US2792546A - Electrostatic generators - Google Patents

Description

May 14, 1957 0. R. CHICK ETAL 2,792,546

macmos'mxc GENERATORS 2 Sheets-Sheet 1 Filed Nov. 28, 1955 May 14, 1957 D. R. CHICK ETAL 2,792,546

ELECTROSTATIC GENERATORS Filed Nov. 28, 1955 2 Sheets-Sheet 2 sqf zgvi INVENTOR Q WA f +Ve 0111/42 8 SOURCE WWQM ATTORNEYS Uni e ate Patent ELECTROSTATIC GENERATORS Application November 28, 1955, Serial No. 549,368

Claims priority, application Great Britain December 1, 1954 4 Claims. (Cl. 3222) This invention relates to high voltage electrostatic apparatus and more particularly to apparatus embodying corona gaps of the kind in which an electric potential applied between a metal point or an array of points and a metal surface causes a high concentration of electric field so that an electric current may flow between the point or points and the surface as a result of ionisation of the gas in the intervening space.

. The invention has an important application in electrostatic generators of the kind in which electrostatic charge is deposited on a moving belt or other insulating charge carrier and is carried to a terminal where a high voltage is developed by the accumulated charge.

In many cases it is required to vary the ionisation current over a large range by varying'the applied potential the range of variation in the current being limited by the permissible range of potential which must lie between the corona onset potential, i. e. the potential at which corona current commences and the breakdown sparkover potential. This limitation is particularly severe at high gas pressures where the corona onset potential approaches the spark-over breakdown potential with the result that it is only possible to vary the applied potential and hence the current over a comparatively small range if risk of breakdown is to be avoided.

The main object of the invention is to provide an improved arrangement in which this limitation is considerably lessened.

According to the present invention high voltage electrostatic apparatus includes apparatus for producing a corona gap or gaps comprising one or more point contacts spaced from the metal surface, means for applying a high voltage between the point contact or contacts and the surface, together with a metal shield insulated from and encircling the point contact or contacts, which shield is gapped or apertured to provide an ionisation current path and is so designed as to increase the space over which a steep voltage gradient occurs.

It has been found that such a shield modifies the electric field around the needle points so that a steep potential gradient can be maintained over an appreciably greater space than will be the case without the shield. This reduces the potential necessary to pass a given current and hence enables a larger current to be passed before the breakdown potential is reached.

If one needle point only is used the shield may be in the form of a metal tube mounted coaxially with the needle so that the point of the needle just protrudes beyond the mouth of the tube.

As above mentioned the invention has an important application in electrostatic generators of the kind in which electric charge is deposited on an insulating charge carrier such as a moving belt and is carried to a terminal where a high voltage is developed by the accumulated charge.

According to one such application of the present invention a high voltage electrostatic generator of the kind referred to, comprises means for applying electrostatic charges to a moving insulating surface forming a charge carrier, including an array of needle points mounted on a bar extending transversely of the direction of movernent of the charge carrier and projecting towards the charge carrier, together with a cylindrical metal shield extending around the bar and having a longitudinal gap through vwhich the needle points protrude and means for maintaining said shield at a potential intermediate between that of the needle points and a second electrode located beyond the charge carrier, e. g. a belt pulley. 4

It will be appreciated that the high gas pressure normally employed to insulate such generators and the current which can be conveyed to the belt without risk of sparking to the needle and carrier is, with existing arrangements, frequently limited to an undesirably low value, as above explained. This limitation can be removed to a considerable extent by arrangements embodying the invention.

It will be appreciated that the corona gap is essentially produced between the needle or needles and the metal surface constituting the second electrode, e. g. the belt pulley, and that the presence of the intervening charge carrier, e. g. a belt, does not materially affect the action of the corona gap or the use of a shield in accordance with the invention. a

In order that the invention may be more clearly understood reference will now be made to the accompanying drawings, in which:

Fig. 1 shows diagrammatically a sectional view of a needle point array embodying the invention.

Fig. 2 is a longitudinal section on the line II--II of Fig. 1.

Fig. 3 illustrates graphically the electrostatic fields within an arrangement not employing the shield and Fig. 4 is a corresponding view showing the electrostatic fields when employing the shield.

Fig. 5 shows diagrammatically an electrostatic generator embodying the invention.

In the arrangements of Figs. 1 and 2 the reference 1 indicates the belt and 2 the belt pulley. 3 is a bar extending transversely of the belt 1, i. e. parallel to the axis of the pulley 2, whilst 4 are the needle points. The invention consists in providing the cylindrical shield 5 around the bar 3, which shield 5 is gapped at 6, the needle points 4 projecting slightly through the gap.

As shown more clearly in Fig. 2 the shield 5, which is metallic, is mounted at one end on the bar 3 by means of an insulating end member 7 and at the other end projects through an insulating bushing 8 such as a perspex bushing.

In Fig. 3, which shows the known arrangement without the shield 5, the equi-potential lines are indicated by dash lines. It will be observed that they are only distorted to a relatively small extent by the needle points 4.

In Fig. 4, on the other hand, the equi-potential lines are distorted to an appreciably greater extent around the needle points and hence the space over which there is a high potential gradient is appreciably increased with the result that there will be considerably enhanced ionisation for a given voltage.

Fig. 5 shows an electrostatic generator embodying the invention.

It essentially comprises a charge conveying belt 1 running over an earthed pulley 2 and a high voltage pulley 2'. A positive charge is applied to the belt from a source indicated by the rectangular block 8 through a corona terminal 9 which may be of the kind above described. The positive charge on the belt is conveyed to the high voltage terminal 10 by a collector 11 which is also connected to the high voltage pulley 2 whilst negative charge is sprayed on to the belt by a terminal 12.

Whilst the invention has been more particularly described with reference to moving 'he'lt type generators it is also applicable to other types of generators, for instance, to machines in which the charge carrier is .a cylinder or disc of glass, plastic or ,other'mat'erial.

-Itis also applicable to apparatus, other than electrostatic generators, embodying corona gaps.

' Whatwe claim as new and desire to secure by Letters Patent of the United States is:

1. High voltage electrostatic apparatus for producing a corona gap comprising a metal surface, a corona terminal said corona terminal having at least one point contact spaced from the metal surface, means for applying a high voltage between the point contact and the metal surface to produce a corona discharge, a metal shieldinsulat'ed'fro'm and partially surrounding said metal contact, edges defining an aperture in said shield through which said contact point protrudes and means ,for maintairiing'sa'id shield at a voltageiintermediate between that of thecontact and'the surface so as to-increase the space over which there is a steep voltage gradient.

' 2. High voltage electrostatic apparatus for producing a corona gap comprising a metal surface, a corona terminal, said coronaterminal comprising abar and an array of point contacts mounted on said bar and spaced from themetal surface, means for applying a'high voltage between the corona terminal and the metal surface to produce corona discharge, a metal shield insulated from and partially surrounding said contact, edges defining an aperture to said shield through which said contact points protrude, .and means for maintaining said shield at a voltage intermediate between thatof the corona terminal and the surface so as to increase the space over which there is a steep voltage gradient.

j ,3. A high voltage electrostatic generator comprising a metal surface, a corona terminal, an insulating charge earrierdisposed between the metal surface and the corona terminal, said corona terminal having at least one point contact spaced from the metal surface and charge carrier, means for applying a high voltage between the point contact and the metal surface to produce a corona discharge on to the charge carrier, a metal shield insulated from and partially surrounding said metal contact, edges defining an aperture in said shield through which said contact point protrudes and means for maintaining a shield at a voltage intermediate between that of the contact and the surface so as to increase the space over which there is a steep voltage gradient and means for transferring electrical charge from said charge carrier to a high voltage terminal.

4. A high voltage electrostatic generator comprising an insulating belt, a'high voltage pulley and low voltage pulley carrying said belt, a corona terminal adjacent the belt passing over the low voltage pulley, said corona terminal comprising a bar, an array of contact points carried on said bar and spaced from thesurface of the belt, means for applying a high voltage between said corona terminal and said low voltage pulley so as to provide a corona discharge onto'the belt, a metal shield surrounding said bar, edges to said metal shield defining a gap through which the contact points protrude, means for maintaining said shield at a voltage intermediate that of the terminal and the low voltage pulley so as to increase the space over which there is a steep voltage gradient and a collector terminal in the neighbourhood of the high voltage pulley for transferring charge from said belt to a high voltage terminal.

References Cited in the file of this patent UNITED STATES PATENTS

Images