3 phase vfd only pulling current on two phases 1

[jsftech]

I have a 50 horsepower heavy duty drive (AB powerflex 700) connected to a 50 horsepower 1185 RPM, 460V 59.2 FLA motor. It spins a 78.75-inch diameter, 3100-pound cage through a 2.0-1 right angle gearbox.
There is also a 250 HP fan pulling air and ground silica through the cage. This airflow will spin the cage without the cage motor being energized. The system is used to classify ground silica particles.
We run the drive around 37 hertz for one product and 21 hertz for another product. We have been having trouble deceling the drive from 37 Hz. to 21 Hz. (decel inhibit fault). I have checked into a dynamic brake and a regen unit. I have managed to eliminate the faults for now by changing the Feedback select parameter from slip comp to open loop and enabling Flux Braking. We only decrease the speed 3 to 4 times a week. The drive shows about 22 amps at 37 Hz. and 17 amps at 21 Hz.
While troubleshooting I decided to check input amps with a clampon and noticed there was no amps on one phase. I switched that input phase with another one and I still didn't have any current on the same input phase of the drive.
I contacted Allen Bradley and they said there was a problem with the drive. They shipped us a new one. I installed it today and it has no input current on the same phase. I contacted our local Allen Bradley distributor and they have never heard of a drive doing this.
Has anyone ever seen this before? Do I have a problem? The drive is in service. Allen Bradley had some concern about the overhauling load while trying to troubleshoot the decel inhibit fault. I don't think the drive is under sized. The drive is only showing 22 amps on the output. It is a 77-amp drive. Does the drive sustain a low amp output by only pulling power from two phases? Allen Bradley doesn't seem to think so. When the drive is accelerating it is pulling amps on all three phases. Thanks.

 

[jsftech]

Update...

I noticed the voltage imbalance at our plant had improved the other day, so I checked the current on this drive again. We were pulling current on all three phases, it's still not balanced but it's closer than it was.

A few weeks ago the drive was running at:
22Hz.
17amps
672BusVDC
The incoming voltage and amps were:
A-B 486v A 3 amps
B-C 476v B 3 amps
C-A 476v C .17 amps
This equates to about a 1.4% voltage imbalance.

The other day the drive was running at:
A-B 471v A 3.5 amps
B-C 468v B 3.8 amps
C-A 466v C 2 amps
This equates to about a .6% voltage imbalance


At a higher speed a few weeks ago the drive was running at:
37Hz.
21amps
647BusVDC
The incoming voltage and amps were:
A-B 484v A 11.5 amps
B-C 473v B 10.5 amps
C-A 471v C 1.3 amps
This equates to about a 1.7% voltage imbalance

The other day the drive was running at:
32Hz.
23amps
632BusVDC
A-B 471v A-G 270 A 9 amps
B-C 466v B-G 272 B 10 amps
C-A 462v C-G 267 C 6 amps
This equates to about a 1% voltage imbalance

As you can see the current on C phase increased significantly. I called the local utility to see if they had found a problem on the line anywhere. They said that they switched our main feed to a different substation.

I checked the amps on several of our motors and the current imbalance on most all have improved especially our 600 HP motors. I guess it was an incoming power issue after all.

Thanks to everyone for your responses, maybe this post will help someone.

 

[ScottyUK]

Thanks for the update - always good to get some feedback.


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