Rotor cage anomalies and unbalanced magnetic pull in single-phase induction motors. I. Analysis

Cast rotor cages invariably have defects caused by porosity and soldering. Porosity occurs when gas bubbles form in the molten aluminium, producing local areas of high resistance in the cage. Soldering occurs when the oxide layer that forms an insulating barrier between the bar and the lamination iron is breached resulting in current flows from bar to bar vis the lamination iron. Both of these defects result in an out-of-balance radial force that rotates at the same speed as the rotor. This out-of-balance loading (or unbalanced magnetic pull) may produce wear on the bearings. Both forms of defect will always be present in all cast rotors to a greater or lesser extent, depending on a wide range of design and manufacturing attributes. The paper presents a detailed development of a mathematical model which can be used to examine both porosity and soldering defects in cage rotors. The model is based on the representation of the rotor cage as a set of current loops. Each rotor bar is notionally divided into segments, and adjacent bars are joined by inter-bar impedances at regular interval along their length. The resistance of each bar segment and each inter-bar impedance can be varied independently to simulate a local area of high resistance in a bar, or a region of bar-to-iron soldering. The rotor loop currents are calculated via harmonic impedances, and this leads to the calculation of radial forces. The work is supported by a practical and theoretical investigation which is described in a companion paper