VFD issues

[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.

 

[Marvo]

I'd be interested to know what transpires if you can find the time to post again in a few days.

If ABB are prepared to swap out the drive at no cost then it would be wise to do so just to rule out that as a possibility.

If your pump is a multistage centrif then low head is not a place where you want to be, even if the head is slightly below specified minimum it can have a marked detrimental effect on mean time between failures so artificially introducing extra head pressure is a close second best to re-specifying a pump with a lower head requirement.

<1 second to >3450rpm is a steep start ramp, it's steeper than the default ramp settings of most drives so I'd double check these settings which may be critical to ensure bearing hydraulic action and possibly seating of the impellers. Also with bearings that rely on water hydraulics for lubrication I'd avoid dry run at all costs, it could lead to the demise of the pump faster than a marginally high run current.

Maybe consider a 0-10 volt level sensor (or something similar that's drive compatible) that can signal the drive to modulate the motor speed between 30-60HZ, it might be simpler and more reliable than PID operation.

Good luck and I'll look forward to reading what happens.

 

[KAG]

FYI I do have a 4-20ma transducer in the well. 0-15psig. Set at 31ft with a setpoint of 10ft (21 from surface). These are submersible open design impellers. Motor is always submerged and the stack can take a ton of run dry without issue. We once ran a pump and motor out of the water for 75 minutes without damage. I think we got lucky! Anyway it may be a programming issue and although I was given parameters directly by the ABB tech people that doesn't mean it's correct. I'll keep you posted once we iron it all out. This has been one of THOSE projects anyway! Again your help is greatly appreciated.

 

[Engineer54]

Now this has been an interesting thread, and i'm also looking forward to what happens on the OP's end....

 

[i=p/u]

If you don't understand I can talk you through what they on About.....

 

[KAG]

I'm certainly open to any suggestions for alternatives to pid control or perhaps programming suggestions. Feel free to email me and I'll give you the blow by blow. I'm thinking I may have to limit the motor to a level where it stays in the curve. Perhaps 50-55hz.