Optimal electromagnetic-thermal design of a seven-phase induction motor for high-power speed-control applications

Induction motors have been traditionally used in industrial applications ranging from a fraction of horse-power up to several Megawatts due to their substantial benefits. Induction drives with more than three-phases are superior to the 3-phase induction drives in terms of overall-volume, torque fluctuations, current passing each stator-winding, ohmic-loss, efficiency and reliability in the case of stator-windings open-circuit fault. These benefits are especially more attractive in variable speed drivers due to the reduced capacity of power-electronics switches. This paper aims to develop an optimal electromagnetic-thermal design procedure of a high-power seven-phase induction motor suitable for variable-speed applications. In this multi-objective design approach, the objective function is defined aiming to increase the efficiency, power-factor, power-to-weight ratio, starting-torque as well as decrease the starting-current. Furthermore, the electrical, mechanical, dimensional, magnetic and thermal limitations are included in this optimization study in order to ensure practical realization of the designed machine. The coupled-circuit method is employed for nonlinear electromagnetic modeling while the current displacement phenomenon is considered in rotor parameters calculations. A lumped-parameter-thermal model is established for calculating heat rises of different parts in each iteration of optimization study. Finally, the performance characteristics of the optimally designed 1-MW 4-pole motor are verified using 2D-FE analyses.