EP0402338A1 - Method and apparatus for neutralising electrostatic charges on a movable web - Google Patents

Abstract

In order to neutralise electrostatic charges on a movable web (2), an alternating voltage of the order of from about 10,000 V to about 15,000 V and a frequency of from about 1000 Hz to about 3000 Hz is supplied to a con­ductive brush (1) spaced from the movable web (2).

Description

• The present invention relates to a method for neu­tralising electrostatic charges on a movable web. The invention also relates to an apparatus for carrying out the method, which apparatus comprises a conductive brush disposed adjacent said web, and means for supplying a high-tension alternating voltage to said brush.
• Electrostatic charges pose a major problem in the ma­nufacture and handling of web-like materials of e.g. plas­tic, paper or rubber. One example of this is a web which is coloured with a paint containing metal particles and which risks being unevenly coloured if the web has a vary­ing electrostatic charge. Another example is a web which is rolled on a core and which may accumulate such high an electrostatic charge that anyone who touches the roll risks getting heavy electric shocks.
• In machines and close to apparatuses for manufactur­ing and handling web-like materials, it is thus desirable to neutralise the electrostatic charges on the web-like material as much as possible.
• Neutralisation can be effected either by means of a passive device, for instance grounded conductor means disposed in the vicinity of the web, or by means of an active device emitting or collecting electrons for neutralising the charges on the web.
• US 4,502,091 describes an active device comprising a bar along which two conductors are arranged. A number of needles have been pressed into each conductor. In opera­tion, a positive high direct voltage is supplied to one conductor and a negative high direct voltage is supplied to the other conductor. By a "point effect", electrons will be collected and emitted, respectively, from the needles for neutralising the charges on the web.
• US 4,486,808 describes another active device for neutralising random electrostatic charges on a movable web, which simultaneously leaves the web with a uniform, positive, negative or neutral charge. In a first embodi­ment, the device comprises a bar spaced from the web and extending transversely thereof. The bar is supplied both with high-voltage direct current and with alternating cur­rent superimposed on the direct current. The frequency of the alternating current is stated to be 600 Hz for both low and high web speeds.
• In a second embodiment, the device comprises a con­ductive brush arranged in contact with the web.
• A common drawback of these known devices is that they allow only a limited web speed to ensure satisfactory neu­tralisation of the charges on the web. Industrial efforts however go towards increasing web speeds for higher pro­ductivity. Today, web speeds of above 1500 m/min are em­ployed. This, in combination with new materials having good insulating properties, such as different plastics, place high demands on efficiency in the neutralisation process.
• The object of the present invention thus is to pro­vide a method and an apparatus ensuring satisfactory neu­tralisation of electrostatic charges on the web at both low and high web speeds and for all types of webs.
• This object is achieved by a method which is charac­terised in that the brush is supplied with an alternating voltage exceeding about 10,000 V and having a frequency exceeding about 1000 Hz.
• The inventive object is further achieved by means of an apparatus of the type described in the introduction to this specification, which is characterised in that the brush is arranged at a distance from the web and that the means for supplying a high-tension alternating voltage to the brush are adapted to supply the brush with an alter­nating voltage exceeding about 10,000 V and having a fre­quency exceeding about 1000 Hz.
• By arranging the brush at a distance from the web, the apparatus of the invention can be used also for wet webs and webs which must not be touched.
• By supplying the brush with an alternating voltage the magnitude and frequency of which have the indicated values, it is possible to achieve highly efficient neu­tralisation, which will be explained in more detail below.
• To prevent accidents when touching parts of the appa­ratus that are under tension, the apparatus further has means adapted to sense the current in the high-voltage circuit of the apparatus and to interrupt the supply to the brush if current above a predetermined limit value is sensed.
• The present invention will now be described in an em­bodiment with reference to the accompanying drawings, in which
• Fig. 1 is a schematic view showing an embodiment of the invention; and
• Fig. 2 is a graph showing the residual charge on the web as a function of the frequency of the alternating vol­tage for different magnitudes of the alternating voltage.
• An apparatus for deionising a web or neutralising static charges on the web comprises, as appears from Fig. 1, a brush 1 arranged transversely of a movable web 2 at a distance of about 10-15 mm therefrom, a voltage transformer 3 and a generator unit 4.
• The brush 1 is of a known type which is commercially available and which comprises an aluminium section 5 to which a multitude of bristles 6 are attached. The number of bristles may amount to about 3000/cm².
• The brush 1 may preferably be a carbon-fibre brush and should be designed in such a manner that the different bristles remain isolated from each other. Each separate bristle 6 forms a tip, and the tips together produce a powerful "point effect" which contributes to improve neu­tralisation.
• The voltage transformer 3 has a primary winding 7 and a secondary winding 8 which is connected to the brush 1 and coupled in series with a resistor 9, in turn connected to ground. The transformation ratio of the transformer is of the order of 1:1000, and the resistor is of the order of 10 kΩ. The point of junction between the secondary winding 8 and the resistor 9 is connected by a line 10 to the generator unit 4. This comprises, in addition to the voltage generator itself, circuits for monitoring ground errors in the high-voltage or secondary circuit.
• The operation of the apparatus appears from the fol­lowing example.
• The voltage generator 4 supplies a 12 V alternating voltage of a frequency of 1000 Hz to the transformer 3 stepping up the voltage to 10 kV. When this high voltage is applied to the brush, this will emit electrons, by a "point effect", during the negative half period of the alternating voltage. These electrons can then neutralise any positive charges on the web 2 which is travelling past the brush 1. Similarly, the brush will collect electrons, also by a "point effect", during the positive half period of the alternating voltage, whereby negative charges on the web are eliminated.
• If a person, directly or through an electrically con­ductive object, touches parts of the secondary circuit that are under tension, a ground error will arise. Current will then flow through the resistor 9. The voltage then produced across the resistor will be sensed by the moni­toring circuits of the generator unit 4, and if the cur­rent corresponding to the voltage exceeds a previously set treshold value, the supply to the voltage generator is interrupted and, hence, the supply to the brush.
• Fig. 2 shows a graph illustrating the result of an experiment in which alternating voltage of different mag­nitude and frequency was supplied to the apparatus accord­ing to the present invention. The ordinate represents the potential the web had after deionisation. The potential is the measure of the residual charge on the web. The abscis­sa represents the frequency of the alternating voltage supplied to the brush. The graph contains four curves A-D. Curve A was measured at an alternating voltage equal to 2500 V, curve B at 5000 V, curve C at 10,000 V, and curve D at 14,000 V. The measurements were made at a web speed of 800 m/min. The curves in the graph show that when the frequency is increased from about 50 Hz to about 400 Hz, the deionisation of the web was improved. A further in­crease of the frequency from about 400 Hz to about 800 Hz gives no further improvement of the deionisation. At fre­quencies above about 800 Hz, the residual charge on the web decreases slightly at 2500 V and 5000 V, but yet re­mains on a relatively high level. At voltages equal to or above 10,000 V and frequencies above 800 Hz, a very sub­stantial improvement of the deionisation is however ob­tained. At about 1000 Hz, the residual charge is very small, and above 1200 Hz it is so small that it cannot be measured.
• Other experiments, which are not reported here, show that the same effect is achieved also at other web speeds.
• Thus, it has been found that improved deionisation is obtained at frequencies above 1000 Hz and voltages above 10,000 V. In practice, however, the frequency should not be increased above about 3000 Hz and the voltage not in­creased above about 15,000 V, since a combination of high frequency and high voltage results in radio interference.
• For comparative purposes, the experiments described above were also carried out with a commercially available charge neutralising device consisting of a copper rod hav­ing tips, but in this case it was not possible to achieve the effect obtained with the present invention.
• Experiments have also been carried out where de­ionising was effected by means of a passive conductive brush arranged at a distance from the web and connected to ground. In this case, a residual potential of about 1200-1400 V was obtained.
• The experiments also show that it is a combination of a conductive brush and the indicated voltages and fre­quencies that produce the aimed-at effect.
• The invention has now been described by way of exam­ple. However, it is not restricted thereby but may be va­ried within the scope of the accompanying claims. For in­stance, one or more brushes may also be arranged at the underside of the web.

Claims (4)

1. Method for neutralising electrostatic charges on a movable web (2), wherein a conductive brush (1) is dis­posed adjacent said web,
characterised by the step of
supplying the brush (1) with an alternating voltage ex­ceeding about 10,000 V and having a frequency exceeding about 1000 Hz.

2. Apparatus for neutralising electrostatic charges on a movable web (2), comprising a conductive brush (1) disposed adjacent said web (2), and means (3, 4) for sup­plying a high-tension alternating voltage to said brush (1), characterised in that the brush (1) is arranged at a distance from the web (2), that said means (3, 4) for supplying a high-tension alternating voltage to the brush are adapted to supply the brush with an alter­nating voltage exceeding about 10,000 V and having a fre­quency exceeding about 1000 Hz.

3. Apparatus as claimed in claim 2, charac­terised by means adapted to sense the current in the high-voltage circuit of the apparatus and to inter­rupt the supply to the brush (1) when a current exceed­ing a predetermined treshold value is sensed.

4. Apparatus as claimed in claim 2 or 3, cha­racterised in that the conductive brush (1) is a carbon-fibre brush.

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