This paper presents the results of a study about the effects of unbalanced voltages on the energy performance of three-phase induction motors. The principal contribution of this paper is that presents a study of the influence of positive sequence voltage on the several characteristics: line currents, losses, efficiency and power factor under different voltage unbalanced conditions. A three-phase induction motor of 3 HP 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 to evaluate the performance of the induction motor.