Feasibility Study of Pump Units with Various Direct-On-Line Electric Motors Considering Cable and Transformer Losses

The high energy intensity of the modern industry determines the high urgency of increasing the energy efficiency of production processes. However, a big number of motor types of enhanced efficiency classes are available on the market. The motor users can be confused about the choice of the right motor solution for a certain application. In this paper, to help with this choice the energy efficiency indicators of various types of electric motors in a low-power pump unit with a constant rotation speed are studied. Moreover, not only power losses in the motor are considered, but also power losses in the cable and transformer, which are influenced by the power factor of the motor. Induction motors (IMs) and synchronous motors powered directly from the grid (direct-on-line synchronous motor with permanent magnet in the rotor, DOL PMSM; direct-on-line synchronous reluctance motor without permanent magnet, DOL SynRM) of IE2, IE3, and IE4 energy efficiency classes are compared. To carry out the analysis, polynomial interpolation of the available catalogue data and experimental data of the motors are used. The main criteria for comparing different motors in this work are the energy savings over the pump’s life cycle and the payback period when replacing an IE2 motor with a motor of a higher energy efficiency class. The article shows that although the DOL PMSM has a lower motor efficiency than the DOL SynRM, it saves more energy due to its higher power factor, which reduces cable and transformer losses. It is also shown that, despite the highest initial cost, when taking into account cable and transformer losses, the payback period of DOL PMSM can be shorter than that of IE3 and IE4 induction motors. DOL SynRM has the shortest payback period in all considered cases, has no troublesome rare-earth permanent magnets, and can also be a valuable solution.