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Looking for transients in 600V 3 phase system, need opinions
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Sterno:
VFD does not brake, it is set to coast.
Gear reduction on conveyor is 780:1  Conveyor moves at 20 feet/min max.  Conveyor uses plastic links on a sliding surface, pretty high friction, conveyor stops very quickly.

Unit has been working fine with line reactor, that said, refrigeration tech hasn't been back to activate his end.

--- Quote from: Sterno on July 05, 2019, 07:16:58 pm ---
I have kicked this up the command chain to get someone with the proper gear / knowledge to solve this as I'm just a Electronics Engineering Technologist, not an Electrician.

--- End quote ---
NiHaoMike:
Is a special order 600V VFD and motor really cheaper than an off the shelf 208V VFD and motor plus a step down transformer?
Sterno:

--- Quote from: NiHaoMike on July 05, 2019, 08:55:19 pm ---Is a special order 600V VFD and motor really cheaper than an off the shelf 208V VFD and motor plus a step down transformer?

--- End quote ---

2 kVA Step-down transformer costs more than the 600V VFD, it would likely increase total cost by $500. I have considered switching to that but it wouldn't address the root issue of the surge.

I should note that this isn't the first one of these we've built.  Five out in the wild as of this year, this is the only one to give us any electrical trouble.  Dozens of other small panels using same vendors VFDs, they've been trouble-free.

As stated above, now it's the Electrician's turn to find the surge.
duak:
This is a low probability conjecture given that the failure looks like it was caused by a transient because it took out the traces to the MOV:

Suppose one of the AC line phases wasn't solidly connected into and out of the MCC or into the VFD or the MCC has a bad contact?  The VFD is basically then single-phasing with all the input power being handled by only two phases, ie., four of the six diodes in the module.  575 V VFDs are rarely rated for full power with single phase input so their input rectifiers aren't rated for the higher current.  Because the L1 and L2 traces are damaged maybe the rectifiers on those two phases overloaded, shorted out and then opened or tripped the MCC.  I'd use a clamp on meter to measure each of the line currents when the VFD is running at load.

I remember an MCC with a screw terminal that could be torqued down but the wire could be pulled out.  I think the setscrew had a damaged thread that didn't allow it to fully clamp the wire so it was a high resistance connection.  This would be the kind of subtle failure cause that would eat VFDs until discovered.

Most 3 phase only VFDs must be derated to 60% of full load when operated on single phase.  How loaded was the VFD in this case?  Since this is a conveyor load, there is a friction load at all speeds and during start up there is the inertial load to accelerate.

Also, when I look at the picture of the enclosure, I see the two modules to the left of the VFD with short wires to ground, whereas the VFD's ground wire looks like it heads off somewhere in the harness.  The GS2 manual calls for a resistance to ground of less than 0.1 ohms. (p.2-11, upper leftish)

When I look at the image of the module, I think see that at least one output phase has also been incinerated.  This makes me think that a transient passed thru the VFD and was ultimately returned to ground in the motor.  I would check the motor, motor cable and all connections for leakage to ground.  I had one CNC connector have its insert's plastic insulator burn between the phase connections because coolant had seeped in.  If the motor is old or is not rated for inverter duty this might be a consideration.

Best o' luck
jmelson:

--- Quote from: Sterno on July 05, 2019, 09:12:02 pm ---I should note that this isn't the first one of these we've built.  Five out in the wild as of this year, this is the only one to give us any electrical trouble.

--- End quote ---
Well, how many of these VFDs have blown?  Pretty crazy that you've built 5 units that were just fine, now they are blowing up!  Is there anything NEW in the plant that could be causing surges?  Might be some other gear in the plant that is using intermittent bursts of current and causing utility tap changers to step up the mains, then when the burst ends, you get high voltage for a few seconds.  It could, of course, even be some other plant on the same feeder.

Jon
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