Eddy Current Losses Analysis and Optimization Design of Litz-Wire Windings for Air-Core Compulsators

Compulsators are popular choices for high-end railgun power supplies. In order to increase the power density, air-core compulsators are proposed by using composite materials instead of ferromagnetic materials. Compared with the conventional iron-core machines, armature conductors of the air-core machines are not surrounded by the ferromagnetic teeth, resulting in the conductors been exposed to highly dense revolving magnetic field, which causes eddy current losses in the conductor strands. On the other hand, high-frequency currents flow through the armature winding during the self-excitation and discharge process, which also causes skin effect and proximity effect losses. Therefore, an effective winding design is essential to ensure proper current density distribution and eddy current losses of armature conductors to avoid local overheating. This paper deduced the calculation model of eddy current losses of a single conductor under alternating magnetic fields and then established the eddy losses calculation model of the air-core compulsator's armature winding. The expression indicates that the eddy current losses of a conductor are determined by both the radial and tangential components of the flux density, which can be obtained by finite-element analysis. Based on the calculation model, the performance of two particular armature designs (a solid conductor and a compacted type-8 litz wire with the same area) are analyzed and discussed. The conductor eddy current losses are estimated and compared for both designs. Finally, an optimization study is carried out to find the best choice of strand diameter to minimize the total losses.