GB637012A - Improvements in or relating to systems for the automatic speed control of electric motors - Google Patents

Abstract

637,012. Automatic control systems for D.C. motors. IGRANIC ELECTRIC CO., Ltd. Aug. 27, 1945, No. 21962. Convention date, Aug. 31, 1944. [Class 38 (iv)] The speed of a D.C. motor is maintained at a predetermined level by a variable resistor, in series with the armature, which is set automatically by a reversible motor, the direction of rotation of which is determined by electronic means, by the change in the main motor speed. In the embodiment, the armature 10a of motor 10 is supplied through resistor 13a from busbars L1, L2, across which the shunt field 10b is connected. Cross-head 13c, moved. on a screw shaft 13g by reversible motor 14, varies the proportion of resistor 13a inserted in series with armature 10a. The motor 14 rotates in a direction depending upon which of fields 14b, 14c, is energized by the relays 15, 16, which are excited from the thermionic switching circuit comprising valves 32, 26, 35, 37. The appropriate switching sequence of valves 32, 26, 35, 37 is determined by the value of a D.C. voltage supplied to the thermionic switch from a tachometer generator 11, driven by motor 10. Valves 26, 32 are connected between lines 23, 22, the former having a positive potential and the latter being earthed. Valves 35, 37 are connected between lines 24, 23, the former being positive with respect to, the latter. To operate the system, push-button 18 is closed and relay 17 energized, thus closing contacts 17b, 17c and energizing coil 12a. Switch 12c opens and 12b shuts connecting armature 10a to the mains. The voltage delivered by tachometer 11 is zero and the voltage of grid 26c is therefore negative with respect to cathode 26a which is connected to tapping 28a of speed-adjusting rheostat 28, and the valve 26 conducts only slightly. The voltage of anode 26b and consequently of grid 32c is thus positive, causing valve 32 to conduct, rendering the grid 37c negative and valve 37 non-conducting. Valve 35 passes sufficient current under these conditions to energize coil 21a closing contacts 21 to operate relay 16 through coil 16a. Contacts 16b close, the field 14b is excited and the cross-head 13c moves to the left as the motor 10 accelerates. The tachometer speed, and therefore the voltage of grid 26c, rises, the valve 26 conducts, the potential of anode 26b falls, reducing the current passed by valve 35, until opening of the contacts 21b, and 16b stops motor 14 when the setting of resistor 13a is such as to permit motor 10 to run at the speed corresponding to the position of tapping 28a. If the acceleration of motor 10 does not cease, the curent passed by valve 37 rises to close contacts 20b which cause winding 14c to be energized and cross-head 13c is moved in the reverse direction. Changes in the speed of motor 10 from the datum result in either valve 37 or 35 conducting to cause the appropriate corrective movement of cross-head 13c along resistor 13a. Hunting is eliminated by the short-circuiting of resistors 27, 29, by contacts 15c, 16c when relays 15, 16 are energized.