Efficiency-based compensations and the mechanical load dependencies of rotary transformer for rotary ultrasonic machining applications

A new rotary ultrasonic machining (RUM) spindle is proposed. It adopts contactless rotary transformer replacing the well-established slip ring technology to supply power for the transducer. Owing to the large leakage inductances, the capacitive piezoelectric transducer and the variable mechanical load, the circuit compensation is crucial for efficient and reliable power transfer. In this study, the mathematical models are presented to identify the power transfer efficiency and capability of the rotary transformer used in RUM. A general optimisation method of compensation for maximum transfer efficiency is proposed. The mechanical load dependencies of the transfer efficiency, transfer capability and power factor are researched and discussed. The efficiency has been found to be dependent of the secondary compensation elements. The series-series (SS) topology is applicable to a wide range of mechanical load variations, followed by parallel-series (PS) topology. For a varying load in RUM, the contactless energy transfer is possible to achieve high efficiency, high power factor and appropriate output power that adapts to load variations.