Stator Winding Connection-Mode Management in Line-Start Permanent Magnet Motors to Improve Their Efficiency and Power Factor

Three-phase line-start permanent magnet motors (LSPMs) are a recent entrance in the marketplace. In general, such motors are designed to meet IE4/Super-Premium Class requirements. As with squirrel-cage induction motors, LSPMs can benefit in terms of efficiency and power factor from a proper magnetizing flux adjustment as a function of their load. This is important to improve the motor performance when it is oversized and/or driving variable-load fixed-speed applications. In this paper, an experimental study on the improvement potential of steady-state partial-load efficiency and power factor in special multiconnection LSPMs with up to six different flux levels is presented. Some novel conclusions are offered regarding the efficiency and power factor gains and connection-mode management as a function of motor actual load, including the delta-star change in conventional six-terminal LSPMs. With the proposed solution, efficiency gains of 6-8% points and power factor gains of 0.4-0.5 points can be obtained in LSPMs at light partial loads, representing a very significant performance improvement. Moreover, it is demonstrated that reactive power injection into the mains grid is possible with LSPMs.