VFD Ground Fault

[Brijm]

I have an odd problem with a Dan Foss FC series VFD getting a ground fault, that I would like to get some opinions on.

The drive is on a powdered metal molding press. Most of the time, the drive runs fine. Occasionally especially during a cold startup (Sunday night after being down), the VFD when trying to start will get a ground fault. They also tell me, that when it is going to fault, that their is a high freq whine coming from the machine (not sure if it is from the motor or the drive). Normally shutting down the drive and restarting it is no problem.

Last year at the advice of Dan Foss, we changed out the power card (CT card). From the customer, the problem went away for a few months, and then returned. I did ask about the ribbon cables that where always a problem on VLT5000's. Dan Foss only pointed to the power card.

My plan is to swap the drive with a similar press with a VLT5000; to prove that the problem is in the drive, and then send it in for repair. I wanted to see if anyone else had any suggestions. It especially threw me, that it the problem is mostly when the starting the drive, after being down for a few hours, and the high pitch noise, that is coming from the machine.

 

[dmargineau]

The vast majority of the VFDs "Ground Faults" are usually related to the motor's "electrical condition"; a "cold" start-up might have to "deal" with condensation moisture within the motor's windings; heat generated by the operation will eventually dry the moisture out until the next start-up.

 

[alexmcsheffrey]

Hi i have had a very similar problem with a vlt5000 i did read somewhere that it could be due to faulty ct,s measuring the output current?

 

[Brijm]

Hi i have had a very similar problem with a vlt5000 i did read somewhere that it could be due to faulty ct,s measuring the output current?

Yes... this is extremely common on the VLT5000's. It is usually either the ribbon cables between the power card and control, or the relay board (CT Board). The CT board (I believe they called this the power board on the FC) was already replaced.

Of course the manufacturer seems to always blame the motor and cables first (when they meggered fine) But normally in my experience with this manufacturer a component change in the drive seems to fix the problem.

Note, the drive/motor and machine is about 4 yrs old.

 

[Brijm]

The vast majority of the VFDs "Ground Faults" are usually related to the motor's "electrical condition"; a "cold" start-up might have to "deal" with condensation moisture within the motor's windings; heat generated by the operation will eventually dry the moisture out until the next start-up.

I had considered that, but none of the other machines have this problem... and it has increased over the winter, when moisture levels are low. Normally my experience with the previous versions of these drives are that it is in the drive. It seems that the ground faults mostly show on Dan Foss drives, short of an actual motor problem.

 

[dmargineau]

I had considered that, but none of the other machines have this problem... and it has increased over the winter, when moisture levels are low. Normally my experience with the previous versions of these drives are that it is in the drive. It seems that the ground faults mostly show on Dan Foss drives, short of an actual motor problem.

The motor is probably totally sealed; the moisture will remain within the case either in vapor or liquid form; winter operation presumes the highest temperature differential which could easily reach the evil dew-point when you start producing liquid.
I am dealing with mostly AB and Siemens VFDs (up to 200 HP) and most of the experienced "Ground Faults" were mitigated with motors replacements.

 

[Bit_Bucket_07]

I've seen a VFD detect a ground fault in a system although it was not on that particular drive's motor. Turned out that a fixed speed screw conveyor had an intermittent fault because the sealtite had broken loose from the peckerhead on it's motor. Whenever that little screw conveyor bumped a leg to ground, it faulted the drive, which was running a large mixer.

If I'm not mistaken, this system was operating on an ungrounded delta 480V transformer at the time. That could have been a piece of the puzzle. We found the ground fault by shutting everthing down except the mixer, and then starting each piece of equipment one-by-one until the mixer drive faulted.

 

[DickDV]

I believe Bit Bucket has the answer. Pretty much all VFD brands will need their output short circuit fault option turned off if powered by floating delta power.

The false ground faults occur when the floating delta makes a sudden shift in balance to ground. This results in large unbalanced discharge currents flowing in the motor leads which fool the drive's ground fault detector.

We routinely turn off the ground fault on all drives powered by floating delta networks just for this reason.

 

[Brijm]

I believe Bit Bucket has the answer. Pretty much all VFD brands will need their output short circuit fault option turned off if powered by floating delta power.

The false ground faults occur when the floating delta makes a sudden shift in balance to ground. This results in large unbalanced discharge currents flowing in the motor leads which fool the drive's ground fault detector.

We routinely turn off the ground fault on all drives powered by floating delta networks just for this reason.

I will have to verify, but this is a newer building, and we don't have too many floating delta systems around here. I also don't believe that the Dan Foss allow you to turn off the ground fault.

 

[OkiePC]

The vast majority of the VFDs "Ground Faults" are usually related to the motor's "electrical condition"; a "cold" start-up might have to "deal" with condensation moisture within the motor's windings; heat generated by the operation will eventually dry the moisture out until the next start-up.

+1
Ground fault practice for me:
Kill power to vfd. read dc bus and study how it drops. When below 10 or so volts (I am low resistance) unhook the motor leads and perform a megger test on the entire circuit. Record all of this in your work request paperwork whether or not a problem is found. Next, perform a phase to phase motor circuit resistance check with a quality Ohmmeter and record those three values too.

Most times you can at least prove the circuit is safe to connect to the VFD and try again, or, more often, identify the exact source of the ground fault with one short stoppage and a systematic approach.