An iterative compensation method for production tolerances in electric drive systems

Like all industrially manufactured products, interior permanent magnet synchronous machines are subject to deviations due to production tolerances. In particular, deviations in the permanent magnet flux and the air gap thickness cause considerably high torque errors and influence efficiency. Especially for traction drives in hybrid or electric vehicles this is not acceptable. In this paper, the impact of deviations caused by tolerances is first discussed. Second, a way to determine the deviation of an individual system is given. Finally, a method for compensation is presented that significantly increases torque accuracy with optimum efficiency: Based on a conventional open-loop torque controller, an iterative torque compensation (ITC) is introduced. Therewith, the commanded torque is adapted based on a torque estimation. The aim of the paper is to reduce torque error by changing the control method. The concept is proven by test bench measurements with machines deviating in their permanent magnet flux. Therewith, the torque error is reduced from 12 % to 2 % with respect to maximum torque.