VFD to Motor Earth

[sunrays]

I intend to connect a 45 kW VFD to 4x7.5kW Motors for Long Travel of a Yard Machine. As per the VFD Manual the Drive output earth terminal has to be connected to the motor earth to ensure perfect grounding.

In the present application the motors as well as the VFD panels are installed on metal frames which are all on the same potential due to the fact that the machine structure is fully metallic.

Is it then safe to ground the drive output and motor locally instead of running a seperate earth conductor for each motor from the VDF?

I might add here that in an earlier instance the same make of VFD connected to an individual motor (mounted on a concrete bed) I could measure a 230 V between motor body and earth in the absence of the VFD to motor earthing.

Apreciate your suggestions and thanks in advance.

 

[controlsdude]

I believe the NEC is clear on bonding and grounding. I would look it up and then you will be very informed.
I am not sure what mobile equipment that you have but there are strict guidelines in the NEC on what would apply.

To get a general ed on grounding and bonding I thought this publication went through it throughly.

http://ecmweb.com/grounding

I am concerned you might have a hazard from what you described with measurements that you have taken from an earlier installation.

It does not take much to kill someone believe me. All the bad electricians are dead, only the good ones stay alive.

 

[itsmoked]

To sum it up you can not use any "mounting hardware" for grounding. Hence all your panel bolts, bolts used to hold the enclosures on, etc, etc are not permitted to be considered grounds.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sunrays]

Thanks Controlsdude & itsmoked for your time.

Controlsdude: The voltage measurement was more of an inqusitiveness to check the effect of non grounding of drive and motor and was not done to kill some one!

The machine is used to stack iron ore in piles along the lenght of the track. The drive and motors are used for the long travel motion on the track.

The entire machine is made of steel.

 

[controlsdude]

I was using that line to get your attention.

NEC article 610 Cranes and hoists should cover it. Grounding is part of it then reference article 250 on specifics.

BTW, if you measured 230vac on the motor frame then the motor is not properly grounded or bonded.

be safe

 

[DickDV]

Controlsdude, just FYI, the copper ground bond that is generally specified between the motor and the drive frame is not a safety issue and therefore not a concern of the NEC. This bond is present only to provide a high-frequency low impedance ground reference between the motor and the drive to avoid nuisance drive faulting.

As mentioned above, a continuous steel conductor would probably serve just as well but bolted connections, fitted but not welded joints, etc. are not good enough. Thus, the continuous copper ground bond is the preferred method.

On a long stacker/destacker like you seem to be describing, it is problematical to run the ground thru sliding rail contactors. Hopefully, some kind of long-reach festooning system is available.

 

[controlsdude]

Ground thru sliding rail contactors

I have done high speed cranes and transfer cars and used the open and closed collector that had the ground in the bus bar. So a long-reach festooning system for grounding would not be necessary.

I believe wampler and vahle as an example, sell this bus bar that would carry the ground through a collector-bus. I never had an electrical inspector fault this type of system.

I think the problems we had was when the you had a collector disengrate on you that it was wise to put in a ground fault detect and missing phase detection. I know this was above code requirments but would point to maintenance problems with the collectors and the bus bar.

 

[sunrays]

The Drive Panel as well as the motors are on the moving equipment. Drive to Motor connection is by cables only. The incoming power to the equipment is by means of flexible cables wound / unwound on a reeling drum as the machine moves forward / reverse on rails.

 

[roydm]

Doesn't NEC call for a copper ground to the motor. I would run the ground in each of the motor cables.
Out of curiosity how do you protect the individual motors, a separate O/L for each motor?
Heres an open question, in Sunrays application with 4 motors on one drive, if the cables are of different length do the reflected waves tend to cancel each other?
Roy