Field-weakening strategy for torque-loss compensation

A method for controlling the direct current and the quadrature current applied to an internal permanent magnet (IPM) motor to achieve a desired rotational speed and a desired torque over a range of internal operating temperatures of the motor without exceeding a maximum stator current, the method comprising:

• applying a voltage to the motor;
• selecting an initial commanded direct current value and an initial commanded quadrature current value based on the desired rotational speed and the desired torque;
• applying the initial commanded direct current value and the initial commanded quadrature current value to the motor;
• comparing a measured terminal voltage of the motor with a maximum terminal voltage, the terminal voltage varying with the internal operating temperature of the motor;
• and
• when the measured terminal voltage exceeds the maximum terminal voltage:
• generating a direct current differential value responsive to a difference between the measured terminal voltage and the maximum terminal voltage;
• generating a quadrature current differential value in response to the direct current differential value;
• adjusting the initial commanded direct current value to a first modified direct current value in response to the direct current differential value;
• adjusting the initial commanded quadrature current value to a first modified quadrature current value in response to the quadrature current differential value;
• and
• applying the first modified direct current value and the first modified quadrature current value to the motor.

A system for controlling the direct current and the quadrature current applied to an internal permanent magnet (IPM) motor to achieve a desired rotational speed and a desired torque over a range of internal operating temperatures of the motor without exceeding a maximum stator current, wherein an increase in stator current causes an increase in a measured terminal voltage, the system comprising:

• an initial current value generator that generates an initial direct current value and an initial quadrature current value responsive to a selected input voltage, a selected speed, and a selected torque;
• an integrator that integrates a difference in the measured terminal voltage in excess of a maximum terminal voltage and generates a direct current difference value;
• a quadrature current difference generator that receives the direct current difference value and that generates a quadrature current difference value;
• a first modified direct current generator that adds the direct current difference value to the initial direct current value to generate a first modified direct current value;
• a first modified quadrature current generator that adds the quadrature current difference value to the initial quadrature current value to generate a first modified quadrature current value;
• and
• an output system that applies the first modified direct current value as an applied direct current value to the motor and that applies the first modified quadrature current value as an applied quadrature current value to the motor.