High-Frequency Brushless Excitation with Bi-Directional Power Flow for Electrically Excited Synchronous Machines

In traditional brushless excitation systems of electrically excited synchronous machines, a diode rectifier is adopted which converts ac current in the transformer to dc current in the field winding. This is a simple and cost-effective solution. However, the losses of the diodes deteriorate the system efficiency. Besides, only positive voltage can be output from the diode rectifier which means the field winding cannot be de-excited as fast as it is excited. Therefore, in this study, a new topology is proposed, in which two back-to-back H-bridge converters with a common dc-link in between are placed in the rotor circuit. The first converter operates in active-rectification mode which controls the dc-link voltage, while the second one can apply bi-polar voltages across the field winding to guarantee uniform control bandwidth during both excitation and de-excitation. MOSFET switches are adopted in the converters, so the losses are reduced compared with those of the diode rectifier.