Precision testing of electric tachometers

The magnetic-drag electric tachometer which was developed especially for the measurement of aircraft-engine speed is inherently a more precise measuring device than those instruments which normally have been used as reference standards of speed measurement. This paper describes an electric-tachometer testing system which meets the requirements of high accuracy and is also suitable for large-quantity manufacture of tachometers. The system provides facilities for testing at 22 scale points in a range of speeds from 500 to 4,500 indicated rpm without referring to a standard instrument. The power to drive the tachometers being tested is supplied from tachometer generators which are driven by a d-c motor controlled by a precision tuning fork. The method of operation and the control system are explained.