Minimization of energy losses in fixed speed cyclic duty induction motor drives

The concept of power factor control, for energy conservation, by reducing the voltage applied to an unloaded or lightly loaded induction motor was first introduced in late seventies by Frank J. Sola of NASA. Since then a number of Products employing phase controlled thyristors for the control of applied voltage with closed-loop power factor regulation have been marketed under various brand names by a number of manufacturers. Further refinements in load and power factor sensing as well as control circuit hardware have been either claimed through patents by Sola and others or reported in the technical literature. Analytical studies of three phase induction motors operating from phase controlled thyristors in the stator circuits have also been well documented. Study of all of these references raises some very interesting questions such as: Do the power factor controllers really improve the system power factor as product manufacturers lead one to believe? Is it really necessary to regulate power factor to minimize no load losses by voltage control? Analysis and experimental data indicate that while the operating power factor of the motor does improve by voltage reduction by phasecontrolled thyristors in the stator circuit, the overall system power factor at the input to the controller does not change. Analytical explanation of this phenomena using circle diagrams and phasor relationships as well as experimental results are presented in this paper. A new controller design based on this research and development effort is introduced.