Traditional air compressors are fixed speed, meaning they run at a constant and consistent speed. This produces a fixed amount of compressed air per minute. There are many benefits to fixed speed compressor technology if your compressed air demand is constant and unchanging. However, this isn’t always the case. As fixed speed compressors are always operating at full-throttle, if all of the output is not required then energy is being wasted. http://www.fscooling.com/compressor/dc-variable-speed-compressor/