The time constants are the times in which the temperature rises or falls by 0.632 times its maximum value ?m, and are provided by the machine manufacturer. The short time rating of a CMR (continue duty) motor varies with its thermal time constant and may differ from one manufacturer to another depending upon the cooling design adapted and its effectiveness. The shorter the thermal constant, the lower will be the short time rating a CMR motor can perform. It is not, however, practical to achieve the thermal time constant infinitely high, which is a compromise with the economics of the motor’s design such as size, wall thickness of the housing, number and depth of cooling fins and efficiency of the cooling fan. 
