Patent GB-602783-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: B23Q15/00
Filed: 1945-12-05
Abstract: 602,783. Automatic control of electric motors. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Dec. 5, 1945, No. 32930. Convention date, Dec. 5, 1944. [Class 38 (iv)] [Also in Group XXXVII] Current; E.M.F.; speed.-A control system for machine drives, for example machine tools, comprises a direct-current motor which is supplied from an alternating-current source through rectifier means of the gaseous discharge type, said rectifier means having a control circuit including an electronic tube for varying the voltage of the rectified current supplied to the motor, and thereby the motor-speed, and load-responsive measuring means comprising an electronic tube connected with said control circuit so that said voltage is varied in dependence upon the tool-load of the machine for reducing said speed at high loads. As shown, an alternating-current motor CM drives a cutter of a milling machine, a bed 2 of which is coupled to a feed motor FM, the armature of which is energized through gas-filled rectifying valves T1 and T2 from a secondary winding 11 of a transformer TR. The grid electrodes of the valves T1 and T2 are connected through a transformer 30 to a phase-shift circuit comprising a transformer 33. The grid circuits of these valves also include a valve T3 of the highvacuum type which provides a direct-current grid bias component for the rectifying valves which varies in accordance with the motorspeed, so that; during normal operation, a deviation of the motor-speed from a pre-set value will effect a corresponding change in the value of the rectified output voltage in such manner as to correct such deviation. The grid potential of the valve T3 is basically determined by the setting of the rheostat 41 and varies in accordance with the difference between a component proportional to the feed motor armature voltage, derived from a resistor 47, and a component proportional to armature voltage drop, derived from a portion of a resistor 24. Further control voltages are super-imposed upon this resultant grid-bias voltage by means of valves T4 and T5 of the high-vacuum pentode type. Normally, the valve T5 is blocked by a high negative potential applied to the grid thereof, which is derived from a rheostat 55, but when the armature current of the motor FM reaches a predetermined value, a positive grid bias component derived from a resistor 23 causes the value T5 to conduct and so affect the grid bias of the valve T3 that the output of the rectifying valves T1 and T2 is reduced. A similar arrangement comprising the valve T4 ensures that when the current input to the cutter motor CM reaches a predetermined high value, the feed motor speed is reduced. The grid circuit of the valve T4 comprises a rheostat 61 which affords a high negative blocking potential for the valve which is opposed by the rectified output of a current transformer in the stator circuit of the motor CM developed across a resistor 63. In a machine wherein different types of cutters are used simultaneously, different valves similar to T4 may be provided, each being allotted to one cutter, so that the feeder motor will operate at that speed which is proper for the cutter motor carrying the greatest proportion of the load.