GB915269A - Improvements in and relating to mechanical braking systems for electrically-driven winding, hoisting or the like engines - Google Patents

Abstract

915,269. Control of A.C. motors. ENGLISH ELECTRIC CO. Ltd. Feb. 26, 1959 [Feb. 27, 1958], No. 6450/58. Class 38 (3). A winding operation is initiated when a control member is moved to operate one or other of the stator contactors 12, 13, for an induction motor 10, and, at the same time, contacts 29 are closed to energize a relay 28 whereby contacts 20 are closed and contacts 23, 24, are opened. A transducer 19 is then energized by a current transformer 22 in accordance with the stator current of the motor, and a rotor resistance 14 is reduced to increase the motor torque. An increase in the energization of the transducer 19 reduces the effort exerted by mechanical brakes 15 upon the motor, the brakes being released by the admission of fluid to a cylinder 17 having a control valve 18 which is, in turn, operated by the transducer. As the brakes begin to slip and the winding engine breaks away from rest, a motion-responsive relay 32 opens its contacts 30 to de-energize relay 28, so causing the current transformer to be short circuited and disconnecting the transducer from the transformer circuit. De-energization of the relay 28 also connects the transducer to a further brake control device 25 so as to release the brakes completely. Temporary de-energization of the transducer is prevented when changing from one supply circuit to the other, a rectifier 21 preventing interaction between the two circuits whilst they are in parallel. As the end-of-wind positions are approached, limit switch 33 is opened to de-energize the transducer and re-apply the brakes. In a modification, the control device 25 varies the braking effort in accordance with the deviation of motor speed from a reference value, so tending to reduce the deviation. Specification 863,262 is referred to.