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EMC Filters - In or Out?


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In many regions around the globe, EMC filters are specified for use with variable frequency drives in order to comply with local regulations.


Many companies now manufacture variable frequency drives with in-built EMC filters, whilst others simply reccommend the use of an external filter with their product.


I would like to know your thoughts re the advantages and dis-advantages of each format.




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If you compare the effect of a particular filter when used inside the drive as compared to used external, then the best results should be obtained by internal usage.

External usage implies that there will be a length of cable between the drive and filter that can radiate and therefore reduce it's effectiveness.

It is also very likely, that there will be a difference between internal and esternal filters. Many internal filters are simpler than external connected ones ans a means of controlling the cost of the drive. Under these cases,and external filter may well be more effective.


Best regards,

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Hello Marke,


Many thanks for your response. As usual it doesn't end there, not with me anyway!


I think the key advantage of internal filters is that they eliminate the need to purchase a seperate device and therefore assist to reduce installed cost ie the installer needs to fit and wire 1 component hence there are space, time and cost savings.


As you quite rightly mentioned with an external filter there is a short piece of cable that can act as an antena and this is therefore another good argument for the use of drives with internal filters.


Some of the disadvantages I see are:


1. As you indicated internal filters are simpler devices and therefore they may not perform quite as well as an external filter.


2. Most internal filters are designed to limit emmissions to levels acceptable for industrial applications. That is, they are of the Class A variety. However many drives find there way into Class B applications. In such cases an external Class B filter will still be necessary and therefore the internal filter becomes a cost penalty.


I'm not sure what the best option is, or whether there is a simple answer to this question.


I guess I'm just looking for more arguments for an against to enable me to make an informed decision and/or establish a position on the subject.




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One thing that is often overlooked is that inherent to the EMC filter design, there is a network of capacitors, resistors and inductors to earth. This means that there is always going to be a true current flow to earth. If there is an ELCB connected, it is more than likely going to trip.

Unless the drive that has an internal filter has a means of disconnecting the EMC filter, it is almost impossible to use it on an ELCB protected circuit.

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