I have (18) Geothermal ground wells, each with a 7.5HP motor at 480V, 3-phase. Each pump is controlled with it's own VFD. The well casing is bonded to the ground system and lightning arrestors are installed on each well pump. The electrician has measured voltages and amperages of the following values on the ground conductors: well#1(6.1V/.25A); well#2(7.7V/.26A); well#3(9.5V/.31A); well#4(11.2V/.38A); well#5(8.3V/.37A); well#6(5.8V/.43A);well#7(6.4V/.48A); well#8(9.2V/.36A);etc. I think you can get the picture here. This doesn't seem right to me. The electrician is telling me that the voltage and current is due to the electromagnetic field created by the three phase conductors. Each well feed has it's own conduit from the building to the well location. I have never seen readings this high before on a ground conductor and these values (especially the amperage) worry me. Can someone offer any suggestions?
Voltage on Ground Conductor
- Thread starter bfletcher
- Start date
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Don't think you have an issue...
Don't think you have an issue...
I am working with some film projectors using VFD's (240v three phase input) and have found that a value of voltage and current you discovered are pretty common. Some VFD's, such as the ones I'm using, even state in the manual that they have "High Leakage current" and that a proper earth (ground) is essential.
Check with the manufacturer of your drives, and look at the manuals if they're available to see if this applies in your case.
IMHO, the current values your electrician found aren't a cause for concern. Unless you're having other issues with the drives or motors I wouldn't worry.
Don't think you have an issue...
I am working with some film projectors using VFD's (240v three phase input) and have found that a value of voltage and current you discovered are pretty common. Some VFD's, such as the ones I'm using, even state in the manual that they have "High Leakage current" and that a proper earth (ground) is essential.
Check with the manufacturer of your drives, and look at the manuals if they're available to see if this applies in your case.
IMHO, the current values your electrician found aren't a cause for concern. Unless you're having other issues with the drives or motors I wouldn't worry.
- Location
- Massachusetts
VFDs are notorious for causing current flow on the EGC. I don't think leakage current is the right term but either way you have current on the EGCs.
I was involved with four 500 HP VFDs, we measured 17 amps on the EGCs This current destroyed the bearings on the motors.
The VFD maker (CH) suggested the following 'cures'
Keep line and load wiring at least 3' apart or shielded
Use shielded VFD cable on the load side to the motor.
Install reactors at the VFDs.
But... they would not say that any of those things would stop it.
Ceramic bearings where installed in the motors and grounding brushes installed against the shaft so even with the EGC current the bearings would not be affected.
I was involved with four 500 HP VFDs, we measured 17 amps on the EGCs This current destroyed the bearings on the motors.
The VFD maker (CH) suggested the following 'cures'
Keep line and load wiring at least 3' apart or shielded
Use shielded VFD cable on the load side to the motor.
Install reactors at the VFDs.
But... they would not say that any of those things would stop it.
Ceramic bearings where installed in the motors and grounding brushes installed against the shaft so even with the EGC current the bearings would not be affected.
Current and Voltage on Ground Conductor
Current and Voltage on Ground Conductor
I've done some research and it looks like the readings I'm getting for current and voltage on the ground conductor are refered to as Bearing Current and Shaft Voltage. These are very common with VFD applications and can cause premature bearing failure. Am I correct in my understanding of these terms? Like I said before this is the first time I've ever run into this.
Current and Voltage on Ground Conductor
I've done some research and it looks like the readings I'm getting for current and voltage on the ground conductor are refered to as Bearing Current and Shaft Voltage. These are very common with VFD applications and can cause premature bearing failure. Am I correct in my understanding of these terms? Like I said before this is the first time I've ever run into this.
Cold Fusion
Senior Member
- Location
- way north
Oh-oh i got a denseness attack. Nope, I'm not getting the picture. When you say you are getting a voltage on the ground conductor, I'm seeing one voltmeter lead on the ground conductor (but I don't know which end - the well end, or the VFD end). And I don't know where you are putting the other voltmeter lead.bfletcher said:
Where are you measuring this voltage? Is the 480V system impedance grounded or solidly grounded?
cf
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I echo cf's question: how was the voltage measured. I was _guessing_ that the voltage was measured by disconnecting the EGC and measuring the voltage from the disconnected end and the rest of the grounding bus, but I would like to see that guess confirmed.
I would suggest the following diagnostics:
1) get an oscilloscope and measure the _frequency_ characteristics of the voltage and current
2) shut down an individual pump and make measurements on that line.
3) shut down all of the pumps and repeat the measurements.
VFDs switch their output at a very high frequency. This switching is 'common mode' (all the legs switch at the same frequency and in phase with the exception of the modulation) and this high frequency common mode signal is capacitively coupled to ground. Some of this current is coupled to the rotor and then through the bearings to ground (which can cause the problem that iwire mentioned), but the bulk of this current is coupled by the motor frame and the conduit itself. (You can also have current magnetically induced in the rotor, again through the bearings.)
My hunch is that if you are measuring this capacitive current, I doubt you have significant cause for concern.
-Jon
I would suggest the following diagnostics:
1) get an oscilloscope and measure the _frequency_ characteristics of the voltage and current
2) shut down an individual pump and make measurements on that line.
3) shut down all of the pumps and repeat the measurements.
VFDs switch their output at a very high frequency. This switching is 'common mode' (all the legs switch at the same frequency and in phase with the exception of the modulation) and this high frequency common mode signal is capacitively coupled to ground. Some of this current is coupled to the rotor and then through the bearings to ground (which can cause the problem that iwire mentioned), but the bulk of this current is coupled by the motor frame and the conduit itself. (You can also have current magnetically induced in the rotor, again through the bearings.)
My hunch is that if you are measuring this capacitive current, I doubt you have significant cause for concern.
-Jon
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