Analysis of Circulating Current Loss for High-Speed Permanent Magnet Motor

High-speed permanent magnet motors, which are characterized by high power density, are used widely in high-speed centrifugal machinery. The magnetic gap is huge in high-speed permanent magnet motors compared with lower speed counterparts. Therefore, the leakage flux in stator slot can dominate the total flux generated by the winding. With the increase of the rated power of high-speed permanent magnet motors, the area that one coil occupies in one slot increases. Because of different positions of strands in one coil, the inductance of each strand differs, sometimes greatly. The impedance unbalance between the strands results in unbalanced distribution of current in strands, which leads to more winding loss and uneven temperature rise in winding. In this paper, following the calculating inductance of each strand, the currents in each strand are derived from loop current equations. The factors affecting the circulating currents are also investigated by a simplified slot topology and winding parameter. Measures to inhibit the circulating current are also proposed to decrease the circulating current loss. Finally, two measurements of ac resistance are introduced.