Electrical Engineering ⇒ Topic : Magnetomotive Force (M.M.F.)
MAGNETOMOTIVE FORCE (M.M.F.)
A current carrying conductor produces a magnetic field around it. The coil should have the correct number of turns in order to produce the required flux density. Magnetomotive force is defined as the product of current and number of turns, i.e.,
Magnetomotive force = Current x Turns
where I is the current through the coil, N is the number of turns of the coil.
The unit of m.m.f. is ampere turns. Since N is dimensionless, its unit can also be written as ampere (A).
Consider a ring specimen of a conductor of mean radius r having a winding of N turns which are wound uniformly as shown in Fig.(a) Let the current in
FIGURE(a) Magnetic flux due to current ring in a coil having N turns
the winding be I A. Then, the product of the current (I) and the number of turns on the winding (N) is known as the magneto-motive force (mmf) of the circuit, i.e. mmf = NI.The mmf in the magnetic circuit is analogous to the emf in the electric circuit. Emf in an electric circuit is responsible for the flow of current. But in a magnetic circuit mmf is responsible for the existence of flux in the magnetic medium. This mmf acts along the magnetic line of force whose length is l = 2π r
.Also the magnetic field intensity (H)= mmf/unit length NIll
where lis the mean length of the magnetic path.
The above result can also be obtained using Ampere's circuit law, as below.
Assuming H to be the same since the coil is wound uniformly, we get
Magnetomotive force (m.m.f.). Magnetomotive force drives or tends to drive flux through z magnetic circuit. It is equal to the work done injoules in carying a unit magnetic pole once through the entire magnetic circuit ; m.m.f, is measured in ampere-turns (AT).
where N = number of turns of a magnetic circuit, and
J = current in ampere in those turns
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