Determination of temperature rise of induction motors

The “American Standards for Rotating Electrical Machinery” (American Standards Association) prescribe that the temperature rise of motors shall be measured by the thermometer method for purposes of rating. Definite limiting values of temperature rise are established for each type of machine. The AIEE Standards No. 1 fix conventional allowances of 15 degrees centigrade, 10 degrees centigrade, and 5 degrees centigrade between the actual hottest-spot temperature and the highest observable value of temperature as determined by thermometer, resistance, and embedded-detector methods, respectively. This paper presents information on the relations between the measured values of temperature rise by different methods as found in tests on several hundred induction motors ranging from 10 to 1,000 horsepower in rating. It is shown that, with modern motor construction, variations of 20 degrees centigrade or more are sometimes obtained by the thermometer method on a given machine depending on location of thermometers or thermocouples, whereas the resistance measurements give relatively consistent values of temperature rise. It is therefore suggested that the standards for temperature-rise measurements be revised, and that the resistance method be adopted for all forms of enclosed or protected machines which are not readily accessible for application of thermometers on laminations, insulated windings, and other adjacent parts.