Brushless excitation system for a class of variable-speed synchronous motors

A new brushless excitation system is described for a synchronous motor such as may be used in some forms of variable-frequency inverter-fed drive with rotor-angle control. Transformer primary and secondary windings are incorporated in the stator and rotor, respectively, and the latter winding feeds the field winding via a bridge-rectifier circuit. The transformer primary winding is connected in a series-resonant self-commutated thyristor inverter circuit and the mode of operation of this system is explained. Variation of the firing rate of the inverter, below its maximum operating frequency, controls the excitation current. A special arrangement of two transformer secondary windings is shown by tests to provide rotor excitation that is independent of angular position of the rotor and of its rotational speed. The scheme has unusual operating conditions, particularly in respect of rectifier conduction period and waveforms, and these are explained by both analysis and experiment.