GB729474A - Improvements in and relating to electrically operated hoists, winders and the like - Google Patents

Abstract

729,474. Control of A.C. motors; controllers. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. July 24, 1953 [Aug. 7, 1952], No. 19932/52.. Class 38 (3). A piston 2, forming part of a fluid-pressure servo system, is coupled by means of a forked rod 3 to the moving electrodes of a liquid resistance which is connected in the rotor circuit of a slip-ring induction motor driving a winder or hoist. Displacement of the driver's lever to reduce the liquid resistance moves sleeve 4 so that ports 6 are closed to cause oil pressure to build up within the cylinder. The piston thus follows the movement of the control lever, until the ports 6 are opened, at a rate dependent upon that at which oil flows into the cylinder. Valves 10, 11 are biased to the closed positions but are operated by solenoids 12, 13, to open ports 14, 15, whereby oil is discharged from conduit 7 to by-pass the cylinder 1. Control circuits are provided for the solenoids to energize them in accordance with the motor current, so that valve 10 is actuable in accordance with this current exceeding a predetermined value when the motor is driving the winder, and so reduces the rate of removal of rotor resistance in this event. The valve 13 is similarly effective during dynamic braking operation. In the control arrangement shown, a transductor 20 has output windings 23, 24, connected in circuit with solenoids 12, 13. A positive feed-back control winding 26, a biasing winding 28 energized with constant direct current, and current-responsive control winding 36, are also provided for the transductor. The winding 36 is connected through rectifier 37 to a current transformer 38 in the rotor or stator circuit of the motor. When a predetermined value of motor current is exceeded, the output of the transductor increases sufficiently to cause solenoid 12 to open valve 10 so that liquid is by-passed from the cylinder 1 and the rate of movement of the liquid controller is reduced. During braking, a resistor 33 is added to resistor 34 to reduce the excitation of biasing winding 28. The solenoid 13 thus opens the valve 11 at a lower value of motor current. When the electrodes of the liquid resistance controller approach the end of their travel, a limit switch 35 opens to de-energize winding 28, and slow movement of the controller is introduced irrespective of the value of motor current. A modified arrangement, Fig. 4 (not shown), comprises two transductors, the output windings of which are respectively connected .to the solenoids 12 and 13. The transductor controlling the solenoid 12 has a control winding energized in accordance with the stator current of the motor. The current-responsive winding of the other transductor is connected across a shunt in the direct-current excitation circuit for the motor during braking. This direct-current excitation is made dependent upon the rotor current of the motor. In a further arrangement, the transductor 20 may be replaced by a relay having an operating winding corresponding to the winding 36 and an opposing winding corresponding to the winding 28. The relay controls a contactor having contacts in the solenoid circuits.