GB738429A - Improvements in and relating to the control of alternating current electric motor equipments - Google Patents

Abstract

738,429. Control of A.C. motors; controllers. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. May 21, 1954 [May 29, 1953], No. 15027/53. Class 38 (3). [Also in Group XXXIX] An induction motor driving a winder is braked by supplying its stator winding 2 with direct-current derived from a rectifier 9 which is controlled by transductor 15, phase-shift circuit 16, and peaking circuit 17. When the dynamic braking operation is initiated, the liquid rotor resistance 6 is at its maximum value, and, during the early part of the lowering movement of the electrodes 7, contacts 21 are open so that the control winding 42 of the transductor is de-energized and the output of the rectifier 9 is dependent solely upon the current supplied to control winding 41. This current is controlled by potentiometer 18 the moving contact of which is coupled to the electrodes 7, the motor excitation being increased as the rotor resistance is reduced. At a predetermined position of the electrodes 7, a mechanical member 50 closes contacts 21 which remain closed until the lowermost position of the electrodes is reached. The control winding 42 is thus energized in accordance with the rotor current of the motor, whereby, as the rotor resistance is further reduced, the excitation of the motor is increased in accordance with the increase of rotor current but independently of the position of the movable electrodes, the potentiometer 18 being arranged so that the voltage applied to the winding 41 is not varied after the contacts 21 have closed. The transductor 15 is of the double-cored type having series-connected output windings 32, 33, connected in the input circuit of the phase-shifting circuit 16. The transductor also has feedback winding 40 and biasing winding 43. The windings 41, 42 act in the same direction with the winding 40 and are opposed by the winding 43. The phase shift circuit is arranged so that increase of the output current of the transductor will increase the motor excitation. The liquid rheostat is controlled by lever 22 through a fluid pressure servo device 23 having control valves and operating means 28. The device 28 may comprise a circuit for maintaining the actual speed of the motor substantially equal to the desired speed at any position of the winder. In a modification, a polarized rectifier circuit may be employed instead of the contacts 21, so that energization of the winding 42 is prevented until the voltage across resistor 49 exceeds a predetermined value. The rectifier 9 may be replaced by an exciter or magnetic amplifier.