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Inverters and conducted emissions: when is it acceptable to rely on internal filters?

As a manufacturer of EMC filters we understandably pay great attention to the reasons that filters are sometimes not used in situations where we would believe them to be necessary.

EMC filters are often needed to ensure compliance with the relevant standards for conducted emissions but their use is also good practise as conducted emissions can lead to system faults when the radio frequency interference damages sensitive electronics. One area where we see the possibility for confusion is around drives supplied with internal filters.

According to EN/IEC 61800-3 (Adjustable speed electrical power drive systems – Part 3: EMC requirements and specific test methods) a power drive system is an adjustable speed AC or DC motor drive consisting of a drive module which when connected to its motor becomes a power drive system (PDS). EN/IEC 61800-3 is the EMC standard applicable to many of the applications where variable speed drives are used and it specifies emissions limits, immunity levels and test methods for a PDS. It does this by defining two different environments and four categories of use.

The first environment refers to premises directly connected to a low voltage power supply which serves buildings used for domestic purposes. This can be domestic dwellings but also commercial and light industrial establishments.

The second environment is establishments other than those directly connected to a low voltage power supply which serves domestic residences. It includes industrial areas and technical areas or any buildings that are fed from their own dedicated transformer.

EN/IEC 61800-3 specifies emissions limits, immunity levels and test methods for a PDS in 4 categories according to its environment and installer competency. Category C1 is for a PDS with a rated voltage of less than 1000V for use in the first environment and permitted to be installed by anyone.

Category C2 is again for a PDS with rated voltage less than 1000V but which is not plug-in or portable. When a category C2 PDS is to be used in the first environment it must be installed and commissioned by a professional. The professional is defined as an individual or an organisation having the required skills for installing and or commissioning PDS including knowledge of EMC issues.

In the area closest to my heart (conducted emissions) C1 comes with a lower, or tougher, emissions standard than C2. It is easy to see why this is: in C1 applications there are domestic dwellings and unqualified users so greater care must be taken to protect them from conducted noise that could damage electrical equipment. In category C2 the building is either isolated by its own transformer or the PDS has been installed by an expert who has properly accounted for EMC issues.

Categories C3 and C4 apply exclusively to the second environment and are less relevant to the point of this article. That point being that modern drives are very often supplied with inbuilt filters. The inverter manufacturer will usually specify the category of emissions that can be expected if a certain motor lead length of a certain specification is used. This could be for example C2 with 5m of cable. The casual user might assume that if their PDS is to be used in a factory that is an industrial setting so C2 emissions are acceptable. However, it might be possible the factory is small and is fed by a transformer that also supplies domestic dwellings or the PDS might have a plug and be portable. In these cases C1 would be needed and the internal filter may not be acceptable.

Another detail to be aware of is that longer motor leads can usually be expected to greatly increase the conducted emissions from a drive. If a drive with an internal filter is specified to achieve C2 with 5m were to use a motor cable 10m long then there is a high probability that C2 would no longer be met.

If a lower category is needed than is achieved by the internal filter or if longer motor cables are used Roxburgh EMC external filters can be deployed to solve the problem. We can demonstrate the performance using our fully calibrated pre-compliance EMC lab.

For technical advice please email or call 01724 273200.

Article first published : 13|11|2018