Method of extending the operating speed range of a rotor flux based MRAS speed observer in a three phase AC induction motor

An apparatus for motor drive control comprising:

• a rectifier adapted for connection to an AC voltage source and producing a DC output voltage;
• a three-phase voltage source inverter connected to said DC output voltage terminals of said rectifier and first, second and third phase output lines for supplying three-phase power, first, second and third current shunt resistors and a plurality of semiconductor switches connected to said DC output voltage, said first, second and third phase output line and connected to ground via said first, second and third current shunt resistors;
• and
• a digital signal processor having first, second and third analog-to-digital inputs, each receiving a voltage at a corresponding first, second and third current shunt resistor, said digital signal processor programmed to calculate corrected instantaneous pulse width modulation drive signals for supply said plurality of semiconductor switches to produce a three phase motor drive at said first, second and third phase output lines, said digital signal processor programmed to
• calculate a reference estimate of rotor flux from applied voltage and measured stator current,
• calculate an adaptive estimate of rotor flux from said measured stator current and a motor speed estimate,
• calculate an error from a cross product between the reference estimate of rotor flux and the adaptive estimate of rotor flux,
• calculating a motor speed estimate from said error,
• produce instantaneous pulse width modulation drive signals to supply said plurality of semiconductor switches to produce a three phase motor drive at said first, second and third phase output lines for an AC induction motor corresponding to the motor speed estimate, and
• wherein said calculation of an adaptive estimate of rotor flux includes altering a cut off frequency and a DC gain of at least one low pass filter dependent upon said motor speed estimate.