Patent GB-966279-A (GB)
Title: Current comparator device
IPC: G01R1/28
Filed: 1962-12-21
Abstract: 966, 279. Voltage, current measurement. CANADIAN PATENTS & DEVELOPMENT Ltd. Dec. 21, 1962 [Dec. 26, 1961], No. 48325/62. Heading G1U. [Also in Division H1] A current comparator device comprises an inner closed magnetic core surrounded by a detection winding toroidally wound thereon; a second closed tubular magnetic core extending coaxially with the inner core and positioned radially outwards of the detection winding; at least two outer windings toroidally wound over both cores, and a further winding positioned radially between the cores and wound toroidally with the same number of turns as a selected one of the outer windings. Fig. 1 shows a toroidal A.C. comparator transformer having an axial primary magnetic core 10 carrying successively a detection winding 11, a split copper electro-static shield 13, a deviation winding 14, an inner portion 15a of an excitation winding, a tubular gapped laminated magnetic core 16 of rectangular sections magnetically shielding the inner core and windings, an overlying outer portion 15b of the excitation winding, equally series opposed to 15a, and primary and secondary windings 18, 19. The excitation windings thus produce no flux in core 10 and induce no voltage in detector winding 11, but magnetise only core 16 to induce voltages in windings 18, 19 which additionally act inter se as a transformer with core 16. In Fig. 2 a unit ratio current transformer T is calibrated by connecting its primary to a supply P in series with comparater primary 19, whose secondary 18 is connected in series with the secondary of the current transformer and standard load Z. A null detector D1 connected to winding 11 indicates errors of the current transformer, and is zeroised by excitation of deviation winding 14 by an error current having a quadrature component adjustable by capacitor C, and an in phase component adjustable by resistor R1 both derived from two series resistors 5 in the current transformer secondary circuit and switchable in phase sense at X, Y. The excitation windings 15a, 15b are energized from a phase and magnitude adjustable voltage source VS linked to supply P, and a further null detector D2 is connected between the join of load Z and series resistors r and the junction of winding 18 and the secondary of T. The excitation supply is initially adjusted in phase and magnitude to inject a voltage into secondary 18 which is equal and opposite to the voltage drop across that winding and resistors r due to the secondary current. Thereafter, resistor R and capacitor C may be calibrated to read the in phase and quadrature errors of the transformer T directly. The load Z may be replaced by a phantom load comprising a resister shunted by a switchable reversible portion  n2 of the winding of an autotransformer, which has a variable tap at n1 returned over a reversing switch to the second winding n4 of a mutual inductor having a,tap connected over ganged contacts of the same reversing switch to the null detector D2. The first mutual inductor winding 3 is connected in series with the resistor; the simulated load having a resistive component  R1 n1/n2 and a series inductive component  j# M n3/n4 where M is the mutual inductance between n3and n4. (Fig. 3 not shown). In a modification of Fig. 2 (Fig. 4 not shown) the primary of T in series with the comparator primary 19 is shortcircuited, and the power supply is connected in series with an ammeter between resistances r and the load Z. The power supply is adjusted for a required ammeter current effecting a larger required power transfer to the short-circuited primary windings, after which excitation is adjusted to null D2, and RC is adjusted to null D1 as before; windings 18, 19 acting as opposed comparator windings in the core 10 and as transformer windings in the core 16. In a further modification (Figs. 5, 6) excitation winding 15b is omitted, and excitation winding 15a and deviation winding 14 are replaced by a compensation winding 20 equal to secondary winding 18 and connected thereto at the junction with the secondary of transformer T. A single resistance 5 is used, and the adjustable CR network is excited by a centre-tapped autotransformer AT, half of which is connected across r.