Implications of 42V Battery Power on the Design of Motors for Electric Power Steering System 2000-01-3068

Advances in electric motors, power electronics and control methods are mainly responsible for the fast growth of electric power steering systems as replacements for hydraulic based systems. The introduction of high energy Neodymium magnets in the early 1980's gave motor designers opportunities to design smaller size, smaller weight and higher performance motors which enhanced the commercial viability of electric power steering systems. With a 12 V power system and limited available power, these high efficiency motors also increased the steering load capabilities. However, with large steering loads, roughly above 10,000 N rack force, the limitations of the 12 V system become apparent. At such large loads, the resistive voltage drops across the different circuit components have a greater influence on the motor design, than the power dissipation capability of the motor. Under these circumstances, a 42 V power system becomes very attractive in the design of the steering motor, both from a cost and size perspective. This paper will address the design considerations of electric motors for electric power steering application, with a 42 V power system in the automobile. Motor requirements, sizing, packaging and thermal issues and other critical design issues of such motors will also be discussed. The paper will also compare various parameters of a 12 V motor design to that of a 42 V motor design for light and heavy steering loads.