Induction motor driver

A method for controlling the speed of an induction motor including a rotor, said method comprising the steps of:

• selecting a desired rotor speed;
• determining motor rotor speed;
• comparing said motor rotor speed to said desired rotor speed, to thereby generate a speed error signal;
• applying said speed error signal to a proportional-integral controller, to thereby generate a torque command signal;
• normalizing said torque command signal to maximum specified torque to thereby generate normalized signals;
• determining the magnetizing current of said motor;
• generating a stator current amplitude command signal from said torque command signal;
• determining ideal volts/Hz for the motor;
• subtracting said actual volts/Hz from the ideal value of volts/Hz to thereby generate volts/Hz error;
• multiplying (528) said volts/Hz error by at least a constant and an integral to thereby generate a phase correction signal;
• generating a current phase precursor signal from said torque command signal;
• adding said current phase precursor signal to said phase correction signal to thereby generate a signal representing the phase of the stator current;
• and
• controlling the switching of current to and from a central capacitor in response to said amplitude command signal and said signal representing the phase of said stator current.

A control apparatus for an induction motor, which induction motor includes a stator, said control apparatus comprising:

• a source of three-phase alternating current;
• a switching circuit including a source port coupled to said source of alternating current and a motor port coupled to said stator, said switching circuit including a central capacitor through which all charge flows between said source port and said motor port and also including controllable switches for routing said charge to create stator current;
• a switching controller coupled to said controllable switches for establishing the timing of said controllable switches in response to digital command signals representing (a) the amplitude of the stator current and (b) the phase of the stator current;
• a source of motor speed command signals;
• a motor control arrangement coupled to said source of motor speed command signals and to said switching controller, said motor control arrangement comprising
• an error detector for generating a motor speed error signal, a torque command controller for generating a torque command signal from said speed error signal, a torque current normalizing arrangement for normalizing said torque command signal to the full-load motor torque to produce torque current signal, for processing said torque current signal for generating signals representing (a) the amplitude of the stator current and (b) the phase of the stator current, both adjusted by the magnetizing current of the motor;
• said motor control arrangement further comprising
• a coupling path for coupling said amplitude of the stator current adjusted by the magnetizing current from said motor control arrangement to said switching controller for control of the timing of said switches, an error detector coupled to said motor port and to a source of nominal motor V/Hz signal, for generating V/Hz error signal, a summing circuit coupled to sum processed V/Hz error signal with said signals representing the phase of the stator current, to thereby generate phase-corrected signals representing the phase of the stator current, and a coupling path for coupling said phase-corrected signals representing the phase of the stator current to said switching controller.