Variable speed controller for an induction motor

Claim 1

A variable speed controller for an induction motor, said variable speed controller resolving a primary current of said induction motor into a magnetizing current and a torque current, said primary current provided by an electric power converter generating outputs at variable voltages, frequencies and phases, said magnetizing current being a component providing a magnetic field parallel to a magnetic flux axis of said induction motor, said torque current being a component in quadrature phase to said magnetizing current, and said variable speed controller regulating said magnetizing current and said torque current independently to control an induction motor torque, said variable speed controller comprising: (a) a magnetic flux phase device for integrating a reference primary angular frequency signal and for obtaining a signal representing the position of said magnetic flux axis; (b) an induced voltage generating device for generating a signal representing the induced voltage of said induction motor; (c) a first primary angular frequency generating device for generating a first primary angular frequency signal based on said induced voltage and a magnetic flux value; (d) a slip frequency generating device for generating a signal representing a reference slip frequency based on a reference torque current value, or an actual torque current value, and a reference magnetic flux value; (e) a first speed generating device for subtracting said signal representing said reference slip frequency from said signal representing said first primary angular frequency, and for obtaining a signal representing a first speed value; (f) a changeover signal generator responsive to a signal representing a sensed operating condition of said motor for generating a changeover signal, said changeover signal representing a first mode of operation and a second mode of operation for said variable speed controller, said first and second modes of operation determined by a predetermined changeover threshold of said sensed operating condition signal; (g) a load torque generating device for generating a signal representing an estimated load torque based on said reference torque current value, said signal representing said first speed value, and said magnetic flux value; (h) a sample holder for holding said signal representing said estimated load torque in said second mode of operation; (i) an acceleration and deceleration torque generating device for subtracting said signal representing said estimated load torque from the product of said reference torque current value and said magnetic flux value, said acceleration and deceleration torque device generating a signal representing an acceleration and deceleration torque value; (j) an integrator for integrating said signal representing said acceleration and deceleration torque value with an induction motor mechanical constant to obtain a signal representing a second speed value; (k) an adder for adding said signal representing said second speed value and said signal representing said reference slip frequency, to obtain a signal representing a second primary angular frequency; and (l) an output device for selecting and outputting, in response to said changeover signal indicating said first mode of operation, said signal representing said first primary angular frequency as said reference primary angular frequency, and said signal representing said first speed value as an estimated speed, and for selecting and outputting, in response to said changeover signal indicating said second mode of operation, said signal representing said second primary angular frequency as said reference primary angular frequency and said signal representing said second speed value as said estimated speed.