Patent GB-731360-A (GB)
Title: Improvements in or relating to electric motor controllers
IPC: H02P25/26
Filed: 1952-05-05
Abstract: 731,360. Control of two A.C. motors. IGRANIC ELECTRIC CO., Ltd., and TAYLOR, J. R. May 5, 1953 [May 5, 1952], No. 11294/52. Class 38 (3). A system for a grab hoist comprises two A.C. motors which operate respective winches, namely a holding line winch which actuates the ropes supporting the grab with the grab jaws open, and a closing line winch which actuates the ropes which when pulled cause said jaws to close and then share the load with the holding line ropes. A controller for each motor comprises voltage relay means adapted to be controlled according to the rotor voltage and thereby adapted to regulate the rotor resistance during lowering according to the speed of the motor in such a way as to afford a controlled low speed. As shown, when a master controller MC for the holding line motor HM is moved to a first hoisting notch, contacts Cl close to shortcircuit resistor R11-R12 so that voltage relay VR2W is operative at 85 per cent of the standstill rotor voltage. Contacts C4 close to energize coils HW, MW and to actuate contacts HC, MC, so that the motor HM is started, and coil VR2W is energized to close contacts VR2LC, VR2RC in circuit with rotor contactors 2RW, 3RW controlling contacts 2RC, 3RC. As the motor speed rises on light loads, contacts VR2RC and VR2LC are opened in succession. Contacts C8 close on the second hoisting notch so energizing coil 4RW through contacts VR3C of relay VR3W and closing contacts 4RC for acceleration of the motor. On the third hoisting notch, contacts C7 energize rotor contactors 5RW, 6RW, but these do not close immediately owing to hold-out coils HO5, HO6, which are maintained energized for a period by the discharge of capacitors EC5, EC6. When the master controller is moved to the first lowering notch the motor is connected in the hoisting direction with all the rotor resistance in circuit, so that the load overhauls the motor. Resistance R11, R12 is in circuit with coil VR2W so that contacts VR2RC, VR2LC are controlled according to the speed of the motor, to keep the speed within a required range of values. On the second lowering notch, the motor is connected in the lowering direction with rotor contacts 4RC closed, and, on the third notch, the contacts 5RC, 6RC, 7RC are closed in succession under the regulation of capacitor-controlled time lags. When the controller is returned to the first lowering notch the motor is connected in the hoisting direction with contacts 2RC, 3RC closed for providing a high retarding torque. Similar operation obtains when the controller is returned to the " off " position, the energization of the contactor Hw being maintained for a period during this movement of the controller by capacitorcontrolled contacts. The motor is finally disconnected at a low speed by means of the relay VR2W. Should the master controller be thrown from the full speed lower notch to the full speed hoist notch, the relay VR3W will prevent closure of contacts 4RC at excessive speeds. The closing line controller, Fig. 2 (not shown), is similar to that described except that the star-point of the rotor resistance may be opened so that the closing line motor may be connected to the supply on the first lowering notch with the starting resistance opencircuited, so producing no torque. Excess rotor voltage will cause closing of the rotor circuit to produce a hoisting torque. Furthermore, a current responsive relay is added to prevent reduction of the rotor resistance when there is excess current in the stator. The arrangement is such that, during lowering when the two controllers are on corresponding notches the grab will be lowered in the closed condition, and when the closing controller is one notch in advance of the holding controller the grab will be lowered in the open condition. The closing line winch is geared to run faster than the holding line when both motors are at the same speed, so that the closing line is paid out at the proper rate.