Proponents of voltage reduction (“optimisation” as they like to call it) have started suggesting that equipment is more energy-efficient at lower voltage. In fact this is quite often not the case. For an electric motor, this diagram shows how various aspects of energy performance vary as you deviate from a its nominal voltage. The red line shows that peak efficiency occurs, if anything, at slightly above rated voltage.

Reduced voltage is associated with reduced efficiency. The reason is that to deliver the same output at lower voltage, the motor will need to draw a higher current, and that increases its resistive losses.