GB932704A - Apparatus for measuring the quotient of two electrical quantities - Google Patents

Abstract

932,704. Measuring impedance. BROWN, BOVERI & CO. Ltd. Oct. 23, 1961 [Oct. 25, 1960], No. 37857/61. Class 37. In an apparatus for measuring the quotient of two sinusoidal electrical quantities, e.g. impedance as a quotient of voltage (u) and current (i), the quantity (i), to be taken as the denominator of the quotient, is supplied to a parallel circuit consisting of a choke coil 1 and two Zener diodes 2 and 3 connected in series in opposite directions, in which it is converted into a quantity iz having a relative phase displacement proportional to the reciprocal value of i. Signals representing i and iz are fed to a phase comparator 8, which produces impulses the breadths of which are proportional to the phase displacement of iz, i.e. to the reciprocal of i. The amplitudes of these impulses are made proportional to the other quantity u, to be taken as the numerator of the quotient, so that the area of each impulse is proportional to the required quotient. Each impulse is then passed through an integrating circuit 10 which produces a triangular pulse whose peak amplitude is proportional to the quotient, and this peak value is indicated on the meter 11. The operation of the parallel circuit may be explained by reference to Fig. 2, where the input current i is shown in Fig. 2 (a), and the voltage that would normally appear across the inductance 1 if the current i passed through it is represented by curve U DR in Fig. 2 (b). This voltage at first is sufficiently large to cause the Zener diode 2 to conduct in its reverse direction, so that the reverse voltage U R2 across this diode is applied across the inductance 1, and the actual current through the inductance builds up as shown in Fig. 2 (c). The current through the diodes (iz = i - i DR ) therefore follows a curve as shown in Fig. 2 (a). At the point where i DR = i and iz = O the voltage across the Zener diode pair reverses, and i DR starts to fall. i DR depends only on the breakdown voltages of the Zener diodes and the value of the inductance 1 so that the largest phase differences #γ between iz and i occur for the smallest values of i (iz returns to zero more rapidly for smaller peak i) and it can be shown that this difference is rigorously proportional to 1/i. Voltages proportional to the currents i and iz are tapped across the resistorr 4 and 5 and converted in the peak limiters 6 and 7 into rectangular form. They are then supplied to a thyratron 8 which delivers an output pulse only when the two inputs are of different polarity, so that the width of this pulse is proportional to the phase difference between i and iz, i.e. proportional to 1/i. This pulse is then amplitude modulated by the voltage u in the unit 9, and eventually the quotient u/i is displayed on the meter 11 as above described.