Prediction of Losses and Efficiency for Three-Phase Induction Machines Equipped With Combined Star-Delta Windings

Combined star-delta windings in electrical machines result in a higher fundamental winding factor and cause a smaller spatial harmonic content. This leads to lower I2R losses in the stator and the rotor windings, and thus to an increased efficiency. However, compared with an equivalent six-phase winding, additional spatial harmonics are generated due to the different magnetomotive forces in the star and the delta parts of the winding. In this paper, a complete theory and analysis method for the analytical calculation of the efficiency for induction motors equipped with combined star-delta windings is developed. The method takes into account the additional harmonic content due to the different magnetomotive forces in the star and delta parts. To check the analysis' validity, an experimental test is reported both on a cage induction motor equipped with a combined star-delta winding in the stator and on a reference motor with the same core, but with a classical three-phase winding.