Loss Reduction in Axial Flux Machines using Magnetic Shielding

Abstract

Abstract– The introduction of compacted insulated iron powder in electrical machines design makes their manufacturing process easy together with high rates of production and the machine parts made from it are stable dimensionally compared to conventional laminated steel. The research work presented in this thesis was carried out with the main aim to improve the overall performance of a three-phase Axial Flux Machine (AFM) using Soft Magnetic Composite (SMC). To realise it, the machine was redesigned in a way to benefit from the unique properties of the material such as low eddy current loss at high frequency, isotropic magnetic properties and simple manufacturing process. Due to the three-dimensional (3D) nature of the SMC material and AFM structure, 3D Finite Element Analysis (FEA) was carried out for accurate prediction of performance and extensive simulation results were provided. Higher fill factor up to 70% was achieve by compacting the pre-formed coils on a bobbin before sliding onto the tooth for final assembly, which offered a significant improvement in performance. AC winding loss analysis was performed due to open-slot stator winding configuration and the higher frequency of operation resulting in skin-depths of the same order of size as the typical conductor diameters. A method of AC winding loss reduction was introduced using a single steel lamination sheet to shield the windings from stray fields due to the open-slot stator construction which encourage an elevated AC loss at AC operation. Moreover, this approach is easy to implement for this machine topology and does not require the use of more complex twisted and Litz type conductors. To validate the 3D FEA, a prototype machine was built which ultimately resulted in 6 machines being tested without and with steel lamination sheet during this PhD. The measured result which includes the back EMF, full load voltage, torque, power and losses are thoroughly presented and agreed with the 3D FEA very well. Depending on lamination type, it is shown that the AC winding loss reduced by up to 48.0%, total loss reduced by up to 31.7%, this method has disadvantages of minor reduction of up to 3.5%, 5.8% and 2.8% in the peak back EMF, torque and output power respectively. The efficiency has increased by up to 10.3%. The research studies signify the viability of designing and producing a highly efficient AFM with SMC and has the potential for mass production, this thesis makes significant contribution by implementing a simple novel method for AC winding loss reduction using steel lamination sheet to shield the stray flux due to open-slot stator winding construction.