Comparison of minimum energy consumption indicators for electric motors powered directly from the mains at part loads

The article presents a comparison of minimum energy consumption indicators for motors powered directly from the mains at part load conditions. The energy characteristics of induction motors and line-start permanent magnet synchronous motors of different energy efficiency classes are compared. As an example, the calculations of proposed energy consumption indicators are provided for the throttle-controlled 2.2 kW pump unit with variable flow-time profile. It is shown that if a motor is used mostly at part load conditions and was chosen by its energy efficiency class only, minimum energy consumption might not be achieved. Universal average efficiency indicator is suggested for motors running at variable loads with output power much lower than the rated power. The comparison shows that the proposed average efficiency does not depend on a specific application or load profile and describes the motor energy consumption enough for practical goals. Thus, this indicator can be used for fast evaluation and selection of a motor by the lowest energy consumption criterion, especially without detailed information about load profile. Calculation of this indicator requires only motor efficiency data per 4 points 25, 50, 75, 100% of rated load. © Published under licence by IOP Publishing Ltd