Patent GB-599852-A (GB)
Title: Improvements in or relating to double screened electric transformers
IPC: H01F19/04
Filed: 1945-04-27
Abstract: 599,852. Transformers. STANDARD TELEPHONES & CABLES, Ltd., and KRAM, W. April 27, 1945, No. 10699. [Class 38 (ii)] [Also in Group XXXVI] A transformer has two mutually insulated screens between the windings with a tapping point on each screen, and any undesired coupling between the input and output sides resulting from E.M.F. induced in the screens is reduced or removed by neutralizing the p.d. appearing at the tapping points or by choosing such points for the tappings that the p.d. is substantially zero. A H.F. transformer has primary and secondary windings 1, 2 separated by two gapped screens 3, 4 as shown diagrammatically in Fig. 1, and in Fig. 4 with all these parts as cylinders arranged around a 'core 16, the whole being enclosed in a case 5. Screen 3 is connected at 21 to terminal 10 and screen 4 at 22 to metal case 5 and terminal 11. It is normally desired that the capacitances between winding 1 and screen 3 or winding 2 and between winding 2 and screen 4 should be very small, but measurements show that the apparent. capacity with H.F. current in the windings is much greater than without. For a method of measuring this effect (see Group XXXVI). The apparent increase in capacity is shown to be an effect of the E.M.F's. developed in the screens which act as turns connected by the capacity between them. The E.M.F. between two points, one on each screen, is calculated and it is shown that it is zero between the tapping points 21, 22 if these are so selected that 2#+#-(alpha+#)=O, where alpha, #, # are the angles so designated in Fig. 5. The E.M.F. e between points 21, 22 on the screens connected through the capacitance k between the screens, may be balanced out by the essentially parallel effect of the input primary voltage Eo connected through the capacitance c effective between the other terminal of primary 1 and screen 3 as shown diagrammatically in Fig. 9A. This, may be done by adjusting the tapping points, 21, 22 to vary e, but for further adjustment a small trimming condenser may be connected! between screens 3, 4. Alternatively, all adjustment may be made by a condenser connected between the end of the primary winding not connected to box 5, and terminal 10. The compensation may also be made by the connection between tapping points 21, 22 of a winding 24 inductively coupled to primary 1 in series with a trimming condenser 25 as shown in Fig. 11. In a further modification, the screen E.M.F's. of two similar transformers are balanced against each other by suitable connection of the screens, the primary windings being connected preferably in parallel opposing, but optionally in series opposing, the secondary winding in parallel or series opposing and the terminals of corresponding screens being connected, Fig. 13 (not shown). Fig. 14 shows a H.F. inductance bridge A, B, C, D in which Z1, Z2 are the ratio impedances and Z3 is the variable impedance to balance Z4 the unknown impedance. Two double screened transformers 26, 27 connected in tandem transmit the supply to diagonal AC, and a detector is connected across B, D. Three screen systems A, B, D are shown ; by the dashed lines connected to A; by the continuous normal weight full lines connected to B ; by the double-weight full lines connected to D and earth. The action of the screen E.M.F's. on the bridge network, Fig. 15 (equivalent circuit, not shown), may be nullified by the methods previously described, but alternatively the method shown in Fig. 16 may be used to correct transformer 26. Conductors 6, 8 and 7, 9 not connected to the B screen join the secondary of transformer 26 to primary of transformer 27 and are connected to the D screen through a differential condenser 32 inside the B screen as shown. It can be shown that a circuit is formed equivalent to a bridge with the B and D screens across one diagonal and the two connected windings in parallel across the other, and in which the four arms are formed by the capacities ; two between the B screen and terminals 6, 8 and 7, 9 and the two of the differential condenser 32. By variation of the latter the bridge can be balanced to reduce the E.M.F. produced by the screens between B and D to zero, Fig. 17 (not shown).