to the VFD vendor.....

 

[Skogsgurra]

You may have experienced some (now outdated) voltage controlled induction motor controllers that actually needed a grounded neutral to work at all.

I have seen that habit carried over to other motor control applications where no grounding shall be done.

Never ground any neutral or star point when running off a VFD!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

Grounding the Wye point of a set of motor windings? I didn't know ANYBODY ever did that! What's the point? It serves no purpose I can think of, unless perhaps the motor winding insulation was so weak as to not be capable of taking the full line voltage. But either way, I agree; do NOT do it on a VFD!

I'm going to knock off for the rest of the day now, I have learned my new thing.

 

[waross]

In a perfect world you may be able to ground the neutral of a three phase induction motor. (Line connected,not VFD driven).
You must be very confident that the supply voltages will always be exactly equal and the phase angles will always be exactly equal.
A three phase induction motor will try to balance unequal supply voltages. That is part of the reason that a motor heats up if the supply voltages are unequal.
If the neutral is connected then the motor will also try to correct unequal phase angles. The motor may also see greater voltage unbalances phase to neutral than phase to phase.
Don't ground the motor neutral unless a specific drive specifically requires it.
By the way, if single phase voltage regulators are used on the distribution circuits in your area, you may see fairly good phase to neutral voltages, but unequal phase to phase voltages and phase angle errors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DickDV]

And anyway, how are you going to ground the center of the wye when the connection is not brought out to the junction box on most motors?

 

[waross]

Yes DickDV. I feel a little foolish for not noticing this little detail. Gotta give you a star.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I assumed he had a motor with the connections brought out, i.e. a Y-Delta motor. Otherwise, how would you evknow what the internal configuration was?

 

[waross]

I was thinking a nine lead motor. They are almost universally star connected, but the star point is buried. On 240/208 there is a neutral but it is only for half the windings.
The leads that form the star point on the low voltage connection are at about 139 volts on the igh voltage connection. You probably are right about the star/delta configuration jraef.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

Interesting - grounding the star point, never seen that either. I wonder what effect that would have on the system voltage if you lost a phase somewhere. The motor should still run feed power backwards eh!
Definite No for VFD
Roy

 

[Skogsgurra]

You never used the UAL three-phase voltage controllers? They were so badly designed that you HAD to create a return path from the motor's star point to ground (or neutral).

But those were voltage controllers based on triacs - no VFDs.

Grounding the star point of a VFD fed motor would probably create a DC path from mains via DC link rectifiers, IGBTs and motor winding to ground. That would not be good at all.

So, as already said, don't do it.

Still very interested to know from where hydropowerengr got this idea. Hello hydropowerengr! Any answer to that?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

I'm with Peebee. Grounding the motor Y point has got to be a violation of most electrical codes.

Connecting the Y point to neutral could be considered OK code wise but is almost never done in practice.

 

[cswilson]

And don't forget that if the VFD is using "third-harmonic injection" to improve its DC bus utilization, it is purposely varying the voltage of the Y's center point from the neutral voltage. You would really screw things up if you tried to tie it directly to neutral.