Patent GB-593057-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: H02P7/298
Filed: 1945-05-25
Abstract: 593,057. Control of D.C. motors. WEST & CO., Ltd., A., and WHITE, F. W. C. May 25, 1945, No. 13150. [Class 38(iii)] In a control system for a shunt motor arranged so that the motor is speeded up to the base speed by increasing the armature voltage to a maximum, and speeded up further by inserting resistance in the shunt field circuit, a master preselector is set to predetermine the speed to which the motor is to be accelerated, this setting determining the setting of two subordinate preselecting elements the first of which, for all speeds below the base speed, predetermines the extent to which the armature voltage is to be increased, and the second, for all preselected speeds above the base speed, predetermines the amount of resistance to be inserted in the field circuit after the armature voltage has reached a maximum. Control in both forward and reverse directions is provided and, for use for example in a planing machine, the " return " speed may be made greater than the " cut " or forward speed. The motor armature 1 is fed from an A.C. supply 3 through a transformer 7 and a mercury arc rectifier 4 with control grids connected to the secondary winding of a transformer 11, the primary of which is connected alternately to phase shifters 5, 6 driven by torque motors 18, 18a respectively. The extent of phase shift is controlled by radial arms 12, 15 carried by the phase shifters, the arm 12 being arranged to operate between stops 13, 14 corresponding to zero and maximum voltage output of the rectifier, and the arm 15, when operated, moving between stops 16, 17 corresponding also to zero and maximum voltage output. The master controller comprises two concentric arcuate contacts 22, 23, and an intermediate contact 21 of smaller extent with a continuation resistance 20, and a co-operating " cut " or forward control arm 24 provided with a contact 25 which bridges the contact 22 and either the contact 21 or resistance 20 according to the setting of the arm. A " return " arm 26 has a contact 27 bridging the contact 23 and resistance 20, and its range of movement is limited to the extent of the resistances. The arm 24 may be preset so that contact 25 is at an intermediate position on contact 21, or at an intermediate position on resistance 20, or on the junction of contact 21 and resistance 20, according to whether the speed required is less than, greater than, or equal to, the base speed. A hand-wheel for setting the arm 24 is geared to a rack and pinion which simultaneously sets a stop 28 in a corresponding position relative to the stops 13, 14 of the phase shifter 5, thereby predetermining the amount of clockwise movement of the arm 12 of the phase changer and thus controlling the bias on the grids of the rectifier 4. When the motor is started with the arm 24 set so that contact 25 is on resistance 20, the field winding 2 is connected over relay contacts 29 direct to a D.C. source 19, and the phase shifter 5 operates to increase the armature supply voltage to a maximum, whereupon relay coil 29w operates to open contacts 29. This inserts the predetermined portion of resistance 20 in the field circuit which is completed through arcuate contact 22 and closed contacts 8b, which close or open with cam of table-operated contacts 8 in the armature circuit and 8a in the phase shifter circuit. The clockwise movement of the arm 12 of phase shifter 5 is accompanied by anti-clockwise movement of arm 15 of phase shifter 6 which, during " cut " movement is on open circuit. The return stroke is effected by the opening of contacts 8, 8a and 8b and the closing of contacts 10 and 10a, whereupon the motor armature current is reversed and the field circuit includes that part of the resistance 20 between contact 27 of arm 26 and a conductor 34.