Powerflex 70/700 Current Lmt Gain Par 149 action?

Am having problems with an occasional overcurrent situation and playing with settings 148,149.

For Par 148.
Some manuals talk of this being a percentage of drive rating.
So is this to be entered directly as amps to current limit at OR as amps percentage of drive rating.

For Par 149.
Can find no information in manuals as how to use this.
Or exactly what action it has on current limiting.

Also tried using the "Auto reset" = 3 with a 30second delay
But this does not reset the drive after an HW overcurrent trip.

Anyone have any experience ith these settings.

Drives normaly running at about 90% FLC.

Most of the time when I see HW Overcurrent, I end up finding a short path to ground between the drive and motor. Sometimes it's an intermittent short. Sometimes I can only find it with a megger.

If the drive is just barely big enough for the application, I could see how you'd get a hardware overcurrent situation if the load suddenly increased. So, your drive is running at 90% of its current rating, or 90% of the motor FLA?

90% Motor FLA.

Had not thought of an intermittant motor ground fault.
Have 18 motors running off the one VSD, so is a possibility.

18 motors?!?

They all start and stop at the same time?

They each have their own independent OL protection?

To troubleshoot that, I would disconnect from the output of the drive, then meg the wiring and all the motors/disconnects all at once. If you find a low reading, open some of the motor circuits until you identify it.

I have seen three and four motors per drive before, but never 18.

my two cents, 18 motors for instance @ 1 HP do not consume the same as one 18 HP motor, 18 motors power consumption must be 1 HP motor consumption multiplied by 18.

The size of the drive wasn't mentioned but I'll bet its too small for the application. The rule I use for multiple motors is to add up the individual FLA's and add 10% more for each extra motor. So, in this case, add up the FLA's and add 170%. That will give you somewhere around a 40 or 50hp drive.

Seems extreme but that's what's needed to avoid overcurrent faults. I have done lots of multi motor applications with one having 48 motors on one drive and this rule has never failed me.

Just to test the principle, pull nine motors off line and run a while with the remaining nine. Then put those motors offline and run with the other nine. I'll bet your faulting goes away. If a motor winding failure had been the cause, one of the nine-motor sets would have still faulted.

This is one unit, there are 500+ of these things running around the world.
Get occasional problems on some of the others, this one has been a headache.
Drive is sized correctly.
Each motor has its own contactor and Overload unit.

It may be the insulation breakdown on motor, seems to happen when drive ramping down or up, when there is some possibility of vibration.
Had thought that if earth fault that drive would of tripped on earth fault.
Can get trip with 25Hz and running amps at 5% of motor FLC.
It not as if the amps are any where near FLC during speed change.

Realy wondering about the Par 149 and what its function is.
Weather or not I could use it to have the Current limit act faster.

Do the megger test check at 500v and 1000v. You should be able to test all the motor circuits individually and quickly from the bottom of your contactors/ol relays.

I have noticed with all a/b drives, a short to ground in a disconnect or peckerhead will cause the HW Overcurrent fault to supercede a ground fault, especially if the ground is not "by the book". We have some systems that are properly grounded, and some that have too many intermediate ground connections...not ideal, but I have seen this fault at least 10 times with all flavors of powerflex drives, and at least 8 times I am sure we found shorted motor windings or wiring. I have only seen a fault to ground properly reported by a Powerflex drive two or three times, and that was with the recently installed and properly grounded motor to drive connection.

I am without manuals, but I think what you'll find is that you can cause the drive to limit the current to a level below the drive rating, and then you'll get a motor OL Fault if it's a a real load/weak short. You'll still have problems, because the drive will reduce voltage and frequency to all 18 motors and keep right on trying to run. Is this okay?

Also, I helped one guy on here who had a similar need and he ended up with a motor stalling due to a real overload condition which he had defeated with the current limit settings, by causing the final calculation for the current limit to end up being less than the programmed motor FLA. Then you get a nice warm stalled out motor and the drive keeps right on trying...

Hope this helps...
Paul

Thanks OkiePC, good advice.
Will carry out some megger testing of all the motors.

Any idea why "Auto Reset" would not reset these type faults.

Let us know how the motors megger out. Just because its been done 500+ times, that doesn't make it right.

Stay tuned! For your sake, I hope you find a bad motor.

Presumably 18 motors means 18 cable runs. Since all of these cables are in parrallel the capacitances will add up and the installation will be prone to problems with transients (just like a single drive with a very long cable run). Do you have any sort of protection to mitigate this?

Originally posted by Greg Fowlds:

Realy wondering about the Par 149 and what its function is.
Weather or not I could use it to have the Current limit act faster.

Click to expand...

The hardware overcurrent detection and the current limit structure are two completely different things. The hardware overcurrent structure can be thought of as a fast-acting fuse and is intended to catch catastrophic current faults before they can blow the cover off the drive.

The whole current limit structure is intended to act more like an overload relay and prevent long-term component damage due to torque overloads. It can also be used kind of like an electronic clutch, which is where the current limit gain comes into play. You can adjust how quickly the current limit structure reacts to an increase in current. However, if you are getting to the point of hardware overcurrent the current limit structure will not be fast enough to drop the current in any case.

Have you swapped out the drive yet? On one project we did a couple years ago we went through 3 PF70 drives on a single axis, all throwing hardware overcurrents. We checked everything we could think of between drive replacements and nothing stuck out. Finally #4 was put in and we haven't seen the issue since. Go figure.

Keith

Andybr said:

Presumably 18 motors means 18 cable runs. Since all of these cables are in parrallel the capacitances will add up and the installation will be prone to problems with transients (just like a single drive with a very long cable run). Do you have any sort of protection to mitigate this?

Click to expand...

I have never installed something like this , I have installed max 8 motors. Normally what I do is to run separate conduit for each motor, just to avoid what Andy said.

Greg, the reason you don't see high current when the drive faults is due to the fact that the drive releases the output extremely quickly to protect its power circuit components. The relatively slow scan on the keypad display will miss it every time.

The fundamental problem here is that 18 small motors in parallel become a far lower impedance that the equivalent hp single motor. So, for starting and speed change purposes, the drive is practically operating into what it thinks is a short circuit. Thus, the overcurrent faults.

While I still believe that the drive is not sized large enough for this configuration, you may be able to get out of the problem by adding a single 5% motor lead reactor right off the drive output terminals and before you branch out to the 18 motors. This will increase the impedance the drive sees and will also compensate to some degree for the lead length and the inductive and capacitive sensitivities that come with long leads. I'd probably try this first since its the easiest and probably cheapest. The only drawback is that the motors will have a little softer speed stiffness when under load changes but, if you've got 18 motors on one drive, that's probably not an issue anyway.

I would go with DickDV's recomendation on this. Just because you have 500 of these installed doesn't mean you aren't living on the ragged edge with all 500. In fact, that fact that you get occasional faults on others would seem to indicate that you are. Unless you send this out with a full engineered installation package that strictly defines conductor placement, all it takes is for a couple cables to be a little closer to each other or a grounded surface to change the L/C relationship enough to cause issues.

A load reactor will always give the drive something that looks like a solid motor load, which it needs both for PWM commutation and motor lead current detection.

Keith