VFDs and EMC

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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?


“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.