The precise estimation of iron loss is the foundation for thermal field analysis and efficiency optimization of electrical machines. In the inverter-fed motor system, the present methods are difficult to realize an accurate calculation of the motor iron loss. In order to improve the accuracy, a modified loss surface model is proposed, and an iron loss calculation procedure based on the loss surface model and finite element analysis is presented. A ring specimen is introduced instead of the Epstein frame to develop the loss surface model. In order to verify the performance of the developed loss surface model, the hysteresis modeling results with harmonics are compared with the measured ones on a square specimen. Furthermore, the proposed iron loss calculation method is applied to the loss estimation of an inverter-fed interior permanent magnet synchronous motor. A post-processing package is developed to compute the iron loss of the interior permanent magnet synchronous motor. Finally, an experiment platform is constructed to measure the iron loss of the interior permanent magnet synchronous motor. The feasibility and accuracy of the proposed method are verified by comparing the calculated results with experimental results.