Brian, et al,
You have received considerable insight into the technical issues
involved in the construction and application of GFCI's from several folks
here.
You should almost have enough information to technically resolve the
issue. VFD motor drives are essentially a SMPS applied to a motor with the
adjustment of the output V/F to give the desired partial speed operation of
the motor. Just compare the FFT from the unit when the motor drive is
running to that of the GFCI trip characteristics (as a function of
frequency). You will then clearly see where the motor harmonics are large
enough to trip the GFCI. (I'm sure that none of this specific data is
readily available; you will have to collect it yourself).
We've seen this type of problem coming for some time. The use of
active electronic switching of mains voltage generates a generous group of
harmonics which all of the other systems connected to the surrounding
circuit have to correctly respond.
The use of appropriate filters to allow for increased HF current
provides the needed protection for users. Unfortunately, the GFCI systems
don't seem to take this into account as far as I can tell. Each protection
seems appropriate but the combined set gives problems.
Looks like your just about set up for the fun engineering part of
resolving your problem.
:>) br, Pete
Peter E Perkins, PE
Principal Product Safety Engineer
PO Box 23427
Tigard, ORe 97281-3427
503/452-1201 fone/fax
[email protected]
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