ABSTRACT

In this work, the modelling and performance analysis of six-phase asynchronous motor under asymmetrical faults based on finite element analysis (FEA) is prioritised. Initially, an experimental test on a three-phase squirrel cage induction motor is performed to provide validation and enable the extraction of necessary machine data to be used in the study. From the extracted machine data, the three-phase induction motor is first modelled and analysed in FEA for proof of concept, after which the same motor is remodeled in six-phase to carry out the analysis under different asymmetrical fault conditions, in both single and double layer winding configurations. For the laboratory experiments, no-load, blocked rotor, load and retardation tests were carried out on a three-phase single layer winding induction motor prototype, and compared against FEA simulation results in ANSYS Maxwell 2D software. The use of a three-phase motor prototype is due to, among other things, the absence of a six-phase drive for operating a six-phase motor prototype. A good agreement between the test three-phase motor and the simulated three-phase motor is obtained in terms of the core loss which were measured at 57 W and 56.6 W, between simulation and experiments, respectively. Based on the comparison done between the three-phase and six-phase single layer winding motors in transient FEA, the percentage overshoot in speed and core loss are 4.95% and 3.72% for speed, 14.04% and 5.74% for core loss, respectively. The reduced percentage overshoot in speed and core loss at transient of the six-phase single layer winding motor is an indication of the six-phase single layer winding motor, consuming less energy than the three-phase single layer winding motor.  On the other hand, the double layer winding layout of three-phase and six-phase motors when compared, gave respectively 4.73% and 3.79% overshoot in speed, and 14.02% and 4.45% overshoot in core loss, under transient performance condition. Again, the lower percentage overshoot on the six-phase motor is an indication of six-phase motor taking less energy at transient than the three-phase motor. Comparison of the six-phase motor in both single and double winding layer configurations, in term of percentage torque ripples, reduction in speed and reduction in core loss, under asymmetrical faults conditions of loss of phase A excitation voltage, indicates that 47.3%, 1.43% and 18.08% for single layer winding as against 3.42%, 1.31% and 18.08% for double layer winding. In terms of implementing the faults on phases A and B voltage excitations, the single layer winding six-phase motor recorded  38.9%, 4.16% and 8.48% while its double layer winding counterpart had 39.96%, 7.68% and 44.86% for torque ripple, speed reduction and core loss reduction, respectively. While implementing the fault analysis at phases A and D, the single layer winding had 32.1%, 7.53% and 8.48% as against 78.37%, 7.69% and 46.98% for torque ripple, speed reduction and core loss reduction, respectively. The performance analysis under these different fault conditions show that six-phase induction motor in both winding configurations displayed better functional capability despite loss of phases unlike the equivalent three-phase motor that malfunctions and stops operation due to loss of phase excitation. To this end, six-phase squirrel cage induction motor, investigated and compared with its equivalent three-phase motor in this work, is found suitable for use in critical operations such as submarine, due to its better performance at transient and fault tolerant capabilty.