A phase-calibrated variable-frequency supply for the testing of servo mechanisms

The paper describes apparatus suitable for measuring the frequency response of servo mechanisms and similar devices. The voltage and waveform of the supply are substantially independent of the frequency used, and there are two outputs of known and variable phase relation. One of the outputs is used to oscillate the servo mechanism, while the other is used to follow the phase shift through the servo transmission, an oscillograph being used as a phase null indicator. The frequencyconvertor of a Schrage motor is used to provide the supply, the phase calibration being obtained either on the brush gear, or on a phase shifter supplied at 50c/s and operating through an induction-type frequency changer. The rotor is independently driven by a d. c. motor at a speed depending upon the frequency required. Two impedance determinations, covering a frequency range 0.5¿150 c/s, illustrate the types of measurement possible. A test on field coil at a constant potential of 20 volts, is an example at low power test on the free-to-run armature of a separately-excited motor, at constant current of 30 amp, is an example at high power. The effects of armature reaction in the frequency convertor are briefly discussed, and tests of the phase calibration of the Schrage machine are described.