In some high performance applications it is desired to suspend a motor without mechanical bearings. Some reasons for such a requirement would be high speed capability, lack of access to perform maintenance, or the motor is in an environment which poses a difficulty for conventional bearings, such as extreme temperatures and pressures.
While separate magnetic bearings and motors can be used in these high performance applications, they suffer some drawbacks. First they increase the axial length of the rotor they support. Secondly using separate systems increases the weight of the machine, because each function has to be sized for their required peak power. Finally separate systems lead to more parts, which increases cost and complexity. It is these issues that have driven the need for a bearingless motor. A bearingless motor is a motor which is capable of producing lateral forces which are used to suspend the rotor.
Currently bearingless motors have separate windings for motoring and magnetic levitation. These motors fully utilize their common stator iron, however winding space is still dedicated to either the levitation or rotation functions. Furthermore these motors are capable of only providing forces in one radial plane, in the case of radial gap motors, or one axial direction, in the case of axial gap motors.
The motors, conceived and presented in this thesis, are wound without internally connecting the pole-pairs. Force is controlled by varying rotor reference frame d-axis current to each pole-pair. Therefore all of the windings can produce torque or levitation force, depending on need. Furthermore the conical shape of the motor allows forces to be created in both radial and axial directions. Therefore a pair of these motors allows full 5-axis levitation.
This thesis presents the theory, simulation and lab results of a fully levitated rotor by two of these conical bearingless motors. The prototype motors presented in this thesis are sized for a high speed flywheel energy storage application. These bearingless motors are simulated with magnetic circuit models, Matlab Simulink control models, and finite element analysis. Measurements taken on the prototype system verify the theory and simulation results.