Induction motor control system

Claim 1

An induction motor control system comprising: a variable-frequency power supply for supplying an output voltage at a given phase for driving an induction motor, having a rotational speed, a magnetic flux phase and a voltage, at various speeds; first switching means, connected to said induction motor, said variable-frequency power supply and a commercial power supply for supplying a voltage at a given phase for selectively connecting said induction motor to said commercial power supply or said variable-frequency power supply; means for calculating a first torque current reference from a speed difference between a speed reference and said rotational speed of said induction motor; means for calculating a second torque current reference corresponding to a phase difference between said phase of said output voltage of said variable-frequency power supply or a magnetic flux reference, having a given phase, for said induction motor, and said phase of said voltage of said commercial power supply; second switching means for selectively outputting said first torque current reference or said second torque current reference as a torque current reference, prior to switching of said first switching means; and vector control means for performing vector control of said variable-frequency power supply in response to said torque current reference outputted from said second switching means, and an excitation current reference calculated from said speed reference and a detected magnetic flux of said induction motor, so that said rotational speed and said voltage of said induction motor are maintained at predetermined values, respectively; whereby said torque current reference outputted from said second switching means controls a slip frequency of said induction motor through said vector control means.