Patent GB-484221-A (GB)
Title: Improvements in control of the speed of electric motors and in their application to distant control systems
IPC: H02P7/26
Filed: 1936-11-19
Abstract: 484,221. Automatic control of D.C. motors; converting. SUBMARINE SIGNAL CO. (LONDON), Ltd. (Submarine SignalCo.) Nov. 19, 1936, No. 31741. [Class 38 (iii)] [Also in Groups XXXV and XXXVIII] Varying E.M.F.-In method of and system for operating at an adjustable constant speed an electric motor having an armature immersed in a constant magnetic field, a grid-controlled rectifier is provided having the motor armature in its anode-cathode circuit so that the applied anode-cathode potential is opposed by the back E.M.F. of the motor, the anode-cathode and grid-cathode circuits being supplied with alternating potentials and the phase of the grid potential with respect to the anode potential being altered in accordance with the desired speed, the change making the grid potential less leading for increased speed. The system is preferably used for adjusting the position of a body driven by a motor which is controlled from a distant point. A pointer at the control station is adjusted by a hand wheel which also rotates a cam 22 carrying contact members 23, 24 moving over resistances 27, 28, only one of which members is in contact with the appropriate resistance at any time, according to the direction of rotation of the hand wheel. The resistances 27, 28 control the phase of the potential on the grids of two valves 33, 34, the anode-cathode circuits of which include contacts 44, 45 and the armature of the motor 7. The cam 22 serves to open or close contacts 46, 47 in the circuit of a relay 42, the contact 43 of which engages one or other of the contacts 45, 44; when contacts 43, 44 are in engagement current impulses flow through the motor armature in one direction and when contacts 43, 45 engage the current flow is reversed, the direction therefore being determined by the direction of rotation of the hand wheel. The phase of the grid potential of the operative valve is also controlled by the hand wheel, and by rotating this wheel so as to decrease the value of the resistance in the grid circuit the grid potential is made less leading. This results in an increase in the motor speed, which continues to grow until the back E.M.F. generated maintains the speed constant. The motor thus develops maximum torque on starting. By means of a self-synchronous generator driven from the motor 7 and connected to a self-synchronous motor which drives the shaft of the cam 22 through suitable gearing the resistance arms 23, 24 may be returned as the motor 7 rotates and in this case the hand wheel may set a pointer at the control station to a desired position, whereupon the motor 7 is started up and rotates a pointer at the distant station at the same time returning the arms 23, 24 towards their initial position, the system coming to rest when the indications of the two pointers agree. An inertia mass may be provided to prevent too rapid rotation of the hand wheel or the resistances 27, 28 can be tapered or arranged in steps to obtain any desired speed at the various settings of the contact members 23, 24 for instance, the resistance variation may be small near the position of balance and larger for greater displacements of the contact members. The hand wheel may be automatically controlled for example by a gyro-compass to maintain the controlled member pointing in a desired direction. In a modification, one valve only may be used, a double-pole, double-throw relay being employed to reverse the direction of current through the motor armature. The relay, when operated by the closing of contacts 46, 47 on one direction of rotation of the hand wheel, serves to connect up the motor armature in one direction, the connection being reversed when the hand wheel is rotated in the opposite direction with the contacts 46, 47 open.