Patent GB-392168-A (GB)
Title: Electric supply for lamps in coal mines and like places
IPC: F21V23/02
Filed: 1931-11-06
Abstract: 392,168. Supply and control systems for consuming devices ; supporting electric lamps. CRAMP, W., 16, Augustus Road, Edgbaston, and SEARBY, N. H., 629, Chester Road, Erdington, both in Birmingham. Nov. 6, 1931, No. 30751. [Classes 38 (ii) and 38 (iv).] Systems depending upon static transformers.- A lighting supply system, more particularly for coal mines &c., comprises a number of transformers 40, 41, 42 the primaries 21 of which are connected to the supply in series, the secondaries 20 being removable from their respective primaries and connected to lamps 6 so that the lamps may be de-energized sparklessly. The connecting cable is of the concentric type the outer sheath forming the earthed return lead and serving to earth the casing &c. of the transformer. The system may be arranged so that one or more of the transformers 54 supplies a further system 55, 56 of series connected transformers. Each transformer may comprise an E-shaped yoke mounted within the flange 17 of a base 15 and surrounded by a highly resistive split-metal casing 25. The middle leg 22 carries the primary winding 21 which is enclosed in a split annular container formed of alloy known under the R.T.M. No-mag filled with insulating compound. The secondary winding 20, which may be similarly enclosed, is movable on the middle leg 22 and is supported on a spacer 121 connected to a bevelled yoke 19 which co-operates with the outer limbs 18 of the main magnetic circuit and is fixed to a removable cover 8 of the transformer. The yoke 19 is, during operation, saturated to a certain extent, e.g. by placing brass or insulating washers between the iron laminations so that removal thereof does not affect the reluctance of the magnetic circuit appreciably, or, alternatively or in addition, the limb 22 may be short enough to provide an air gap between it and the yoke 19. Alternatively, the yoke 19 may comprise a plate of cast iron of suitable section. The coil 20 is thus removable with the cover 8 which is secured to the casing 25 by wing nuts 10 and has a depending flange 11 which may be split and extended so as to surround the coil 20. Instead of the split container for the coil 21, the casing may be divided transversely by a diaphragm of high resistance non-magnetic material placed over the primary coil, bored for the passage of the core limbs and split to prevent circulating currents. The diaphragm may have a facing of insulating material. When the secondary coil is protected by the spigot 11, a similar diaphragm may be provided for it also. The lamp 6 may be mounted under a cover 1 on a base 5 secured to the cover 8 by rivets 7. The cover 1 may be arranged to be only removable after removal of the cover 8 of the transformer together with the coil 20, or alternatively the nuts 3 may be removable only by authorized persons. In a modification the lamp 6 may form a separate unit connected to the transformer unit by flexible armoured cable. The design of a suitable transformer is given in the Specification. Earthing arrangements for the primary and secondary sides of the transformer unit are described, spring contacts on the cover 8 being arranged to engage the casing 25 to ensure earthing of the cover whenever it is likely to be energized. Instead of running the system from a constant voltage main transformer, said transformer may be arranged to give constant current at a variable voltage.