POCO vs VFD

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VFDs for use in AG irrigation and small town water supply has taken off in the last few years. One of my more knowledgeable suppliers was lamenting that the POCOs practice of undersizing transformers is causing problems when used with VFDs. He was refering to requirements for line side harmonic filtering but what else could should we be looking for, if anything?
 

Besoeker

Senior Member
Location
UK
VFDs for use in AG irrigation and small town water supply has taken off in the last few years. One of my more knowledgeable suppliers was lamenting that the POCOs practice of undersizing transformers is causing problems when used with VFDs. He was refering to requirements for line side harmonic filtering but what else could should we be looking for, if anything?
A couple of other things.
Ensure that the motor is designed to be used with a VFD.
Correctly shielded cables but I can't offer advice on that. I'm in UK and our installation practices are different from those generally used in USA.
 

Jraef

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2 more, especially if the project involves retrofitting drives to existing motors:

1) Do a search on "VFD Standing Waves" and read up on this phenomenon if you are not already familiar with it. Part and parcel to the FM that makes VFDs work is this nasty little side effect that can destroy motor insulation, especially on existing older motors. New motors designated as "Inverter Duty" will usually have insualtion material that takes this into account, but if not, you need tpo address it in the system design, i.e. output filters and shielded cable as Besoker mentioned.

2) Motor bearing damage can occurr as a result of voltage being created and building up in the rotor of the motor, then jumping across the bearings and races to find a path to ground. Again, new Inverter Duty motors often will have grounding shaft seals / bushings so that there is a less destructive path for it. But when retrofitting or if not accounted for in submersible motors, the damage can take place in a matter of months. There is not a lot that can be done aside from the grounding, although some types of motor termination filters claim to help, so sometimes the thing to do is either premptively address it in the motor or at least let the user know they may need to budget for that in the near future. I know on deep well submersibles this has turned out to be a very nasty surprise for a lot of people and although it's expensive to pull a pump, it's often less expensive than losing a crop.
 

Besoeker

Senior Member
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UK
2 more, especially if the project involves retrofitting drives to existing motors:

1) Do a search on "VFD Standing Waves" and read up on this phenomenon if you are not already familiar with it. Part and parcel to the FM that makes VFDs work is this nasty little side effect that can destroy motor insulation, especially on existing older motors. New motors designated as "Inverter Duty" will usually have insualtion material that takes this into account, but if not, you need tpo address it in the system design, i.e. output filters and shielded cable as Besoker mentioned.

2) Motor bearing damage can occurr as a result of voltage being created and building up in the rotor of the motor, then jumping across the bearings and races to find a path to ground. Again, new Inverter Duty motors often will have grounding shaft seals / bushings so that there is a less destructive path for it. But when retrofitting or if not accounted for in submersible motors, the damage can take place in a matter of months. There is not a lot that can be done aside from the grounding, although some types of motor termination filters claim to help, so sometimes the thing to do is either premptively address it in the motor or at least let the user know they may need to budget for that in the near future. I know on deep well submersibles this has turned out to be a very nasty surprise for a lot of people and although it's expensive to pull a pump, it's often less expensive than losing a crop.
Both of which ought to be covered if the motor is designed to be used with a VFD.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
I can attest to using non VFD rated motors with VFDs. Some years back my first one was a duplex 250 HP verticle turbine for a golf course. It ate the motor in pretty short order. Of course back then they were so new that a lot of things were not well understood as they are today. Lucky for us we did not specify or sell them the equipment so we were off the hook. Since then we have learned a lot to reliably apply VFD's, but still always learning new things in this area.

Back to th OP question, does the transformer size enter into this?
 

iwire

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Location
Massachusetts
Both of which ought to be covered if the motor is designed to be used with a VFD.

I was involved with four 500HP VFD duty motors that had fluted bearing falures due circulting currents.

There was much finger pointing between the two engineering groups, the motor manafacturer and the drive manafacturer. I don't know what the final soultion was. They had us trying a few difrent things to test options.
 

Besoeker

Senior Member
Location
UK
Back to th OP question, does the transformer size enter into this?
The supply transformer does.
Its impedance has a direct bearing on the secondary side voltage distortion. If that is a point of common coupling with other connected loads then those loads will be subjected to the voltage distortion
Whether that presents a problem needs to be looked at on a case by case basis.
 

Besoeker

Senior Member
Location
UK
I was involved with four 500HP VFD duty motors that had fluted bearing falures due circulting currents.

There was much finger pointing between the two engineering groups, the motor manafacturer and the drive manafacturer. I don't know what the final soultion was. They had us trying a few difrent things to test options.
Possibly insulated bearings? At least at one end, maybe.
It's a problem I've heard about and read about but not one that I've come across.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
The supply transformer does.
Its impedance has a direct bearing on the secondary side voltage distortion. If that is a point of common coupling with other connected loads then those loads will be subjected to the voltage distortion
Whether that presents a problem needs to be looked at on a case by case basis.

For this example, we have little or no connected loads other than the VFD and associated control. The POCO would typically install a transformer half the size or less than what we would if connected per NEC. Just the way it is. At what point does this size reduction start affecting the VFD or the transformer? I see it eventually being a two edged sword but that is from my less than educated WAG.
 

Besoeker

Senior Member
Location
UK
For this example, we have little or no connected loads other than the VFD and associated control. The POCO would typically install a transformer half the size or less than what we would if connected per NEC. Just the way it is. At what point does this size reduction start affecting the VFD or the transformer?
By affecting, I suppose you mean adversely affecting. As long as the transformer is at least adequately rated to supply the drive, probably neither will be adversely affected.

The VFD input is usually a plain rectifier feeding a fairly big bucket capacitor so generally won't mind a dirty input waveform. Might be a different matter with an active front end using IGBTs instead of diodes to take sinusoidal current from the supply but then you wouldn't have a dirty waveform resulting from distortion of the transformer voltage.
 
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