GB557916A - Improvements in or relating to the control and protection of electric arc furnaces - Google Patents

Abstract

557,916. Control systems for arc furnaces ; protective cut-out arrangements. REYROLLE & CO., Ltd., A., LEYBURN, H., and ALDERSON, R. June 4, 1942, No. 7605. [Class 38 (iv)] [Also in Group XXXVII] In an arc-furnace control system A having its supply volts reduced in operation from 160 or 200 volts down to 100 volts by a variably-tapped transformer primary B, and the circuit breaker C opened at each tap-change, the duty on the breaker is minimized by lifting the electrodes by means not shown and reducing the current so that underload relays G1 ... G3 close and, with the operator's switch F engaging contacts F<SP>2</SP>, energize the breaker trip coil C2. The dosing coil C' is energized by moving handle F on to contacts F<SP>1</SP>. An emergency trip switch M is also provided. In Fig. 2, when F is moved to tripping position, a contactor N with holding contacts N' is energized to prepare the trip circuit and supplies over circuit O a motor for lifting the electrodes, the contactor N being de-energized by the opening of the breaker. Protective systems.-The circuits of the undercurrent coils G1 ... G3 include also the windings of overcurrent relays, of which J1 ... J3 have low settings and long time delays and K1 ... K3 have high settings and short delays, the former being mainly employed during the initial period when small overloads of relatively-short duration would produce frequent tripping. A resettable earth-breakage relay L has trip contacts L1 in parallel with the parallel-connected overload contacts, and a contact L2 in the closing-coil circuit to prevent reclosure until attention has been called to the fault.