New Trends in Efficiency Optimization of Induction Motor Drives

Scientific considerations presented in this paper are related to the methods for power loss minimization in induction motor drives. The induction motor is without doubt the most used electrical motor and a great energy consumer. Three-phase induction motors consume 60% of industrial electricity and it takes considerable efforts to improve their efficiency (Vukosavic, 1998). The vast majority of induction motor drives are used for heating, ventilation and air conditioning (HVAC). These applications require only low dynamic performance and in most cases only voltage source inverter is inserted between grid and induction motor as cheapest solution. The classical way to control these dives is constant V/f ratio and simple methods for efficiency optimization can be applied (Abrahamsen et al.,1998). From the other side there are many applications where, like electrical vehicles, electric energy has to be consumed in the best possible way and use of induction motors in such application requires an energy optimized control strategy (Chis et al., 1997.). The evolution of the power digital microcontrollers and development of power electronics enables applying not only methods for induction motor drives (IMD) control, like vector control or direct torque control, but also development of different functions which make drives more robust and more efficient. One of the more interesting algorithm which can be applied in a drive controller is algorithm for efficiency optimization. In a conventional setting, the field excitation is kept constant at rated value throughout its entire load range. If machine is under-loaded, this would result in over-excitation and unnecessary copper losses. Thus in cases where a motor drive has to operate in wider load range, the minimization of losses has great significance. It is known that efficiency improvement of IMD can be implemented via motor flux level and this method has been proven to be particularly effective at light loads and in a steady state of drive. Also flux reduction at light loads gives less acoustic noise derived from both converter and machine. From the other side low flux makes motor more sensitive to load disturbances and degrades dynamic performances (Stergaki & Stavrakakis, 2008). The published methods mainly solve the problem of efficiency improvement for constant output power. Results of applied algorithms highly depends from the size of drive (fig. 1) (Abrahamsen et al., 1998) and operating conditions, especially load torque and speed (Figs. 2 and 3). Efficiency of IM changes from 75% for low power 0,75kW machine to more then 95% for 100kW machine. Also efficiency of drive converter is typically 95% and more.