What are the practical problems for designing low freq. chokes or EMI filters
I designed a 100 Hz power line filter comprising a 300 mH choke, an 8 uF capacitor and some series resistance. The design worked on 60 Hz AC at 230 volts for a Canadian oil platform - it was used to prevent massive voltage spikes on the AC connection to a small private telephone exchange. The voltage spikes were just resetting the phone exchange every ten seconds and only occured when the machine that drove pipes into the oil well was operating.


  What makes it difficult or a challenge to design such filter with 200
  Hz cut off for instance?


Well, I had to get an inductance of 300 mH and given that for a single turn the inductance was 10 uH, it meant I needed about 173 turns. This, stretched out end-to-end, was about 3 inches per turn or 43 foot of wire. So, to carry the bit of current to the phone exchange without too much volt drop I needed average sized connection wire of 7/0.2 (= 0.22 sq mm or about 24 AWG). If you do the math, the wire alone has the volume of a larger ferrite core so we are not talking suitable for a PCB because the core probably weighed around 300 grammes.

It soon mounts up but, the applications for this sort of filter are very few and far between unless you design HVDC switch yard reactor smoothing chokes: -



Note the choke is bottom left and compare this with the building middle right!