Patent GB-829239-A (GB)
Title: Electrical test instrument
IPC: G01R1/22
Filed: 1957-08-29
Abstract: 829,239. Electric measuring instruments; frequency and power factor measurements. WELWYN ELECTRICAL LABORATORIES Ltd. Aug. 29, 1957 [Sept. 7, 1956], No. 27257/57. Class 37. An electric test instrument comprises an A.C. generator 51 driven by a synchronous motor 60 energized by an A.C. source and means for measuring the output voltage magnitude of the generator; whereby a direct frequency measurement of the A.C. source is obtained. As described, the test instrument is adapted to measure voltage, current, frequency, power factor, and waveform distortion, by movement of the ganged rotary switches 40a, 40b from the number 1 or OFF'position to the positions 2, 3, 4, 5, 6, respectively. Voltage measurement. The voltage is applied, across terminals 25, 26, through range switch 30 and transformer secondary 33, to one diagonal of rectifier bridge 11, 12, 13, 14; and is measured by moving-coil meter 10 connected across the other diagonal. Current measurement. A current carrying conductor 19 is encircled by legs 16, 17 of mechanically-operated tongs which provide a closed magnetic circuit through core element 15. The voltage induced across coil 21 encircling this core is fed through range switch 22 to the diagonal of the rectifier bridge and thence to meter 10. Frequency measurement. An A.C. voltage is applied across terminals 25, 26 through range switch 30 to transformer secondary 32 and thence to synchronous motor 60 which rotates one revolution per frequency cycle of the applied voltage and is coupled to A.C. generator 51, the output of which is proportional to the generator speed, and thus to the applied voltage frequency, and is fed from stator coil 50 to one diagonal of the rectifier bridge. The meter 10 thus provides a measure of the applied voltage frequency. Power factor measurement. The voltage across a circuit of which the power factor is to be measured is applied across terminals 25, 26 to drive motor 60 and thus develop a voltage across the stator coil 50 of generator 51. Tong legs 16, 17 encircle a conductor carrying the circuit current, thereby inducing across the secondary of the transformer 43 a voltage in series with, but opposed to, the voltage across the stator coil. Potentiometer 41 is adjusted to match these voltages, and the stator coil is rotated manually about the rotor axis of generator 51 thereby varying the relative phase of these voltages until a null reading is obtained in meter 10. The angle through which the stator coil is rotated is equal to the phase angle between the supply current and voltage. The coil is mounted on a non-magnetic gear-wheel which meshes with a sprocket attached to a control knob on the test instrument. The periphery of the gear-wheel is graduated and is visible through a window of the instrument case, thus enabling the phase angle, or power factor, to be read, Figs. 1-3 (not shown). Distortion measurement. A true null reading is only obtained in the power factor measurement if both current and voltage have the same waveform. Generator 51 produces a pure sine wave output, and the distortion of the current waveform from sinusoidal is indicated by the residual reading of meter 10 at the null position; the meter sensitivity being increased for this purpose by using position 6 of switch 40 whereby the dropping resistor 45 is shorted.