Magnetomotive Force (mmf )

Chapter 4.5 Magnetomotive Force (mmf )

Fundamental Electrical and Electronic Principles Third Edition Book
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Fundamental Electrical and Electronic Principles Third Edition Book

  • 116Fundamental Electrical and Electronic Principles Worked Example 4.1 Q The pole face of a magnet is 3 cm by 2 cm and it produces a fl ux of 30 μ Wb. Calculate the fl ux density at the pole face. A A 3 2 10 4 m 2 ; 30 10 6 Wb BABteslaso mT 30060506411Ans Worked Example 4.2 Q A magnetic fi eld of density 0.6 T has an eff ective csa of 45 10 6 m 2 . Determine the fl ux. A B 0.6 T; A 45 10 6 m 2 Since tesla, then weberso Wb BABA06450276.1μAns 4.5 Magnetomotive Force (mmf) In an electric circuit, any current that fl ows is due to the existence of an emf. Similarly, in a magnetic circuit, the magnetic fl ux is due to the existence of an mmf. The concept of an mmf for permanent magnets is a diffi cult one. Fortunately it is simple when we consider the fl ux being produced by current fl owing through a coil. This is the case for most practical magnetic circuits. In section 4.2 we saw that each turn of the coil made a contribution to the total fl ux produced, so the fl ux must be directly proportional to the number of turns on the coil. The fl ux is also directly proportional to the value of current passed through the coil. Putting these two facts together we can say that the mmf is the product of the current and the number of turns. The quantity symbol for mmf is F (the same as for mechanical force). The number of turns is just a number and therefore dimensionless. The SI unit for mmf is therefore simply ampere. However, this tends to cause considerable confusion to students new to the subject. For this reason, throughout this book , the unit will be quoted as ampere turns (At). Thus mmf, ampere turnFNI(4.2)