Localized rotor overheating of large Direct on Line (DOL) motors

Most often, serious reliability problems with large motors particularly synchronous motors may simply go unnoticed especially if the industrial facility´s operation is a priority. The nature of such problems is not very well known in industrial applications and can be difficult to detect in the early stages of its development until a motor failure occurs. Localized rotor overheating commonly known as hot spots or black marks for large motors and specifically synchronous motors is one example of such a scenario. This paper will provide insights to the nature of the localized rotor overheating for Direct On Line (DOL) large motors in general, problem detection methods and will propose possible rectification means in order to resolve the problem definitively.