Improvements in and relating to electric circuit arrangements for compensating undesired voltages

Abstract

Abstract 580,005. Voltage measuring. MORRIS, D. G. O., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. April 5, 1943, No. 5447. [Class 37] [Also in Group XXXV] To derive from a load impedance a control voltage independent of unwanted voltages occuring also in the impedance as for example to obtain an accurate measure of the back e.m.f. of an electric motor, the load impedance is connected in series with an equivalent impedance wherein is produced a voltage equivalent to the unwanted voltage which is then caused to act in opposition to the voltage across the load impedance so as to compensate wholly, partly or in excess, the undesired voltage. Fig. 1 shows a Wheatstone bridge arrangement in which the load impedance and the equivalent impedance R1 (L1) are associated with resistances R3, R4 to provide a potential on conductors 3, 4 in which the unwanted potential, e.g., the resistive or inductive drop in the armature G of a motor, is eliminated. Fig. 2 shows an arrangement to compensate for the voltage occuring due to a change in current in an inductive load, the equivalent impedance including an inductance P having a secondary winding S in circuit with the conductor 4. Fig. 4 shows the application of this arrangement to a four pole machine a wherein the circuit 3, 4 is arranged to balance out the potential drop in the armature brushes. Fig. 3 shows means by which a control quantity dependent in the velocity of an electric motor may be obtained. An amplifier C, such as a vibrating contact device controlling, for example, the voltage applied to conductor 1, 2 in accordance with the difference between the said velocity and a desired velocity defined by a controlling winding w<2> has a winding w<1> connected across auxiliary armature brushes and opposing the winding w<2>, a compensating winding w<3> opposing the winding w<1>, being connected across the equivalent impedance R, L1. Specifications' 566,882 and 577,315, [both in Group XXXVII], are referred to.

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