GB687871A - Improved method and apparatus for controlling a magnetically vibrated screen - Google Patents

Abstract

687,871. Reciprocating machines. SHIELDS, R. (Tyler Co., W. S.). July 5, 1950, No. 16728/50. Class 35. The supply of current from the A.C. supply to an electromagnetic vibrator is controlled by a gas-filled discharge tube whose grid is energized from an oscillator circuit with alternating current of half the supply frequency, the phase of the grid control voltage being shifted to control the A.C. supply current passing through the tube. The phase of the grid controlling voltage can be adjusted to allow the passage of only alternate positive half-waves of the supply, or to allow parts of the intermediate waves to pass in addition, or to allow all the positive half-waves to pass. The natural frequency of oscillation of the vibrator is such that these adjustments produce a successive reduction in the amplitude of the vibration whilst maintaining a quick action to prevent clogging. The invention is described as applied to the control of a screening machine of conventional plunger, spring - supported armature type. The vibrator energizing coil 27 is fed through the discharge tube RT from the supply 50, 51, from which the power transformer P of the control circuit is also supplied. The grid circuit of the tube RT is connected across the secondary winding of a transformer 60 whose primary winding is part of an oscillatory circuit including a condenser 68, a transformer 74 and an energizing valve VT. This circuit oscillates at half the supply frequency. The grid of the valve VT is excited from the secondary winding of the transformer 74 across which is connected an inductance 76 and manually variable resistor 75 by means of which the phase of the oscillation with respect to the main supply may be adjusted to modify the amplitude of the vibration.