Abstract :
VARIOUS American standards for electrical apparatus provide, in addition to continuous ratings, certain short-time ratings, based on 5-, 15-, 30-, and 60-minute runs with the apparatus starting cold. Usually these short-time runs do not correspond to conditions met in service; for example, a motor very seldom starts cold, operates for a limited period of time, and then remains at rest until it has cooled to ambient temperature. Often these short-time ratings are applied to intermittent loads, such as shown in figure 1, in which T represents the total time for each cycle of operation; Tr = kT, the running time; and Ti = (1−k)T, the time the motor stands still. Since the short-time ratings do not indicate the type of operating cycle with varying values of T, k, and operating current I, for which these ratings are suitable, suggestions have been made from time to time to establish standard intermittent load ratings, especially for motors. Efforts along these lines have so far not led to any definite action because it is difficult to find practical standard provisions applicable to the many variations encountered in operating cycles. Furthermore, the present standard short-time ratings are so simple and convenient for acceptance-test ratings that there naturally is considerable hesitancy in adopting a complicated system of intermittent ratings. This raises the question of the possibility of devising various reasonably simple means for applying short-time ratings to intermittent load conditions without resorting to time-consuming temperature calculations, for instance, those used in the application of railway motors. In many industrial applications the cost of these calculations would be entirely out of proportion to the total cost of the motor and any slight saving in the size and cost of the motor resulting therefrom.
