Inverter earth fault 1

[panou]

Hi to all. I have noticed some tripping of a Danfoss inverter VLT5000 with alarm 14 EARTH FAULT. After recycling the power and resetting the inverter it resumes normal operation. Is there any logical explanation for this ? Earth fault is a short circuit between phase and earth and is something more or less permanent. Should i suspect the screnned cable between inverter and motor or should i expect a possible motor insulation failure ?
Thanks in advance for your time.

 

[itsmoked]

Sure you can concider both those causes... May also be some sort of transient causing it too. Anything changed lately around the system? Added anything new?

 

[DickDV]

If you do not have a continuous copper ground between the motor frame and the drive frame, false ground faults are often the result.

If you do have the grounding in place, an intermittent motor insulation failure is probably the most likely cause. Usually, it just gets worse until the motor fails. In the meantime, most drives have a provision for turning this fault off. You may want to do that to avoid the nuisance.

If your power supply to the inverter is one of those nasty floating delta or high resistance grounded wye systems, the drive will often confuse imbalance problems on the supply for motor lead grounds. Your only recourse in that case is to turn the fault off in the software.

Don't you just love those floating networks!!!

 

[jraef]

To add to DickDV's excellent response, moisture in the conduit can be an intermittant cause as well. Wire insulation is supposed to be non-hygroscopic, but even the tiniest of nicks can result in a buildup of moisture in the insulation, and the VFD GF detection will pick that up if it tracks to ground. I have seen this time and time again, especially where conduits travel through areas with large temperature differentials, such as from a mechanical room to a refrigerated room. Condensation forms on the warm conductors. Then when the VFD is off for a while as someone trouble shoots it, the moisture dissipates enough to allow operation for a while.

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[aarthoogerwerf]

one question first: Do you have a long motor cable or a cable type with high capacitance. This can cause earth fault trips.
Also some motors have very high capacitance to earth, because of their construction.

You can measure this (disconnect cable from converter)with an appropiate measuring instrument.

Check for allowed cable length in danfoss documents.

 

[alexit]

How long has the system been operating? Is this the first problem with motor/drive system? If yes, its time to start testing. Lets say your system was working fine for 6 months:

The VLT software will provide the same error for detected line to ground shorts and load to ground shorts. It can also supply this error in the event that your line earth is changing impediance.

Start with the motor, use a motor winding analyzer and be sure to measure loaded turn to turn and turn to ground impediance for each winding do this including any connectors used. Surge test at the repeatable hi-pot voltage, do this only twice per turn.

Now check the cable, you can bridge the ends at the motor connector and use the winding analyzer if this is a long run...otherwise just replace the cable with something acceptable for your motor. Be sure this cable is rated for the controller frequency of your VLT5000, this is usually 6-10kHz but somtimes up to 24kHz (I like the olflex products from Lapp.)

Does the system have an inductor or transformer between drive and motor? Check this with the winding analyzer too.

Now check line to the drive, does this have a inductor or transformer. Test it. Check the earth ground impediance, does this change when the machinery is running? When OTHER machinery is running? Now with everything running scope the input voltage/current waveforms from the mains, record for a week or so...is your local utility "helping" by playing with the voltage during peak hours? Does other machinery cause a spike during startup/shutdown? Be sure to check phase to phase synchronus error, is this less than 3%? Is it even close to 3%?

IF you still cannot find a reason, perform EMF "electrosmog" analysis...is the TV station satellite broadcast farm next door properly grounded? How about the airport weather radar sitting 300' from your parking lot? Are you located under imbalanced 1200kV high tension lines?

There is a reason for your error message, it means something is wrong...