VFDs and EMC
It is very important that VFDs are installed correctly in order to minimize interference to other equipment.
There
are a number of basic rules that should be applied to the installation
procedure to minimize problems and these are usually covered in the
instruction manual for the VFD.
I have recently come across a
number of VFD installations that are causing severe interference with
flow meters and other equipment to the extent that some equipment has
been damaged by the induced noise levels. These have not been installed
in accordance with the manufacturers instructions.
I am looking
for a simple test and/or equipment that can be used by an installer to
see if the VFD installation is satisfactory. This does not need to be a
full compliance test, but just a reasonable indicator that the
installation is not "BAD", and that it is safe to install sensitive
equipment near by.
Has anyone got any experience or suggestions?
You are treading on sore toes here! And you probably know it, too!
"Static"
in an AM radio is a sure sign. It is being used a lot by guys checking
for bad installations in apartment houses. It is also easily understood
by anyone. Switch the VFD:s off and static disappears. Easy. No
discussion.
This problem is what makes me spend lots of hours on the road. This and bearing EDM.
I
have been using a very simple circuit to detect and quantify conducted
emissions for many years. It consists of a couple of blocking
high-voltage, high-frequency capacitors to block off mains frequency,
followed by an HF transformer that is good up to 50 MHz. There is also a
50 ohms resistor parallel to the secondary of the transformer.
With
this simple circuit, I get a flat frequency response from about 20 kHz
to 10 MHz. Response then drops 3 dB at just above 20 MHz and 10 dB at 30
MHz. 30 MHz is where the conducted emission standards stop.
I
use this device together with an oscilloscope with FFT so that I can
check for emission spectra in the range 150 kHz - 30 MHz. Very, very few
installations meet the EMC directive. And there is very little to do
about it. This is an official secret.
You can ask any supplier
of VFD:s to show the curves for conducted emission. They will not show
it to you. The reason is simply that most (I hesitate to say all, but I
think that would be correct) PWM based VFD:s emit a lot more than they
are allowed to. I regularly measure 20 or 30 dB above the "home, office
and light industry" limits. Sometimes more than that.
Filter or
no filter, the complete installation always radiates a lot more than it
is allowed to. The reason is that there is no good ground plane to
connect the filter drain wire to. So it is connected to the PE, which
conducts the HF noise all over the building. It is a sad thing. And
denied by all VFD suppliers.
Might could also use a Canary, like in the coal mines of the past. Animals seem to be very sensitive to this stray voltage from VFD's. I have replaced VFD controls with ATL starters on dairies, and within three days the milk production had increased by 50%. I have replaced VFD's with ATL starters on chicken facilities and the egg production increased dramatically. Same thing on cattle and hog feed lot facilities, and the weight gain increased almost immediately. A canary might just break out into an opera or a bad rap song if in the proximity of a VFD.
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