Patent GB-527143-A (GB)
Title: Improvements in or relating to a method of and apparatus for comparing alternating currents
IPC: G01R1/30
Filed: 1939-03-31
Abstract: 527,143. Current, voltage and phase-differ,- ence measurements. ELLIOTT BROS. (LONDON), Ltd., and PETCH, H. S. March 31, 1939, No. 10153. [Class 37] Two alternating currents of the same frequency are compared in magnitude and phase difference by connecting the sources differentially to a winding on a magnetic core, applying a neutralizing current to another winding' on the core, and measuring the magnitude and phase of the current required to neutralize any flux produced by the first winding. The invention is shown applied to the determination of the ratio and angle errors of a current transformer. The secondary windings 1,2 of a standard transformer and the transformer under test are connected in series with their electromotive forces acting in the same sense round the circuit, the primary windings being connected in series. Tappings from the connecting wires 3, 4 between the ends of the secondary windings are taken to a coil 5 on a ring 6 of high permeability material. In series with the secondary winding 1 of the standard transformer is an auxiliary auto-transformer 7, across the output terminals of which are two calibrated potentiometers 8, 9. A neutralizing winding 10 is connected to the potentiometer 8 through a reversing switch 12 and a second neutralizing winding 11 is connected through a reversing switch 13 and a condenser 14 to the potentiometer 9, so that the current in the winding 10 is in phase.with the current in the secondary winding 1 whilst that in the winding 11 is in quadrature therewith. A fourth winding 15 on the ring 6 is connected to an indicating instrument 16 such as a vibration galvanometer. If the voltages in the secondaries 1, 2 are identical in magnitude and phase no potential difference is developed across the leads 3, 4 and no flux is set up in the ring 6. If, however, there is a difference between the errors of the transformers 1, 2 a flux is set up in the ring 6 and a deflection is produced on the galvanometer 16. By adjusting the potentiometers 8, 9 until the deflection is reduced to zero the in-phase and quadrature errors are neutralized and may be read off from scales attached to the potentiometers. In order to provide for testing transformers having different secondary current ratings, the auto-transformer 7 may be provided with input tappings corresponding to each rating. The winding 5 is also divided into three sections each of which is subdivided into a pair of windings of unequal numbers of turns, either member of which may be connected to the leads 3, 4 by a range selector switch. In order to compensate for any departure from pure quadrature in the condenser 14, a compensating winding (not shown) may be reversely wound on the ring 6 and connected in parallel with the condenser 14 and its winding 11 through a resistance. The winding 11 may alternatively be supplied through an inductance. An ammeter may be connected in series with the output of the transformer 7 to check the output, and may be shortcircuited by a link when not in use. Provision may be made for inserting any desired burden in the secondary of either transformer. According to the Provisional Specification the windings 1, 2 may be connected in opposition and, in series with the coil 5.