Patent GB-419822-A (GB)
Title: Improvements in electrical control systems for the control of electric circuits
IPC: H01H43/30
Filed: 1933-05-15
Abstract: 419,822. Electromagnetic switches. GOWAN, S., 4, Wardle Road, Sale, Cheshire, HESKETH, H., 16, Alderley Road, Flixton, Manchester, SYKES, C., 2, Dartmouth Road, Chorlton, Lancashire, WALKER, C. H., Mabrouk, Lynmouth Avenue, Flixton, Manchester, and ASSOCIATED ELECTRICAL INDUSTRIES, Ltd., Crown House, Aldwych, London. May 15, 1933, Nos. 14106 and 33209. [Class 38 (v).] An electrical control system in which a circuit at certain times, such as upon the movement of a control switch, is required to be energized to a predetermined extent and after a short interval to a predetermined lesser extent, has the pilot or control switch adapted in its running position to close the control circuit through an impedance which requisitely reduces the energization of the circuit, which impedance is adapted to be short-circuited by an electrically-operated switch which automatically operates to open its contacts and insert the impedance a predetermined time interval after the control or pilot switch has been moved from its " off " position to its " on " position, the closure of said impedance short-circuiting switch being effected electrically through a circuit which is completed when the control or pilot switch occupies its " off " position. As applied to the system described in Specification 378,180, the control switch 24a, Fig. 1, when in the " off " position completes the circuit of an electromagnetic switch 23<a> through a rectifier bridge 61, the switch 23<a> being of the telephone type and being provided with a copper slug to make it slow-acting. The contacts controlled by the electromagnetic switch normally short-circuit a resistance 25 in the circuit of the closing coil 19 of the main relay so that when the control switch is moved to the " run " position the main relay is fully energized and operates immediately. After a time interval, however, the contacts of the switch 23<a> are opened as this relay is no longer energized and the resistance 25 is inserted in the main relay circuit. The current flowing through this relay is sufficient to maintain it in the closed position, but if the supply fails the reduced current is insufficient to re-close the main relay when supply of current is re - established. Under such circumstances closure of the main relay can take place only after the control switch 24<a> has been moved to the " off " position. In a modified arrangement, Fig. 2, the short-circuiting of the resistance 25 is effected by thermally-operated contacts of a switch 23<b>, the heating coil 62 of which is supplied through a transformer 63 when the control switch is in the " off " position. Alternatively, a separate heating coil may be dispensed with by passing current directly through the thermal element, e.g. a bimetallic strip, Figs. 4 and 5 (not shown). To compensate for variations in ambient temperature, the fixed and movable contacts of the thermal switch may consist of similar bimetallic members, Fig. 3 (not shown), and the thermal element may be arranged to be heated rapidly when the control switch is moved to the " off " position by connecting it directly across the heating transformer the deflection of the strip when it has reached a predetermined temperature and short-circuited the resistance 25 causing a resistance to be inserted in the heating circuit, the reduced current in which is sufficient to maintain the deflection of the movable strip until the control or pilot switch is operated.