Rotor-to-stator reduction factor evaluation for externally-excited synchronous machines

An electrical drive for driving an externally excited synchronous machine that has a rotor and a stator, the drive being configured to:

• set a value for a stator flux linkage reference of the externally excited synchronous machine, wherein the value for the stator flux linkage reference is selected to be small enough to enable operation in a linear region of a d-axis magnetizing inductance of the externally excited synchronous machine without saturation;
• drive the externally excited synchronous machine with a speed within a linear modulation range using said value for the stator flux linkage reference;
• adjust, during the driving, a rotor excitation current reference to determine a minimum-load value of a rotor excitation current corresponding to a minimum stator current and thus to a minimum load;
• cause, during the driving, increasing or decreasing of the rotor excitation current until a stator current is within a pre-defined range;
• determine, when the stator current is within the pre-defined range, an operating value for the stator current and an operating value for the rotor excitation current;
• and
• calculate a rotor-to-stator reduction factor of the externally excited synchronous machine based at least on the minimum-load value of the rotor excitation current, the operating value for the rotor excitation current, the operating value for the stator current and the value for the stator flux linkage reference.

A method comprising:

• setting a value for a stator flux linkage reference of an externally excited synchronous machine that has a rotor and a stator, wherein the value of the stator flux linkage reference is selected to be small enough to enable operation in a linear region of a d-axis magnetizing inductance of the externally excited synchronous machine without saturation;
• causing a drive to drive the externally excited synchronous machine with a speed within a linear modulation range using said value for the stator flux linkage reference;
• adjusting, during the driving, a rotor excitation current reference to determine a minimum-load value of a rotor excitation current corresponding to a minimum stator current and thus to a minimum load;
• causing, during the driving, increasing or decreasing of the rotor excitation current until a stator current is within a pre-defined range;
• determining, when the stator current is within the pre-defined range, operating values for the stator current and the rotor excitation current;
• and
• calculating a rotor-to-stator reduction factor of the externally excited synchronous machine based at least on the minimum-load and operating values of the rotor excitation current and the operating value of the stator current.

A computer program product comprising program instructions stored in a non-transitory computer readable medium, the program instructions being configured to cause an apparatus, when executing the program instructions by at least one processor, to perform at least the following:

• setting a value for a stator flux linkage reference of an externally excited synchronous machine that has a rotor and a stator, wherein the value of the stator flux linkage reference is selected to be small enough to enable operation in a linear region of a d-axis magnetizing inductance of the externally excited synchronous machine without saturation;
• causing a drive to drive the externally excited synchronous machine with a speed within a linear modulation range using said value for the stator flux linkage reference;
• adjusting, during the driving, a rotor excitation current reference to determine a minimum-load value of a rotor excitation current corresponding to a minimum stator current and thus to a minimum load;
• causing, during the driving, increasing or decreasing of the rotor excitation current until a stator current is within a pre-defined range;
• determining, when the stator current is within the pre-defined range, operating values for the stator current and the rotor excitation current;
• and
• calculating a rotor-to-stator reduction factor of the externally excited synchronous machine based at least on the minimum-load and operating values of the rotor excitation current and the operating value of the stator current.