This article presents a simple model for the calculation of core losses in permanent magnet machines based on the total flux linkage of the stator winding. In reality, the total core losses will vary as a function of permanent magnet field and the combination of $d-q$ current components even in the case when different $d-q$ currents produce the same amount of stator winding flux linkage at a given speed. The classical model with constant equivalent core loss resistance predicts constant core losses for constant flux linkage which leads to incorrect results. The model presented in this paper expands the classical core loss resistance based model with a novel additional term which corrects the classical model and calculates an additional core loss component as a function of $d$ axis current, total flux linkage of the stator winding, and two constant parameters. The accuracy of the model is confirmed by finite element simulations and core loss measurements for the cases of an interior permanent magnet traction motor and an industrial surface permanent magnet motor (only simulations).