Patent GB-715340-A (GB)
Title: Improvements in or relating to liquid resistances and to electric motor control equipment including such resistances
IPC: H02P3/22
Filed: 1951-11-16
Abstract: 715,340. Control of A.C. motors; controllers. GENERAL ELECTRIC CO., Ltd., and FRIEDLANDER, E. S. Nov. 11, 1952 [Nov. 16, 1951]; No. 26996/51. Class 38(3) [Also in Group XXXVI]  In an arrangement comprising a liquid resistor the means for altering the effective separation or area of the electrodes so as to vary the resistance value of the resistor is responsive not only to a control signal but also to the'specific resistance of the liquid in the liquid resistor. The invention is described with reference to A.C. wound rotor induction motor mine winding equipment. In Fig. 1 the dynamic braking and motoring of induction motor 10 is controlled by hand-lever 24 through a twopart variable resistor 29, 30 and there is provided in the liquid resistor 13 in the rotor circuit of induction motor 10 a metering section 44 in shunt with solenoid 20 controlling fluid servomotor 14 so that any change in the specific resistance of the liquid (e.g. caused by temperature change) varies the voltage applied to solenoid 20 whereby servo-motor 14 produces a compensating adjustment of resistor electrodes 12. If the lever 24 is displaced for motoring or driving the motoring contactor 40 closes from switch 28 and induction motor 10 drives control generator 22 which feeds servo-motor solenoid 21 through variable resistor 23 so that dependently on speed of motor 10 solenoid 21 through valve 17 controls servo-motor 14 to adjust electrodes 12. The lever 24 also controls motoring through variable resistor section 30 changing the reactance of a transductor 35 in the circuit feed to solenoid 20. If the lever 24 is displaced for dynamic braking a.c. is cut off and d.c. is fed to the field windings of motor 10 and generator 22 with operation of braking contactor 39 controlled. by switch 28. The motor 10 receives d.c. through polyphase bridge rectifiers 37 and transductor 36 and the latter is varied through resistance section 29 so as to vary the d.c. excitation of motor 10. Transductors 35, 36 are jointly controlled by the same d.c. so that liquid resistor 13 is adjusted simultaneously with d.c. excitation of motor 10. Under braking conditions the solenoid 20 is energized by two signal components from different phase lines: a variable component via transductor 35 and a fixed component fed through resistors 50 and 45. In a second embodiment. Fig. 2, there is provision for braking by plugging. Control of speed of the induction motor 10 .is effected by servo-motor 14 changing the value of the liquid resistor 13 when the voltage from a tachometer generator 59 driven by induction motor 10 and applied to the control winding of a transductor 61 is not equal to the voltage obtained from variable resistor 29 or 30 representing the speed set by the control lever 24 and which is applied to the control winding of a comparison transductor 60. If the motor 10 is e.g. rotating in the forward direction and the forward contactor 40 is accordingly closed and plugging contactor 74 is as shown and the speed substantially exceeds the value set by lever 24 then the d.c. output from transductor/rectifier 61/65 is above that of transductor/rectifier 60/64 and a polarized relay 120 is closed to operate change-over contactor 121 and bring in reverse running contactor 40a and reverse the plugging contactor 74 through polarized relay 76. The electrodes 12 are adjusted according to braking required through transductor 104 controlled by plugging contactor 74 and connected to solenoid 20 through metering section 44 and variable'resistor 123. Specification 625,950 is referred to.