Comparison of Different Tapped Windings for Flux Adjustment in Induction Motors

In single-speed three-phase squirrel-cage induction motors with rated power above 3 kW, although the use of electronic soft-starters is increasing fast, the conventional star-delta starters are still the most used, requiring three contactors. In this paper, motor soft starting and/or magnetizing flux adjustment solutions based on two-flux-level, delta- and star-connected, tapped windings are proposed, providing an alternative to the conventional star/delta windings. The motor flux adjustment in the proposed tapped winding is based on the change of the winding factor and/or per-phase number of turns. The connection-mode change system for the proposed tapped windings requires only two contactors, and no extra cost is expected for the motor manufacturing or rewinding. For the proposed tapped winding arrangements, a theoretical analysis is made, and simulated and experimental results are presented. The presented solutions can be used to improve the part-load efficiency and power factor of fixed-speed variable-load motors, particularly if they operate long periods with load lower than 35%-45% and short periods with load close to 100%, and an automatic load-based winding connection-mode management system is used.