Saturation-Related Losses in Induction Motors for Star and Delta Connection Modes

Abstract

The ferromagnetic core of three-phase, squirrel-cage, induction motors is typically designed to operate within the knee region of the B-H magnetization curve, in order to achieve a good tradeoff between the saturation level, core losses, and amount of material used. Nowadays, in the industrial sector, the use of electronic soft-starters and variable-speed drives is wide and increasing. Therefore, the star-delta starting possibility for a given line-to-line mains voltage is no longer a requirement for a large part of the induction motors with rated power over 4 kW. Hence, induction motor manufacturers can design motors to operate with permanent star connection mode at rated voltage and frequency. In this paper, the difference between the star and delta stator winding connection modes in terms of motor losses and efficiency for different levels of core saturation is experimentally investigated for a 4-kW, 4-pole, 400-V, 50-Hz, three-phase, squirrel-cage, induction motor with reconfigurable windings. It is demonstrated that, for full-load operation, the star connection mode leads to lower motor losses. This is due to the inexistence of homopolar saturation-related phase currents in the star-connected stator windings, contrarily to the delta-connected stator windings, in which those currents can exist and achieve significant amplitudes for high core saturation levels. This fact may be used by motor manufacturers to, on the one hand, reduce the motor losses for the same core saturation level, or, on the other hand, reduce the amount of iron in the core for the same motor losses. The latter option, associated with the use of a terminal plate with only three terminals, may lead to a motor manufacturing cost reduction, without reducing the respective rated efficiency.