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Star- and delta-connected windings tolerance to voltage unbalance in induction motors

With the growing use of electronic soft-starters for starting fixed-speed three-phase squirrel-cage induction motors with a rated power higher than 4 kW, the six-terminal star/delta-connection stator windings with delta as the nominal connection, used for the typical contactor-based star-delta starting, are not mandatory. In fact, when a motor is to be started with an electronic soft-starter, it can have a permanent three-terminal delta- or star-connected winding. After starting, the soft-starter is usually bypassed, and the motor operated directly from the grid. In motors with rated power lower than or equal to 4 kW, the stator winding can be either designed for delta or star connection. A question arises: which connection mode is more tolerant to voltage unbalance? An answer to this question is provided in the paper, on the basis of simulations and experiments, offering a new perspective on this issue. Some short outcomes are provided on the per-phase equivalent circuit parameter adaptation for induction motor simulation in star and delta connection modes. A brief voltage unbalance tolerance comparison between single-layer full-pitch and double-layer short-pitched windings is also presented. A polar representation of the per unit stator winding phase Joule losses is proposed as a tool to assess the voltage unbalance impact on stator thermal asymmetry impact evaluation.