Rotary transformer with ferrite core for brushless excitation of synchronous machines

Abstract

Salient-pole synchronous machines (SPSYM) combine a high power density with high efficiency over a wide speed range. In contrast to permanent magnet synchronous machines (PMSM), the field excitation can be controlled actively. Due to the adjustable excitation, flux-weakening is possible with reduced losses. The efficiency at partial load and at high speed can be improved. Furthermore, faults, such as short circuit currents, are easier to handle. These attributes qualify SPSYM as a promising alternative to PMSM for electrically powered vehicles. In order to achieve this additional degree of freedom, electrical power needs to be transferred to the rotating part of the motor. Established slip ring transmission systems are sensitive to ambient conditions such as humidity or impure contacts. Moreover, the limited reliability of contact based systems amplifies the desire to develop contactless transmission systems. In this paper, the electromagnetic design of a rotary transformer with a ferrite core intended to transfer the excitation current is discussed. High frequency effects (skin- and proximity effect), leakage inductances and the waveform of the transformer currents are considered in an automated design process. This process combines analytical calculations and 2D FEM simulations to determine an optimal design. In a final step, the calculated results are compared with measurements performed on a prototype.