Performance Comparison of Bearingless Motor Topologies in Exterior Rotor Construction

This paper presents several feasible slot/pole combinations for a bearingless motor in exterior rotor construction. Due to the limited available stator space, the slot number of the stator has to be chosen small in order to provide sufficient winding space. The characteristics of each possible topology are discussed in great detail and for each setup the optimal design parameters are derived using 3D-FEM analysis. In the end, a fair comparison between the presented topologies is undertaken and the optimal motor topology is implemented in a real-size prototype setup. Introduction For the chemical, pharmaceutical and biotechnological industry sector, the qualitative refinement of high-purity mixing is a mandatory prerequisite to improve both research and production processes. State-of-the art mixing systems either use magnetic couplings or a long shaft passing through a seal to transmit the rotation energy from an outer motor into the process tank. With a bearingless motor [1-8], the common drawbacks of additional bearings inside the process tank and of pinch-off areas due to mechanical contact can be eliminated. Moreover, due to the large possible air gap, clean-in-place and sterilization-in-place processes [9] are facilitated. The bearingless motor is completely free of wear as well as free of lubrication and thus promises low maintenance cost and long life time. The bearingless motor in exterior rotor construction can be advantageously employed for high-purity mixing when only the rotor is placed inside the process tank, whereas the stator and all control and power electronics are located outside, separated by the tank wall (cf. Fig. 1). The impeller can then be mounted directly onto the rotor, which levitates and rotates inside the tank in a contactless manner. With this construction type, flow-low zones are avoided and complete tank drainage through the bottom outlet is not disturbed. Moreover, high mixing torque can be produced with a compact setup. Tank bottom Tank outlet Stator Tank wall Rotor with impeller (levitated) Air gap Fig. 1: Open view of the tank bottom. The stator is placed inside an indentation reaching into the tank bottom, but still outside of the tank wall. Only the rotor with the impeller blades is then mounted inside the process liquid and is levitated around the stator. This paper will focus on different exterior rotor bearingless motor topology possibilities. In a first step, three feasible topologies are discussed and compared analytically. In a second step, their torque and bearing performances are compared based on a 3D-FEM analysis. The goal is to come up with high-torque motors that show stable bearing behavior under working conditions. In the end, the simulation results of the most promising topology are verified with the performance of prototype setup. Exterior rotor bearingless motor topologies The most characteristic topology differences for bearingless motors result from the choice of the rotor pole pair number p and the stator slot number q. Once this choice is made, mainly geometrical variations influence the final design of each motor. For the exterior rotor construction type, the outer rotor diameter (which usually results from the required tank volume and the specific application) limits the motor size in the radial directions. Therefore, the stator space will be very limited since the whole stator iron and the windings need to be placed inside the hollow rotor ring. For this reason, only stator configurations with small slot numbers can be considered because the ratio of iron material to winding material would become unfavorably large otherwise. The proposed topologies are assembled with combined concentrated windings [10], so that there is one separate coil per stator tooth and it contributes to the generation of both torque and bearing forces. The required drive and bearing currents can be digitally controlled independent of each other and will then be superimposed mathematically prior to applying them to the stator coils. The rotor of the bearingless motor consists of a back iron ring and of permanent magnets which are magnetized in radial direction in alternating order. With a very small slot number of three, a permanent magnet motor could be built. However, it is not possible to come up with a bearingless motor with this design, because drive and bearing would influence each other so that they cannot be controlled independently. The four-slot/twelve-pole motor from Fig. 2(a) consists of a two-phase bearing combined with a single-phase drive. It can be controlled with an inverter consisting of four full-bridges,