Drag-Cup A-C tachometer with constant-current excitation

AS A consequence of recent publication of a general analysis of the drag-cup a-c tachometer,1 numerous inquiries have been received relative to the desirability of operating such a device from a current source. Hence, a brief examination of the possibilities of such operation seems worth while. In these communications the ideas expressed about constant-current operation have been to the effect that with fixed exciting flux extreme linearity of output voltage with respect to speed should be achieved, and that troubles caused by change of winding resistance with change of temperature should be eliminated. Mathematical examination shows that extreme linearity indeed is theoretically possible. However, design difficulties in the tachometer itself, temperature effects other than change in winding resistance, and the difficulties of designing and constructing an adequate current source make doubtful the practical realization of the theoretical advantages of operation from a current source to a sufficient extent to show superiority over operation from a voltage source.