Harmonic current command with four degrees of freedom for electric motor

A method of operating a propulsion system in a device, the propulsion system having an electric motor configured to generate torque, and a controller having a processor and tangible, non-transitory memory, the method comprising:

• selecting at least one harmonic order for a harmonic current command, the electric motor having a motor speed;
• defining a plurality of operating regions, including a first operating region, a second operating region and a third operating region each associated with a respective objective function for generating the harmonic current command;
• operating the harmonic current command with four degrees of freedom by selectively varying a q-axis current magnitude, a d-axis current magnitude, a q-axis phase and a d-axis phase as a function of the torque and the motor speed;
• selecting the respective objective function for the first operating region such that a harmonic current magnitude is minimized, the first operating region extending over a lower value torque region;
• and
• selecting the respective objective function for the second operating region such that a radial force density is minimized, the second operating region extending over an intermediate value torque region.

A propulsion system for a device, comprising:

• an electric motor configured to generate torque to propel the device, the electric motor having a motor speed;
• a controller in communication with the electric motor and having a processor and tangible, non-transitory memory on which instructions are recorded, the controller being adapted to select a first harmonic and a second harmonic for a harmonic current command;
• wherein the controller is adapted to define a plurality of operating regions, including a first operating region, a second operating region and a third operating region each associated with a respective objective function for generating the harmonic current command;
• and
• wherein the controller is adapted to operate the harmonic current command with four degrees of freedom by selectively varying a q-axis current magnitude, a d-axis current magnitude, a q-axis phase and a d-axis phase as a function of the torque and the motor speed;
• and
• wherein the controller is adapted to:
• select the respective objective function for the first operating region such that a harmonic current magnitude is minimized;
• select the respective objective function for the second operating region such that a radial force density is minimized with no constraint on the harmonic current magnitude;
• and
• select the respective objective function for the third operating region such that the radial force density is minimized, and the harmonic current magnitude is partially constrained.