Gear Ratio and Its Effect on Traction Motors

The mechanical power delivered by the motors of electrically propelled vehicles is almost always transmitted through gears to axles. The few exceptions are some gearless electric locomotives designed many years ago. Although the top speeds of vehicles and locomotives have increased materially since the inception of electric drive, the very nature of the service requires high tractive efforts in starting from standstill and at low speeds. In the case of transit vehicles, frequent stops require operation at relatively low speeds a large percentage of the time. The history of traction-motor design discloses a trend toward higher and higher motor speeds. These higher speeds have resulted in reduced motor weight and space requirements, which are two most important considerations in traction apparatus. In order to use motors of higher speed economically, greater gear reductions are required. As developments in gear design, gear materials, and heat treatments progressed, greater gear reductions became practicable, and the higher-speed motors came into general use. Modern double-reduction gear units and hypoid gears permit greater gear reductions and, hence, higher motor speeds than the single-reduction spur gears which were once employed in all traction applications. The reductions in weight and size of traction motors are not only advantageous in themselves, but they permit features in vehicle construction which add to the comfort and convenience of passengers and to over-all economy of operation.