The use of fast-switching power semiconductors increases the problems associated with operating three-phase A.C. motors at the frequency inverter. Motor reactors provide basic protection for this between the frequency inverter and motor. Differences in voltage are smoothed while the insulation strength and lifetime of the winding wires in the motor are improved. The motor is also protected by reducing the edge steepness. Motor reactors ensure increased power quality in the overall drive system through the positive impact on electromagnetic compatibility with other system components.