EMI from a VFD

[Keith1029]

I am using a 1/2HP motor controlled by a VFD to rotate an electronics package ~2 ft away as part of a testing fixture. The motor is fixed to an aluminum plate as is the fixture for rotating the electronics package. For *some* of my packages I am getting garbled output when the drive is engaged (but fine when it is off). When the package is moved to ~4 ft away there is no problem. I am suspecting EMI from the drive as the cause of the problem, but I would like to not have to re-build the base of my fixture and drive to create the increased separation. Is there any simple way to shield my electronics from the EMI to avoid the signal problems?
I do not know the frequency of the EMI (if that is indeed the problem).

Thanks for any help.

 

[dpc]

Seems like a good bet. You'll get better advice from some experts shortly, but the first step for me would be to make sure the motor feeder cable from drive to motor is either shielded cable or in metal conduit.

VFD enclosure should well grounded and could be enclosed in a metal enclosure if possible.

Stand by for better answers...




"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[Skogsgurra]

dpc's advice is good enough. One very simple and effective thing to do is to put the motor cable through a ferrite core. Select one with at least 1 MHz corner frequency and put the motor cable through it several times. Put the ferrite core close to the VFD, not close to the motor. It may be a good idea not to put the PE conductor through the ferrite core. Test both versions.
Also, keeping the VFD in an HF tight box which shall be bonded to the test rig with short and wide straps is good. It will reduce air-borne emission.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

I agree. The single most common mistake is that people use regular flexible portable cord (i.e. "SO" or "SJ" cable) for the motor leads coming off of a VFD. Without shielding, those leads are essentially a nice local FM transmitter. If that's what you have done, simply replace that cable with a version that has shielding. It's readily available from industrial supply houses. Belden, Lapp, Alphawire among others all have product available.


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[Keith1029]

Thank you everyone. I will try these out and see if I can get my interferance down to an acceptable level.

 

[Marke]

Hello Keith1029

The problem is due to high frequency noise that is capacitively coupled from the motor windings to the frame of the motor. You need to provide a very low impedance return path for that noise from the frame of the motor to the frame of the VFD. This requires surface area on the return earth path rather than cross sectional area and this is best achieved by the use of a screened cable with the screen well bonded both at the motor end and the VFD end.
The screen termination should be in the form of a clamp or gland that connects all the way around the screen. Do not use pigtails as you have then reduced the surface area and created a constriction.

If the noise level is still too high, the next step is to fit an EMC filter on the input AC supply right at the VFD input. A ferrite ring on the output three phase (but not the screen) may help also.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd