Fitzgerald & Kingsley's Electric Machinery (IRWIN ELEC&COMPUTER ENGINERING)

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materials involved. In fact, the situation is more complex. 8 This section will expand upon these issues. Example 1.9 shows that there is an immense difference between permanent- magnet materials (often referred to as hard magnetic materials) such as Alnico 5 and soft magnetic materials such as M-5 electrical steel. This difference is characterized in large part by the immense difference in their coercivities He. The coercivity can be thought of as a measure of the magnitude of the mmf required to demagnetize the material. As seen from Example 1.9, it is also a measure of the capability of the material to produce flux in a magnetic circuit which includes an air gap. Thus we see that materials which make good permanent magnets are characterized by large values of coercivity He (considerably in excess of 1 kA/m).

Fitzgerald, A. E. (Arthur Eugene), 1909- Electric machinery / A. E. Fitzgerald, Charles Kingsley, Jr., Stephen D. Umans. - -6th ed. Here the ~" = Ni is the mmf applied to the magnetic circuit. From Eq. 1.10 we see that a portion of the mmf, .Tc = Hclc, is required to produce magnetic field in the core while the remainder, f g = Hgg, produces magnetic field in the air gap. Note that, normalized in this fashion, the rms exciting voltamperes can be seen to be a property of the material alone. In addition, note that they depend only on Bmax the magnetic circuit is illustrated in this section and will be seen to apply quite well to many situations in this book. 2m A chapter has been added which introduces the basic concepts of power electronics as applicable to motor drives. i ~ 0.2 . . . . . . . . . . . . . . [ . . . . . . . . . . . . . . . . . . . . . ' . . . . . . . . . . . . . . . . . . . .

because Hrms is a unique function of Bmax as determined by the shape of the material hysteresis loop at any given frequency f . As a result, the ac excitation requirements for a magnetic material are often supplied by manufacturers in terms of rms voltamperes per unit weight as determined by laboratory tests on closed-core samples of the material. These results are illustrated in Fig. 1.12 for M-5 grain-oriented electrical steel.where Bg and Bm are the magnetic flux densities in the air gap and the magnetic material, respectively. which corresponds to a point on the second quadrant of the hysteresis loop. As can be seen from Eq. 1.56, the product of B and H has the dimensions of energy density (joules per cubic meter). We now show that operation of a given permanent-magnet material at this point will result in the smallest volume of that material required to produce a given flux density in an air gap. As a result, choosing a material with the largest available maximum energy product can result in the smallest required magnet volume.

Notice that, because the hysteresis loop "flattens out" due to saturation effects, the waveform of the exciting current is sharply peaked. Its rms value I~0,rms is defined by Eq. 1.51, where T is the period of a cycle. It is related to the corresponding rms value nc,rms of n c by the relationshipon both physical insight and analytical techniques. Mastery of the material covered will provide both the basis for understanding many real-world electric-machinery applications as well as the foundation for proceeding on to more advanced courses in electric machinery design and control. T he chief objective of Electric Machinery continues to be to build a strong foundation in the basic principles of electromechanics and electric machinery. Through all of its editions, the emphasis of Electric Machinery has been