Evolutionary Characteristics of Magnetic Field for the Permanent Magnet In-Wheel Motor Under Time-Dependent Demagnetization

The permanent magnet (PM) of synchronous motor is prone to demagnetization faults due to factors such as high temperature, reverse magnetic field, and impact vibration, which directly affect the operational performance of machine. To accurately predict the air-gap magnetic field distribution accounting for time-varying demagnetization effects, an improved analytical subdomain model is proposed to reconstruct the residual magnetic field after demagnetization. The degree and distribution of demagnetization are innovatively described and defined using orthogonal design and statistical methods. Taking the 20/24 PM in-wheel motor (IWM) as an object, the variation of magnetic field and back electromotive force (EMF) under different demagnetization cases and degrees are compared and analyzed according to the Maxwell's theory and field-circuit coupling time-stepping finite element (FE) method. And the evolution rules and reconstruction characteristics of unbalanced electromagnetic force (UEF), cogging torque, and output torque are explored. The results demonstrate that the improved model predictions agree well with that of simulation results, and the amplitude of flux density and back EMF in demagnetized region are significantly lower compared to healthy conditions. Under the uniform demagnetization, the waveform of UEF, cogging torque, and output torque generated by the residual magnetic field is almost consistent with the healthy condition, with relatively lower peaks. In other cases, with the expansion of demagnetization degrees and demagnetized PM's numbers, the resulting UEF increases in a stepped pattern, and the fluctuation range of cogging torque and output torque both increase to varying degrees.