GB677358A - Improvements relating to electric motor control systems - Google Patents

Abstract

677,358. Control of A.C. motors. ENGLISH ELECTRIC CO., Ltd. Dec. 22, 1950. [Jan. 2, 1950] No. 30/50. Class 38 (iii). [Also in Group XXXV] A control system for a slip-ring induction motor comprises a speed regulator in the secondary circuit of the motor, an electromagnetic brake mechanically coupled to the motor and arranged to be excited in response to increase of motor speed above a desired value dependent on the setting of the speed regulator, and saturable reactor means having A.C. windings in each phase of the supply to the motor arranged to reduce the applied voltage in response to said increase of speed. Fig. 1 shows a hoist motor 4 of the threephase slip-ring induction type which is supplied through a reversing switch 6 and has a speed regulator 7, actuated by handle 10, connected in its rotor circuit. An eddy-current brake 11, 12, is coupled to the motor and has its exciting winding 11b supplied through rectifier 18 from exciter 13. A field winding 13b of the exciter is energized from a constant voltage machine 16 through a rheostat 19 which is coupled to handle 10 so that, as the rotor resistance of the motor is reduced, the excitation of winding 13b is increased. The exciter 13 has also a field winding 13a which opposes winding 13b and is supplied from tachometer generator 20 through bridge rectifier 21. The rectifier 18 prevents energization of brake winding 11b unless winding 13a overcomes winding 13b, that is, unless the speed set by the handle 10 is exceeded. A suitable reactor 23 is connected in the stator circuit of the motor 4 and has its D.C. windings supplied from an exciter 24. This exciter has field windings 24a, 24b, which are respectively supplied from the machines 16, 13. Normally the reactor 23 is saturated and full voltage is applied to the motor, but if the excitation applied to the machine 13 due to winding 13a exceeds that due to winding 13b, indicating that the motor speed is above the desired value, the windings 24a, 24b, oppose and the output of exciter 24 is reduced. The reactor 23 will then become less saturated to an extent dependent upon the amount of overspeed and the voltage applied to the motor will be reduced. A variable resistance coupled to the handle 10 may be placed in circuit with the D.C. windings of the saturable reactor. In the modification of Fig. 2 (not shown) the A.C. windings of two phases of the saturable reactor are each in two sections connected to form a bridge circuit one diagonal of which is connected to the supply and the other to the motor. Adjustment of the regulator unbalances the bridge circuit and reduces the reactance of the third phase winding of the reactor in such a manner that the direction of rotation of the motor, as well as the applied voltage, is dependent upon the direction and magnitude of movement of the regulator. Specification 590,576, [Group XXXII], is referred to.