The insulation of the electric machine to a large extent determines its overall reliability. This necessitates the development of more accurate methods of evaluation of the motor thermal state than the indirect techniques used in most cases in practice. We obtain expressions for the comparison of different modes of operation of electric motors under the condition of equal thermal insulation deterioration. The results of the calculation the change of the residual thermal insulation resource stator induction motors when operating in S3 and S6 modes with different duration of the cycle are presented.