US2181724A - Electrostatic machine - Google Patents

Description

Nov. 28, 1939. I w. D. COOLIDGE El AL I 2,181,724

I ELECTROSTATIC MACHINE Filed Sept. 21, 1958 Inventqrs: Wil Ham D.Coo| idge William K. Kearsley.

Their Attorney.

Patented Nov. 28, 1939 UNITED STATES EIECIROSTATIO MACHINE William D. Coolidge and William K. Ke'nrsley,

Schenectady, N. Y., assignors to General Electric Company, a corporation of New York Application September 21, 1938, Serial No. 231,086

4 Claims. (01. 171-329) The present invention relates to improvements in electrostatic apparatus of the type which employs a continuous insulating belt as a means for conveying electric charges from a charging device to a high potential charge accumulator.

It is an object of the invention to increase the amount of charge which can be carried by a belt of given size and to decrease the likelihood of sparkover from the various parts of the apparatus. This object is accomplished by maintaining the opposed sides of the belt in direct contact in the region between the charging device and the accumulator.

The features which we desire to protect herein are pointed out in the appended claims. The

invention itself may best be understood by reference to the following description taken in connection with the drawing in which Fig. 1 illustrates an electrostatic machine suitably embodying the invention, and Fig. '2 is an enlarged view showing in detail the belt-charging electrodes employed.

Referring particularly to Fig. 1, the electrostatic machine shown comprises an endless belt 25 ill, for example of paper, which is supported in the form of an elongoted loop by means of rollers H and I2. The rollers are mounted insuitable bearings and the belt is-driven by means of a motor l4 connected to the roller II. The belt is 30 given an electric charge by means of charging devices provided adjacent to the rollers, and the charges are conveyed to a spherical accumulating device I 6 which is provided intermediate the ends of the belt. An insulating standard I! sup- 35 ports the accumulator and insulates it from ground.

For charging the belt we may employ any Under these conditions, assuming that a poten-.

tial on the order of several thousand volts is 50 employed between the electrodes l8 and, I9, a brush discharge occurs from the electrode I. The negative elements of this discharge tend to flow to the electrode IE but are intercepted by the upper surface of the belt III, which thus receives 55 a negative charge. The rounded contour of the electrode l9 prevents a similar positive discharge from taking place from that electrode.

As a result of the motion of the belt 10 the negative charges referred to are'carried toward w and into the interior space of the accumulator l6 negative charges borne by the incoming belt por:

through an opening 24 provided therein and are caused to impart a corresponding charge to the accumulator. Additional charging 'electrodes 25 and 26, identical in flmction with the electrodes i8 and I9, cause additional negative charges 5 to be brought to the accumulator by the lower portion of the belt moving inwardly from the right.

The negative charges on the belt are transferred to the accumulator by means now to be 10 described. .In order to take these charges from the incoming portion of the belt there is provided a comb-like electrode 30 similar to the electrode l8 previously described. This'is positioned adjacent to the surface of the belt and receives charges from it by means of a. brush discharge. These charges are communicated to a cylindrical electrode 3| which is directly conuected to the electrode 3. and which is also positioned closely adjacent to the belt surface. The

electrode 3| is further arranged to cooperate with another comb-like electrode 32 positioned adjacent to the bottom half of the belt loop and Y connected to the accumulator l6. By virtue of the negative charge of the electrode 3|, a high gradient is established between this electrode and the electrode 32'. This gradient is in such direction as to cause a brush discharge which carries positive charges from the electrode 32 toward the electrode 3|. Such charges are,.however, intercepted by the surface of the lower belt half and are thus carried out of the accumulator. This withdrawal of positive charges from the accumulator obviously increases its negative potential with respect 'to ground. A negative po- 35 tential of one million volts or more may readily be built up in this way. I y

In connection with the right-hand side of the belt loop and within the accumulator l6 there is provided a second electrode system identical in 40 operation with thesystem which includes the electrodes 30, 3| and 32. This second system includes an electrode 34 for picking up negative charges from the incoming lower portion of the belt and an electrode combination 35, 36 for causing positive charges to be transferred from the accumulator to the outgoing surface of the upper belt half. In general, the positive charges borne away from the accumulator by the outgoing,

portions of the belt are substantially equal to the tions.

The accumulator Ii may be connected to the negative terminal of a load device, such, for example, as an X-ray tube, and may be causedto 55 I energize the device. The success in this connection of an electrostatic machine of the type under consideration depends primarily on the amount of current which it can supply. This, in turn, depends partly upon the rate at which electric 6o charges can be conveyed to and irom the accumulator. For open belts this latter quantity is limited mainly by sparking from the collector to the oppositely charged half of the belt and thence to ground and by the tendency of charges imparted to the belt to escape to surrounding objects when their value is increased unduly. Such escape is apparently a result of ionization of the surrounding air by a corona or brush discharge proceeding from the belt.

We have found that the total current carried by the belt may be substantially increased by operating the two sides of the belt loop in direct contact or in substantially direct contact in the regions between the charging means and the accumulator. While we do not wish to be bound to any particular theoretical explanation of this result, we believe that it is due to the fact that the close adjacency of substantially equal and opposite charge on the upper and lower belt halves tends to decrease the voltage stress which can exist between the belt as a whole and any external object. .Stated in another way, it appears that with the arrangement specified the negative charges are bound to the belt by the close ad glaclency of the positive charges'on the other belt It might appear that placing the belt halves in contact as specified would increase the tendency to direct spark-over between them. That this is not the case is due to the fact that with a given charge density the voltage gradient between the belt-halves is independent of their spacing. This result follows directly from1the known relationship that the voltage between two spaced surfaces is proportional to the quotient of the charge on the surfaces and the capacitance between them. Since the capacitance of two elements is inversely proportional to the distance between them, it will be seen that for a fixed charge, decreasing the spacing of the belt-halves necessarily decreases their potential difierence. Consequently, the potential between the belt sides may actually be very small when they are in direct contact, so that the tendency to sparkover between them is not increased by operating them in this condition.

In order to maintain the belts in the desired relationship during operation there are provided rollers 31, 38, 39 and 40 which serve to force the two belt' sides into direct contact in the regions between the charging devices and the accumulator IS. A fifth roller ll maintains the belt halves in spaced relation within the accumulator so as to facilitate the removal of charges.

While we have stated that the belt-halves are in direct contact it may be that when in motion they are separated by a thin film of air. Obviously no great difference in operation should be expected even if this is the case.

With the arrangement shown the current which may be conveyed from the charging devices to the accumulator is substantially increased. The close adjacency of'the positively and negatively charged belt halves appears to lessen materially the likelihood of a discharge occurring along the surface of the belt. Furthermore due to the decreased corona the total belt length which must be provided between the accumulator and the charging electrode system may be lessened without danger of spark-over between the accumulator and the grounded portion of the apparatus,

. The latter effect becomes especially important in an arrangement in which the apparatus as a whole is to be immersed in a gaseous dielectric other than air, In a dielectric such as vaporous carbon tetrachloride or a fluorine substitution product of carbon-tetrachloride (such as CClzFz) the over-all dimensions of the apparatus may be substantially reduced by decreasing the belt length as specified. Consequently a relatively compact apparatus can be built by utilizing the principles of the invention.

While we have described our invention in connection with an arrangement in which the belt extends completely through the accumulator this is by no means essential. Ihe invention is equally applicable in connection with other arrangements such as that in which only one end of the belt projects into the accumulator. Numerous further modifications may be made by those skilled in the art without departing from the invention, and we aim in the appended claims to cover all such variations as come within the true spirit and scope of the foregoing disclosure.

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

1. An electrostatic machine of the type which employs a charge accumulator, a charging device, and a continuous belt for carrying charges between the charging device and the accumulator, wherein the opposed sides of the belt operate in substantially direct contact in the region between the charging device and the accumulator.

2. An electrostatic machine including a continuous belt, means for charging the belt, a charge accumulator for collecting charges from the belt, means for driving the belt, and means for causing the opposed sides of the belt to operate substantially in direct contact in the region between the charging device and the accumulator.

3. An electrostatic machine including a continuous belt, means for supporting the belt in a closed loop, means for driving the belt, means associated with the belt at one portion thereof for" charging the belt, a hollow accumulator spaced from the charging means and so arranged that its interior is traversed by a portion of the belt, means within the accumulator for receiving charges from the belt, and means for maintaining the opposed sides of the belt loop in mutual contact in the region between the charging'means and the accumulator and for maintaining the sides of the belt in spaced relationship within the accumulator.

ii An electrostatic machine including a continuous belt, means supporting the belt in a.

closed loop, means for drivingthe belt, means WILLIAM D. COOLIDGE. WILLIAM K. KEARSLEY.

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