Permanent-Magnet Synchronous Machines with Non-Overlapping Concentrated Windings for Low-Speed Direct-Drive Applications

University dissertation from Stockholm : KTH

Abstract:  Many geared electric drives can benefit from removing the gearbox and driving directly the load at low speed using a permanent-magnet synchronous machine (PMSM). PMSMs with non-overlapping concentrated windings are particularly attractive in low-speed direct drives since they allow, among other advantages, a high number of poles with a limited number of slots. Therefore, this thesis is dedicated to PMSMs with non-overlapping concentrated windings designed for low-speed directdrive applications.First, the design features specific to PMSMs with non-overlapping concentrated windings are presented and illustrated with examples based on finite-element (FE) simulations. Especially, it is shown that the selections of the stator core manufacturing method, the number of winding layers, the combination of pole and slot numbers, and the geometry of the tooth tips are crucial during the design stage of the machine.Second, the benefits of removing the gearbox in a 4.5 kW, 50 rpm mixer used in a waste-water treatment plant are investigated. With its PMSM having buried ferrite magnets and concentrated windings, the designed direct-drive mixer has a higher efficiency than the commercialized geared mixer, but it is also heavier and more expensive due to the larger required housing, shaft and seals. In addition, the cost of the stator core and coil assembly was also higher than expected. Therefore, the difficulties in manufacturing a stator core with concentrated windings are investigated. Mainly due to economical aspects, no prototype motor has been built for the direct-drive mixer.Finally, measurements on a prototype motor at disposal having a large constantpower speed range are performed. Three methods to measure the inductances without using a position sensor are provided and compared. One of the methods is a new test at load conditions to determine the inductance without the knowledge of the rotor position. Results from this method agree well with a conventional blocked rotor test, as well as with results from 3D-FE simulations. Furthermore, the measurements of the magnetic flux obtained with search coils placed in different parts of the stator are analyzed. The analysis allows highlighting the zigzag flux flowing from one magnet to another through a tooth tip, which is characteristic of PMSMs with non-overlapping concentrated windings. It is shown that this zigzag flux leakage causes high iron losses in the tooth tips that represent approximately 50% of the stator iron losses under field weakening operation. Using these measurements, the 3D-FE model of the prototype is also validated thoroughly.