Design of an efficiency optimization controller for inverter-fed AC induction motors

Simple steady-state controllers have been developed and tested for reducing input power and improving the efficiency of AC motors operated with scalar adjustable speed drives. Voltage perturbation control reduces input power and a second controller changes frequency to correct rotor speed loss caused by voltage drops. Neither controller requires the use of a tachometer, machine model or machine circuit parameters. Both controllers are shown to work well in the laboratory for pump/fan type loads and other loads simulated with an eddy-current brake. A third controller produces an initial commanded frequency which compensates for the variation in slip with changing load and speed, producing the desired shaft speed as a fraction of rated speed, rather than the commanded fraction of rated frequency at an unknown speed. It is shown that motor energy savings using the energy optimizing controllers can be greater when operating near rated conditions than at significantly reduced load and speed, depending upon motor design characteristics.