VFD False Shutdowns

Perhaps a mechanical tremor loosens some conductive dust across the mains, enough to trigger a fault ?

Perhaps aging equipment is no longer able to sense proper leakages ?

Perhaps intermittent arcs are being induced by voltage spikes in insulation of old cables ?

For starters, you must assume there is a fault, and search for the most likely cause.

Intermittents are very hard to diagnose, you must consider the environment as well as the age of components and stresses at fault times. Are the motors being started one at a time or is the sequence random, are motors being switched on and off when fault is triggered ?

Careful observation of the environment and operating conditions are sometimes the only way to find such problems. Do you have access to monitoring equipment which can verify the fault ?

Perhaps the only way to find such an intermittent is to take an educated guess and replace a component, and if the intermittent stops then you call it a solve.

Good Luck

L- I -T.

If push comes to shove, you might try installing an appropriately sized line reactor. Or you could just wait until the short becomes permanate and then you will know that the VFD was trying to help you.

Reminds me of the old story, "Can you spell the VAN in vanilla?"

The topology of a 45kW low voltage VSI drive and whatever type of 6kV drives you have can be so vastly different that making a cursory assumption of any kind of common causality is almost ludicrous. But if I had to pick anything that MIGHT be a connecting link, it would be grounding (earthing) practices. If this happens often, I would take a look at how you are running and shielding your output conductors. One misstep and you can create capable cacitance issues that will not show up in common cable installation testing, but can be interpreted by the drive as a Short Circuit, which is typically defined as too rapid of a rise in current over time. If the cables are allowed to store too much energy as a quasi capacitor, the capacitor charging current can look like a short circuit to the VFD protection algorithms.

Have you checked the motor bearings and the bearings of whatever that motor is trying to spin yet?

Hi,hi, this problem has been happening for quite some time now, we have replaced the motors, adjusted the switching frequency of the drive..

The 6 KV motors are switched on directly.. DOL... but the problem occurs when I shutdown these motors and not when starting..

On shutdown......that makes a big difference to your query sport!

That there then would/might be a back EMF problem.

If there's a VFD in there then it's not DOL start then is it?

Read C a r e f u l l y sweetheart !!.... the 6kv 2500 volt motos are DOL, the 45 kW are fed from the drive ...

Probably you have long cable between VFD and motor.

And if there are harmonics produced in VFD - at higher harmonics, Xc is very low (1/2 pi * f *C) - resulting in increased leakage and tripping of relay.

you can locate temporarily VFD close to motor,thus shortening length of cable and see if it does not trip. May solve your problem.

Please explain what you mean by "shutting down" the motor. Specifically, are you decelerating using a controlled method or are you simply opening the circuit?

There is a world of difference between the two.