EMC/EMI filter question

[Voicecoil]

Anything that has lots of power switching going on in it like a VFD ought to have a mains filter as part of the design, it likely won't meet the relevant EMI regs without it. Note that a filter to stop stuff creeping OUT of the device may have a slightly different design to one aimed purely at stopping crap coming in. And yes, it's pretty certain it will contain an inductor or two along with some capacitors and if it's an input filter ideally a surge protector (VDR).

[john swift]

the EMI filter could be like this



the choke has a low impedance at the mains frequency but
a high impedance to the high frequency noise your trying to block

the capacitors are just the opposite
a high impedance at mains frequency and a low impedance to high frequency noise

so the mains supply passes through uneffected but any high frequency noise between live & neutral are short circuited by capacitors C1 & C2

any noise between the supply lines ( live & neutral) and earth will pass through capacitors C3 & C4 to be grounded

the filter may be potted in a metal enclosure

if VDR's are added to provide transient voltage protection like this



you need a fuse in the live connection as VDR's very often go short circuit when they fail


John