Detection of the Tuned Point of a Fixed-Frequency LCL Resonant Power Supply

Inductor-capacitor-inductor (LCL) series-parallel resonant networks are commonly employed in inductive power transfer systems as they allow a voltage source inverter to generate a constant ac track current independent of the loading, while also allowing operation at unity power factor. To function correctly the LCL network must be tuned properly since any deviation in the track inductor will cause an unwanted reactive load to be placed on the inverter. This letter demonstrates how a simple frequency sweep at power-ON (at reduced output voltage) can be employed to identify and protect the power supply from track tuning errors that can easily arise at installation.