Analysis and simulation of vector controlled bearingless induction motors
The concept of bearingless motors, which combine both motoring and rotor bearing capabilities, is appealing especially in high speed and high power machine applications. Although extensive research has been carried out on permanent magnet and reluctance types of bearingless motors, studies on the induction motor type are less successful. This thesis addresses the bearingless induction motor based on the concept of dual-pole windings, one controlling the motor torque and the other the generated radial forces. A modelling approach is undertaken to investigate the effect of induction machine design on radial force generation and motor levitation under both steady state and transient conditions. The simulation is based on the dynamic reluctance mesh model embedded in vector control systems for the decoupled control of torque, flux and radial force. This is achieved through modification of a previously developed computer software for modelling induction motors in order to model the control of bearingless induction motors. Both the squirrel cage and wound rotor induction motors are investigated and their suitability for generating controlled bearing relief forces assessed. Vector control schemes for the bearingless cage and wound rotor induction motors were also designed and simulated. A mixed field oriented vector control scheme, which incorporates the simple rotor field orientation for motoring control and an airgap field orientation for rotor levitation control, is introduced and found to be advantageous in bearingless induction motor control. Apart from investigating totally bearingless conditions, the study also investigates bearing relief capabilities for a vector controlled cage and wound rotor induction motor in which the rotor movement is restricted by bearings but with the bearing load cancelled by suitably directed radial force. The effects of real winding topologies, stator and rotor slotting and iron saturation on the performance of bearing relief and bearingless induction motors are also presented. Finally, suggestions for future work is included In order to further investigate bearingless induction motors and its applications.