VFD Wiring Question

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ElecTech83

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Hello All! I work in an R&D Lab that uses several 480V pumps of varying sizes, being used to pressurize water. The one that is giving me headaches right now is a 7.5HP 480V 3 phase motor that is wired to an ABB ACS310 VFD. We are using it as a booster pump that is used to feed a much larger pump. We had the VFD wired to the motor just using standard SO cable (I know, this is a no-no) just to prove it would work, which it did. Then we had an electrician come in and mount the VFD enclosure to a wall and install permanent wiring. The electrician ran flexible steel conduit from the VFD to the motor and used solid core 12 AWG THHN wires for both the enclosure power and the VFD output to the motor. This seemed very strange to me, as in all my 12+ years of working with industrial controls I have never seen solid core wire used on a motor. The drive is randomly giving us "Overcurrent" faults and I am certain that the wiring is to blame - I believe we are seeing the charged capacitance of the extremely thin THHN insulation on the VFD/motor conductors discharging back to the VFD. I know that using an actual shielded VFD cable is the correct thing to do here, but I am wondering if I could just run (3) 12 AWG stranded RHH/RHW-2 wires from the VFD output to the motor through the existing steel conduit? I believe the wire I bought was intended to be used for PV (photo-voltaic) applications, but I think it should work. Thoughts?
 
It most certainly is overcurrent, but only for short spikes, which immediately trip the drive. It will run for hours drawing about 4.5A to 5.5A (PID control, maintaining a 40 psi setpoint - which is running at less than 50% of the motors rated torque). Then it will, out of nowhere, spike over the instantaneous trip point of the drive which is 21.9A. The pump is drawing water from a source tank, and nothing on the inlet or outlet side is changing much when these spikes occur. I'd bet my paycheck that these overcurrent trips are due to Corona inception voltages.
 
ElecTech83 said:
It most certainly is overcurrent, but only for short spikes, which immediately trip the drive. It will run for hours drawing about 4.5A to 5.5A (PID control, maintaining a 40 psi setpoint - which is running at less than 50% of the motors rated torque). Then it will, out of nowhere, spike over the instantaneous trip point of the drive which is 21.9A. The pump is drawing water from a source tank, and nothing on the inlet or outlet side is changing much when these spikes occur. I'd bet my paycheck that these overcurrent trips are due to Corona inception voltages.
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I’d take that bet.

My money is on noise getting introduced to the PID loop. When the drive was relocated, the control wiring had to be changed as well!
 
retirede said:
I’d take that bet.

My money is on noise getting introduced to the PID loop. When the drive was relocated, the control wiring had to be changed as well!
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It's not entirely out of the question, but we didn't have this problem when we had it hooked up with standard unshielded S.O. cable, which has considerably thicker insulation. The problem showed up when the wire with thinner insulation was installed. If anything, the addition of the steel conduit should have improved the overall EMI shielding.
 
SO is not cable. It is cord. It will likely work fine for your situation although probably not code.

I don't see how solid wire would make any difference.

I am guessing you have some intermittent issue that you need to work out. Might be that the downstream pump is causing your vfd to trip. Could be some kind of PID loop issue where it goes nuts for a short time.

One thing I will say about using PID loops to control pressure in this type of situation is that it is probably not necessary with centrifugal pumps. The pump curve is going to define the output pressure of the pump and that will be based on the pump RPM, so for the most part to get the right output pressure you need to set the pump speed to a fixed number unless your suction pressure is varying substantially.

ETA:

It could also be that you have a problem with the drive. They do fail in odd ways sometimes. What fault code are you getting when it trips?
 
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I think what you say is possible, but think it is going to take some time to develop.

Might want to check if there is insulation damage causing an intermittent problem? Though I'd expect ground fault message instead of overload message to be more likely on that.
 
petersonra said:
SO is not cable. It is cord. It will likely work fine for your situation although probably not code.

I don't see how solid wire would make any difference.

I am guessing you have some intermittent issue that you need to work out. Might be that the downstream pump is causing your vfd to trip. Could be some kind of PID loop issue where it goes nuts for a short time.

One thing I will say about using PID loops to control pressure in this type of situation is that it is probably not necessary with centrifugal pumps. The pump curve is going to define the output pressure of the pump and that will be based on the pump RPM, so for the most part to get the right output pressure you need to set the pump speed to a fixed number unless your suction pressure is varying substantially.
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I appreciate all of the troubleshooting tips, but my question is whether or not stranded 12 AWG RHH/RHW-2 wire that is advertised as PV (photo-voltaic) wire will suffice as a replacement for the solid core 12 AWG THHN wire that the electrician installed.
 
ElecTech83 said:
It most certainly is overcurrent, but only for short spikes, which immediately trip the drive. It will run for hours drawing about 4.5A to 5.5A (PID control, maintaining a 40 psi setpoint - which is running at less than 50% of the motors rated torque). Then it will, out of nowhere, spike over the instantaneous trip point of the drive which is 21.9A. The pump is drawing water from a source tank, and nothing on the inlet or outlet side is changing much when these spikes occur. I'd bet my paycheck that these overcurrent trips are due to Corona inception voltages.
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Something is not right here. The instantaneous trip point is 3.25X the rated current of the drive. That would mean your rated current is about 6.75 Amp. If is is running at 5.5 A that is pretty close to full load.

The instantaneous trip is fault code 0001. Is that what you are seeing?
 
ElecTech83 said:
I appreciate all of the troubleshooting tips, but my question is whether or not stranded 12 AWG RHH/RHW-2 wire that is advertised as PV (photo-voltaic) wire will suffice as a replacement for the solid core 12 AWG THHN wire that the electrician installed.
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I don't see why you could not use it as a replacement. It is a chapter 3 conductor rated 90 deg C just like THHN that you want to replace.
 
petersonra said:
Something is not right here. The instantaneous trip point is 3.25X the rated current of the drive. That would mean your rated current is about 6.75 Amp. If is is running at 5.5 A that is pretty close to full load.

The instantaneous trip is fault code 0001. Is that what you are seeing?
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Yes, that's the fault code I am seeing. And I had an ammeter set to Max installed on one leg of the VFD output, and it was indeed seeing random spikes over 21A.

The driver's rated continuous current I believe is around 12A, while the motor's FLA is 8.75A. I'd attach pictures of the nameplates, but the site is telling me the files are too large.
 
petersonra said:
Something is not right here. The instantaneous trip point is 3.25X the rated current of the drive. That would mean your rated current is about 6.75 Amp. If is is running at 5.5 A that is pretty close to full load.

The instantaneous trip is fault code 0001. Is that what you are seeing?
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OP said 7.5 Hp motor. Didn't look up anything on the drive he mentioned, but maybe undersized drive for the motor? Figures you are giving are closer to what you might see for a 5 hp motor.
 
ElecTech83 said:
Yes, that's the fault code I am seeing. And I had an ammeter set to Max installed on one leg of the VFD output, and it was indeed seeing random spikes over 21A.

The driver's rated continuous current I believe is around 12A, while the motor's FLA is 8.75A. I'd attach pictures of the nameplates, but the site is telling me the files are too large.
Click to expand...

I made the files smaller...
Screenshot_20210424-093236_Gallery.jpgScreenshot_20210424-093202_Gallery.jpg
 
Something don't seem right about the booster pump for a much larger pump.

Is it carrying all the same media and flow as the much larger pump?

Only thing that makes sense to me so far if that is the case is if the larger pump exists because it is maybe positive displacement and is putting out at much higher pressure.

If bigger pump has a sudden surge of some type and creates rapid drop in pressure or even creates vacuum, then the speed of this pump which is controlled by pressure would get a sudden demand for acceleration- centrifugal pumps are loaded by volume of product being moved - so there would be a sudden increase in load demand as well
 
kwired said:
Something don't seem right about the booster pump for a much larger pump.

Is it carrying all the same media and flow as the much larger pump?

Only thing that makes sense to me so far if that is the case is if the larger pump exists because it is maybe positive displacement and is putting out at much higher pressure.

If bigger pump has a sudden surge of some type and creates rapid drop in pressure or even creates vacuum, then the speed of this pump which is controlled by pressure would get a sudden demand for acceleration- centrifugal pumps are loaded by volume of product being moved - so there would be a sudden increase in load demand as well
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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.
 
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