grounding and bonding

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djim8

Member
there is a 480v feed to a control cabinet that didnt have a groung pulled in the feed. This control cabinet controls variable fequency drive motors that trip from time to time. the fault indicator says that undervoltage has occurred. when we check voltage the voltage is good and we reset the VFD. We had the electrical contractor come back out and ground the control panel. they took a ground from the control cabinet to the building steel. My question is, is this sufficient or does there need to be a ground pulled in the feed conduit and why?
 

djim8

Member
Re: grounding and bonding

We have other control cabinets with grounds pulled in the feeders that also control other VFDs and we dont have this problem. I wasnt here when these feeders were pulled and I dont no why they didnt pull one to this particular cabinet.
 

bphgravity

Senior Member
Re: grounding and bonding

An equipment grounding (bonding) conductor needs to be installed with the circuit conductors to provide an effective fault path back to the branch circuit panelboard. Any of the methods in section 250.118 are permitted to be used as an egc. I do not see building stell in this list.
 

petersonra

Senior Member
Re: grounding and bonding

I don't see how grounding, bonding, or anything to do with green wires is going to solve an undervoltage problem.

I don't have a problem with bonding to the building steel. I don't see that it is likely to solve this problem, or any other problem for that matter, but it won't make it worse.

It seems likely there is some form of EGC coming out with the power conductors. If not, that needs to be done as a safety measure. But that won't fix the undervoltage problem either.

[ May 19, 2005, 09:57 AM: Message edited by: petersonra ]
 
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bthielen

Guest
Re: grounding and bonding

This is an uncanny coincindence. I work for a packaging machinery manufacturer and we recently made some changes to the speed control of a machine's VFD. The machine seems to run fine until we try to command a slightly higher speed. To my knowledge the commanded frequency is not excessive, the VFD is setup typical to other applications we have, and there is nothing unusual about the application but when we execute the command we get a bus undervoltage fault. It seems to work fine at all other speed commands. We have not been able to isolate the cause of this problem. Input voltage to the drive seems stable. My first thought was possibly regenerative voltage due to load inertia during deceleration but I have been told this problem occurs upon acceleration not deceleration.

Anyone have any insight?

Thanks,
Bob
 

petersonra

Senior Member
Re: grounding and bonding

First off, deceleration might cause a bus overvoltage, but I don't see it causing an undervoltage.

My first shot at this would be to put a meter on the bus and see what the voltage is actually doing when this happens.

It's also possible you could just change the parameter in the VFD setup that decides when to trip on undervoltage. Maybe someone set it too high.

Could be that they told the drive the line voltage was 480 and it's really more like 450.

[ May 19, 2005, 04:27 PM: Message edited by: petersonra ]
 
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bthielen

Guest
Re: grounding and bonding

Bob,

Thanks for the input. I had a slip of the fingers. I meant over-voltage not undervoltage and that is why I was thinking of a possilble regenerative situation. You have given me some possibles however and I'll have our technicians check some of these parameters.

Thanks again,

Bob
 
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