Functional Safety Torque Estimation Methods for Position Sensorless Control Applied to Synchronous Machines in Electric Vehicles

For the use of position sensorless control in automotive traction drives, the standard functional safety concept of power inverter modules is not applicable due to omission of the rotor position sensor. To overcome this issue, a modified functional safety concept is proposed. In this context, torque estimation methods known from literature based on flux tables and power comparison are compared by sensitivity analysis with a new developed method based on magnitude of stator current and flux to ensure reliable torque monitoring over complete operating area without using the rotor position information. While methods based on stator flux and power comparison only show adequate accuracy at mid and high motor speed, the new developed method fills the gap down to zero motor speed. This results in a combination of three methods for an optimized strategy depending on the motor speed. Measurements on a series production electric machine verify the proposed approach and gives, from a functional safety perspective, the possibility to omit rotor position sensor in automotive traction drives.