Application of Temperature Ratings for Industrial Electric Induction Motors

Standards for motor temperature ratings have been in existence for many years and are generally well recognized and applied by manufacturers of electric motors and experienced users. However, there are also frequent misunderstandings about the basic motor ratings and capabilities resulting from the application of these standards. This paper attempts to offer a clear presentation of the basic temperature classifications of the primary motor standards in NEMA (2001) and IEC. These building blocks of motor design will be correlated to application issues to provide an insight to the variables of temperature ratings on induction motors. It will cover low and medium voltage motors commonly available, and will relate to both electrical and mechanical factors affected by temperature ratings. The principles and discussion within apply primarily to continuous duty induction motors defined by NEMA (2001) as medium and large three phase alternating current (AC) induction motors ranging from one horsepower to as high as 50,000 horsepower