Can Soft-Starters Help Save Energy?

Electric motors, which are predominantly of the three-phase, induction, squirrel-cage type, consume about 60% of the electrical energy generated in industrialized countries [1]. In many cases, a motor runs on full load for only a small fraction of the operating time. Often, motors are so much overrated with respect to the actual load that they never utilize their full power capabilities. Average efficiency of such drive systems is inferior, since the motors, like all electric apparatus, are designed to operate most efficiently when fully loaded. There are two basic approaches to the minimization of energy consumption by ac drives. One possibility is to employ an inverter that adjusts both the supply frequency and the voltage, so that the motor speed can be controlled and the energy usage reduced [2], However, many applications do not require the expensive frequency control, and motors can be equipped with power electronic soft-starters instead. The supply frequency is fixed and only the stator voltage is controlled, in dependence on the load, to limit the stator current at starting and reduce the motor losses [3-5].