JPS55157992A - Speed control system for induction motor - Google Patents

Abstract

PURPOSE:To minimize a loss of an induction motor to occur at the time of low speed operation by a system wherein voltage waves of an AC power are thinned out by a given number by stages according to a speed set point, and they are added to an induction motor. CONSTITUTION:Number of revolutions of an induction motor 5 is detected by a speed generator 6, and a switching circuit 9 to control power waves through thinning out is added to a circuit to control automatically an AC switching element of thyristors 7, 8 through controlling a conducting phase control angle of a conduction controlling circuit 3 according to a deviation with an output of a speed setting circuit 1. Then, a switching circuit 9 controls the conduction controlling circuit 3 according to a section in which a speed setting voltage of the speed setting circuit 1 corresponds to a set point in the switching circuit, and applies power voltage waves to the induction motor 5 through thinning out every other wave and then at every two waves from the full wave. Thus a small input power is acceptable as compared with a conventional system, thereby minimizing a loss to occur at the time of low speed operation.