Brushless AC field system for stable frequency variable speed alternators

摘要

An AC generator whose field winding (4) on the rotor is excited by AC so that the resultant speed of the rotating field is the algebraic sum of the rotor speed and the speed of the field relative to the rotor itself. This makes it possible to generate AC at a frequency different from that of conventional DC excited generators by the appropriate choice of the frequency of the AC input to the rotor field circuit. It enables the stabilization of the output frequency of the alternator when speed changes occur, by the adjustment of the AC frequency of the rotor input. The AC fed to the rotor is from an auxiliary winding (3) which may be housed on the rotor itself, thereby eliminating the need for any brushes or slip rings. The AC is induced in the auxiliary rotor winding by having an auxiliary winding on the stator (2), distinct from the main stator winding (1). The input to the stator auxiliary winding may be from an inverter (5) whose frequency may be adjustable. By varying the frequency of the inverter output, the frequency of the AC fed to the main rotor field winding may be varied to achieve the desired output frequency of the generator or to stabilize the generator frequency when speed changes occur. Voltage control of the alternator may be implemented by adjustment of the inverter output voltage. The need for a separate exciter machine may be avoided by housing the main and auxiliary winding in the generator itself By the appropriate choice of different pole numbers for the main and auxiliary windings, these two sets of windings can function independently without magnetic coupling, making the machine both exciterless and brushless The restriction on the choice of pole numbers will not apply if the auxiliary windings are housed in a separate exciter machine, in which case it becomes a brushless machine with exciter.