This paper presents a method for selecting a source of comparable electrical power with a test of stability of an autonomous power supply system, using the example of a process of a grain's preliminary cleaning. The factors of violation of stability due to the feature of starting modes of the node of the electric motor load are considered. An assessment of factors that are taken into account in various approaches to determine source of the power supply is given. The calculated dependencies that

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... consider the power ratio of the powered electric motors and electric generator are proposed, regarding the overload capacity of the generator and function of changing voltage value of the generator when a load is applied. The voltage drop in system due to the large starting currents of short-circuited asynchronous electric motors is determined for the given parameters of the drive internal combustion engine, and automatic control of the generator excitation. By checking fulfillment of conditions for stability of the power supply system, the obtained analytical dependencies in relation to determining parameters of an autonomous power supply source for a multi-motor load are solved. The methodical apparatus given here allows the calculation of minimum required power of the power supply source, and determination of the system's performance by checking for stability.