Exact THD Calculations Applied to Minimize Space Harmonic Content of Winding and Rotor Magnetic Fields

Abstract

As a metric for measuring the combined space harmonic content of electric machine winding magnetomotive force (MMF), the winding total harmonic distortion (THD) is defined with an infinite series. Without truncation, the exact THD can be evaluated via a “family grouping” facilitated by a “family sum” over the fewest possible winding factors. Uniformly applicable to distributed windings and fractional-slot concentrated windings (FSCWs), this procedure also explains the inherent difference in harmonic content between both winding categories. Furthermore, the minimal THD (mTHD) that can be achieved by optimizing a winding is revealed with a closed-form expression. While intended for windings, the mTHD concept also benefits the design of low harmonic content rotor magnetic fields. Examples of both winding and rotor mTHD designs are provided.