A process for permanent magnet (PM) demagnetization control for optimal and reliable flux weakening control in permanent magnet synchronous machines (PMSM) is presented. A physics-based model was developed to estimate the magnetic flux operating point of the PM poles during the operation of PMSM. The model is enabled to dynamically estimate the average and the partial PM demagnetization due to reverse armature field as well as the PM temperature rise. The model is a function of physical geometry and material of the PMSM, physics of demagnetization and the ambient temperature. The demagnetization assessment and control is verified on a V-type PMSM using the modified FE-based coupled thermal-field-circuit phase variable model and the time step FE analysis.