GB619765A - An improved electromagnet relay chiefly designed for recording electric surges - Google Patents

Abstract

619,765. Electric tests. LANDIS & GYR SOC. ANON. Nov. 30, 1945, No. 32463. Convention date, Nov. 30, 1944. [Class 37] An electromagnetic relay responsive, selectively, in one of two ways to current impulses of varying magnitude and duration, comprises a three-limbed iron core, Fig. 4, having an armature 12 pivoted at 10a on the central limb 10 and biassed by a tension spring 15 against a stop 16 adjustable to vary the air-gap L1. The core leg 9 and corresponding armature limb are made of smaller sectional area than the adjustable core leg 11 and armature limb corresponding thereto, so that, of the two parallel magnetic circuits energized by the relay coil 13, that through leg 9 saturates at a lower flux value than that through leg 11. In operation, when the coil 13 is energized, nearly all of the magnetic flux generated thereby flows through leg 9 until saturation, after which the flux builds up in the leg 11. With a large surge current the flux build-up is so rapid that the impulse, i.e. the time integral of the attractive force, acting to rock the armature 12 against the action of spring 15 is insufficient to rock the armature because of the rapid build-up of the opposing impulse produced by the flux through core limb 11. During flow of the follow current, however, the opposing impulse is insufficient to prevent rocking of the armature which actuates the follow current counter 3 mechanically through a pallet 18. As shown in Fig. 1, a surge voltage in a line 8 breaks down the diverting conductor 7 and a surge current passes to earth through the resistor 4, the resistance of which is voltage dependent. In parallel with resistor 4 are a resistor 5 which actuates a surge counter 6, and a follow current relay 1, as above described, which responds only to the " follow " current subsequent to the surge, i.e. the current produced through the resistor 7 by the normal line voltage, and which persists for a maximum of one half a cycle of the line frequency. The impedance of the relay is such that it carries substantially all of the follow current.