GB475640A - Improvements in and relating to electromagnetic devices - Google Patents

Abstract

475,640. Electromagnets. BRITISH THOMSON-HOUSTON CO., Ltd. June 25, 1937, No. 17779. Convention date, June 25, 1936. [Class 35] [Also in Group XXXVII] An electromagnetic device for use in an A.C. circuit comprises a magnetizable frame 16 and an armature 17 biassed to an unattracted position by a spring 18, a winding 22 energizable from the circuit for producing an A.C. flux in the frame'to attract the armature, and means for selectively opposing the flow of opposite half-cycles of the flux in portions of the frame to provide a substantially unidirectional flux in the armature. In the arrangement shown which is intended for use in an undervoltage protective system, the frame 16 is E-shaped, the operating winding 22 being wound on the middle limb 21. The outer limbs 49, 50 carry windings 51, 52 which are short-circuited through thermionic valve rectifiers 25, 26, the cathode heating circuits of the valves being supplied from a secondary winding 42 on the middle limb 21. On closing a switch 23, the winding 22 is energized and the armature 17 is attracted to close a circuit through the load 11 at contacts 14, 15. At the same time, an extension 19 of the armature engages contacts 20 to establish a holding circuit for the winding 22. The windings 51, 52 are oppositely connected to the rectifiers and oppose the flow of opposite half-cycles of the alternating flux in the outer limbs 49, 50, so that the flux through these limbs and through the armature is substantially unidirectional, and the armature will be maintained in its closed position without chattering or hum. If the voltage in the winding 22 falls, the decay of the unidirectional flux is opposed by the currents induced in the short-circuited windings 51, 52, so that the armature is maintained in its attracted position for a predetermined interval. The load 11 therefore remains connected during a momentary drop in the supply voltage, but if the drop is prolonged, the armature is released and the load is disconnected. The load may be disconnected substantially instantaneously at any time by opening a normally closed switch 24, which simultaneously opens the circuits of the windings 22, 51 and 52. Specification 441,723, [Group XXXVII], is referred to.