[RFI] RFI consultant

[Jim Brown]

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