Abstract :
Before it is possible to put a group of motors into service, it is necessary to provide means for generating and transmitting the energy which probably will be required to operate them. To select the proper and economical capacity of such generating and transmitting equipment, requires that some reasonably correct estimate be made of the probable amount and probable time distribution of the combined load due to the motors when in service. When the motors all operate in a definite sequence on a definite time schedule the problem is simple. But, in the great majority of cases, such as tool, hoist, elevator or traction drive, the motors comprising the group do not in general operate in any such connected sequence. They operate more or less at random. It is usual to estimate the probable load under such conditions of random, or approximately random, operation by guess work controlled only by experience and by comparison with load records obtained from similar installations already in operation. In general this method of estimating from records is not satisfactory. In the first place such records, if sufficient for the purpose, are expensive to obtain and are not always available. In the second place, they are generally records of effects and not of causes. Whereas, as is shown in the paper, if the causes are known, the effects can be foretold with surprising accuracy. If certain average duty cycle characteristics of the motor application are known, as they must be known to make an intelligent selection of the motors for the required duty, then, by a simple application of the formulas of probabilities, it is possible to determine the characteristics of the energy input to any group of such motors. No very great refinement in determining the average duty cycle for the individual motors in the group is required, for, if the number of motors or the number of observations of actual duty cycles be reasonably large, the average deviation from the mean will be small. It is fou- d that the method proposed gives calculated results that check quite closely with measured results. Not only can the average input, and from this the probable power consumption in kilowatt-hours be determined, but also the r. m. s. or heat generating value of the load as well as the probable amount and frequency of the peaks. Furthermore, data may be obtained for plotting in advance the most probable time distribution of load similar to a graphic instrument record. The whole method is based on a determination of the average value of the operating factor for the group of motors under consideration. The operating factor is the ratio of the average time any motor is running to the average duty cycle period. The operating factor may be averaged over any group of motors or, its average value may be determined from a graphic instrument record, such as obtained from a graphic wattmeter or ammeter. The value of the average operating factor is a better measure of the variations of load or input to a group of motors than either demand factor or diversity factor. In fact, it is shown that the value of the demand factor is generally meaningless and misleading. The value of the operating factor seems to lie within fairly narrow limits over numerous cases of the same kind of motor application. The demand factor does not possess this character. Having thus established a method of rating the current-carrying and protective devices handling the input to a group of motors, it is necessary to establish a method of similarly rating conductors and protective devices handling the input to individual motors in the group. In other words, it is necessary to determine the heating value of any given intermittent duty cycle in terms of a continuous duty having equal heating value. The ratio between these two duties is found as a ratio of duty cycle factors expressed in terms of the operating factor for the individual duty cycle. In the continuous duty the operating factor is unity, and the du
