Induction Motors With Rotor Helical Motion
Performance analysis of the twin-armature rotary-linear induction motor, a type of motor with two degrees of mechanical freedom, is the subject of this dissertation. The stator consists of a rotary armature and linear armature placed aside one another. Both armatures have a common rotor which can be either solid or cage rotor. The rotor can move rotary, linearly or with helical motion. The linear motion generates dynamic end effect on both linear and rotary armature. Modeling such an effect in rotary armature is a significant challenge as it requires a solution considering motion with two degrees of mechanical freedom. In other words, the rotor in the model would have to move between two space coordinates (rotary direction as a regular operation of rotary armature and axial direction). Neither of the available FEM software package is currently capable to solve such a problem. The approach used to address linear motion on rotary armature is based on the combination of transient time-stepping finite element model and frequency domain slip frequency technique. An equivalent circuit model is also developed to address helical motion in rotary armature that is based on the model suggested by Duncan to consider dynamic end effects in linear motors. The results obtained from FEM modeling and equivalent circuit model are partially verified by test carried out on experimental model of the motor to validate the theoretical modeling of the motor. FEM performance analysis of the linear armature is rather straightforward as it only involves motion in one direction similar to a conventional linear induction motor. However, due to the finite core length and open magnetic circuit in the direction of motion, the back electromotive forces induced in the three-phase winding are asymmetric and the air gap flux density distribution can still be distorted, even at zero axial speed. This distortion is known as static end effect. Such an effect does not exist in rotary armature. New viewpoint on the end effects in linear armature, which classifies them into two groups, namely as speed-independent (static) and speed-dependent (dynamic) ones, is presented. Static end effect is modelled by quasi-static finite element analysis coupled with equivalent circuit via the lumped parameters. The analysis of the motor with the combination of static and dynamic end effects is done with time-stepping finite element analysis. The approach makes the contribution of each type of the end effect on the performance of the linear motor more visible, than it is possible within approaches presented to this date. Based on this classification, a novel equivalent circuit steady-state model for linear induction motors is also presented. The novelty resides in classification and inclusion of end effects in the equivalent circuit. Duncan model is used to address dynamic end effect and appropriately modified to account for the saturation of back iron and skin effect. The static end effect, which manifests itself by the alternating field component, is represented by additional circuit branch similar to the ones of motor with alternating magnetic field. Each type of the end effects is modeled and included separately. The results regarding determination of basic performance characteristics are compared with those obtained from finite element simulation. The proposed equivalent circuit is more accurate than the conventional equivalent circuit available in literature by considering all major phenomena in linear motors such as static end effects, dynamic end effect and skin effect. Finally, the performance of the motor with solid layer is compared with the one with cage rotor. It is concluded that, in general, motor with the cage rotor has the better performance compared to the solid double layer rotor.
End Effects, Equivalent Circuit, Finite Element, Induction Motors
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