Method for operating a variable speed drive for driving an electric motor and variable speed drive for executing the method

A method for operating a variable speed drive for driving an electric motor, the method comprising:

• providing a look-up table for a motor, the look-up table associating torque input values and/or flux input values with current vector control values comprising a current magnitude and a current angle, including values at max torque per ampere (MTPA) points and at angular deviations from the MTPA points;
• generating, using the look-up table, a current magnitude and a current angle as control variables as a function of a torque input value and/or a flux input value while maintaining a flux limit;
• and
• outputting the generated current magnitude and current angle to control operation of the motor;
• wherein generating consecutive current magnitude and current angle values comprises:
• storing a current index and an angle index corresponding to a last-used current magnitude and a last-used current angle in the look-up table;
• and
• limiting a search for a next current magnitude and a next current angle to a portion of the look-up table around the stored current index and the stored angle index;
• wherein the current magnitude and current angle values at angular deviations from the MTPA points are increased such that for each increment of current magnitude of 2%, the current angle is varied in steps of 1°, wherein the current angle is within a range of 80° to 180°.