An Analytical Model of a Radially Magnetised Longitudinal-Flux Permanent Magnet Air-Cored Linear Oscillatory Machine

  • Peter Jan Randewijk (Supervisor)

Activity: Examinations and supervisionSupervisor activities


The research presented in this dissertation is focused on the pre-optimisation-design stages of the design optimisation of air-cored radially magnetised longitudinal flux permanent magnet (LFPM) linear oscillatory machines. Electrical linear machine design optimisation involves the determination of the magnetic field, the output force and the inductance as the pre-optimisation parameters. Once the pre-optimisation parameters are solved, the design moves into the optimisation design stage and finally the experimental testing stage. The pre-optimisation stage can be achieved by employing either the finite element method (FEM) or the analytical model method. The pre-optimisation stage is as critical as the optimisation design stage for the final design of the machine to be accurate. While FEM is accurate, the determination of the pre-optimisation design parameters is time consuming and in addition, the commercial packages are expensive. This dissertation therefore focuses on the development of an analytical model to accurately simulate the pre-optimisation design parameters of an air-cored radially magnetised longitudinal flux permanent magnet linear oscillatory machine. The developed analytical model takes into consideration the finite permeability and finite thickness of the yoke materials. This has not been not been done for this kind of machine according to literature. This allows for the capability to monitor the flux concentration in the yokes to determine the saturation levels. The development of the analytical model is approached by firstly developing a model that assumes the machine to be infinitely long, while taking the machine yokes to have a finite permeability and thickness. The model is then extended to model the actual finite length of the machine and its end effects. Again the yokes are considered to have the finite permeability and thickness. The analytical model has the advantage of being much faster than FEM and can be adopted as a cheaper option for the pre-optimisation design stage. The topology of the linear oscillatory machine for which the analytical model has been developed has an air-cored tubular structure machine with surface mounted radially magnetised permanent magnets and with longitudinal flux pattern. The machine is singlephase and is intended to operate at resonant frequency (with free-piston Stirling engines as the prime mover). The analytical model for the presumably infinitely long machine as well as the analytical model for the finite (actual) length of the machine have been validated by the commercial finite element method package MagNet by infolytica inc.. The results for the analytical models correlate well with the results obtained from the FEM package. This indicates that the analytical model can be adopted in the pre-optimisation design stage as a design tool.
Period31 Jan 201513 Jun 2019
ExamineeDickson Kanungwe Chembe
Examination held at
  • Stellenbosch University
Degree of RecognitionInternational