Approximate analysis of the drag cup A-C tachometer

Temperature coefficient, output voltage, and speed linearity error of output voltage are indicated in terms of relative changes in tachometer cup impedances when the output winding is essentially unloaded. By replacing the drag cup of a precision drag-cup a-c tachometer with one having a higher resistivity and a lower temperature coefficient of resistance, the following improved characteristics were observed: 1. The output voltage temperature coefficient was reduced to -0.06%/1 C (degree centigrade) 1/4 its former value. 2. Up to synchronous speed, the output voltage speed linearity error was reduced to ±0.05% of its synchronous speed output, 1/12 its former error. And 3 output volts per 1,000 rpm was reduced to 0.8 volt, 2/7 its former value. A simplified basic tachometer performance equation is derived, and curves are presented to show the required change in drag cup parameters to realize a specified performance. Frequency compensation is achieved by an R-L-C network in the tachometer excitation circuit.