Powerflex 70 Stray Voltage to ground

[Mac 1]

I'm not even sure how to start this one off but We have a polishing unit that uses a PF70 to drive a belt motor. This machine has some suction ducting that connects to it. We recently had the duct disconnected to do some work on it and with the machine powered up one of the guys felt a little buzz when he touched the duct and the panel door. I checked and sure enough there is 50VAC from the ground lug on the panel to the frame of the dust collector? I've isolated it down to the PF70 base that is putting out the voltage to the ground in our machine through the base of the drive. Anyone else ever seen this before? When we hook up the duct the voltage goes away? We have a spare unit and tried swapping it out but we get almost the same voltage to ground? This only happens when we power up the drive.

[Michael Lloyd 85]

My first thought was ground fault on the motor (winding bad, etc) or conductors (bad insulation). Does your drive have a ground fault detection system on it? Have you meg'd the leads and motor?

Second thought - Maybe an SCR is leaking and starting to go bad? Scratch that... the drive swap would have eliminated that...

[Mac 1]

The Motor was my first thought but I disconnected the motor feed wires right at the drive so it's eliminated from the picture. Nothing connected to the drive but the 3 Feed wires in from a fuse block. Just as a test I unhooked the incoming ground wire from the main feed to the panel and sure enough I get 50VAC from the base of the drive to the incoming ground wire! Got me stumped..... 

[Michael Lloyd 85]

You may have already tried this but what happens if you disconnect the ground wire from the drive? I'm wondering if "ground" is being fed by something else. 

Other than the unlikely possibility that both drives have issues I'm equally stumped...

[Mac 1]

I've isolated the drive completely. It's sitting on an absorbent pad in the bottom of the panel with the ground wires disconnected from the backplane. When I measure between the base frame of the drive to my ground in the panel I get the 50Vac? If I pull the fuse block out that feeds the drive the Voltage goes away. I even tried disconnecting the feed wires from the drive and pushing the fuses back in and I get nothing to ground. Only when the drive is powered???

Edited 4 Mar 2016 by Mac

[Michael Lloyd 85]

Wow... it sounds like the drive is leaking to ground but that would mean that your spare drive is doing the same thing???

Very strange...

[Mac 1]

Might have to call Rockwell on this one..... As much fun as that sounds !! 

[Michael Lloyd 85]

Yeah... prepare for the sound of page turning and suggestions to do what you already did :)

[Mac 1]

Thanks for your help.I need to step back from it a bit just to see if I missed something simple. Everything points to the Drive..

[Michael Lloyd 85]

If you can easily pull the SCR's and test them you may may one that is leaking. But... there's that @$_*@ spare drive doing the same thing,

A schematic would be helpful but they don't send those out anymore...

Best of luck. If you find something I'd be curious to know what it is 

[Ken Roach 194]

The PowerFlex 70 ships standard with MOV's that protect against high voltage spikes, as well as common mode capacitors connected to ground.

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/20a-in010_-en-p.pdf

It should be a straightforward experiment to remove the MOV jumper and determine if your 50V AC is being conducted through that circuit.

[Mac 1]

So I got to the bottom of it tonight. I knew some time away from it would be good...... It turns out that whoever wired the machine initially had not made a good ground connection on the 600Vac machine feed. The 50Vac was induced voltage through the ground whenever the drive was powered up. Thanks for the help guys.

 

[Ken Roach 194]

Thanks for posting the followup !

[Michael Lloyd 85]

Ditto thanks and great job finding a not so easy problem

[kaiser_will 65]

I ran into a similar situation some years ago.  You found the same culprit...poor grounding practices.  Take some free time and survey other pieces of equipment in your facility as if it happened once it has happened before.

On a related note, I keep my trusty old analog Tektronix scope under my desk for similar scenarios.  Sourcing out ground noise happens on every machine build.  If you are lucky enough to inherit a Dranetz power meter at work, put the sucker to work once in awhile.