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Can an EMC filter overload with interference?

EMC filter overload

In theory no, but in reality yes! However to understand this totally we need to define ‘interference’.

Generally interference, at least as regards main conducted, can be defined as those phenomena which interfere with the normal operation of connected equipment.

Typically, when we talk about EMC we are concerned with RFI interference occurring in the 150 kHz- 30 MHz region. Common-mode chokes are constructed so they do not saturate in normal operation, however a significant amount of interference in this area can reduce filter effectiveness and actually exacerbate EMC issues that exist with the system, by utilizing chokes and capacitors which are conservatively rated with large cores and short connection cables REO reduce the stray effects which help to cause the saturation which reduces filter effectiveness.

However, interference can appear in many forms. The proliferation of non-linear loads, such as Variable Speed Drives, can create harmonic distortion of the applied voltage. This can mean that the filter can be subject to power losses at frequencies above the 50 Hz fundamental. This coupled with resonant points within the filter can cause interaction which result in much higher than expected power losses.

If your REO filter seems to running excessively hot or even ‘buzzing’ then it’s possible that you have an issue with harmonic distortion or high crest factor.

The crest factor, or peak factor of a waveform is simply its peak value divided by its rms value. The ideal AC mains voltage (and subsequent current) is a sine wave, which has a crest or peak factor of 1.41. The non-sinusoidal current draw of the capacitors that follow a mains rectifier in a VSD increases the crest factor of the current and so increases the saturation of the cores in the filter inductors that carry this current. Selecting a core with a higher permeability will usually solve this problem, either by allowing a higher initial inductance, ensuring that the value at saturation is high enough to maintain filter performance or (for a common mode choke) by allowing a reduction in the number of windings employed hence decreasing the filters leakage inductance. REO can provide assistance if a particular application is likely to require a special choke construction.

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