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Discussion Starter · #1 ·
As I am trying to troubleshoot this wonderful device, I feel like I am getting nowhere. The ground fault issue is gone and now it will not ramp up over 4Hz. Have the command power set to 60Hz and it continues to tell me "Current Limit". Going behind operators who have done a ton of shutting it on and off to see if they can reset it to work. Ran through the parameters and everything is back to the default settings. Any advice out there?
 

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Can you please supply a little more information? HP of motor? Voltage of supply? Type of supply? ( ungrounded delta, grounded wye, high resistance grounded system ).
 

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Discussion Starter · #3 ·
Only have two out of the three. It is a 125 hp motor, 3 phase, 480v. The shaft is not locked up. Upon trying to run the vfd it will get to 4Hz and go back down to 2.5Hz. I can get the grounding info tomorrow if that will help. Thank you!!
 

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I ask because I have run into similar problem with 100 hp PF700. In my instance, the IGBTs went bad creating the same type of problem. In the end, it was discovered that the MOVs and caps were not properly removed for a high resistance grounded system.
 

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Have you verified the motor FLA parameter is set correctly?

Will it ramp up to speed without the motor connected?
 

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In programming, you can tell the drive what you want it to do in response to hitting the current limit. One of those options it to reduce the speed, overriding the commanded speed. The net result in theory will decrease the load and work performed, but keep the machine running.

However the drawback to this strategy is what you are witnessing. If there is high current flow for some reason OTHER than motor load, it still responds in the same way, but the speed cut is extreme. So when you see something like this, it's a tell-tale sign that you have a high resistance short somewhere, either phase to phase or phase to ground. It's high resistance because it is not high enough current flow to trigger other protection circuits, so the current limit system is seeing it.

A cracked transistor is one possibility, it is leaking DC to the heat sink (ground), but only when turned on. So as the drive ramps up the output power, the leakage increases, the drive responds by driving down the speed, which stabilizes the current leakage below the limit level, but of course does no useful work. You can get the same effects from other forms of high resistance shorts as well, but those are going to have to be in the motor leads or windings, so they are easier to find (with a megger, not an ohmmeter by the way). When you say "The ground fault issue is gone now." what do you mean?
 

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I hate the powerflex drives, we always have issues with them. Just call the technical help line in the manual, they are good at helping fix issues, or finding problems.
 

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Powerflex 700s often end up with start boost settings that are too aggressive. I see this exact problem all the time. The boost settings are auto calculated by the motor model. Do an autotune and try again.
 

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Discussion Starter · #9 ·
Appreciate the help! I have megged the motor and the leads. Everything was fine. Two nights before the problem it ran but would only run at 30Hz. At 60Hz it would run for 3 minutes the ground fault. Now I am where I am.
 

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Appreciate the help! I have megged the motor and the leads. Everything was fine. Two nights before the problem it ran but would only run at 30Hz. At 60Hz it would run for 3 minutes the ground fault. Now I am where I am.
Were all 3 of the motor and leads disconnected from the VFD during the megger test?

What I'm asking is at any time was the megger feeding any voltage back into the output of the VFD?
 

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I hate the powerflex drives, we always have issues with them. Just call the technical help line in the manual, they are good at helping fix issues, or finding problems.
I find them easy to work with.
 
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can you copy cat it from another drive??
 

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ALWAYS meg the motor and wiring BEFORE connecting to a VFD. Sometimes energizing a VFD into a ground fault will toast a drive. Is this VFD new or has it been in service for a while? Is the supply voltage to the VFD anything other than a wye system? If so, you must remove the MOV jumpers from the VFD in 2 or 3 locations on a 700.

From your comments, here are some possibilities:

VfD damaged by connection to other than wye supply without MOV jumpers disconnected. (Usually fatal to the Pflex drives)

High resistance ground fault in motor or wiring.

Mechanical load excessive.

One phase open in motor or wiring. (single phasing) this should generate a drive fault.

No output from one phase on the VFD. This should generate a drive fault.
If all of the parameters are correct, The drive could be faulty.
 
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