PowerFlex 753s programming question

Question/Issue: I have PF753 running a 150hp motor (piston pump). For years the techs/operators have had an issue starting this motor. It appeared as if it was single phasing. The rotor would turn a few degrees then slam back the opposite direction, back and forth and not actually start. The simple solution for the longest time was to simply rotate the motor by hand about 90 degrees and then it would restart just fine. Not long ago the drive was replaced for a different issue, but also hoped it would cure the starting issue we would see sporadically. It did not. Recently we could not get the motor started even after a manual spin of the rotor. Decision was made the swap motors even though electrically everything tested fine. New motor did exact same thing, unfortunately. So long story short and a few dollars later it appears to be a setup/programming issue in the VFD all along. We have made a few changes and are able to start the motor, but there is still a short period of what looks like single phasing before the motor finally spins up. Any suggestions on parameters to take a look at to achieve a smooth start? FYI the motor only starts a few times per week, and runs pretty much continuously. Also, never faults VFD.

Thanks

I don't have an answer for you, but have you tried contacting your local AB rep?

That's the next step. Just thought I'd throw a dart here and see what happens. Didnt find anything on knowledge base. Thanks.

It would help to know what control mode the drive is in (Parameter 35). If it is in volts/hz mode, then this is really confusing, because the drive would ramp the motor to speed in an open loop manner. If it is in one of the vector control modes, then this type of behavior would be typical of a drive that has not been tuned to the motor or tuned improperly.

If you find that PRM 35 is set to anything other than 0, I would try changing it to 0 and checking performance. You should also check the motor set up parameters to ensure you have the right nameplate info loaded.

drforsythe said:

It would help to know what control mode the drive is in (Parameter 35). If it is in volts/hz mode, then this is really confusing, because the drive would ramp the motor to speed in an open loop manner. If it is in one of the vector control modes, then this type of behavior would be typical of a drive that has not been tuned to the motor or tuned improperly.

If you find that PRM 35 is set to anything other than 0, I would try changing it to 0 and checking performance. You should also check the motor set up parameters to ensure you have the right nameplate info loaded.

Click to expand...

+1

It would also be a good idea to make sure the motor is starting with the pump unloaded. Depending on the design of the pump, piston pumps can be very difficult to start while loaded.

You could try running the auto tune function. We had a 500 HP 750 series drive that the AB rep suggested we run because of an overheating issue. It seemed to help, but then the summer like heat wave ended in the fall. Looking forward to July to see if he was right...��

Check the value of P75 Flux Current Ref, is this is to big it can cause problems when starting, depending on the number of poles it should be from 33% to 66% of Full Load Amps.

Alan

What is the start method for the drive? Hardwired? 2 wire? Does it do the same thing when started from the keypad? Is reverse disabled? Is flying start enabled? What does it do when it doesn't start- does it just go forward and reverse back and forth forever?

What amps is the motor pulling at statup? Could it be going into current limit mode?

Have you done an auto-tune?

Can you connect your laptop to it and post all non-default parameters?

In my experience even if you are using V/Hz, you need to do the Autotune on the drive. The latest generation of high speed IIGBTs (transistors) used in these drives (and others like them) are much more sensitive to what used to be minor differences in the motor equivalent circuits, rendering the motor less capable of developing torque if the tuning is not done. the rise time of the new IIGBTs is so fast that the VFD actually has to go into current limit immediately to avoid a dI/dt problem because it doesn't "know" that the high rate of rise is just because of an unexpected Xc ratio (technobabel, I know). Often times if the load is a centrifugal pump or fan, starting torque requirements are low so you don't notice. But a piston pump is a high starting torque application so not tuning it will result in the motor not acting right.

For a piston pump though, I would not even try using V/Hz mode on it, I would use SVC,which absolutely requires the autotune function be performed.

This may be a little off topic but I am tired of replacing AB drives. PF700s and PF750s. 5hp all the way to 1200hp. Old reliance drives seem to be excellent IMO and steadily hum while replacing newer AB drives. If I had my way I would probably use ABB for new drives.

/Rant

Honestly, it sounds very much like a compressor unload issue that you have been fighting forever. Especially with a piston compressor, it is trying to start against a "loaded" piston, you could be fighting a very high load to get the motor turning.
I would start with that, before changing anything electrical again.
Just saying this as a maintenance guy, that's my gut feeling.