Characterization of induction motors in adjustable-speed drives using a time-stepping coupled finite-element state-space method including experimental validation

This paper describes how a comprehensive time-stepping coupled finite-element phase-flu-linkage-based state-space modeling approach can be used for characterization of induction motors in adjustable-speed drives. The model implemented the faster indirect and iterative coupling approach based on the curl-curl nondiffusion equation rather than the direct coupling approach, which is based on the curl-curl diffusion equation. The model presented in this paper is capable of rigorously modeling the effects of magnetic nonlinearities and space harmonics due to the machine magnetic circuits' topology and winding layouts, time harmonics resulting from electronic switching of inverters, as well as the synergistic interaction between these time and space harmonics. This includes the presentation of a unique approach for the calculation of electromagnetic torque by means of a concept of energy balance computed from instantaneous magnetic field solutions calculated at each time sample in an ac cycle. This approach reveals higher content of torque ripples than given by more conventional approaches to torque calculations. The results of motor drive performance simulations and corresponding test results are shown to correlate well here with current waveforms and torque averages. This includes sinusoidal excitation, and six-switch inverter excitation with pulsewidth modulation switching pattern, respectively, The analytical/computationaI soundness, robustness, and capability for design assessments and modifications are detailed in a companion paper.