Compressor ground problems

We have three 1500 cfm compressors. Two were installed together and have identical Siemens circuit breakers (800Amp) sharing a common
panelboard. The third was installed later and is wired and fed separate. Of the two sharing the common panelboard one has extremely high currents (170 Amps) on the motor ground wires in the starter enclosure. All motors are started by Siemens Sikostart starters "in line" without bypass contactors.
All conductors have been thoroughly megged showing high resistance to ground.
Here is the name plate data off the problem motor; MFR:Baldor, ENCL:TEFC, Frame:449T, HP:300, PH:3, Volts:460, Des:B Amps:328, Code:H Hz:60, Rating:40C amb cont, NEMA Nom Eff:95.8%, PF:88% SF:1.15
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snipped-for-privacy@usfenceonline.com wrote:

The motors as well?
With 170 A ground currents, you might be able to see a winding to ground fault with an ohm meter.

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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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All conductors from motor end of starter and motor have been meggered and show high resistance to ground. I do not have any historical data as the company does not have a program for tracking insulation breakdown over time. There are no obvious shorts at the starter through the motor.
What else can cause such a high potential across the ground?
First indication there was problem was when motor connections were inspected and found insulation had melted off ground wires at the motor. Thermal camera image showed ground wires leaving starter enclosure at 150 F.
Thought there might be a harmonic problem but power meter at MCC showed now harmonic problems. To be sure we rented a meter to verify at starter which confirmed no large harmonic problems. Verified each conduit had one wire of each phase and ground wire. Grounds were added from compressor frames to building, insuring good metal to metal contact.
Who ever wired starter enclosure did a poor job of removing paint from panel for grounds. New grounds have not been pulled from motor to starter yet.
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snipped-for-privacy@usfenceonline.com wrote:

What type of power system is this on... Delta? Wye? 3-wire? 4-wire? Where is it grounded... corner? center tap?
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Benjamin D Miller, PE
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snipped-for-privacy@usfenceonline.com wrote:

Potential? Or current?
How and where are these parameters being measured?

Are these VFD systems by any chance?

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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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The current is not flowing from motor to ground but probably from the starter via the ground wire to the motor.
If you megged the wiring it could be that the electronic filters inside the vfd are broken. However this is not easy to check as the max voltage on this filter with some elcos and coils has a maximum. If possible change over the two compressors and do same check.
On Tue, 4 Nov 2008 14:00:44 -0800 (PST), snipped-for-privacy@usfenceonline.com wrote:

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