Control system for induction motor

Claim 1

A control system for an induction motor comprising a VVVF controller supplied with a.c. current input voltage and current and having a converter and an inverter composed of self-arc extinguishing elements to supply a variable voltage, variable frequency a.c. voltage to the induction motor which controls speed of the induction motor; control means for controlling elements of said converter by using a pulse-width modulation (PWM) of gating signals to gate input current through said elements to produce a rectified sinusoidal current having a given phase relationship with the input voltage; and dead time generating means provided in said control means for introducing a phase shift to produce a phase difference between the output current and the input voltage to said converter by varing the width of the PWM gating signals in accordance with speed command values to produce a non-unity power factor whereby current to the induction motor is controlled to vary motor speed over a wide range from stop to full speed without requiring full voltage output from said inverter, thereby minimizing motor noise.