GB337907A - Improvements relating to automatically varying series resistances for use with alternating current - Google Patents

Abstract

337,907. Ferranti, Ltd., and Porter, G. L. Sept. 23, 1929. Current and pressure meters; frequency meters ; contacting -solid resistances; sets of resistances.-In order to increase or decrease, as required, the response of an alternating-current instrument to small changes of voltage, there is connected in series with the winding c, Fig. 1, of the instrument or relay a resistance comprising two oppositely connected metal rectifiers a, b, as described in Specification 309,514, the value of which resistance rapidly falls with increase of current. In a modification, Fig. 2, an instrument or relay has two oppositely-connected windings d, e, the current in the winding c being controlled by a constant resistance f and that in the winding d being controlled by a variable resistance a, b as described above. The resultant flux follows a curve as shown in Fig. 3, having a maximum value in one direction at the point m at which the ampere-turns of the windings d, e are increasing at the same rate. By choosing optimum values of resistance and turns, this portion of the curve can be made approximately horizontal for a range of voltage corresponding to that occurring on a supply system. An instrument of the induction type, or other type sensitive both to frequency and current, will therefore act as a frequency indicator. If an element operating on the decreasing portion of the curve shown in Fig. 3 is mechanically opposed to one having a rising characteristic as shown in Fig. 1, the resultant torque varies very rapidly with change of voltage, passing through a zero value at the voltage at which the opposed torques are equal. Such an arrangement is suitable for a relay for maintaining constant the voltage of a system. In another modification a double winding as described above forms one of the windings of a dynamometer, which in consequence has no torque at the critical voltage and torques of opposite sign for voltages above and below this value. A similar result may be obtained by connecting two of the above-described variable resistances in opposite arms of a Wheatstone bridge connection, the other two arms being constant resistances. One pair of opposite points are connected to the supply and the other pair to the winding of a dynamometer.