|Hybrid and electrical vehicles (HEV) are the next evolutionary step in automobile technology. However, the electrical systems which propel HEVs are fundamentally different from conventional technologies. Therefore, a few limiting technologies will delay widespread HEV success such as the battery, power electronics and connectors. The high temperatures, vibrations, humidity, and contamination in a vehicle can reduce the reliability of these technologies.
HEV electrical connectors conduct much more power and are more susceptible to failure and reliability problems than the connectors in conventional vehicles. Thus, in this work, a 40A high power electrical connector, used in HEVs, has been studied extensively by both modeling and experimental approaches.
In this work, a multi-physics (involving structural, electric and thermal coupled fields) finite element model considering multi-scale rough surface contact of the 40A high power connector is created. This cutting-edge model includes the coupled effects of nano to macro-scale surface roughness, contact pressures, electrical and thermal contact resistances, stresses, displacements, applied currents, electric potential (voltage drop), current density, temperature, Joule heating and thermal expansion. It is a powerful tool that can be used for fundamental connector characterization, prototype evaluation and design.
A few prominent findings were made from the results of the 40A connector model. It appears that the current flows mostly through very small regions that are usually near the contacting surfaces in the connector, thereby suggesting that the available conducting material can be more efficiently used by developing optimized connector designs. Interestingly, from the 40A connector model, it was found that the temperature rise (ΔT or change in temperature) in the bulk material is not very high, although ΔT values measured experimentally indicate otherwise. Through analytical calculations and experimental measurements of ΔT for the cable and the connector, it is believed that a large portion of the temperature rise in actual 40A connectors is due to the Joule heating in the supply cables. However, the local asperity temperature is also theoretically calculated and should be very high at the contact, which could cause an increased oxidation rate and surface melting.
Coming to the experimental investigation in this work, 40A connectors were tested under both stationary as well as vibrating situations. For stationary tests, an increase in connector resistance and connector temperatures with an increase in applied currents is noticed. Also, more importantly, the same increasing trend of connector resistance with respect to applied currents is observed in both the 40A connector model and the stationary connector tests.
An environmentally controlled and accelerated 40A connector test methodology was designed and created to characterize connector degradation and fretting under vibrating conditions. This includes the necessary hardware to control connector conditions and to monitor current, voltage drop, connector resistance (R) and connector temperature (T) in real time. A series of parametric tests were completed where the effects of vibration direction, amplitude, frequency, temperature and humidity on R and T were studied.
Based on the accelerated test results, larger increases in the values of average and maximum R occur from vibrations in the Y direction (perpendicular to cable axis (Z)). Significant change in R and T (either, average or maximum) occurs at the highest vibration frequency of 200 Hz and in the Y direction. Increase in R could be due to an increase in T, fretting corrosion and wear. Also, frictional heating and increased Joule heating lead to an increase in T.