Patent GB-585832-A (GB)
Title: Improvements in or relating to alternating current electrical load indicating instruments
IPC: G01R15/00
Filed: 1944-05-19
Abstract: 585,832. Current measurements: maximum demand indicators. SHOTTER, G. F., and HILL, E. W. May 16, 1944, No. 9711. [Class 37] In an instrument for measuring the peak value of an intermittent alternating current, the grid of a gas filled discharge tube is biassed positively in accordance with the current to be measured, and negatively by a potential which is increased by means of relay connected in the anode circuit until the gas filled tube becomes non-conducting, the variation of the negative biassing potential which is a measure of the peak value of the current in question being indicated by suitable means. The invention is particularly applicable to resistance welding apparatus. The current to be measured flowing in a single phase line 14 supplies, through transformers 13a, 13 and rectifier bridge 15, direct current to resistance 11. The potential across resistance 11 which is proportional to the current being measured biasses positively the grid 10 of a gas discharge tube 1. Negative bias is applied to the grid by the potential drop across a resistance 8 fed with D.C. through potential transformer 6 and rectifier 7, a varying proportion of this potential being applied to the grid by a wiper arm 12. The anode of tube 1 is fed through transformer 3 and rectifier bridge 4 and has a relay 5 in series therewith. At zero condition with no current flowing in the line 14 the negative bias supplied by resistance 8 is just sufficient to prevent flow of anode current. When a current flows in line 14 the potential produced thereby across resistance 11 reduces the negative potential on the grid 10 whereupon anode current flows to energize relay 5. Contacts 32 thereof close to energize an impulse counter 29 which through ratchet 30, advances wheel 28 one step. The wheel 28 through the crown wheel 27 advances the differential planet wheel 22 which through spindle 19 and gearing 18 advances the wiper arm 12 over resistance 8 to increase the negative potential on the grid 10. A tuned circuit 35, 36 across the relay 1 permits release thereof at the cessation of the current peak. This operation is repeated for each successive current pulse in the line 14 until the negative potential on the grid 10 applied by resistance 8 just over-rides the positive potential produced over resistance 11 by the line current pulse so that the tube 1 does not strike. The movement of the arm 12 is then proportional to the maximum peak value of current in line 14 and is indicated by a pointer 20 on spindle 19 which may read current direct on a scale 21. At each actuation of counter 29 a meter 34, for example as described in Specification 168,339, [Class 37], is energized over contacts 33 so that should the current in line 14 flow for an extended period the disc 26 of the meter 34 drives the arm 12, through worm gearing 25 and crown wheel 23, to increase the negative potential on the grid until the balance condition is reached, and the tube 1 is kept extinguished. Zeroizing means are provided for the arm 12 and pointer 20. For three phase working three current transformers will be used each supplying potential to resistance 11 through a separate rectifier. The impulse counter 29 may, alternatively, be connected directly in the anode circuit of the gas tube 1 and relay 5 omitted. The instrument will function as a normal maximum demand indicator in the case of a continuous load. According to the Provisional Specification the meter 34 is connected to register the line load continuously and the relay 5, upon operation, puts the worm gearing 25 into mesh whereby the pointer 20 functions as above to indicate maximum demand or peak value.