The power draw characteristics of air compressors are primarily determined by the type of compressor control and the relationship between the compressor’s output capacity and the compressed air demand in the plant. In this paper, we review the most common types of compressor control for small reciprocating and rotary air compressors, and derive relations for estimating compressed air output as a function of the type of control and motor loading. Using these relations, we develop a method to estimate the ratio of compressed air output to energy input, which we call the average operating efficiency, based on input power to the compressor.

The measured power draw signatures of industrial air compressors operating in different control modes and with different loads are discussed as case studies. The case studies illustrate how to identify the type of control, and potential problems such as inadequate compressed air storage, over-sized compressors, and compressed air leaks from the power signatures. Average operating efficiencies for the case studies are compared to suggested guidelines for energy-efficient air compression.