Axial field permanent magnet machines with high overload capability for transient actuation applications

This thesis describes the design, construction and testing of an axial field permanent magnet machine for an aero-engine variable guide vane actuation system. The electrical machine is used in combination with a leadscrew unit that results in a minimum torque specification of 50Nm up to a maximum speed of 500rpm. The combination of the geometry of the space envelope available and the modest maximum speed lends itself to the consideration of an axial field permanent magnet machines.
The relative merits of three topologies of double-sided permanent magnet axial field machines are discussed, viz. a slotless toroidal wound machine, a slotted toroidal machine and a yokeless axial field machine with separate tooth modules. Representative designs are established and analysed with three-dimensional finite element method, each of these 3 topologies are established on the basis of a transient winding current density of 30A/mm2. Having established three designs and compared their performance at the rated 50Nm point, further overload capability is compared in which the merits of the slotless machine is illustrated. Specifically, this type of axial field machine retains a linear torque versus current characteristic up to higher torques than the other two topologies, which are increasingly affected by magnetic saturation.
Having selected a slotless machine as the preferred design, further design optimization was performed, including detailed assessment of transient performance. A key feature of this design is the use of a solid (i.e. non-laminated) toroidal stator core. This provides a stator with increased mechanical robustness, improved heat transfer and a ready means of incorporating fixing points into the core. However, these advantages are gained at the expense of a significant eddy currents in the stator core. A series of three-dimensional, magneto-dynamic finite element simulations were performed. Although the resulting eddy current losses are excessive for continuous operation, the reduction in transient performance which results from the eddy currents is shown to be manageable. The loss analysis is supplemented by transient three-dimensional finite element thermal modelling. Three-dimensional mechanical analysis is performed in combination with analytical equation to analyse the stator and rotor plate deflection subject to axial attractive force.

The construction of a prototype double-sided axial field machine is described in this thesis which contains several interesting design features including a profiled rotor core to reduce mass, radially magnetised rotor magnets to produce torque from the axially oriented conductors on the inner edge of the toroidal winding. The testing of the machine is performed under a series of load points up to 75Nm to validate the predicted torque versus current density characteristics.