Patent GB-490857-A (GB)
Title: Improvements in and relating to electric motor control systems
IPC: B66C13/18
Filed: 1936-11-23
Abstract: 490,857. Master control of two A.C. motors. IGRANIC ELECTRIC CO., Ltd., and TAYLOR, J. R. Nov. 23, 1936, No. 32005. [Class 38 (iii)] Braking.-A lift or hoist motor and its electromagnetically operated brake are controlled by a controller having hoisting and lowering positions, and means are provided which ensure that, subsequently to the motor being driven in the lowering direction, the controller is ineffective to apply the brake unless the motor has first slowed down to a predetermined speed under the control of electric braking connections, and these braking connections have been disestablished. The invention is described in connection with a grab hoist having closing and holding lines each operated by an alternating current motor under the control of its own controller. Upon operation of either of the controllers to their final hoisting positions 4h through intermediate notches 1-h 3h, the corresponding motor is connected for hoisting and secondary resistance is short-circuited in sections by the sequential energization of coils rw1-rw6 and closure of contacts rc1- rc6, the contacts rc3-rc6 being time controlled, and the contacts rc5 and rc6 closing in interlocked time sequence on the final notch. The contacts rc7 of the holding line motor 2 do not close during hoisting. The coils rw4-rw6 actuating contacts rc4-rc6 of the closing line motor 1 are controlled by the contacts jmc of a relay jrw to interrupt the control of the closing line motor if it should take too great a share of the load. For lowering each controller must be moved first to its fifth lowering notch 5l to make lowering connections of the motors and energize a relay winding to close contacts rc3 to short-circuit sections of secondary resistance. After a time interval contacts rc4 close to short-circuit further secondary resistance. If the holding line controller, Fig. 4, is moved to its sixth notch 6l contacts rc5, rc6, rc7 close in timed sequence to shortcircuit further secondary resistance. Also a relay R1w is energized to close contacts R1c which permits closure of contacts rc5 and rc6 of the closing line motor to shortcircuit steps of secondary resistance in timed sequence. If either controller is returned to its fourth lowering notch the line connections are reversed and all secondary resistance inserted. At the third notch the first step of secondary resistance is cut out to increase the braking. At the second notch a further step of secondary resistance is cut-out, and a relay Rw is de-energized to open contacts Rc2 and place the energizing circuit for the hoisting contactor hw under the control of a relay vrw having contacts vrc energized by the motor secondary voltage. Thus when the motor speed falls to say 20 per'cent synchronous speed, the braking circuit is opened and mechanical brakes applied. The mechanical brakes are set by the de-energization of a coil bw de-energized when the hoisting contactor is opened. The transmission, between the closing line and its motor is higher, e.g. 7¢ per cent higher than that between the holding line and its motor so that in lowering an empty grab the closing line is paid out at least as fast as the holding line. The secondary resistance compensates for this difference in operating a full grab. The two controllers are situated with their operating handles towards one another so that they may be operated over corresponding notches by one hand.