Development of reliable designs of squirrel cage rotors for traction motors used in locomotives working in arduous operating conditions

Indian Railways (IR) introduced 6000 hp (4476 kW) electric freight locomotives with asynchronous traction motors during 1990s to haul heavy freight trains (5200 tons) at steeper gradients. Arduous operating duties such as frequent wheel spinning and stalling have led to breakage of rotor bars within three years of service. Optimum use of locomotives contributes significantly to less life of rotors. Shorter life span of standard designs of squirrel cage rotors made us to develop two new designs to meet the requirement of IR type arduous operating conditions. This paper discusses failure analysis of rotors along with development of two new designs duly validated with finite element analysis, used for the first time in traction applications. Design 1 has copper laminations, mounted on the shaft in place of floating solid resistance rings, whereas Design 2 has side support to the resistance rings through end plates and they are in service. Both take care of excessive torsional stress and their estimated life is 18 years. Zirconium-copper alloy is used for material of rotor bars and resistance rings because of its superior mechanical properties for the first time. IR adopted both the designs. New designs save USD 4 million annually for IR.