Method and apparatus for estimating rotor position in a sensorless synchronous motor

A motor drive system for a multi-phase synchronous motor having an input voltage source comprising:

• a power conversion section configured to convert an input voltage to an output voltage for the synchronous motor;
• at least one current sensor configured to provide a current signal corresponding to a current output to the motor;
• a memory device;
• a program stored in the memory device;
• a processor receiving the current signal, the processor configured to execute the stored program to:
• obtain a motor resistance and a q axis inductance for the synchronous motor;
• obtain an amplitude of the output voltage at a predetermined time interval;
• obtain the current signal at the predetermined time interval, wherein the predetermined time interval is selected such that a change in speed of the synchronous motor during the time interval is approximately zero;
• apply each of the current signal and the amplitude of the output voltage as an input to a model of the synchronous motor to obtain an observed value of motor current output to the motor, the model being a function of the motor resistance, the q axis inductance, and a controller gain and being independent of motor speed and d axis inductance;
• determine the estimated rotor position based on the difference between the observed value of motor current and the measured current signal;
• and
• provide at least one command signal to the power conversion section to control the conversion of the input voltage to the output voltage based on the estimated rotor position.

A method of estimating a rotor position in a synchronous motor comprising the steps of:

• measuring a current drawn by the synchronous motor while commanding rotation of the motor at a desired speed;
• determining a voltage applied to the synchronous motor while commanding rotation of the motor at the desired speed;
• obtaining a q axis inductance for the synchronous motor;
• determining an observed value of the current output to the motor during motor rotation as a function of the current drawn by the synchronous motor, the voltage applied to the synchronous motor, and the q axis inductance;
• and
• determining the estimated rotor position as a function of the observed value of the motor current output to the motor, wherein the estimated rotor position is independent of the commanded speed to the synchronous motor and independent of a d axis inductance of the synchronous motor.