Towards high-speed bearingless disk drives

Bearingless disk drives unite the functionality of magnetic bearings with electrical drives in a highly compact form with reduced mechanical complexity. The rotor field of a permanent magnet synchronous machine is used for passive stabilization in axial and tilt directions and, additionally, serves for the active creation of radial forces. This special magnetic levitation topology is being used in high-purity pumps in the pharmaceutical, medical, and the semiconductor industry which operate at low speeds. The research activities leading to this work proposed the use of bearingless disk drives for high rotational speeds for the first time. Many aspects of high-speed operation such as increased winding and stator core losses, or the special challenges in mechanical rotor design due to the high centrifugal forces are, therefore, being considered in this work. Equally, the ability to provide sufficient active radial forces and drive torque independently in an efficient fashion is a key specification for the bearingless operation. The design process includes the definition of suitable criteria which reflect the abovementioned drive properties and demonstrates the advantageous application of multiobjective numerical optimization to the problem. Special attention is also given to the problem of tilting motion in a dedicated chapter since many possible applications involve high-speed impellers or turbines with tight housing tolerances. As tilt-triggered gyroscopic whirl can lead to the destruction of the system, a novel electrodynamic stabilization coil is proposed and documented based on a corresponding experiment. As a result of the design process, several prototypes have been constructed, each realizing either a 5-phase double coil or a 6-phase single coil toroid winding concept using litz-wire and a low-loss slotless stator core. The prototype rotors comprise diametrically magnetized, ring-shaped rotors with high-strength support bandages and have a total outer rotor diameter between 30mm and 32mm. The developed radial position and rotational speed control loop with an underlying current controller features several non-linear extensions for reducing either losses, undesired rotor motion, or computation time. Different experiments for demonstrating the operational behavior, including rotor orbits during run-up and input power measurements have been conducted using this control scheme which was implemented at a standard DSP which controls a 10 half-bridge power electronic module. The maximum rotational speed reached during these experiments was 115 000 rpm which corresponds to a surface speed of 192.6m/s. To the knowledge of the author, this surface speed constitutes a world record for magnetically levitated disk drives. A detailed analysis of the losses provides analytic models for the iron losses, the eddy current winding losses, and the windage losses. Existing models have been matched to the loss measurements conducted at a customized test rig in order to allow an extrapolation of the expected losses at a speed of 200 000 rpm.