Equivalent thermal circuit for nonventilated motors

ALTHOUGH heating is one of the chief limitations in the rating of induction machines, present methods of predicting heating leave much to be desired. Articles have been written on the subject of heating of very large motors and generators,^1,2 but relatively little appears to have been done toward developing accurate and comprehensive methods for calculating the heating of National Electrical Manufacturers Association (NEMA) frame size machines. Unfortunately, the methods developed for large machines are not well suited, as for smaller machines.