Field weakening control of PMSM used in an electric power steering system

Presently, the direct current motor is widely used as an actuator in electric power steering system (EPS) for compact cars. When it comes to mid-size saloons, the permanent magnet synchronous motor (PMSM) is more suitable for numerical advantages. To acquire the smooth steering feeling, the motor´s torque and speed should match well. However, the rated speed of the motor used in EPS is usually round 1000r/min, while under the condition of fast operating steering wheel the motor speed may be higher than 2500r/min. Therefore, it is necessary to carry out field weakening control of PMSM. In most cases, the stator´s resistance is ignored when the field weakening control strategy is carried out. However, the stator´s resistance should be taken into consideration in EPS application for high current and low voltage. This paper proposes a novel field weakening law of PMSM with consideration of motor stator resistance. The proposed law is verified by experiments on EPS test bench. And it is found that the heavy steering feeling problem under the fast operating steering wheel condition is solved by using the field weakening law proposed in this paper.