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I am converting a floor standing drill press to variable speed. I bought a KB VFD and a 1 hp, 3 phase motor. The VFD takes a single phase 120 VAC input and produces the 3 phase output. The KB is old school, no keypad programming, instead is a simple speed knob and jumper wire configuration.<br />
<br />
I wired it up on the bench and everything functions fine. The motor case is ungrounding as it it still on the workbench. I checked for stray voltage from ground to the motor frame and found 80 VAC. Not my first rodeo so I figured it was capacitive coupled leakage; low current, and would not be a concern once everything was mounted and grounded. To verify I clipped in a 3 ohm resistor from line ground to the motor frame and tested again. The voltage on the motor frame dropped as expected, reading 150 mVAC.<br />
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The calculation with 150mV across a 3 ohm resistor works out to 50 mA. I guess that surprises me. 50 mA seems high for a current coming as a result of leakage. <br />
<br />
On one hand, if I never checked this I would have grounded the motor frame when it was bolted up to the machine frame; and the VFD has a solid connection from it chassis enclosure to line power ground, so I would have been blissfully unaware.<br />
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What is your take? Does the 50 mA seem reasonable? Do I ignore it and proceed to wire it in properly and not look back?
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I think the suggestions above are great.<br />
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I would think about using 240V<br />
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Personally, I would bolt it up and see where things are. Inverter driven motors have a tendency towards leakage current from the usual sources, plus of course the stray current induced in the rotor. I've seen larger motors with grounding brushes to keep the stray voltage out of the bearings.<br />
<br />
I would be inclined though not to suspect the motor. Rather, I would suspect the VFD for not producing a clean (enough) sine wave. I think it is good practice to put input and output snubbers on VFDs as a general precaution, but especially if you are not using an inverter rated motor or 120V power. A couple of appropriately sized ferrite toroids and you are in business, or you can buy a reactor. I saw someone once used some black pipe elbows and unions to make a square that he wrapped his incoming power line around. He'd measured the inductance and figured out how many wraps he needed to get it to 60Hz resonance. Pretty slick, I thought. More generally you can buy EMI suppressors to put on the input side. E.g.<br />
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If you can, I would try to use 240VAC as input so the VFD only has to create one phase. That is the nature of a VFD. Using two hots will cut down on the (harmonic) noise generated.<br />
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All the best,<br />
<br />
Peter</div>
