GB645931A - Method and apparatus for the control of electrostatic fields - Google Patents

Abstract

<PICT:0645931/III/1> <PICT:0645931/III/2> In apparatus for controlling the strength of an electrostatic field used in a paint-spraying apparatus such as that described in Specification 591,474, means are provided whereby the potential difference between spaced-electrodes is automatically reduced so as to prevent spark-over, in the event of the electrode spacing becoming less than a predetermined minimum. In Fig. 1 the one electrode 17 is connected through a rectifier 19 to the high-voltage winding of a transformer 20 and is associated with a paint-spraying gun 16, whereas the other electrode, which is grounded, is formed by the object 15 to be sprayed, or by a conducting-backing therefor. The primary of the transformer 20 is connected to a source of supply 40 <PICT:0645931/III/3> through the contacts 38, 39 of an electromagnetic switch 35, which has its holding-coil 36 connected in series with maintaining-contacts 37 and the contacts of a relay 30 across the supply 40. The switch 35 is closed by a manual switch 41 having contacts 42 in parallel with the contacts 37. In the event of the electrode-spacing becoming less than a predetermined safe minimum, the increased field-current causes an increased voltage drop across a resistor 22, thereby decreasing the bias on the triode 25 which conducts, so as to energize the relay 30, open the switch 35 and de-energize the transformer 20. The switch 35 may include a pair of contacts 43 connected to a signalling-device 45 which indicates when the transformer 20 is de-energized. Normally-closed contacts 47 in the triode circuit, open on actuation of the starting-switch 41, so as to prevent the energization of the relay 30 in the event of a starting surge. In Figs. 2, 3, a succession of objects 50 suspended from a grounded conveyer 51, are passed through the field created by a cage-like electrode 52 supporting a plurality of wires 56, the paint being introduced by spray-guns 53. In the event of an object 50 becoming displaced so as to reduce the spacing between itself and the electrode 52, it interrupts one or more light-beams 61 . . . 64 projected from sources 60 and reflected by mirror-systems 65a, 65b, on to photo-electric cells 66 . . . 69. The photo-electric cells energize relays 71 . . . 74 which in turn control the energization of electromagnetic switches 76 . . . 79, which are so connected that as the light-beams 61 . . . 64 are successively interrupted and the relays 71 . . . 74 open, the potential applied by an auto-transformer 100 to the primary of the transformer 20 is progressively reduced, being finally cut off on interruption of the beam 64. The apparatus may be re-set manually by a switch 110 when a displaced object 50 no longer interrupts the beam 64, or it may be re-set automatically by eliminating the contacts 90 from the switch 79. Specifications 604,567 and 644,593 also are referred to.