SIMILARITY RELATIONS IN ELECTROMAGNETIC MOTORS - LIMITATIONS AND CONSEQUENCES FOR THE DESIGN OF SMALL DC MOTORS

摘要

In theory, the size of electrical motors can be varied simply by rescaling all the dimensions of a given motor by a constant factor. The influence of a proportional downsizing on motor performance data such as power dissipation capabilities, produced torque and torque losses is investigated to give a first glance on the theoretical laws of rescaling. These considerations are verified by investigating existing small brushed and brushless permanent magnet motors from a diameter of 40 mm downward. We concentrate on slotless motor designs which are most promising at very small dimensions due to the absence of magnetic detent torque. Extrapolating the motor data down to smaller diameters and comparing to the theoretical behaviour gives quantitative predictions about the torque characteristics of smallest dc motors. Combining these findings with reasonable assumptions about achievable tolerances and bearing technology results in minimum motor sizes of 1 to 4 mm depending on design (brushed, brushless), manufacturing processes and technology, as well as achievable tolerances.