The DM currents produce fluxes in opposite directions and in the ideal case they
exactly cancel each other; the mutual inductance is therefore negative and cancels
the self inductance part. Therefore the choke does not present any impedance to
DM currents. In the case of CM currents, the fluxes set up by the individual currents
add up and therefore the mutual inductance is positive. The choke presents a series
impedance of ZCM ¼ jv(L þ M) ¼ jv2L per winding to CM currents. This type of
choke is called CM choke.