Patent GB-704295-A (GB)
Title: Improvements in and relating to control systems for electric motors
Filed: 1952-02-06
Abstract: 704,295. Control of A.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. Feb. 6, 1952 [Feb. 6, 1951], No. 3146/52. Class 38(3) [Also in Group XXXV] In a system for operating a synchronous type motor, which may be used for ship propulsion, at variable speed from a substantially constant frequency source a synchronous generator has its phase windings 1, 2, 3, respectively connected through valves 1A-1C, 2A-2C, 3A-3C, to the phase windings A, B, C, of the motor. The valves are of the immersion-ignition type, each having an igniter element 19 excited through a phase-change transformer 23 having a fixed primary 25 and an adjustable secondary 26. The grids. 21 may be excited so that the valves are operated in the proper sequence by means of a magnetic distributer 27 having a primary winding 30 and secondary windings WA, WB, WC, wound on its stator 28. The winding 30 is energized from the generator through a frequency multiplier 31. The rotor 33 of the distributer is driven by the motor and has projections which, when disposed opposite a pair of stator poles, increases the linkage between the windings 30 and that secondary winding which is wound on those poles. The rotor 33 carries a non- magnetic member 35 which decreases the linkage between the primary winding and the secondary windings on the other stator poles. The secondary windings WA, WB, WC, of the distributer are connected with primary windings TA, TB, TC, of tranformers 35a, 36, 37, which supply control signals to the appropriate grids 21 of the ignitrons. A predetermined half-wave of the voltage supplied to the windings TA-TC is by-passed through rectifiers 39 and resistors 40. When the counter electromotive force of the motor reaches a predetermined value the voltage output from rectifier 50, which is supplied through transformer 41, becomes sufficient to operate a relay 32 and de-energize the winding 30. Control of the grid transformers 35a 36, 37, is thus transferred to their other primary windings MA, MB, MC, which are supplied through auto-transformer 48, phase shifter 45, and transformer 41, in accordance with the motor voltage. Capacitors 38 which are connected in series with windings TA, TB, TC, have a high reactance at the relatively low frequency at which the windings MA, MB, MC, are supplied. Saturable reactors SLA, SLB, SLC, are interconnected with windings MA-MC to prevent undesired phase shift of the grid voltage of the ignitrons, control windings FAFC of the reactors being supplied with current which varies with the motor voltage through rectifier 53. An increase in inductance of the windings MA-MC due to an increase of frequency is thus offset by a decrease of reactance of the windings SLA-SLC. This arrangement also causes the grid transformers to produce peaked wave forms, so improving the accuracy of the grid firing. Reactors 54 and capacitor 55 are effective during the starting period to prevent the windings MAMC from acting as secondary windings with relation to the windings TA-TC. After the relay 32 operates the elements 54, 55 are short-circuited. A negative bias source represented by batteries may be required only during the starting period. Speed control may be effected by regulation of the field or armature excitation of the motor, or through the agency of the phase-shifters 23, 45. Specification 565,148, [Group XXXV], is referred to.