Since the development of the thyristor, many varieties of power conversion equipment have been utilized as adjustable frequency power supplies for AC motors. Variable frequency drives with AC machines are now commonly used where speed control is required and substantial effort has been devoted toward the development of drives that are efficient and practical. However, the now common use of variable frequency drives has thus far not resulted in a significant change in the design of the AC machines which are used in such systems. In general, where AC machines are to be operated with solid state, adjustable frequency power sources, this fact has only influenced the selection of the design values of the conventional parameters of the machine--for example, rotor resistance and leakage reactance in the case of an induction motor or q-axis rotor resistance and magnetizing reactance in the case of a reluctance motor.
Considerable effort has been directed to the development of solid state converters in which the harmonic content of the output voltage is minimized. The object of these designs is to produce a rotating field within the machine having a minimum of time harmonics. This objective is pursued because conventional AC machines generally have sinusoidally distributed windings which are optimally suited to be driven by pure sinusoidal power sources. In machines designed in this manner, space and time harmonics are minimized, and the air gap field becomes essentially sinusoidally distributed. However, even converter systems which seek to provide a driving wave form which simulates a sinusoid normally produce only a step-wise approximation, resulting in the introduction of unwanted harmonics and a reduction in power conversion efficiency from that obtainable with pure sinusoidal power. In addition, in machines driven with sinusoidal power only about one third of the active iron core is near saturation at any instant and the iron is effectively underutilized.
Similar inefficiencies are encountered where AC generators are used to provide power to a DC load through a rectifier. AC generators are conventionally designed to provide a pure sine wave output voltage which, when rectified, yields a DC voltage having a substantial "ripple" component. If the ripple component must be eliminated or reduced, additional filtering elements are required and energy may be lost in the filter, reducing the overall generating efficiency.