Patent GB-390409-A (GB)
Title: Improvements in and relating to electric motor control systems
IPC: G01G1/38
Filed: 1932-09-02
Abstract: 390,409. Control of motors; actuation of switches. BRITISH THOMSONHOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Jones, B. W. ; 1392, Glenwood Boulevard, Schenectady, New York, U.S.A.) Sept. 2, 1932, No. 24519. Convention date, Sept. 3, 1931. [Classes 38 (iii) and 38 (v).] Travelling to predetermined limits.-In a control system for an electric motor performing adjustments, the electric motor has its armature connected in series with a current-limiting device across an electrical supply source and in parallel with a pressure-variable resistance the pressure on which is mechanically increased whereby the resistance and therefore the motor speed, are decreased as the adjustment required decreases. The invention is described in connection with a weighing machine. When the commodity is placed on the scale platform (not shown) a draw rod 42 displaces the scale beam 12 and closes contacts 17, 18 to start a motor 10 supplied from lines 21, 22. A resistance 20 in series with the armature of the motor limits the current supply. The motor propels the poise 11 along the beam by means of a screw 13. The contact 18 rocks upon a pivot 44 and, acting against a spring 25, gradually increases the pressure upon a carbon pile 23 as the poise moves outwards, thereby gradually decreasing the resistance of the pile. The pile is shunted across the armature of the motor 10 and consequently reduces the speed of the motor so as to bring the poise to equilibrium position at a gradually decreasing speed until at the balancing position the contacts 17, 18 open and the motor stops. If the load on the platform is reduced the motor is controlled in a similar manner by a contact 19, reversing field windings 15 and carbon pile 24 so as to reverse the movement of the motor and poise. The weight is indicated on a dial 31 rotated from the poise by a cord 27 and gearing 32. After balance has been attained and after a further predetermined time, a lamp 34 controlled by a synchronous motor 35, as described below, is arranged to illuminate the dial. The closure of the circuit of the motor 10 causes a coil 52 to open a switch 40 which in opening frees a spring-actuated plunger 41 to open a switch 57, and close a switch 39. When the scale beam is balanced the motor 10 and coil 52 are deenergized permitting the switch 40 to close and establish a circuit through the switch 39 and synchronous motor 35. This motor rotates two cams 36, 37. The cam 37 re-sets the plunger 41 and closes the switch 57 and finally opens the switch 39 to stop the synchronous motor. The cam 36 closes the switch 38 to illuminate the lamp which remains illuminated until another cycle of operations is instituted by loading the scale platform. In a modified control for the lamp 34 a solenoid connected across the armature of the motor 10 opens the lamp circuit whenever the motor is energized and closes the circuit whenever the motor is stopped at the balancing point, a dashpot delaying the closure for a predetermined time after equilibrium is reached. Instead of using two carbon pile resistances a single resistance compressible from both the top and the bottom may be employed.