A typical way for obtaining the characteristics of the electric machines are the methods based on the theory of the magnetic circuits. These methods contain assumptions that lead to significant errors in the calculation results. Modern software allows to perform calculations of the magnetic fields using the numerical methods, and, it is thus possible to obtain more adequate characteristics of the electric machines, which are indicative of their working properties in operation. Hence, the aim of this study is to describe the method of obtaining the characteristics of a high power turbogenerator by the calculation of its magnetic fields. The proposed methods for determining the characteristics of the electric machine under study are based on the iterative methods using the automated numerical calculations of the magnetic fields. The calculations were carried out using the FEMM software finite element method and the Lua script. The paper presents characteristics of turbogenerator such as the idle speed and short circuit, angular, stator winding excitation, adjusting and U-shaped. The adequacy of the calculations was checked by comparing the obtained characteristics with those calculated by classical methods. The most important results of the work are the obtained differences in determining the characteristics by numerical calculation and calculations using the theory of magnetic circuits. Their significance lies in the fact that the modernization of turbogenerators often takes an increase in their power by an amount commensurate with the error by using the classical method of obtaining characteristics based on the theory of magnetic circuits.