GB617424A - Improvements in or relating to apparatus for converting a direct potential into an alternating potential - Google Patents

Abstract

617,424. Electric control systems. JOHNSON, D. C. March 21, 1946, No. 8833. [Class 40 (i)] [Also in Group XXXV] In apparatus responsive to the polarity of a direct potential which is connected across a coil wound on part of a magnetic circuit, the reluctance of which is cyclically variable at a steady frequency, a discriminating relay compares the phase of the A.C. induced in the coil with that of a standard A.C. of the same frequency. In a system for measuring or using the temperature registered by a thermocouple 1, the thermocouple terminals 2, 3 are connected to a circuit comprising a choke 6, a potentiometer 7, and a coil 5 wound on a high permeability core 4. A horseshoe core 10, which completes the magnetic circuit of coil 5, is rotated by a synchronous motor 11 at half the frequency of the supply current which is also fed through a transformer 20 to anodes 17, 18 of two triodes. The reluctance of the magnetic circuit of coil 5 thus varying cyclically twice in each revolution of core 10, A.C. of the supply frequency is induced in the coil circuit and the A.C. volts produced across choke 6 are applied through a valve amplifier 14 to grids 15, 16 of the triodes. In one triode, the potentials of grid and anode are in phase and in the other they are 180 degrees out of phase. The phase of the grid volts is determined by the direction of current in the thermocouple circuit, and as the thermocouple is initially balanced against the voltage of potentiometer 7, it depends upon whether the thermocouple temperature is above or below the datum. Thus, one or other of the triodes will conduct according as the temperature is above or below datum, and the magnitude of the anode current will be proportional to the A.C. volts and therefore to the D.C. volts of the thermocouple and thus to the temperature. If 21, 22 are coils in a differential ammeter or a split field motor, the temperature may be indicated or used to control mechanism. The residual magnetism in core 10 produces an A.C. having a frequency of half the supply frequency superimposed on the other A.C. Since this means that each grid voltage varies similarly with respect to the anode volts, each valve is rendered conductive by the residual magnetism effect to the same extent during each full cycle of the main A.C. so that it produces no effect in the differentially wound ammeter or motor. Choke 6 may be replaced by a resistance or transformer, or a second coil may be wound on core 4.