Considerations when modernizing drives for buildings based on an energy efficiency and cost perspective

The efficiency consequence of keeping over-dimensioned Induction Motors (IM) in Heating Ventilation and Air Conditioning (HVAC) applications, when modernizing such systems, using the potential that modern power electronic drives gives, is investigated in this paper. Five different Eff1 IM ratings in the range 1.1-4 kW have been the target of the analysis. It is a common assumption that the motor efficiency drops rapidly as the load decreases below 75% its rated operation, which is the case for grid connected motors or motors operated with constant V/Hz ratio by an inverter. As a result, over-dimensioning of the IM will result in increased energy cost compared to a smaller rating. However, by using a variable V/Hz ratio, it is shown here that the efficiency is in general highest for the largest IM regardless of the loading, despite the increase of the mechanical losses caused by increased cooling and friction. A conclusion found is that it is in principle always worth to keep an over-dimensioned existing motor instead of replacing it with a smaller motor having a better efficiency class.