VFD Wiring Question

[kwired]

It is carrying the same media (water). It's feeding a 30 GPM pump that is running around maybe 5,000 psi? It's being used for life cycle testing of a high pressure water nozzle, so it turns on/off at a set interval. I don't know the specifics - I'm not the one that runs the tests.

While I appreciate all the angles, I think some are going down the wrong path. The facts are: this process was working fine for three months, right up until the wiring was changed. While I understand that coincidences do happen, the basics of troubleshooting tell us to identify what changed between when it was working and when it stopped working, and that leads us to wiring. That's literally the only thing that changed.

So it is doing sort of exactly what I had in mind, and that was based off a process I am familiar with. Except in my case there is no VFD on the "feed pump" but the High pressure pump involved does have VFD, but runs at fixed speed and only gets adjusted when running different products that need different pressure or flow rates on the high pressure side of things.

I imagine in my case they could probably increase energy efficiency if they would vary the speed of the feed pump but otherwise the high pressure pump don't really care what incoming pressure is as long as there isn't shortage of available product, but once it is up and running it runs at a steady rate.

When you say this is only thing that changed was wiring I'm presuming you talking about changing from the SO cord to the THHN.

What about changes in loading? Are we still moving same amount of product at same same pressure, rate of flow and of same product consistency?

If this is just plain water then product consistency likely the same, but is pressure and flow rates still same? Process I deal with that I am comparing to there is difference in solids content from one product to another as well as different flow or pressure rates needed for different products, so things change depending on what product they happen to be making.

 

[ElecTech83]

So it is doing sort of exactly what I had in mind, and that was based off a process I am familiar with. Except in my case there is no VFD on the "feed pump" but the High pressure pump involved does have VFD, but runs at fixed speed and only gets adjusted when running different products that need different pressure or flow rates on the high pressure side of things.

I imagine in my case they could probably increase energy efficiency if they would vary the speed of the feed pump but otherwise the high pressure pump don't really care what incoming pressure is as long as there isn't shortage of available product, but once it is up and running it runs at a steady rate.

When you say this is only thing that changed was wiring I'm presuming you talking about changing from the SO cord to the THHN.

What about changes in loading? Are we still moving same amount of product at same same pressure, rate of flow and of same product consistency?

If this is just plain water then product consistency likely the same, but is pressure and flow rates still same? Process I deal with that I am comparing to there is difference in solids content from one product to another so things change depending on what product they happen to be making.

Yep, the process has not changed. Same pressure, same rate of flow, same product consistency. Only change is the wire, and specifically the insulation thickness of said wire, which is why I want to replace the THHN with RHH/RHW-2.

 

[zbang]

Nothing stopping you from replacing the wire, and that might make the problem "go away", but history and many peoples experience says that THHN insulation is unlikely to be the cause of these shutdowns.

To pursue the wiring, I'd be more concerned with using solid conductors and whether vibration or heat has loosened some of the connections. The fact that it worked with the SO connection is almost irreverent here since the drive was moved and rewired. Heck, moving/mounting the drive itself could have created an intermittent in the drive.

Regards the PID loop- it may have worked, or at least nobody saw problems then the drive and the PID wiring were moved. There could be an intermittent connection there, too. (Are you logging the PID with enough resolution to catch an event when/if the drive drips?)

Step back and look at the system, not just the suspect.

 

[ElecTech83]

Nothing stopping you from replacing the wire, and that might make the problem "go away", but history and many peoples experience says that THHN insulation is unlikely to be the cause of these shutdowns.

To pursue the wiring, I'd be more concerned with using solid conductors and whether vibration or heat has loosened some of the connections. The fact that it worked with the SO connection is almost irreverent here since the drive was moved and rewired. Heck, moving/mounting the drive itself could have created an intermittent in the drive.

Regards the PID loop- it may have worked, or at least nobody saw problems then the drive and the PID wiring were moved. There could be an intermittent connection there, too. (Are you logging the PID with enough resolution to catch an event when/if the drive drips?)

Step back and look at the system, not just the suspect.

I like to take one step at a time when troubleshooting, and when I saw that NFPA79 2018 had called out specific wire types (which did not include THHN) to be used when wiring these types of drives, for this specific reason, it made the most sense.

 

[kwired]

I like to take one step at a time when troubleshooting, and when I saw that NFPA79 2018 had called out specific wire types (which did not include THHN) to be used when wiring these types of drives, for this specific reason, it made the most sense.View attachment 2556324

That information basically applies to about any drive. There are many applied to existing circuits as well as new circuits using THHN/THWN that go for years with no trouble as well though. And as I mentioned earlier, other than if there is a problem from installation damage it likely takes quite a bit of time before insulation breaks down to be an issue here.

 

[ElecTech83]

That information basically applies to about any drive. There are many applied to existing circuits as well as new circuits using THHN/THWN that go for years with no trouble as well though. And as I mentioned earlier, other than if there is a problem from installation damage it likely takes quite a bit of time before insulation breaks down to be an issue here.

It's my understanding that older VFD's chopped DC output didn't fire as rapidly as newer drives, and as such did not see such large dv/dt values, so it would follow that on older drives (and larger drives with higher current ratings for that matter), that perhaps THHN would not cause these faults. Also, it's not the insulation being damaged that causes the faults. It's the dielectric constant of the wire itself, which is directly related to insulation material and thickness.

 

[ptonsparky]

Get the vfd cable and be done with it.

 

[zbang]

Does NFPA 79 even apply here? I doubt it. Art 430 part II covers motor circuit conductors.

 

[petersonra]

View attachment 2556324

note that this says cable. you are proposing to use individual conductors. in any case I doubt that NFPA79 is entirely appropriate for your application, although following it will not hurt anything.

while changing the wiring may "fix" the problem, it is almost certain that neither the type of insulation or whether it is stranded or not is the problem. If there is something wrong with the wiring, like an intermittent short or open, or a bad connection, replacing the wiring may cause the problem to go away because as part of the process of replacing the wiring you removed the actual problem as well. You might also have an intermittent problem with the motor.

Maybe your plant electrician has an insulation tester and would be willing to come test the existing wires and motor for you.

You might as well go and change out the wires and see if it makes any difference, since you seem so determined to do so. If the problem goes away it is likely there was some kind of problem with the wires, or the connections. If not, you can move on to other things to look at.