VFD issues

[KAG]

I have a vfd that is giving me some ramping problems in PID control. It was running smoothly and as it was reaching it's setpoint went into high current overload. After some wire switching and moving to local control it ran smoothly at a 40hz and above. At 60hz current readings using a fluke ammeter show dead on manufacturer specs but the VFD shows at least 15% higher readings hence the overcurrent fault. ABB believes it is a faulty VFD. Just looking for some input from those of you out there who have experience with these things.
Also Just wanted to say this is a great forum and I am a new member!

Regards,

 

[Marvo]

Welcome to the forum KAG.

The problem with measuring the run current of a VFD is the carrier frequency. Many ammeters can't give an accurate reading at those kinds of frequency ranges. Can you state exactly which VFD and exactly which ammeter you have please.

As a rule I'd always take the run current calculated and displayed by the VFD as being the real life run current at least up to the point where it's proven without doubt to be inaccurate. The only times where I've encountered a VFD that was giving inaccurate readings it was down to the inaccurate motor information that was inputted into the VFD. I've never come across one that was inaccurate because it was actually faulty.

 

[KAG]

With the VFD running at 60hz, ABB ACS310, the display reads 8.4amps, the output from the vfd reads 7.4, and the current from the disconnect ahead of the VFD is 7.5 approximately. Measured with a Fluke True RMS clamp on. Can't remember the exact model. This thing was running fine at 60hz in PID mode then began to show current excess on the display and went to high current warning. Runs fine in local control. Strange?

 

[KAG]

By the way thanks for the help. I would expect the current readings to be stable at 60hz? Also the voltage was fluttering up and down 10-30volts on the motor side of the VFD as well.

 

[KAG]

AT 60 hz I would expect the readings to basically be motor name plate. At one point during the running of this motor it ramped up to max drive current and stalled to 22.5hz. after a bit of looking and wire switching it took off and ran fine in hand mode.

 

[Marvo]

You're going to have the same problem measuring the output voltage of the VFD, again it's very possible the carrier frequency is beyond the range measuring capability of your Fluke.

Have a quick read of this if you're not sure how the modulated output of a motor drive is made up of carrier frequency (can be way over 10kHz for some drives) as well as the output frequency (0-60hz for USA systems).

Just out of interest what is the motor doing?

 

[KAG]

The motor is powering a submersible water pump. Even at SFA the current draw should be well under the vfd 10.8a limit. Itis under its greatest load when the water level in the dewatering well is high and motor current should drop as the water level falls. I spoke with the ABB tech support rep and he feels there is too much difference in the clamp meter reading and the vfd amp readings. It is possible the motor could be at fault (it is brand new) but it runs fine in hand operation!

 

[KAG]

Excellent read. Makes sense.

 

[Marvo]

Do the pump manufacturers state specifically that the motor is suitable for VFD running? Some motors don't have good enough insulation on the windings and the VFD output can cause tracking internally, symptoms of which could present as high and unstable run current.

 

[KAG]

Meter was a Fluke 30/32 True RMS - What would you recommend for an accurate (as accurate as can be expected) short of a scope.

 

[KAG]

Yes they do. They are wound specifically for that purpose with the caviat that they never be run below 30hz. Below 30hz the bearings (water lubricated) do not receive sufficient flow to properly lubricate. It is certainly possible that the pump intake picked up a small amount of debris however at 3450rpm it would simply chew it up and spit it out without hesitation. A millisecond spike would be all I would expect.

 

[Marvo]

If you're a Fluke fanboy you could go for a Fluke289. Here's a US link where Fluke explain the problems of taking readings from VFD outputs with standard instruments. I'd normally suggest you try our forum sponsors who offer members a decent discount but they're UK based so unfortunately you're out of luck.

 

[KAG]

Much appeciated

 

[Marvo]

Obviously without seeing the installation in the first person it's difficult to go through the normal proceedure of fault finding. Rereading the thread in its entirety a few thoughts spring to mind.

For now at least I'd ignore what your Fluke is telling you. There's a good chance it can't give you accurate readings so rather take the run currents from the VFD itself as gospel. By all means beg or borrow another instrument and confirm them but I don't see why the drive would be incorrect if you've inputted correct motor info during setup. I think the current reading discrepancy is introducing unnecessary confusion and leading you astray from finding the root cause of the tripping.

Pumps don't always follow that lower RPM = lower shaft power requirement = lower running current. There's many things that can produce an unexpected run current, 1. lock rotor as you mentioned a blockage may have occurred 2. The pressure across the pump, if it's too low you can get unexpected cavitation or impeller lift which can increase the run current. Most pumps have a minimum head pressure requirement to prevent this. It's worth noting incorrectly sized pipework or an elbow too close to the inlet or discharge can have a similar effect. Also excess static pressure ie a blockage on the discharge side can either increase or possible decrease the run current outside of expected limits depending on the type of impeller.
These are all pump problems that could give the kind of symptoms you're seeing.

I think the PID mode might be a large part of the problem. With an unpredictable load it can be hit and miss at best. PID can also be very complicated to set up correctly on a VFD, if we ever have an application that requires PID control we tend to stick to known combinations of VFD and machine where someone has done the painstaking work of setting up the hundreds of parameters for that particular combination. At least to start with I'd go through the entire commissioning procedure without PID mode. Once you're sure everything else is behaving to expectations then last of all recommission in PID mode and at least you'll have baseline figures to work from when testing.

 

[KAG]

Agreed. I have another meter, Extech EX830, which I'll use to confirm readings. When I left the installation is was running at a smooth 40hz with no issues in local mode. I tend to agree with the PID part of your response. The pump manufacturers like a gain of 4 on these things with an integral setting for @ 20 seconds. Other issue is submersible pumps typically like quick starts so the builders say 1 sec max start time. The problem here is varying volumes so the PID will be necessary unless there is another way to control the speed as the water rises. These 4" submersibles typically draw max current during the lowest head conditions and this particular installation is just outside the curve for the model. Even in the service factor however the amp draw is within the confines of the vfd's maximum. I may allow it to run a few days at a reduced manual speed and see how it all shakes out. If all goes well then I will try the PID macro again and see what happens. ABB says the drive is faulty so we'll replace it and see if that makes any difference. A few seconds of dry run periodically is better than losing a motor. I may also induce some artificial head on the discharge in the form of a valve! Last resort is to go to a smaller pump and overspeed. Franklin electric allows up to 80hz on their submersible motors, with reduced overall life of course. I will however respond when I get to the bottom of it but I believe you are correct about the PID.