GB806840A - Improvements relating to transformer circuits - Google Patents

Abstract

806,840. Transformer arrangements. SOC. GENERALE DE CONSTRUCTIONS ELECTRIQUES ET MECANIQUES ALSTHOM. Oct. 12, 1956 [Oct. 12, 1955], No. 31226/56. Class 38 (4). The voltage supplied to a traction or other load by the secondary winding 6 of a main transformer 2 is varied by adding to or subtracting from it the voltage of secondary winding 44 of an auxiliary transformer 38, the primary windings 37, 41 of which are energized from variable tappings on the primary winding 3 of the main transformer, the ratio of the auxiliary transformer 38 being altered on passing between buck and boost. The primary winding 3 is connected across the main supply and has tappings connected to circularly spaced contacts 14-20. With them co-operate rotating contacts 29, 30 to form an on load tap changing switch connected to resistor 47 through switches 33, 34. With these closed, Fig. 1 then shows the circuit arranged for lowest output voltage, the voltage of winding 44 being at maximum opposition to that of winding 6. Windings 37, 41 of the auxiliary transformer are energized in series from main winding 3 at terminal 5 and contact 14 through contacts 23, 25 of change-over switch 24, and contacts 39, 40 of switch 36. To increase the output volts by one step switch 33 is opened contact 29 is moved to contact 15, switch 33 is closed, switch 34 is opened, contact 30 is moved to contact 15 and contact 34 is closed. During each change, resistor 47 first develops a voltage drop in series with contact 30 to reduce the auxiliary transformer voltage, and then limits current between the connected taps. When contacts 29, 30 have stepped round to contact 22, windings 37, 41 are short-circuited, the output volts being those of winding 6. Contact 23 is then moved to 26 and further rotation of contacts 29, 30 connect increasing amounts of primary winding 3 to the auxiliary transformer in opposite sense so that the auxiliary voltage assists the main voltage. Before this boosting takes place, contact 40 of switch 36 is moved to engage contacts 35, 49. Only part 41 of the auxiliary transformer primary winding is now in circuit, thus changing its ratio and the value of each voltage step, and part only of resistor 47 is used to reduce correspondingly the voltage drop introduced into the circuit during each step. The rotary arrangement of contacts 14-20 allows the voltage steps to progress in value if desired, as the order of selection is the same both for reducing buck and increasing boost. The change in ratio allows the progression to continue smoothly. In a modification the tapping contacts are arranged linearly, Fig. 2 (not shown), the voltage steps then being equal. Various modifications are described including the following: resistance 47 may be replaced by a variable saturable choke; the switches may be dry, or oil-immersed, and may be assembled with or separate from the tap changer; the auxiliary transformer may comprise two separate primary windings or may be replaced by two transformers; and several circuits may be combined for polyphase operation.