Basics of multi-stress aging tests: survey of actual operating conditions of large industrial motors

A study of operating conditions of 563 large industrial motors was made in Finland by visiting the factories and sites and observing various parameters on the site. From the statistics gathered, the distributions of various parameters like ambient temperature, humidity, and percentual loading can be determined. Also, the distribution of actual temperatures of windings can be calculated, showing, e.g. that the average operating temperature of a Class B industrial motor is only 70°C or that only 0.6% of motors operate above 110°C. When multistress aging tests are planned, actual operating conditions like those collected in this study should be used as starting points or design basis values. Thermal acceleration should be estimated using the observed actual temperatures, mechanical stresses should be estimated using the observed starting frequencies, etc. In this way, the combinations of aging stresses and levels can be selected so as to produce reliable test lives in multistress aging tests. For most parameters, including operating temperature, the 95% limit value is proposed as the design basis value. Based on these design basis stresses, the actual life of the insulation can be found without using a reference insulation system