It has been more than a century since the principle of the operation of induction motors was discovered. The first industrial motors of this type were constructed at the end of the Nineteenth Century and soon gained the favor of users who appreciated their raggedness, simplicity and relatively low manufacturing cost compared to DC commutator motors. The design and performance of induction motors were improved during the Twentieth to Century, reaching maturity around the late 1930s. Further improvements date principally concern the use of modern construction material, specially for winding isolation, and deeper knowledge of certain phenomena regarding harmonics and noise reduction, more efficient cooling techniques, and so on. Nevertheless, fed by fixed-frequency and fixed-magnitude mainlines, induction motors do not lend themselves to easy speed regulation, and in this respect they have always seemed inferior to DC commutator motors, which had no serious competitors in the field of variable speed applications.