Investigating the Failure Mechanism of Wet eMachines in the Presence of Transmission Oils

Abstract

This paper investigates the failure mechanisms in electric machines used in heavy trucks and buses exposed to transmission oil. Five machines from field operations were examined. Subsystem tests—copper corrosion testing, conductive deposit tests, material compatibility tests, and oil analysis—were conducted. Copper sulfide, known for its conductive properties, was found in failed machines and during subsystem tests with lubricants containing reactive sulfur components in contact with exposed copper. Two prerequisites for the chemical reaction—the presence of reagents and sufficient activation energy—were met at normal operating temperatures. Analysis revealed the sulfur presence in damaged machines and identified cracks in the polymer coating on copper wires, allowing corrosive substances to reach the copper surface. The proposed failure mechanism involves active sulfur reacting with copper to form copper sulfide. These layers can create contact bridges between phases in the eMachine, leading to short circuits, sharp temperature increases, electrical arcing, and total machine failure. Coating aging experiments showed that fluids can form conductive films on the polymer layer. Oil conductivity measurements showed minor changes within the static dissipative regime, and contamination by transmission and axle oil elements was found in failed eMachines but not in functional ones. Activation energy experiments demonstrated that copper sulfide forms at normal operating temperatures, with damage likely within 10–100 days depending on conditions. These findings highlight the risks associated with certain oils in eMachines and the need for further research to develop strategies to mitigate these risks and ensure long-term reliability.