Self excitation of a variable-speed constant-frequency machine

Consider a machine having a 3-phase stator and a wound rotor with commutator and brushes; the brushes rotate at a constant speed. The rotor is driven at a variable speed and fed from the stator through a transformer and a rectifier. This machine, thus excited through the rotor by a direct voltage, produces an alternating stator voltage of constant frequency which is independent of the rotor speed. This could be applied to electric-power generation using the wind energy. It is shown here under which conditions this machine charged with an inductive load becomes self-excited. Equations in ¿¿ co-ordinates for the machine and the load are derived. A linear mathematical model for the system is obtained and a known stability criterion is applied. Conclusions are: (a) it is not necessary to use capacitors for self-exciting the machine; (b) it is not necessary, but it is convenient, to work at velocities higher than synchronous; (c) for each relatively high velocity, especially if it is higher than synchronous, there is a range of the feedback gain which makes the system unstable; this range is greater at higher velocities; (d) the self excitation is more easily accomplished if higher frequencies are used. Experimental results, obtained in the laboratory within the limitations of the equipment available, are also presented. They show the same tendencies, thus checking, in general, these conclusions.