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Emc From Incorrectly Installed Vfds


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All VFDs can generate EMC issues if not correctly installed.

The high frequency noise results from the high speed switching on the output of the VFD and can be minimised by the correct connection of the motor to the output of the VFD.

see : http://www.vfd-emc.co.nz


Best regards,


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  • 2 weeks later...

There are many different theories out in the industry as to how the motor and VFD should be connected, what is important and what is not, and what applies to different companies and VFDs.

When you study all the installation guides, one thing that is clear, is that all recommend the use of a screened cable for EMC compliance.

Some manuals provide a higher level of detail than others into the hows and whys of the screen terminations, but there is limited documentation on the effects of different terminations and departures from the recommendations.


In New Zealand, there is currently work under way to create a code of practice covering the installation of VFDs and this is aimed at reducing the issues created by poorly installed devices. Those that are interested should make sure that they get onto the discussion group when the next draft is published.


One of the major issues that we have in terms of EMC compliance, is that the tests prescribed in the IEC standard do not determine the level of conducted noise emanating from the frame of the motor into the connected structures.

In essence, provided that there is a good low impedance decoupling between the frame of the VFD and the DC Bus, it is possible to pass the conducted test with hundreds of volts of noise on the frame of the motor.


Any noise voltage on the frame of the motor relative to the frame of the VFD will cause High Frequency noise to be conducted from the frame of the motor into the connected structures around the motor. This noise current will follow every path possible to find it's way back to the DC bus of the VFD.


The critical point in minimising this "stray" current, is to minimise the nooise voltage on the frame of the motor, and the best way to do this is to ensure that the high frequency impedance between the frame of the VFD and the frame of the motor is as low as possible.


I have set up a web site at http://www.vfd-emc.co.nz which covers some of the technology and also some of the tests that I have done recently. This gives some definitive relative numbers for the different ways of terminating the VFD to the motor and show the importance of using a good EMC cable and no pigtails.

I intend to slowly test more VFDs and add them to the results, but you can see that there are quite major variations between VFDs also.

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