Following the passing of EPAct 92 in the U.S., and similar energy initiatives around the globe, a significant number of energy-efficiency projects were initiated relating to electric motor repair. The BC Hydro motor repair study, performed in 1993, covered 11 energy-efficient 20 hp electric motor models. One of each was held as a "standard" for dynamometer testing, and two more of each model were shorted and sent blind to various repair shops across a large geographical area.
Findings from the BC Hydro study showed the lowest decrease in efficiency to be 0.5%, with the most significant being around a 4% loss of efficiency. The cause of the highest losses? Bearing replacement. An increase in friction and windage because contact sealed bearings were used resulted in an average loss of 3% of efficiency. This was a surprise as the researchers expected the most significant losses to be the result of core and I2R from rewinding. Instead, rewind losses accounted for an average of 1% per rewind, while mechanical problems resulted in much higher values. The solution? Use non-contact sealed bearings when such applications are required.
Other areas that can increase friction loading include overgreasing and improper mechanical fits through repair. Your average electric motor (C3) bearing is not designed to be packed full of grease. In conditions where you see an increase in bearing temperature after greasing the bearings, you are identifying the lost efficiency (heat) due to increased friction. Through repair, improper mechanical fits—including the use of peening or fillers instead of proper machining practices—will also increase the friction in your bearings. The lesson? Follow proper greasing practices and ensure that your repair facility is performing quality machining. MT