On 7/13/2021 7:20 PM, Don Kirk wrote:
Hi Scott,
I can't answer your question about a consultant, but let me make some
comments based on a few cases of VFD and AC Servo Drive interference I've
had to deal with as a ham as well as some of my day job experience with
VFDs and the likes (over the years I've done a fair amount of electrical
control system design for some of the manufacturing equipment that we use
in development as well as production).
In the cases of RFI I've encountered as a ham, the biggest issue is that
the system integrator (company designing the electrical controls package
that included a VFD in the design) did not include the input power line
filter that the VFD manufacturer said was was required to make their VFD
EMC compliant (most system integrators totally ignore the recommended power
line filter in their design). Turns out that a lot of the VFD
manufacturers know what it takes to make their VFD EMC compliant and
document this in their manuals, but users of their drives are clueless and
often ignore their recommendations.
One case involved a VFD that was driving a commercial kitchen ventilation
fan motor at a large apartment complex near my house. After I identified
the VFD as the source of my RFI they went back and installed the power line
filter recommended by the VFD manufacturer, and this fixed my RFI issue.
Yes, and it's critical the the line filter be properly integrated into
the unit.
Note: in cases where the motor is located a considerable distance from the
drive I know there are techniques that should be considered regarding the
cables going from the drive to the motor (well bonded conduit, special
shielded cabling, etc.).
Yes, and this can be a VERY big deal. A colleague who specialized in
troubleshooting noise problems with sound systems in large public
facilities ran into RF trash from VFDs for their elevator motors. The
controllers were at considerable distance from the motors, and poor
wiring techniques resulted in stong magnetic fields that coupled into
audio system wiring that ran in proximity. The fundamental principle is
that each circuit carrying these large pulsed currents must be treated
as as transmission line, and should be twisted pair. It's also critical
that returns NOT be bonded to equipment ground, so that return current
cannot divide between the return conductor in the transmission line and
the equipment ground. That's because for the fields from a transmission
to cancel effectively, the current in the two conductors must be equal
and opposite.
It's also highly desirable that all of that wiring be in rigid steel
conduit, because it provides magnetic shielding.
For any VFD that is part of the design I would
have someone review the VFD manual to see what filter is specified to make
the drive EMC compliant and if this information is not included in the
manual then someone needs to contact the VFD manufacturer for this
information. If they can't provide this information then I would be
hesitant in using their drives in the design
Agreed. I view any "blank stares" or lack of concern from ANY
contractor, vendor, or mfr of stuff that has a good chance of being a
problem to be a sign that I should look elsewhere.
or I would at least specify a good power line filter.
Not a solution if the above-noted problem with control wiring exists.
I've also seen toroid cores available for use with
VFD systems as chokes (on the input and/or output side of the drive) in
addition to using traditional power line filters.
They would need to be used as common mode chokes.
73, Jim K9YC
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