When calculating the thermally unsteady state of electric machines, apart of the parameter on which depends the heating of machines in the thermally steady state there is still a parameter related to the thermal inertia of the individual parts of the machine. The aim of the paper is an analysis of the influence of the parameters affecting the thermal variations in the machine. The most convenient for this analysis is the assumption that the machine consists of two thermally inert elements because this assumption allows to perform the analysis analytically. The conclusions of the analysis also concern the machine treated as a system composed of many thermally inert elements. As a result of the analysis a view has been obtained what role is played by time constants and what by thermal coefficients in heating and cooling of machines. It was shown that the time constants depend on that whether the machine rotates or not.