Motors need to be sized according to system and process requirements. In most applications, oversized motors can be in use, installed as a safety factor to manage excessive loads. Therefore, by limiting the occurances of excessive loadings, and remembering that most motors can operate around 15% above their capacities for shorter periods, larger motors can be substituted with smaller ones providing savings (Worrell et al., 2008. p.54).
However, selecting the correct size requires careful analyses of multiple factors. Most motors maintain a relatively high efficiency if operated above 50% of full load capacity, and efficiencies usually drop significantly if the load factor drops below 40%. Although this may promote the selection of a smaller motor, it does not automatically lead to optimal solution. Large motors often have a higher efficiency rating and experience lower loss in efficiency at lower load factors. Consequently, in order to identify the optimum size selection the quantitative and temporal dynamics of load fluctuations, and the top and partial load efficiencies of alternative motor sizes need to be taken into consideration and need to be assessed for different operational scenarios in the plant. This also involves paying attention to special requirements, such as ability to meet a potential short term high-load, as is the case with the starting torque of certain refrigeration compressors.
Graphs, like the one below, showing the efficiency loss experienced by different motors in various load factors will be useful in making the optimum selection.