The Magneto-motive force (mmf) is the cause due to the presence of the a magnetic flux in a magnetic circuit,
mmf,Fm=NI amperes
where N is used to represent the number of conductors (or turns) and I represents the current in amperes. We sometimes express the unit of mmf as ‘ampere-turns’. As we know, since ‘turns’ have no dimensions, the SI unit of mmf is the ampere. Magnetic field strength (or magnetizing force),
H= NI/l ampere per metre
where l stands for the mean length of the flux path in metres.
Thus mmf =NI= Hl amperes.
