Motors that are designed to operate at several voltages operate most efficiently at the highest rated voltage. The difference in efficiency at the different voltages may be as high as several percent. Therefore,check whether the transformer that supplies power to the motors has taps that you can reset to the higher voltage. This is often the case. However, this may not be practical if the transformer serves loads other than the motors. See whether the other equipment can operate satisfactorily at the higher voltage. Or, if possible,rearrange the power circuits to allow the motors to be operated by higher-voltage transformers.