On the star, delta and star-delta stator winding connections tolerance to voltage unbalance

When a three-phase squirrel-cage induction motor is to be started with an electronic soft-starter, the respective stator winding can be designed for nominal power with delta, star or star-delta connection. If the motor has two sets of windings sharing the same slots, which may be connected in series or parallel, more connection modes are possible. In this paper, on the basis of a theoretical approach, simulations, and some experiments, a comparative analysis of the tolerance to voltage unbalance of the delta, star and star-delta stator winding connections is presented, offering a new perspective on this issue. It is demonstrated that, under unbalanced voltage supply condition, these stator winding connection modes have a different performance in terms of phase voltage and current unbalance ratios.