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Old March 19th, 2017, 01:03 AM   #1
dburnum
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Very odd Powerflex 525 behavior

Just got back from a call-out to a plant that was having issues with a AB 525 drive. Ethernet control(start/stop/reference), and the speed reference was a static set-point....I guess it was just there so an operator could speed up the conveyor as needed.
It runs a dynamic motion conveyor that swings large weight via a gearbox, to convey product without damaging its structure.
Started last Thursday evening go an "OverVoltage" fault. Checked it out and everything seemed fine. Checked out a few parameters to see if there was an issue, and didn't see any. the drive reset without issue and ran almost exactly 24 hours.
Friday evening, again the drive faulted on "OverVoltage" again. Checked logic to make sure there was no possibility to chatter the start/stop on the drive. Decided to enable "Flying Start" in case that might have been it, checked the regulator compensation and it all seemed fine. Had some trouble resetting the drive, as it would run the conveyor about 5 minutes and fault again. Eventually it reset, and ran almost exactly 24 hours, before faulting again.
Saturday evening I tried to reset it, and again it would run maybe 5 minutes or so, then fault "OverVoltage". Watching the DC bus, and I never see it rise above 706VDC. I think it has to reach over 800VDC to trip on overvoltage. I've noticed that they have been running this conveyor lately at 60Hz continuously, so I set a lower set-point into it to slow it down a bit (55Hz).

This is where it gets weird. I can see that the "Commanded Frequency" is 55Hz, but the drive output hangs around 58-59.5Hz...and the drive status hangs on decelerating, and never changes from that.
We are currently oversold on product, so I've been getting an earful from higher-ups. I couldn't figure it out, and didn't have a spare 525 in that size, so moved it to a across-the-line starter as a temporary fix.

Anyone seen this behavior? I searched the knowledge base, and all the suggestions I saw related to a regenerative action from the motor, but I had "stop" set to coast, not ramp down the motor....and there was no wild fluctuation of reference, but a digitally set set-point.
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Old March 19th, 2017, 04:19 AM   #2
AustralIan
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Because of the sway of the load, it is entering regen mode when the load swings in front of the hoist.

I suppose you could implement a sway controller or bigger drive or add a braking resistor.

I am not sure if just altering Accel rates or shapes will get you out of this.

Weird that it happens at a specific time of day. Maybe they only lift that load at that time? Maybe there is higher AC voltage at that time of day? Maybe a load elsewhere switches off, causing a high AC voltage briefly, raising your DC bus?

Maybe someone else can weigh in with different chokes etc as a solution.
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Old March 19th, 2017, 07:17 AM   #3
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It appears you have a generator instead of a motor.......

As AustralIan says, you are operating in regen mode for some reason, and will need a regen resistor.
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Old March 19th, 2017, 09:26 AM   #4
jraef
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You likely have your Bus Regulator mode enabled (A550). It attempts to prevent OV trips by modulating speed changes automatically, which explains the observation of the speed being higher than commanded. Read the precautions about this on page 12 of the manual. Most likely someone is attempting to change speed (lower) too rapidly.

Even if you have the Stop mode set to Coast, if you have an aggressive decel time set and a high inertia load, the drive will attempt to bring the speed down, but if the inertia keeps it overhauling, it will regenerate. Try setting your decel time much higher than it is. If you DO want it to decelerate faster on a speed change, you need braking. When you then enable braking, it disables the bus regulation.
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Old March 19th, 2017, 10:27 AM   #5
DickDV
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There is a little detail in the above conversation that is worth highlighting. Note that setting the drive in Coast-to-Stop mode does NOT eliminate the possibility of regenerative braking and the inevitable High DC Bus Voltage Fault if braking hardware is not installed.
As jraef points out, if you decel rapidly from a higher speed to a lower speed WITHOUT STOPPING, that often causes regeneration to occur because the motor is trying to pull the load down faster than it would naturally decelerate.

This oversight is easily one of the most common errors I encounter in the field.
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Old March 19th, 2017, 12:51 PM   #6
dburnum
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Quote:
Originally Posted by jraef View Post
You likely have your Bus Regulator mode enabled (A550). It attempts to prevent OV trips by modulating speed changes automatically, which explains the observation of the speed being higher than commanded. Read the precautions about this on page 12 of the manual. Most likely someone is attempting to change speed (lower) too rapidly.
You are correct, A550 is enabled, and I am thinking that (A547) Compensation is set to Elctrical+Mechanical. It supposedly slows down the regulator to not react quickly to change in speeds. I had this set to electrical only, but would still get the fault, and when I disabled (A550), the drive would immediately trip on OverVoltage.

Even though the Conveyor's gearbox swings weight, it is not an oscillatory action on the weights. They are timed so that the conveyor travels (swinging) in one direction at a more rapid rate, than the other.

http://www.sssdynamics.com/equipment...ick-industrial

Anyway....I can also trend the selected speed of this conveyor, and it reference was never changed through these instances. I installed this Intellicenter about 8 months ago, and have no issues with this VFD until now.
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Old March 19th, 2017, 01:47 PM   #7
GaryS
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Just a thought
if this VFD was running good a long time then all of sudden started tripping on and overvoltage fault with no other changed to the drive parameters or the load conditions
I would check the buss caps they may have lost there value and can no longer filter the buss ripple correctly. I have seen this before the measurable buss voltage will not show the spike but if you can check the cap values you may be able to find it or just change the caps.
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Old March 19th, 2017, 04:32 PM   #8
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the conveyor, runs 24/7....as so does our facility. I wonder if 7.5HP on the drive might be slightly undersized, and the bus may have been stressed. It turns a 7.5HP motor, but maybe sizing for HD, may be a better sizing, or even slightly over that.
Either way, I am becoming more convinced that something within the VFD is failing. I may just replace it, with something slightly larger. Before the Intellicenter install it was run by a 7.5HP(ND) Powerflex 70. Never had problems with it.
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Old March 19th, 2017, 10:18 PM   #9
jraef
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Quote:
Originally Posted by GaryS View Post
Just a thought
if this VFD was running good a long time then all of sudden started tripping on and overvoltage fault with no other changed to the drive parameters or the load conditions
I would check the buss caps they may have lost there value and can no longer filter the buss ripple correctly. I have seen this before the measurable buss voltage will not show the spike but if you can check the cap values you may be able to find it or just change the caps.
Good thought, but if the DC bus ripple gets too high, the drive interprets that as an UNDER Voltage condition. And the caps on a 520 drive are not made to be replaced or repaired, it is a "component class" drive, made to be less expensive and smaller, but everything is SMT and if anything breaks you toss the section (the 520 does separate into a control section and a power section). But that does bring up another possibility: if there is another 525 of the same size, swap out the power section and see if the problem goes away. If it does, it was a defect in the power section somewhere.

Another potential cause of OV faults is severe line transients. Could be that something else in the facility (or a neighbor's facility) that creates a lot of line notching, is turning on sporadically. Unlike the PF7 family of drives, the 520 series do not have DC bus chokes until you get to >20A. Although not as good for this as Line Reactors, DC chokes do provide some dampening of transient pulses and can help avoid nuisance OV trips. If you don't have a line reactor ahead of this drive I would strongly recommend it, but if it's in an MCC bucket, you may not have room. Still, a PF70 would need more space anyway. One point on this however is that if you have other 525 drives in this area, they would likely also be tripping.

But first, try extending the decel time out a lot longer as your first trick, it's easy and free...

Side note: There is no ND or HD rating of 520s that small by the way, they are all HD rated until you get >20A as well.

Last edited by jraef; March 19th, 2017 at 10:25 PM.
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Old March 20th, 2017, 10:50 AM   #10
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I've had this occur also. Had to extend the Decel time AND change from Coast to Ramp Stop. More controlled stopping than Coast.
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Old March 20th, 2017, 01:44 PM   #11
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At 1AM I reprogrammed the Intellicenter to run the conveyor from an across-the-line starter. Turned into a bit of a headache as my module properties on my IO tree somehow became corrupted, and RSL5K wouldn't open them. The Intellicenter communicates via OPC back to WinCC for operator control, so making on-the-fly changes were not happening. Entire process line was down for 4 hours while I did what I could to get them going again.
Since I am able to read back current from the MCC, I placed some logic in to read back current.
On that 7 1/2 HP motor, I can see rapid load changes from 5.53A-8.76A in less than a second, which would account for the status of the VFD rapidly changing status from decelerating/accelerating/at reference.
Would rapid changing loads like that be stressful on the DC bus/bus caps? The drive is rated at 13Amp.
Talking to my AB supplier, I am going to go ahead and order another one, since I need a spare anyway. I'm going to bench-run this one for a while, and possibly warranty it, if my scope shows excessive ripple on the bus.
I have no way of testing it back to the original conveyor for now that has rapid changing loads, to see if increasing the decel would fix this.
I'm still perplexed why it showed a commanded frequency of 55Hz that night, but would never get there, and was constantly on "decelerating" trying to slow down, but never doing so.
I'm also going to place a call to AB to see what they say.
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Old March 20th, 2017, 09:02 PM   #12
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I had a 525 that had a similar issue caused by another piece of equipment starting and stopping, that was connected to the same power source. Not saying that is your problem just throwing it in there.
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Old March 20th, 2017, 10:29 PM   #13
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Quote:
Originally Posted by dburnum View Post
At 1AM I reprogrammed the Intellicenter to run the conveyor from an across-the-line starter. Turned into a bit of a headache as my module properties on my IO tree somehow became corrupted, and RSL5K wouldn't open them. The Intellicenter communicates via OPC back to WinCC for operator control, so making on-the-fly changes were not happening. Entire process line was down for 4 hours while I did what I could to get them going again.
Since I am able to read back current from the MCC, I placed some logic in to read back current.
On that 7 1/2 HP motor, I can see rapid load changes from 5.53A-8.76A in less than a second, which would account for the status of the VFD rapidly changing status from decelerating/accelerating/at reference.
Would rapid changing loads like that be stressful on the DC bus/bus caps? The drive is rated at 13Amp.
Talking to my AB supplier, I am going to go ahead and order another one, since I need a spare anyway. I'm going to bench-run this one for a while, and possibly warranty it, if my scope shows excessive ripple on the bus.
I have no way of testing it back to the original conveyor for now that has rapid changing loads, to see if increasing the decel would fix this.
I'm still perplexed why it showed a commanded frequency of 55Hz that night, but would never get there, and was constantly on "decelerating" trying to slow down, but never doing so.
I'm also going to place a call to AB to see what they say.
I've seen incline conveyors generate loads like this, and one that was on a level plane but had a low gear ratio. In our cases, using a bigger drive and a DC brake got us out of trouble. I think your earlier instincts were probably on target. I would recommend you get a 10HP or 15HP VFD and a braking resistor. You might get a brake resistor alone and get away with it, but if you have the opportunity to oversize the drive, it cannot hurt.
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