OPTIMUM VALUES FOR MAGNET AND ARMATURE WINDING THICKNESS FOR AXIAL-FIELD PERMANENT-MAGNET BRUSHLESS DC-MOTORS

When the motor thickness and the required starting torque are set for an axial-field permanent magnet brushless dc motor, the optimal ratio of the permanent magnet thickness to armature winding thickness is 2:1 because the copper loss of the armature windings is kept to a minimum. This conclusion is based on a new method presented here, in which factors such as loop resistance, a radial air gap magnetic flux density approximated by curves of second order, and an experimental formula for the leakage coefficient, are incorporated. The demagnetization curves of magnets are assumed to be linear, as Nd-Fe-B, Sm2Co17, SmCo5, and SrO . 6Fe2O3 magnets with the linear demagnetization curves are widely used for this type of motor. The new method is also applicable to estimating torque, current, and input power at the start. These items can be more promptly estimated without a computer program, and the physical insight is more easily gained by the aid of the new method than a 3-D finite element method. This is true despite the fact that the loop resistance and torque correction factor must be measured, using a base motor. The calculated values of torque, current, and input power agree well with the measured values.