A Compensation Method for Production Tolerances in Electric Drive Systems Using an Extended Open-Loop Torque Control

Interior permanent magnet synchronous machines are unavoidably subject to production tolerances. Quantities like permanent magnet flux or air gap thickness of a machine may deviate with regard to their nominal parameter values. Driven with a conventional torque control strategy this leads to inaccuracy of the output torque. Especially for traction drives in road vehicles, significant torque error is not acceptable. In this paper, a method for compensation of such deviations is presented. It is based on maximum-torque-per-current (MTPC) open-loop torque control. The conventional torque controller is doubled leading to a Parallel Torque Compensation (PTC) structure. It is capable of adapting to an individual machine increasing torque accuracy and efficiency. The concept is proven by simulation as well as test bench measurements with an exemplary machine, showing a significant improvement of torque accuracy with the error reduced from 12 % to 2 % with respect to maximum torque. At the same time, the maximum torque available is increased. With PTC, a purely software-based solution is given that optimizes torque accuracy and efficiency in permanent magnet synchonous drives.