Predicting motor and generator maximum torque as a function of mass

There is currently no universally accepted way to predict the maximum input/output torque of electrical generators/motors as a function of the machine's mass. A new correlation is proposed for this relationship that collapses the data from machines with masses ranging over more than three orders of magnitude and torques ranging over five orders of magnitude. It is shown that, at least for machines operating below 3600 rpm, this relationship is only weakly dependent on other parameters of the machine design such as operating rpm, motor type (inductance vs. permanent magnet), operating voltage etc_ Physical arguments are given to explain the correlation and its independence from other operating parameters. This new correlation allows a system designer to have a simple one variable equation to calculate the mass of a motor or generator given a desired torque capability. Because the correlation is so predictive, it can also be used as a metric to compare different machine designs. Families of machines that have slightly higher torques than predicted by the correlation typically are “better” designs.