This chapter focuses on motor performance. More specifically it focuses on providing guidance in the selection of magnet pole count Nm/ slot count Ns, and the ratio of the outside rotor radius Rw to the outside stator radius Rso. In addition, it develops procedures for computing cogging torque, radial force, core losses, and AC winding resistance.
9.1 Motor Constant As has been demonstrated in earlier chapters, motor constant describes the torque production efficiency of a motor that is independent of the number of turns per coil and the motor current. Therefore, motor constant provides valuable insight into maximizing motor performance. General Sizing Before maximization of the motor constant is discussed, it is beneficial to identify how the dimensions of the ferromagnetic portions of the motor are related to the number of magnet poles, the number of slots, and the rotor outside radius. To simplify this discussion consider the motor cross section shown in Fig. 9-1, and let the air gap magnetic field distribution be ideal as shown in Fig. 4-5. If one ignores the alternating direction of flux flow over alternating magnet faces, the total flux crossing the air gap in Fig. 9-1 can be written as Âżtotal = B A = Bg2nRr0Lit
(9.1)
where B ? is the amplitude of air gap flux density and L st is the axial stack length of the motor. This flux divides among the teeth and travels outward or inward depending on which magnet polarity faces each tooth. As a result, the magnitude of the flux flowing in each tooth is