System for variable speed operation of induction motors

摘要

A system for operating an induction motor at variable speeds, which is particularly effective when an induction motor is to rotate at a very low speed of several r.p.m. or less. The system produces polyphase sinusoidal signals of a frequency which corresponds to the difference between a command speed and the actual speed of the induction motor, the number of phases of the sinusoidal signals being equal to the number of phases at which the induction motor operates, a sinusoidal carrier wave having a constant frequency of from 50 to 400 Hz and a constant amplitude, and polyphase induction motor drive signals formed by modulating the carrier wave by the polyphase sinusoidal signals. The induction motor drive signals are amplified up to a prescribed value and then applied to the induction motor to energize the stator for each respective input phase so that the rotor of the induction motor will be caused to rotate at a very low speed.