Patent GB-735916-A (GB)
Title: Improvements in or relating to electrical bridges employing transformer ratio arms
IPC: G01R17/10
Filed: 1953-04-10
Abstract: 735,916. Measuring bridges. WAYNE KERR LABORATORIES, Ltd., and CALVERT, R. April 2, 1954 [April 10, 1953], No. 9870/53. Class 37. An A.C. bridge for determing Q, loss angle, power factor, or dissipation constant of an impedance comprises an excitation transformer having two secondaries in series as the ratio arms, a detector transformer a first point on whose primary is connected to the junction of the ratio arms, means for connecting the test impedance between the free end of the first ratio arm and a point on the detector transformer primary remote from the first point, an adjustable low impedance potential divider across the second ratio arm developing at its slider a known fraction of the voltage thereacross, a standard reactance connected between the slider and a point on the detector transformer primary remote from the first point, an adjustable potentiometer between the slider of the potential divider and the junction of the ratio arms, and a known resistor connected between the potentiometer slider and a point on the detector transformer remote from the first point; the adjustable potentiometer being calibrated in terms of the quantity to be determined. In Fig. 2 the ratio arms are constituted by the series secondaries 31, 32 of an A.C. energized transformer 30, the free end of winding 31 being connected through unknown capacitative impedance 33 to the free end of the primary 34 of transformer 35 whose secondary energizes an unbalance detector (not shown), the junction of windings 31, 32 being directly connected to the junction of primaries 34 and 58. Winding 32 is provided with a uniform range of tappings adjustably connected over sliders 40, 41 to a tapped autotransformer winding 43 connected over sliders 44, 45 to a similar winding 46 connected over sliders 47, 48 to a slidewire potentiometer 49 having a slider 50, the successive tappings constituting a decade potentiometer for deriving accurately any fraction of the excitation voltage across winding 32. Slider 50 is connected over standard capacitor 51(C) to the free end of the primary 34 of transformer 35, and adjustable potentiometers 52, 55 are connected between slider 49 and the ratio arm junction. The slider of potentiometer 55 is connected over resistance 57 to the free end of the primary 58 of transformer 35 while the slider of potentiometer 52 is connected over standard resistance 54(R) to the upper end of the primary connected to the unknown impedance. In operation the decade potentiometer is adjusted to balance the bridge for reactive components of the impedance while potentiometer 52 is adjusted to balance for resistive components, and it is shown that for the capacitative impedance 33, Q = #CR/K2, where K2 is the fractional setting of potentiometer 52, which may accordingly be calibrated to indicate Q, loss angle, power factor, or dissipation constant. The signal from potentiometer 55 developed across primary 58 in opposition to that across primary 34 compensates for the loss resistance of standard capacitor 51, and where the unknown impedance is inductive the connections from potentiometer 52 and capacitor 51 and from potentiometer 56 to the unbalance transformer primary must be interchanged. A standard inductance may be substituted for standard capacitance 51 with appropriate circuit reversals. The decade potentiometer may be replaced by a simple potentiometer and the circuit compensating for the standard reactance loss resistance may be omitted (Fig. 1, not shown).