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For very high temperature applications Alnico or rare-earth cobalt magnets must be used. 36 Electric Machines Two important qualities of a permanent magnet (PM) are defined below with reference to the second quadrant of its hysteresis loop. Permanent Magnetization or Residual Flux Density (Br) It is the flux density trapped in closed magnetic structure if the applied mmf (and therefore the magnetic field intensity, H) were reduced to zero. e. it would demagnetize the material. Its value is negative and in units of kA/m.
The electrical analog of the magnetic circuit is drawn in Fig. 8(a). 796 ¥ 106 4 p ¥ 10- 7 ¥ 2 ¥ 10- 4 From Fig. 75 mWb N Rg 2 Rg 3 Ni Rg 1 R g || R g 2 3 (a) Fig. 8 1 Rg (b) Electrical analog of Fig. 7 (b) mr = 5000. This means that the reluctance of magnetic core must be taken into consideration. The analogous electric circuit now becomes that of Fig. 9. 159 ¥ 10 R c3 6 R g2 R g3 The equivalent reluctance is Req = (Rc1 + Rg1) || (Rc2 + Rg2) + Rc3 + Rg3) R c1 R g1 Fig. 4 The magnetic circuit of Fig.
These can be classified as ferromagnetic and ferrimagnetic. Ferromagnetic materials can be further subdivided as hard and soft. Hard ferromagnetic materials include permanent magnet materials, such as alnicos, chromium steels, certain copper-nickel alloys and several other metal alloys. Soft ferromagnetic materials are iron and its alloys with nickel, cobalt, tungsten and aluminium. Silicon steels and cast steels are the most important ferromagnetic materials for use in transformers and electric machines.