GB446848A - Improvements in electric motor control systems - Google Patents

Abstract

446,848. Automatic control systems; converting. BRAILSFORD, F., 3, Truro Avenue, Stretford, Manchester, DANNATT, C., 95, Grove Lane, Hale, Cheshire, and ASSOCIATED ELECTRICAL INDUSTRIES, Ltd., Crown House, Aldwych, London. Feb. 4, 1935, No. 3636. [Classes 38 (ii) and 38 (iv)] [See also Group XXXVII] Arcs.-A control system for driving a D.C. motor 1 in either direction from an A.C. source 4 comprises two grid-controlled rectifying valves I, II, reversely connected in parallel, with the grid 13 of the valve I so connected to the circuit of the valve II that the valve I is rendered conducting or non- conducting according to whether the valve II is nonconducting or conducting respectively, the grid 17 of the valve II being provided with control means to render it conducting or non-conducting when desired. When the valve II is conducting the motor runs in the reverse direction, while when the valve I is conducting, it runs in the forward direction. In the arrangement shown, the motor 1 is used for the automatic control of the feed of welding electrode 19 to maintain constant voltage across the welding arc. The arc is supplied from D.C. mains 22, 23 through the usual resistance 20 and choke coil 21, the grid 17 of the valve II being connected to the electrode 19 through a resistance 18. The cathode of the valve II is connected to a tapping 26 on a potentiometer 25, so that when the electrode 19 is at a distance from the work 24 and no arc is passing, the grid 17 is negative with respect to the cathode, and the valve II is nonconducting. Current then passes through the valve I and the motor rotates in the forward direction to feed the electrode 19 towards the work. As soon as the electrode contacts with the work, the grid 17 becomes positive with respect to the cathode and current flows through the valve II, the valve I becoming at the same time nonconducting owing to the connection of its grid with a resistance 6 in the circuit of the valve II. This connection includes a tapped resistance 14 and condenser 16, the discharge of which maintains the negative voltage on the grid 13 of the valve I during the half-cycles when no current is flowing through the valve II. The motor now rotates in the reverse direction and moves the electrode 19 away from the work to strike the are, the reverse movement being continued until the voltage across the arc reaches a value corresponding to the tapping point 26 on the potentiometer 25. Thereafter the two valves act intermittently and alternately to feed the electrode to the work at the correct speed to maintain the desired voltage across the arc. In a modification, Fig. 2 (not shown), for use where the welding arc is supplied from a source of fluctuating potential, the potentiometer 25, instead of being connected directly across the supply, is energized from a separate source of substantially constant D.C. voltage, which may be obtained by rectification from A.C. mains. Each of the valves 1, II may be replaced by a pair of valves arranged for full-wave rectification.