On the optimal efficiency of split-phase induction motors under light load conditions

It is a known fact that when operating at light load condition, the efficiency of induction motors can be improved by reducing the supply voltage. This paper investigates the optimal efficiency of split-phase motors when loaded below their horsepower rating. An accurate equivalent circuit model that takes into account stray load loss and rotational loss, in addition to core loss, is proposed for efficiency calculations. The model is used to determine optimal values of the source voltage for different shaft output powers. The process involves solving a set of nonlinear equations at each load level. When neglecting rotational losses, however, the optimal solution becomes straightforward as this is reduced to constant power factor operation. The work is illustrated by both simulations and laboratory measurements on a commercial 1/3 hp motor.