GB704481A - Improvements relating to the control of electrically operated hoists and winders - Google Patents

Abstract

704,481. Control of A.C. motors. METROPOLITAN-VICKERS ELECTRICAL CO, Ltd. May 1, 1952 [Oct. 3, 1951], No. 23080/51. Class 38(3) A control system for electrically operated hoists or the like has the speed of an alternating current driving motor regulated by a liquid controller which is adjusted by a positional servo mechanism during both the driving and dynamic braking operations, the servo mechanism having two phases in its operation during both driving and braking; firstly, a rapid movement to adjust the liquid controller to a value to give maximum starting or maximum braking torque as the case may be and secondly, a much slower movement over the remaining portion of the motor operations. During driving the driver's control lever is moved to the " full on " position thus moving the hollow sleeve 3, by means of a lever 6, so that the port 7 is closed by movement of the sleeve 3 into the piston 2. Pressure, fluid thus acts on the piston via the port 10, hollow sleeve 9 and the cylinder 8; the valve 16 being open and the valve 17 being closed during the driving operation. Since the full fluid pressure acts on the piston 2, it moves rapidly to move the lever 1 to adjust the position of a liquid controller to give maximum driving torque. At some point in the travel of piston 2 and the sleeve 3 a link 11 becomes operative to move the sleeve 9 to close the port 10 so that the pressure fluid can now only act upon the piston 2 by way of a reduced orifice 13. The pressure on the piston is therefore reduced and consequently the piston moves more slowly over the remaining part of the motor controller range. To initiate dynamic braking the driver's control lever is moved to the " braking " position when the valve 16 will be closed and the valves 17, 18 will be opened thus admitting fluid under full pressure to the piston 2 by means of a port 14, thus the piston 2 moves rapidly to reduce the value of the liquid controller in the rotor circuit. When the rotor current reaches a pre-determined value, means, such as electromagnetic relays, or a saturable inductor, energized by current transformers associated with the rotor phases, become operative to close the valve 18 so that the fluid supplied to the piston 2 has to pass through an adjustable valve 21 which limits the fluid pressure so that the piston again moves at a slower rate. The means operating to close the valve 18 may alternatively be associated with the stator circuit. Specifications 651,379 and 690,090 are referred to.