Optimization of rotary transformer for high-speed applications

In grinding applications the processing of hard ceramics can be improved, if an ultrasonic axial vibration is superimposed to the rotation of the grinding tool in the grinding spindle head. The ultrasonic vibration is generated with a piezoelectric transducer which is attached to the rotating part of the spindle. Usually, the electrical energy is transferred with a rotary transformer consisting of a rotating secondary and stationary primary winding. There, the design of the rotary transformer is very crucial, since the available space in the spindle is limited and the cooling of the transformer is difficult. In addition, for high-speed applications the mechanical stresses on the transformer are high. Therefore, in this paper an optimization procedure of the rotary transformer with respect to minimum losses is presented, where magnetic, electrical and mechanical constraints are considered. In general, for a robust operation of the ultrasonic system an LLCC-filter is inserted between the piezoelectric transducer and the power converter, which makes the system insensitive to thermal and mechanical parameter variations. In order to keep the system complexity low, the needed LLCC-filter is magnetically integrated into the rotary transformer. In the literature, typically the components of the LLCC-filter are selected in such a way that the resonances of the LLCC-filter are matched to the mechanical resonant frequency of the piezoelectric transducer. This filter design method, however, doesn't result in minimum transformer losses. Therefore, in this paper also a new design method is proposed, which results in minimum transformer losses while keeping the ultrasonic system behavior stable and the control scheme simple. With a built prototype the proposed optimization procedure is experimentally verified based on a calorimetric measurement setup.