Influence of the positive sequence voltage on the derating of three-phase induction motors under voltage unbalance

Abstract

This paper presents a study of the influence of the positive sequence voltage on the derating of three-phase induction motors under voltage unbalance and its comparison with the NEMA derating curve. Three criteria for derating induction motors under unbalanced voltages are presented: the rated current, the rated equivalent current and the rated losses. These criteria are theoretical and experimentally analyzed. A 4 kW three-phase induction motor was used in the study. The paper suggests that the positive sequence voltage must be considered together with the Voltage Unbalance Factor VUF (or Percent Voltage Unbalance PVU) index for derating purposes. If only the VUF (or PVU) index is taken into account, a facility user cannot appropriately derate the motor.