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VFD Problem

VFD Problem

(OP)
The Power to the 1050kW, 6kV motor is fed from VCB-->VFD-->Motor. We heard from the operator that the motor stops suddenly and starts after the 30s~1min. I feel that this might be due to the voltage fluctuations from the power supply side. The Under voltage protection at the VCB Panel has 50ms time delay but the VFD has only 30ms. Therefore the under voltage protection at the VCB panel may not have operated but the under voltage protection at the VFD may have operated and restarted again as the VCB which is feeding the VFD is still ON. The important point is to note that the VFD did not trip the VCB on the operation of under voltage function. There no other alarms/trips while this happened. I have not heard about the VFD showing any fault in it's display.

Has any experienced this type of problem? any comments?

RE: VFD Problem

I'm probably the least experienced person here to give an opinion on this...

the 30s to 1min tripping sounds like it might be related to overheating. It's worth checking to see if there's a zero sequence path somewhere on the motor, which could cause heating. This could be the result of a grounding issue.

RE: VFD Problem

The variable start-up time you're stating is very similar to a problem I created with a VFD in the last year. It was quite vexing as what I did seemingly had nothing to do with "time delays".

I set up a lathe with a VFD and thinking it would make sense to have the drive in the "catch mode" where if the drive was shut off and the motor was still spinning when the operator turned it back on it would smoothly re-accelerate the motor.

What happened though was that the drive then insisted that it know what the motor was doing before ever starting which makes sense. This caused a horrifying result that was actually deadly. When you walked up to the lathe and it was off and hit run - nothing would happen - for a while. From 1 second to a minute or more. This meant the operator could press go and nothing would happen at all, could decide he hadn't pressed start and possibly start chuck-keying the chuck and have the lathe surprisingly start spinning.

I figured it out after much head scratching. It was the "catch" parameter waiting to figure out what the motor was doing and with the motor stationary it took an extended RANDOM time to figure it out and actually start the motor. Manually turning the shaft, like to align a chuck key, would allow some sort of input to appear and the VFD then knew what was happening and would've started. Without any shaft turning the VFD cannot receive any speed information and waits until noise is seen.

Turning off the catch parameter instantly solved the issue. I think the software people who wrote the VFD SW didn't realize what trap they were setting.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD Problem

Could you have a loose connection that causes the shut-down and then have "catch ON" causing a random restart.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD Problem

If there is no fault displaying in the VFD, then the first thing I would suspect is that the VFD is being COMMANDED to turn off, then back on again. From the VFD's standpoint, it would just doing what it is told, hence no fault display. I have run into this with both VFDs and Soft Starters, where the machine / process has some sort of sensor that has a time delay involved and if flow/pressure/movement/torque etc. is not at some predetermined value within xx seconds, it assumes something is wrong and removes the Run command. Then because something else is still calling for it, the VFD is re-initialized and because the motor was likely still spinning, the sensor is satisfied the second time because its condition was met within the allotted time frame.

Bottom line, start by looking at your command signals. Many VFDs allow you to observe and capture that data.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: VFD Problem

A similar thing hapened in a ventilation system where several fans were working into a common exhaust (high chimney. There were fans that were motoring as they were supposed to and under certaing circumstances one of them overspeeded (kind of an ejector effect sucking air through the fan), which caused DC link to get higher than expected. Which shut the VFD down.

We got fault indications so it was not such a mystery, after all. Everyone knew it was overvoltage. The problem was WHY? Nothing to be seen on the grid. And no OV trips on the other VFDs

All I can say is: do not speculate. Record.

Record grid, DC link (yes, you need special fuses and probes), speed (a simple inductive pick-up will do), possible fault signals. Then sit back and see what happens. And then, when it happens, draw the right conclusion and just fixt it. It aint necessarily electric.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: VFD Problem

That size of VFD should be recording the under-voltage trip if it's actually tripping. So, check for that again.

No log of the trip means it's likely being told to stop.

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