GB854782A - Improvements in or relating to electrically operated tumbling grinding mills - Google Patents

Abstract

854,782. Automatic control systems for grinding-mills; magnetic couplings. UNION CORPORATION Ltd. Aug. 19, 1957 [Aug. 31, 1956; Feb. 19, 1957], No. 26069/57. Classes 35 and 38 (4). A method of controlling a tumbling grinding- mill comprises the steps of generating a control signal responsive to variations of the mill motor input power, modifying the control signal and utilizing it for a control operation on the mill to keep the motor input power at a maximum. The control is directed to obtaining optimum operation by varying either the motor speed or the feed of grinding mediums to the mill, too much or too little resulting in a reduction of input power. The mill motor current is monitored by transformer 1, Fig. 2, the rectified output charging capacitor 17. Double triode 18 forms with transformer 20 an A.C. bridge circuit for comparing the P.D. across capacitor 17 with a variable reference voltage from potentiometer 10. The output is amplified at 30 and applied to phase-sensitive power amplifier 32, 33 which in turn energizes two-phase motor 8. This drives through gearing the slotted rotor 43, Fig. 3, of a magnetic clutch. Mounted on bearings on the rotor spindle are pairs of arms, such as 52, 53, which carry permanent magnets 56, 57 and 58, 59 adjacent the rotor rim. Rotation of disc 43 drags the arms through a small angle until the movement is restrained by one or the other, depending on the direction of rotation, being engaged by lever arm 69, pivoted at 74. This lever rocks, operating one or the other of mercury switches 77, 78. A follow-up control is provided by motor 8 also varying the setting of potentiometer 10 in a sense such that the reference voltage tend to balance the voltage of capacitor 17. Operation of switch 77 denotes a decrease of input power and, depending whether the mill is being operated just above or just below the optimum point, the feeder motor is stopped or started. In Fig. 5 thyratron 80 anode circuit is switched periodically by timer 92. If switch 77 is closed when the anode circuit is complete the valve conducts and energizes relay 91 to control the feeder motor as required. A further mode of operation utilizes timer 92 to give a time between the control pulses sufficient for the operative condition to alternate between points just below and just above the optimum. In another embodiment (Fig. 7, not shown) the switches operated by the magnetic clutch are arranged as a reversing switch between an amplifier output lead and one winding of a twophase motor, so that the feeder motor is subject to continuous and reversible control.