April 22, 2005
4 Motor Testing and CBM
Understanding the term ‘motor failure.’
VP, Electrical Reliability Group
T-Solutions, Inc.
What is Failure?
Technology, at least most of them, can be used to detect some stage of impending failure. The question then becomes hwo to define failure and how to you select ‘condemning’ criteria. If you expect an answer that relates to an exact science, then you will be disappointed.
The act of selecting condemning criteria requires understanding your system and the art of probability. As such, reliability engineering requires the study and experience of probability. An incorrect question to pose to a reliability professional is: “How long will it last?” The correct question is,”What is the risk that it will not last this long?” Just as when a doctor provides a chance for success when related to your health, the reliability professional provides the chance for success of your system health.
There are, of course, two basic types of failure: Evident, which are observable to the operators and hidden failures, which do not appear until other systems fail. For instance, a fuse that will not open would be a hidden failure, as would be a faulty pressure relief valve.
In this case, however, we are going to focus on evaluating systems in which the condition can be evaluated. The interesting part is that we are going to provide pass/fail guidelines in this series. Guidelines are, of course, not exact. For instance, the IEEE Std 43-2000 states that the minimum insulation resistance for a 480 Volt motor is 5 Megohms to ground. However, I have seen motors start with insulation resistance far less than 1 Megohm. Would this then be a failure?
This topic will be explored in more detail in the future.
