Reducing Cogging Torque with Rotor Surface Notches in Double-Layered IPMSMs

In this study, a design method is proposed for forming notches on the rotor surface to reduce the cogging torque of interior permanent magnet synchronous motors (IPMSMs) with double-layered magnets. The proposed method is predominantly based on theoretical equations and can be applied to general double-layered IPMSMs, regardless of the magnet arrangement. First, the waveform of the square of the air-gap flux density (Bg2) for the reference model is approximated using a two-step trapezoidal waveform. Using the approximation, the harmonic components of Bg2 are formulated when notches are fabricated on the rotor surface. Additionally, we derive a Bg2 waveform that minimizes the main harmonic components of the cogging torque. The position and width of the notches are determined based on the derived waveform, and only the notch depth is adjusted by finite element analysis. The finite element analysis reveals that the cogging torque for the model with the optimized notch placement is significantly reduced. Furthermore, prototypes were manufactured and their cogging torque waveforms were measured. The measurements confirm that the amplitude and major components of the cogging torque are reduced.