Performance and heating curves for motors on short-run duties

The paper shows how curves may be prepared that facilitate the design or selection of motors for services with short runs such as the operation of cranes or urban transport. From these curves, with little calculation, the values of such quantities as mean speed, power consumption and temperature rise of motor when operating on a statistically specified schedule, may be estimated for various combinations of motor windings, gear ratios, accelerating currents and the other quantities open to choice. The procedure is based on the recognition of the conditions under which the characteristic curves of the motor and the speed/time curves in operation are of similar shape. One condition for this is that the magnetization curves of the motors should be of the same shape. For classes of motor and operation satisfying these conditions for similarity, various `required¿ quantities may be given by single curves with scale factors. It is shown that the curves as drawn (i.e. with scale factors of unity) are the characteristics of a hypothetical sample or `model¿ of the class of drives. Such models are not necessarily realizable physically. The principles are illustrated by the development of curves giving run times, current consumption, motor-heating coefficients and other quantities that result from the operation of series-wound motors having magnetization curves of the shape found to be characteristic for modern mill-type motors. The application of these curves is briefly illustrated by consideration of the choice of a motor for a long-travel motion on a crane. The data presented are intended to be illustrative rather than complete and to suggest means by which users and designers may prepare data adapted to their particular needs and to the particular classes of motor in question.