The article examines the validity of the assumptions made in the derivation of the relations for average losses method used to evaluate the thermal state of the induction motor. Defined the boundaries of applicability of this method. Average losses method is based on the assumption that the average rate of insulation aging linearly related to the average winding temperature. At the same time, the dynamics of the temperature of the stator winding is substantially different from the dynamics of the average motor temperature. Therefore, in the intermittent mode oscillations of stator winding temperature can go far beyond the boundaries in which the definition of the average rate of insulation aging based on the average temperature remains valid. The article analyzes the dependences establishing the ratio between the average rate per cycle of insulation aging and aging at an average rate per cycle temperature for the simple intermittent mode cycle, including the period of work and pause. On the basis of these relations defined the area for such combinations of relative operation time and cycle time at which will be the correct use of the average losses method, since in this area quite accurately performed assumptions underlying this method. However, it is shown that there is a large area with such combinations of relative operation time and cycle time under which the average losses method gives an estimation of the motor thermal state with a substantial error. The charts of the area of applicability of the average losses method in the plane with coordinates «relative operation time» and «cycle time» are shown. Defined combination of motor parameters, which determines the position of the boundaries of the area of applicability in these coordinates. Gives the values of this parameter combinations for asynchronous motors with insulation class B and F.