# Power drop caused VFDs to reverse; powerflex 525 and compactlogix



## Breakfasteatre (Sep 8, 2009)

we had a small overhead conveyor system with 2 motors on 525 vfds controlled by a compactlogix plc.

control of VFD is through ethernet

customer called and told us they had a short blackout and when the power came back on, the two motors were now running backwards. 
Maintenace plugged into the system and said the VFD parameters were either backwards or possibly they defaulted? he corrected the direction bits and the system is running as normal again.

any ideas? we havent fully commissioned the system but if anyone has any experience with someone like this happening, it wouldnt hurt to not be totally blind when i go back with our integrator


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## gpop (May 14, 2018)

What probably happened is the motor was running forwards (backwards in the field) and someone got cleaver and simply toggled the bit in the plc. Power glitch cleared the plc memory and it now runs forward again. 

Instead of toggling the bit add a rung so the bit is always on or simply rewire the motor then disable reversing. I have seen this done a few times especially if the motor was reversed from the push buttons on the vfd hmi.


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## just the cowboy (Sep 4, 2013)

Good reason why we power down then retest on startup. We also load the latest and greatest program before we are done to make sure all variables are there. 
We just had a drive startup when PLC powered down, oddball drive used pulses to start and stop not held signals.


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## JRaef (Mar 23, 2009)

Along these lines, if you have a Logix processor and a PowerFlex drive being controlled over Ethernet, they may have enabled a feature called *"Automatic Device Configuration" or ADC*. ADC has the VFD programming stored in the memory card of the Logix processor, so if the VFD has to be changed, the person changing it does not need to reprogram it, ADC will do that automatically for you overt he Ethernet connection.

But here's the rub... When you enable ADC, the processor essentially takes a snapshot image of the VFD programming at that moment. If someone makes changes to the DRIVE after that takes place, the PLC does not know about those changes unless someone manually told it to store the new image. If that didn't happen, then the first time power is cycled, ADC puts the OLD programming back into the drive.

So if, per what gpop said, someone "fixed" the reverse operation by flipping the bit rather than changing the wiring, *but didn't reset the PLC's ADC image of the drive*, the PLC put it back to the original state. If they fixed it again the same way, it will happen again when power is cycled. So they need to either change the direction permanently with the output lead swapping, or find out how to update the ADC image in the PLC for that drive.


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