Development of the lead-free brush material for the high-load starter

Carbon brushes for automotive starters are used under severe conditions of high electrical current density, high contact pressure and high sliding velocity. Lead has traditionally been added to carbon brushes to improve their performance and durability. Because lead is an environmental hazardous substance, after January 2005 the EU has prohibited adding lead to carbon brushes for electric motors installed in new vehicles. This paper reports the development of the lead-free brush material for the high-load starter. Those brushes are used in much more extreme conditions - at a PV-value (the product of brush contact pressure and sliding velocity) approximately three times that of other starter brushes, and double the electrical current density. The major technical requirement of this development was to decrease the electrical wear in brushes caused by commutation sparking. We developed the brush material that reduces electrical wear by adding a phosphorous compound. Because the phosphorous compound can improve the lubricity at high-temperature and the contact stability of brushes, the developed brush reduces commutation sparks. The life of the developed brush is about 1.5 times longer than that of conventional brushes containing lead.