Solving locomotive wheel polygonisation by reducing variable frequency drive interharmonics

Severe vibrations and an increase in structural component fatigue failures have been observed on a freight locomotive class operating in South Africa. The cause of the severe vibration was linked to polygonal wheel wear dominated by the 20th order. This locomotive is powered by an alternating current traction system, which is controlled by a variable frequency drive. The variable frequency drive generates interharmonics that produce torque pulsations in the torque output from the motors. Interharmonics are determined by the alternating current supply frequency ( f g ) and the fundamental frequency ( f m ) of the variable frequency drive. The interference of the torque pulsations with a coupled resonance mode is reaffirmed as the root cause of the polygonal wheel wear. An experimental investigation of this phenomenon was performed by means of on-track testing. It was found that the 5 f g - 7 f m interharmonic excited the coupled resonance mode at approximately 42 km/h, which fixes the 20th-order polygon on the wheel. Two approaches that may eliminate the polygonal wheel wear were introduced and discussed. It was found that the forced excitation of the coupled mode by the 5 f g - 7 f m interharmonic could be avoided by postponing the speed where the alternating current traction motor's pulse mode changed.