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There are essential differences in the design of systems for water/air flow kinetic energy conversion into mechanical energy. First of all, speeds of rotation of the turbine rotors are different: the speed of those which operate in open current does not exceed a few rev/min, for example, flow turbines (water current turbines); hydraulic turbines of low pressure rotate at speeds of tens to hundreds rev/min; small power wind turbines (up to 50 kW) rotate at speeds between 100 and 500 min 1 ; highpower wind turbines -20 to 40 min 1 . However, both systems are classified as low-speed systems having important similarities: the physical processes that occur at the interaction of the turbine rotor with an open water or air current are similar; due to variations in water or air current speed, the systems function under random; the power cubic dependence of water or air current speed, etc. Therefore, studies on electrical generators, which follow below, refer to both conversion systems, hydraulic and wind as well.New terminology appeared in the literature with the exploitation of renewable energy sources: "solar pump," "solar battery", "solar generator" "wind generator", "hydro generator", etc. The use of the adjective nearby traditional designation does not necessarily mean that the principle of operation of a certain component of energy conversion system differs considerably from that known. The purpose is to emphasize some technical and design features of this component, arising from the specific technical requirements imposed.One of the most important technical requirements imposed on all components of the renewable energy conversion systems, in particular, on electric generators is their efficiency. Electricity produced from renewable sources is still expensive because of high initial investment. Efficiency increase of energy conversion is a real way of reducing the investment (to obtain a unit of electricity will require a smaller area of hydro turbine blades or a wind turbine with a smaller diameter, a more robust and easy hub, a lower tower height, etc.).

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