Patent GB-606502-A (GB)
Title: Improvements in and relating to voltage indicating means
IPC: H02H3/20
Filed: 1945-02-07
Abstract: 606,502. Current and voltage measuring apparatus. GENERAL ELECTRIC CO., Ltd., and MENZLER, M. L. Feb. 7, 1945, No. 3089. [Class 37] [Also in Group XXXVII] Means for indicating when a voltage exceeds or falls below a predetermined value comprises a relay adapted to be actuated at a voltage substantially equal to this value and upon actuation serving to close a circuit to a second relay, the second relay serving to give an indication and to introduce impedance in series with the winding of the first relay, which impedance has a value slightly less than would be necessary to cause the actuation of the first relay to cease with the actuating voltage remaining constant. The apparatus is shown as applied to the control of arc welding. A welder's cable 1 is connected to an A.C. supply and to a welding electrode 2, the work 3 being earthed. Maximum and minimum permissible values of the arc voltage are given by red and green lamps 6R, 6G, both at the control unit 4 and at the welder's shield 5. The primary winding of a transformer 9 is connected across the arc and its secondary winding is centre tapped, the upper half being connected in a circuit. controlling the red lamps 6R and the lower in a circuit controlling the green lamps 6G. Corresponding components in the upper and lower circuits are indicated by the same numerals, but are distinguished by the suffixes R, G respectively. The output from the transformer 9 is passed through a rectifier 10 to a potential divider 11, 12 across one arm of which a resistance 13 is shunted to increase the range of movement of the tapping point over a small voltage range. Two relays 14, 15 are provided, of which the former has a single contact 16 and the latter two contacts 17, 18. A resistance 19 is connected across the contacts 18. The relay 14 is connected across the potential divider 11, 12 and is therefore always traversed by a current proportional to the voltage across the arc. The contact 16 operates the relay 15. The tapping on the potential divider 11R is adjusted to the upper limit of arc voltage and the potential divider 11G to the lower limit. The condition of the relays is shown when they are not energized and the arc voltage is zero. Contact 17G is closed and 17R is open so that only the green lamps 6G are alight, indicating that the arc voltage is too low. When the current to the welder's cable 1 is switched on and before the arc is struck, the full voltage appears between the welding electrode and earth. This causes the relays 14R, 14G to operate and close the circuits of the relays 15R, 15G which also operate and close the contact 17R of the lamps 6R and open the contact 17G of the lamps 6G. The red lamps only light, indicating that the voltage is too high. The contacts 18R, 18G are also opened, introducing the resistances 19R, 19G into the circuits of the relays 14R, 14G. When the arc is struck and the correct length has been established, the arc voltage assumes a value which varies over a small range. The potential dividers 11R, 11G are then adjusted until no indication is given by either of the lamps 6R or 6G over this range. If the voltage rises, the relays 14G, 15G remain operated with the green lamps extinguished and the relay 14R operates, thus operating the relay 15R which lights the lamps 6R and introduces the resistance 19R in the circuit of the relay 14R. This is of such a value that the relay 14R remains operated, but the decrease in voltage necessary for its release is lessened, so that the operating and release voltages are brought closer together. Condensers 20R, 20G are connected in parallel with the windings of the relays 14R, 14G and are of such value that the indication given when the voltage is close to the predetermined value is intermittent at a convenient frequency. If it is required to indicate when the current exceeds or falls below a predetermined value the resistance 19 is connected in parallel instead of in series with the relay coil 14.