Accelerated life testing of a traction inverter

An IGBT based traction inverter drive system has been developed for the new generation of Northern Line trains for London Underground. New inverter designs may experience reliability problems associated with new manufacturing build as well as design integrity. In this case the use of IGBTs introduces a major change from the switching components used previously in this type of application and the cost implications to the equipment supplier of unreliable operation would be extremely large. Assurance that a number of known areas of concern with reliability of the equipment has been adequately covered in the design process is therefore essential. A particular concern is with the IGBT wire bonded construction with the wires becoming detached under conditions of severe thermal cycling. The particular concern was the station to station running where the substrate temperature might rise and fall through more than 20°C, due to load current variation, whilst the heat-sink temperature would stay constant. To ensure that the whole inverter circuit will be sufficiently reliable in service a test system was built which would allow an accelerated life test programme to be carried out on the new inverter design prior to its delivery. An innovative test circuit was used in which the electrical operating conditions are matched to those which would be experienced in service but at an accelerated rate. This is achieved by operating the test circuit continuously under an automatic and unmanned test regime