Strategies to improve the performance of three-phase induction motor driven systems

Abstract

Electric motor driven systems (EMODS) account for over 40% of global electrical energy demand, i.e., 7400 TWh per year, and for about 70% of the demand for industrial electrical energy, being by far the most important electrical load. Furthermore, EMODS offer today large efficiency improvement potential, on average within 20% to 30%. If the energy saving potential associated with EMODS becomes effective, the consumption of a huge amount of fossil fuels can be avoided. The greenhouse gas emissions can significantly be reduced at zero or even negative costs, since the efficiency improvement measures in EMODS are, in general, cost-effective. Therefore, EMODS can play a key role in helping many countries’ efforts to meet the post-Kyoto targets. This can strongly contribute to combat climate change and fossil fuel dependency of modern economies, providing that, at the same time, other measures are implemented, namely the promotion of renewable energies. If all EMODS were optimized, energy cost savings could reach 65-100 G€ per year worldwide and a huge amount of greenhouse gas emissions could be avoided. Nowadays, in the scope of world energy issues, the importance of the performance improvement of EMODS is evident. However, in most cases, the investment in energy efficiency is not yet a major concern in new plants or during refurbishment. This thesis is meant to be a contribution on that scope, offering several comprehensive overviews and novel contributions on different EMODS-related topics. Briefly, recent developments and strategies to improve the performance of EMODS integrating three-phase induction motors, which account for the largest majority of the industrial electric motors, are addressed, including technical, economical, and policy aspects. The focus is on the efficiency and reliability of EMODS, being discussed several related topics such as motor performance, variable-speed drives, power quality, mechanical transmission, and system design. The end-use or mechanical-load devices are not analysed in terms of intrinsic performance, although the speed variation impact on the respective required power is taken into account. Motor efficiency and life-cycle cost related aspects are analysed, and information regarding motor standards, eco-design and market transformation, is presented. The impact of motor speed variation and inverters on EMODS efficiency and reliability is also addressed, including a comparison between 2-level and 3-level voltage-source inverters. Regarding three-phase induction motors, useful methods for in-field motor load estimation are analysed, being proposed a number of improvements in some well-known methods. Novel considerations on stator winding specifications and connectionmode change, as a function of the motor actual operating conditions, for both two-connection (delta or star) and multi-connection motors, are presented, including the discussion of theoretical, simulation (motor models), and implementation issues. Regarding the stator winding connection-mode management, besides the general proposed methodology, it is also proposed a novel electronic device for that purpose. Methodologies for stator winding optimization and/or customization, particularly useful for the rewinding process, including a tutorial and a user-friendly stator winding redesign software to help rewinders to improve motor performance for each particular situation, are proposed. Due to its present relevance, power quality impact on line- and inverter-fed motors is also discussed. A number of considerations on motor and EMODS reliability, including a comprehensive and extensive analysis on bearing currents and on voltage transients associated with inverter-fed motors, are presented. The most important contributions were previously published in national and international journals and/or conference proceedings. Some of the topics addressed in this thesis are still under research and future publications on them are expected.