VFD Over Current Trip with VTPs 1

[Munequip]

We have experienced nuisance trips at two unrelated locations with similar equipment. Looking for opinions on the cause.

Location 1 - vertical turbine pump with a VHS motor installed outdoors. Pump is started with a VFD, and the unit operates successfully. However, the VFD would occasionally ( at least four occurrences) trip on "over current", and we would find that the pump would turn freely (by hand). It could be re-started, and there would not be an issue. System has function properly since lubrication was added to the ratchet on top of the motor. (yes, the non-reverse ratchet)

Location 2 - vertical turbine pump with a below grade discharge pumping water that is at near freezing temperatures. The pump is equipped with a mechanical seal that drains back to the basin. The pump would operate successfully until it was turned off, and the water that remains in the seal would freeze. The VFD would try to re-start and trip on "over current". Water (at near freezing temperatures)would be sprayed onto the seal area of the pump, the VFD would be energized, and the system would operate. This scenario occurred dozens of times until a "shed" was constructed over the pump heads and motors, and it stopped. One of the VFDs failed (cause unknown at this time), and the new VFD trips on "over current". System turns freely by hand, and it was re-started.

My question is if anyone has experience nuisance tripping with VFDs. We have installed hundreds of pumps on VFDs, and have experienced this nuisance tripping in the last couple of months. Not sure if the VFDs are too sensitive, or what may be causing it. Not sure why the overcurrent fault of the VFD would cause it to shut down during initial start up.

 

[jraef]

In the first case, if, after experiencing the OC trip, the shaft could still be rotated easily, it may have been a problem with some other condition in the circuit, i.e. the conductors or even the components in the drive conducting in ways they were not supposed to, as in line-to-line shorts (GF would typically be displayed as a different fault). But another possibility is that although you can turn the shaft by hand, when you start to accelerate a VT pump, there is what is called "uplift" of the shaft, which goes into a thrust bearing on the top of the pump. It could be that in the process of greasing the backspin ratchet, you were also greasing the thrust bearing. You might want to speak to your pump supplier about regular required maintenance.

IN the second instance, icing could cause an OC trip, but its more likely that the same conditions causing the icing might be causing some other electrical issue, again, a short somewhere. The mysterious failure of the previous VFD is a big clue toward that.

"Will work for (the memory of) salami"