GB720289A - Improvements relating to the control of electrically operated hoists and winders - Google Patents

Abstract

720,289. Control of A.C. motors. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Oct. 21, 1952 [Oct. 22, 1951], No. 24593/51. Class 38 (3) An alternating current induction motor equipment for driving a hoist or the like has provision for dynamic braking in which the load resistance is of metallic form and variable by the operation of a driver's control member to control the braking torque, the control means for the resistance including a plurality of mercury switches rotatably mounted at a speed dependent on the motor speed to control a number of circuits which are operative to prevent reduction of the load resistance to values reducing the braking torque below a maximum irrespectively of the operation of the driver's control member. The mercury switches SRS1, SRS2, SRS3 are mounted on a rotating table which is driven at a speed corresponding to the speed of the rotor of the induction motor so that the mercury in the switches is displaced against gravity bias to open-circuit the switches. Movement of the driver's control member during dynamic braking energizes the operating coil OCDB of the dynamic braking contactor thus supplying the stator of the induction motor with direct current and further movement of the control member energizes operating coils OC1 ... OC10 to operate the contactors to remove resistance from the rotor circuit. The latter sequence of these coils OC8 ... OC10, however, cannot be energized until the motor speed has diminished to values which allow the mercury in the switches SRS1, SRS2, SRS3 to remake the circuits therein; these switches being calibrated to close at differing motor speeds. Thus the closure of these contactors is dependent of the motor speed irrespectively of the position of the control member. In a further embodiment the mercury switches are replaced by a tachometer generator driven by the motor, the output of which sequentially operates a series of relays which control the respective contactor circuits. Specifications 571,700 and 604,819, [both in Group XXX], are referred to.