Brushless Permanent Magnet Motor Design- P6: You've just picked up another book on motors. You've seen many others, but they all assume that you know more about motors than you do. Phrases such as armature reaction, slot leakage, fractional pitch, and skew factor are used with little or no introduction. You keep looking for a book that is written from a more basic, yet rigorous, perspective and you're hoping this is it. | 146 Chapter Six Figure Geometry for the torque calculation in the dual axial flux topology. To understand this phenomenon consider the magnet shown in Fig. . The flux entering the stator from a differential slice is r BgOpr dr. In the stator this flux splits in half in the back iron to return through adjacent magnets. Therefore if Bmax is the maximum allowable flux density in the back iron the back iron flux is 7 Bm wblkstdr from which the required back iron thickness is Be6 r i r 6-55 It is not practical to build stators with a linearly increasing back iron width. Therefore a constant back iron width equal to the maximum of is chosen B TD0 Wbi 656 Using the same argument made earlier in the required tooth bottom width is . . 2 . iafi r ia6 r which is Wtbi N k 6 57 smomaxK st at the inner radius. Since the slot width is constant the tooth bottom width increases linearly with radius. This agrees with the topology shown in Fig. . Thus even though the tooth width is smaller at the inner radius the flux density in the stator teeth is uniform with respect to radius. That is the narrow teeth at the inner radius are not any Design Equations 147 more saturated than the teeth at the outer radius. Moreover since the stator back iron thickness is wider than necessary at the inner radius the net steel reluctance at the inner radius is much lower than that at the outer radius. Given the slot bottom width is sb Tsi W tbi and the slot aspect ratio at the inner radius is sh asi ---------- W bi IVS5 Electrical parameters The derivation of electrical parameters closely follows the radial flux topology analysis conducted earlier in this chapter. Therefore the derivations for this topology will not be justified as thoroughly. Torque. The torque produced by the axial flux topology requires some development because the torque is produced at a continuum of radii from R to Ro. Rather than develop the torque expression from the basic configuration .
