Patent GB-630313-A (GB)
Title: Improvements in or relating to crane hoists driven by alternating current motors
IPC: B66C13/26
Filed: 1946-07-18
Abstract: 630,313. Control of several A.C. motors. IGRANIC ELECTRIC CO., Ltd., and TAYLOR, J. R. July 18, 1946, No. 21523. [Class 38(iii)] A crane hoist is driven by two alternat-- ing-current motors, namely a main motor and a smaller auxiliary motor which tends to drive the hoist at a higher speed than said main motor, whereby, with both motors operating together in the hoisting direction, the auxiliary motor will in the case of light loads, take most of the load, a controller being provided having a plurality of hoisting and lowering notches, and at certain of said notches the resistance in the main motor secondary circuit is controlled in accordance with the voltage of said main motor secondary, so as to provide a substantially constant speed for the notch. When a main controller D is moved to a first hoisting position 1h a main contactor closes its contacts M to connect the primary winding of the motor A to a supply source. Voltage relay windings VR1w- VR4w are connected across two secondary termintls of the motor A in series with a. resistor r, so that, according to the value of the secondary voltage, one or more of contacts VR1-VR4 will close to energize one or more of relay windings 1Rw-4Rw which control contacts 1R-4R in the main motor secondary circuit. The secondary circuit resistance is thus reduced or increased according to an increase or decrease of motor load, and a balance is established at which an approximately constant hoisting speed is obtained on the first notch. At the second hoist position, 2h, the winding 4Rw is positively energized through contacts of the master controller, contacts 4R are therefore closed, together with contacts 3R, and the voltage relays VR1-VR4 are rendered ineffective. At the third hoist position contacts H are closed to energize the auxiliary motor B for hoisting. At the same time contacts 12R are closed to reduce the auxiliary motor secondary resistance. When the controller is moved to the fourth hoist position, provided that the main motor secondary voltage is sufficiently low for the contacts VR2 to be opened, a resistor r1 now being in the voltage relay winding circuit, a relay winding 5Rw is energized through normally-closed contacts of the relay 2Rw. Contacts 5R are accordingly closed, as are contacts 6R and 7R after a time-delay. If the current in the main motor primary circuit falls below a given value, contacts LR, corresponding to a relay coil LRw which is energized through a current transformer CT and a rectifier K2, will close to energize a relay winding DRw and open contacts DR1. The relay windings 1Rw- 7Rw are thereby de-energized to open the main motor secondary circuit. At the same time the contacts 13R are closed to remove the whole of the controllable resistance from the auxiliary motor circuit. At the first lowering position, 1l, the main motor is again connected to the supply in the hoisting direction, the voltage relay windings being connected in series with resistors r and r1 across two terminals of the main motor secondary. The contacts L are closed to connect the auxiliary motor for lowering, and the load, together with the auxiliary motor, overcomes the main motor. The speed of lowering is controlled by variation of the main motor secondary resistance through the voltage relays. At the second lowering notch a resistor r2 is additionally inserted in series with the voltage relay windings VR1w-VR4w so that an increase of speed is obtainable. At the third and fourth lowering notches resistors r3 and r4 are sequentially inserted in series with the voltage relay windings. At the fifth lowering notch, the contacts 12R, and thereafter the contacts 13R, are closed and the resistor r5 is inserted in the series circuit of the voltage relay windings. The arrangement is such that at the last lowering notch the auxiliary motor runs at above synchronous speed, the capacity of said motor being such as to prevent "running away " if the main motor should fail. When the controller is moved towards neutral for reduction of speed, so long as the voltage relay VR2 remains energized, contacts 2R2 will be closed to maintain the contacts M thereby allowing the main motor to provide braking, the auxiliary motor being disconnected. The contacts 2R2 drop out at substantially zero speed. The resistance in the main motor secondary circuit is controlled by the voltage relays to provide an appropriate braking effort. The voltage relay may alternatively comprise a common relay with a plurality of contacts operable at different voltages. The auxiliary motor may be cut out during lowering, the hoisting effort of the main motor being such that the load is sufficient to prevail over it.