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 regulation as a function of 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 multi-connection 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 rules, including the delta-star change in conventional 6-terminal LSPMs. With the proposed solutions, efficiency gains of 6-8 percentage points and power factor gains of 0.4-0.5 points can be obtained at light partial loads, which represents a very significant performance improvement. Moreover, it is demonstrated that reactive power injection into to the mains grid is possible with LSPMs.