Patent GB-512949-A (GB)
Title: Improvements in or relating to grab hoists
IPC: H02P5/68
Filed: 1938-03-21
Abstract: 512,949. Motor control systems. IGRANIC ELECTRIC CO., Ltd:, and TAYLOR, J. R. March 21, 1938, No. 8613. [Class 38 (iii)] In a grab hoist system in which a closing line, when tensioned, closes the grab and supports the weight and a holding line, when tensioned, supports the weight, the two lines are operated respectively by direct-current motors fed from respective generators. The speed of each motor is controlled by controlling the excitation of the appropriate generator. The motors and generators have independently excited fields and the armature of each motor M1, Fig. 1, is connected with the armature G1 of its respective generator. The speed of the motors is controlled by the insertion of resistance R in the field circuit GF1 of the appropriate generator. The two generators are controlled by individual controllers of which the holding controller D1, Fig. 3, is shown. In the off position of the controller D1, Fig. 3, a winding w1 is energised through drum contacts 1, 2, locks up through 3c1 and overload contacts c2, and places an economising resistance r2 in series by opening contacts 2c1. The winding w1 also closes contacts 1c1, Fig. 1, to connect together the two armatures M1, G1. In the first hoisting position h1, drum contact 3 is engaged and winding w3 is energised through closed contacts 4c1, and contacts 1c3, Fig. 1, are closed to energise the generator field GF1 in the direction for hoisting, and winding w4 is energised through contacts 2c3 to cut out a series resistance r3, Fig. 1, in the motor field. At the second notch h2 a drum contact 4 causes energisation of a winding w6 which provides a holding circuit for w3 by contact 2c6 and short-circuits a section of series resistance R by contacts 1c6. At the third, fourth and fifth notches windings w7, w8, w9 are successfully energised to decrease the field resistance R by closing 1c7, 1c8, 1c9 and in turn to close holding circuits for the preceding windings by means of 2c7, 2c8, 2c9. At the fifth notch also a winding w10 is energised to cut out the last section of field resistance R but this is delayed until a contact 3c9 has been closed by the preceding winding w9. Similarly the energisation of each of the preceding windings w6 to w9 is delayed until the preceding winding has been energised. At the sixth hoisting notch h6 a winding w11 opens a contact c11, Fig. 1, to insert series resistance r1 in the motor field MF1. The auxiliary contacts 2c6 to 2c11 in the circuits of their preceding windings cannot be opened until their operating windings have been de-energised, a process which is delayed by shunt resistances r6 to r10, and thus prevents the too rapid insertion of field series resistance R if the controller is quickly returned to the " off." In the lowering positions l1 to l5 the operation is similar except that, at the first notch l1 a winding w12 is energised instead of w3 so that, by closure of contacts 1c12, the generator field GF1 is energised in the direction for hoisting. Further there is no sixth position in the lowering direction and thus the additional speed provided by weakening of the motor field is not available. The controller for the closing motor, Fig. 4 (not shown), is identical except that the sixth additional notch is provided in the lowering instead of the hoisting direction. The additional speed provided by these notches enables the grab to be maintained closed when raising at full speed and to maintain it open when lowering at full speed. An overload relay is provided to de-energise the controller on the occurence of an overload. The closing motor is also provided, between windings w7 and w8, with a normally closed contact of a jamming relay. This contact opens to render movement of the controller beyond the third position inoperative to prevent excessive current being passed to the motor if the grab is unable to close, as by meeting an obstacle.