Patent GB-735089-A (GB)
Title: Improvements in or relating to electric arc furnaces
Inventor: VOKAC CHARLES WILLIAM
IPC: H05B7/156
Filed: 1953-08-31
Abstract: 735,089. Fluid-pressure servomotor-control systems. WHITING CORPORATION. Aug. 31, 1953, No. 24006/53. Class 135. [Also in Group XXXV] Apparatus for regulating the arc gap of an electric arc melting furnace 10 comprises liquid confining means such as the ram 29 and the cylinder 28 for varying the position of the furnace electrode 18, a device such as the pump 38 for forcing liquid into the liquid confining means, and another device such as the pump 46 hydraulically in series with the first device for withdrawing liquid from the liquid confining means, one of the two devices comprising a positive displacement hydraulic pump controlled by means responsive to variations in the arcing gap so as to vary the discharge volume of the last-mentioned pump over a range which includes an intermediate point at which the discharge volume of the first device equals that of the second device. In a three-phase direct arc furnace each electrode 18 is supplied through a conductor 22 from a power transformer 23. The electrodes 18 are each supported by an arm 19 from the ram 29, each arm being guided by a channel member 24 moving between rollers 26 on a bracket 27. The furnace containing the charge 12 has a refractory lining 11 and an earth connection 14. Liquid is supplied to the cylinder 28 through a pipe 39 and withdrawn by a pipe 43, these pipes being connected on accumulator 31 having an air inlet valve 32. Check-valves 36, 37 are associated with the accumulator outlet 33 and spring-loaded relief valves 44, 47 with the accumulator inlet 34. The pump 38 which may be of the gear or vane type is driven by a constant speed alternating current motor 58, Fig. 4. The pump 46 which is similar to the pump 38 is driven by a direct current motor 54 having a shunt field winding 56 energized by a constant source of current. The current to the armature 53 is varied by the secondary winding 51 of a transformer 48, the primary 49 of which is shunted across the electrode and the earth connection 14 so that the winding 49 receives maximum current when the arc is broken. A rectifier 52 is arranged in circuit between the secondary winding 51 and the armature. The desired arc gap is determined by the resistor 57. The pump 38 supplying at a constant rate may be replaced by a fixed orifice between the cylinder 28 and the constant pressure tank or accumulator 31. A stop-valve is provided on the outlet side of the orifice. Alternatively the pump 46 may be replaced by a fixed orifice associated with a stopvalve. In the latter case the pump 38 is driven by a variable-speed motor 63, Fig. 5. Fig. 5 shows an electric control system for use where the pump 46 has a constant speed and the pump 38 a variable one. The pump 46 is driven by the constant speed alternating current motor 62 and the pump 38 by the variable-speed motor 63 which has an armature 64 energized from a constant source of direct current through an adjustable resistor 66, and a field winding 67 energized partly from the constant source through a rectifier 68 and partly from the secondary winding 49 through the rectifier 52. A reduction in the arc gap increases speed of the motor 63 in this case so that the pump 38 speeds up to raise the electrode. The speeds of both pumps may be changed simultaneously but in .opposite senses. Fig. 6 shows a control circuit for this purpose. The pump 38 is driven by a motor 74 having a constant armature current from the rectifier 68 and a varying field current from the rectifier 52. The motor 79 driving the pump 46 on the other hand has a varying current in its armature 82 and a constant current in its field winding 81.