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Particle conveying with travelling, curvilinear electric field generated with plane electrodes (Part I)
Journal of Electrostatics, 17 (1985) 95--98
95
Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands
PARTICLE CONVEYINGWITH TRAVELLING, CURVILINEAR ELECTRIC FIELDS GENERATED WITH PLANE ELECTRODES (PART I) I . I . Inculet and G.S.P. Castle Faculty of Engineering Science, The University of Western Ontario, London, Canada, N6A 5B9
ABSTRACT The authors present the theoretical background of a novel electrostatic conveyor for particles of various materials and sizes as large as 500 um mean diameter. The conveying action involves the generation of t r a v e l l i n g , curvilinear, AC fields from planar electrodes. The particles moving under the electrostatic forces are exposed to unidirectional, centrifugal forces. The conveyor ought to operate on both conductive and non-conductive materials. Many industrial applications are envisaged in the conveying, classifying and separating of materials. The experimental results w i l l be published in a subsequent paper (Part I I ) . GENERAL The e a r l i e r neglect of the electric f i e l d forces in industry in preference for magnetic f i e l d forces has been primarily a result of the fundamental difference in their relative effectiveness. Magnetic f i e l d s , which are capable of generating mechanical forces on surfaces in the range of 500 kPa, have dominated the electric fields which, on surfaces in a i r , can barely produce 40 Pa.
The progress that we have
witnessed in the engineering of magnetic fields has been extraordinary. Developments, such as the linear induction motor, offer continuing new challenges and opportunities for industrial applications of magnetic f i e l d forces. Furthermore, while with magnetic fields one can engineer a continuous rotational or linear motion, the electric fields have not been able to be u t i l i z e d for the realization of a pure electrical force system which can propel charged particles.
This is a result of the physical law which requires
electric f i e l d lines to originate and terminate on an e l e c t r i c charge.
Thus
i t is impossible to generate an electric f i e l d l i n e which is closed on i t s e l f , such as the magnetic f i e l d lines surrounding an e l e c t r i c current.
However,
forces generated by e l e c t r i c fields have unique properties unmatched by those in magnetic f i e l d s , namely: (a)
they work on a l l materials, magnetic or non-magnetic, as long as an
0304-3886/85/$03.30
© 1985 Elsevier Science Publishers B.V.
96 e l e c t r i c charge is placed on t h e i r surface or i n t e r n a l p o l a r i z a t i o n
occurs; (b)
they move unipolarly charged, free material along the general path of the electric f i e l d lines and in the direction determined by the polarity of the charge and the direction of the electric f i e l d ;
(c)
although they are very small on a macroscopic scale, they are capable of generating enormous accelerations on particles in the micron and submicron range.
Many attempts have been made to achieve effective particle conveying by electric f i e l d s .
One notable achievement, the so-called "electric curtain",
was originated in the early 1970s at the University of Tokyo by Masuda (ref.1).
Specially spaced and shaped cylindrical electrodes energized by a
two-phase or three-phase AC system of voltages were shown to be capable of moving fine particles over rotational trajectories basically advancing in one direction.
In this system centrifugal forces were used to hold the charged
particles in suspension while the electrical f i e l d reversals caused the rotation which led to unidirectional movement. The non-uniform, t r a v e l l i n g electric f i e l d was theoretically described, in an approximate way, as being composed of an i n f i n i t e number of modes of advancing and retarding rotating fields having the same angular velocity and different amplitudes and linear velocities.
The equipment found application in spray painting booths, in
particular where fast colour changes are necessary (ref.2). THE NEW CONCEPT On the basis of today's knowledge, electric fields alone are not capable of achieving continuous rotational or linear motion for a charged particle. Additional forces are necessary. Mechanical forces, such as centrifugal forces due to vibration on curved trajectories, in combination with the electrical forces may be made to work to propel particulate material in one direction. It has already been shown (refo3) that curvilinear electric fields may be generated from a f l a t surface when a potential gradient is established along the electrode.
When small charged particles are placed on such a surface, they
begin to vibrate along curvilinear paths.
As the curvature of the f i e l d lines
is oriented in the same direction, the resulting centrifugal forces also act in the same direction. In addition, i f a system is arranged for an alternate generation of curvilinear fields from two surfaces which are interspersed and energized independently from an AC source, particulate material placed on such surface w i l l be continuously conveyed for any distance.
97 PROPOSED EXPERIMENTAL CONVEYOR A two-phase energized conveyor is shown schematically in Fig. 1.
The
curvilinear fields are achieved, for example, by a potential difference of approximately 20 kV between edges AI and BI on Phase I and A2 and B2 on Phase I I , and at the same time a potential of approximately 10 kV is maintained between edges BI and B2 and the nearest ground by means of resistors, R. The plates are connected to Phase I I identically to those in Phase I. All plates of the conveyor must be made of resistive material such that a current w i l l flow between edges AI and BI and A2 and B2.
This arrangement w i l l
result in a curvilinear AC electric f i e l d as shown schematically in the figure.
The degree of curvature of this f i e l d w i l l be dependent upon the
length of the plates AI-B I and A2-B2 relative to the separation distance, D, to the ground plate. A time lag of (~/2) radian between the two phases w i l l generate a form of t r a v e l l i n g , curvilinear electric f i e l d and resulting centrifugal forces at a l l points on the surface of the conveyor.
T
=.,
CORV,L,NEARAC
,1
I
CONVEYING
~E PHASE I I
"l PHASE I I
i
!~R
, HASEI ACUPLY J
Fig. 1. Schematic diagram of a two-phase t r a v e l l i n g , curvilinear AC e l e c t r i c field.
98 The speed at which the material is conveyed w i l l depend on the length of the d i f f e r e n t plates and the frequency at which the AC p o t e n t i a l is applied a l t e r n a t e l y to one or the other set of plates,
Each phase potential has i t s
own constant frequency which could be anywhere from 10 to 60 Hz.
Certain
materials may be conveyed most e f f i c i e n t l y at predetermined frequencies (ref.4).
I t is worth noting that the frequency of the two sets of plates or
phases need not be the same.
The frequency of alternation of the two phases could be varied widely (e.g. 0.1 to 5 Hz). CONCLUSIONS The proposed novel electrostatic conveyor is believed to offer considerable promise for continuous conveying of particulate materials up to a size which can be l i f t e d and vibrated by an electric f i e l d .
As they advance, the
particles move up and down on curvilinear paths.
It is also envisaged that for
larger particles the system could work well in combination with simple mechanical vibration.
REFERENCES 1 S. Masuda and T. Kamimura, Approximate Methods for Calculating a Non-uniform Travelling Field, Journal of Electrostatics, I (1975), pp. 351-370. 2 S. Masuda, S. Mori and T. Itoh, Applications of Electric Curtain in the Field of Electrostatic Powder Coating, Speech at 1975 Conf. of the Electrostatic Society of America, June 24-26, 1975, Univ. of Michigan. 3 I . I . Inculet and R.K. Rowe, Generation of Curvilinear Electrostatic Fields from Parallel and Inclined Plane Electrodes, Conf. Rec. 1983, 18th Annual Meeting, IEEE Industry Applications Soc., Mexico City, Oct. 3-7, pp. 1122-1124. 4 I . I . Inculet, Y. Murata and G.S.P° Castle, A New Electrostatic Separator and Sizer for Small Particles, IEEE Trans. on Industry applications, IA-19, 1983, pp. 318-323.
95
Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands
PARTICLE CONVEYINGWITH TRAVELLING, CURVILINEAR ELECTRIC FIELDS GENERATED WITH PLANE ELECTRODES (PART I) I . I . Inculet and G.S.P. Castle Faculty of Engineering Science, The University of Western Ontario, London, Canada, N6A 5B9
ABSTRACT The authors present the theoretical background of a novel electrostatic conveyor for particles of various materials and sizes as large as 500 um mean diameter. The conveying action involves the generation of t r a v e l l i n g , curvilinear, AC fields from planar electrodes. The particles moving under the electrostatic forces are exposed to unidirectional, centrifugal forces. The conveyor ought to operate on both conductive and non-conductive materials. Many industrial applications are envisaged in the conveying, classifying and separating of materials. The experimental results w i l l be published in a subsequent paper (Part I I ) . GENERAL The e a r l i e r neglect of the electric f i e l d forces in industry in preference for magnetic f i e l d forces has been primarily a result of the fundamental difference in their relative effectiveness. Magnetic f i e l d s , which are capable of generating mechanical forces on surfaces in the range of 500 kPa, have dominated the electric fields which, on surfaces in a i r , can barely produce 40 Pa.
The progress that we have
witnessed in the engineering of magnetic fields has been extraordinary. Developments, such as the linear induction motor, offer continuing new challenges and opportunities for industrial applications of magnetic f i e l d forces. Furthermore, while with magnetic fields one can engineer a continuous rotational or linear motion, the electric fields have not been able to be u t i l i z e d for the realization of a pure electrical force system which can propel charged particles.
This is a result of the physical law which requires
electric f i e l d lines to originate and terminate on an e l e c t r i c charge.
Thus
i t is impossible to generate an electric f i e l d l i n e which is closed on i t s e l f , such as the magnetic f i e l d lines surrounding an e l e c t r i c current.
However,
forces generated by e l e c t r i c fields have unique properties unmatched by those in magnetic f i e l d s , namely: (a)
they work on a l l materials, magnetic or non-magnetic, as long as an
0304-3886/85/$03.30
© 1985 Elsevier Science Publishers B.V.
96 e l e c t r i c charge is placed on t h e i r surface or i n t e r n a l p o l a r i z a t i o n
occurs; (b)
they move unipolarly charged, free material along the general path of the electric f i e l d lines and in the direction determined by the polarity of the charge and the direction of the electric f i e l d ;
(c)
although they are very small on a macroscopic scale, they are capable of generating enormous accelerations on particles in the micron and submicron range.
Many attempts have been made to achieve effective particle conveying by electric f i e l d s .
One notable achievement, the so-called "electric curtain",
was originated in the early 1970s at the University of Tokyo by Masuda (ref.1).
Specially spaced and shaped cylindrical electrodes energized by a
two-phase or three-phase AC system of voltages were shown to be capable of moving fine particles over rotational trajectories basically advancing in one direction.
In this system centrifugal forces were used to hold the charged
particles in suspension while the electrical f i e l d reversals caused the rotation which led to unidirectional movement. The non-uniform, t r a v e l l i n g electric f i e l d was theoretically described, in an approximate way, as being composed of an i n f i n i t e number of modes of advancing and retarding rotating fields having the same angular velocity and different amplitudes and linear velocities.
The equipment found application in spray painting booths, in
particular where fast colour changes are necessary (ref.2). THE NEW CONCEPT On the basis of today's knowledge, electric fields alone are not capable of achieving continuous rotational or linear motion for a charged particle. Additional forces are necessary. Mechanical forces, such as centrifugal forces due to vibration on curved trajectories, in combination with the electrical forces may be made to work to propel particulate material in one direction. It has already been shown (refo3) that curvilinear electric fields may be generated from a f l a t surface when a potential gradient is established along the electrode.
When small charged particles are placed on such a surface, they
begin to vibrate along curvilinear paths.
As the curvature of the f i e l d lines
is oriented in the same direction, the resulting centrifugal forces also act in the same direction. In addition, i f a system is arranged for an alternate generation of curvilinear fields from two surfaces which are interspersed and energized independently from an AC source, particulate material placed on such surface w i l l be continuously conveyed for any distance.
97 PROPOSED EXPERIMENTAL CONVEYOR A two-phase energized conveyor is shown schematically in Fig. 1.
The
curvilinear fields are achieved, for example, by a potential difference of approximately 20 kV between edges AI and BI on Phase I and A2 and B2 on Phase I I , and at the same time a potential of approximately 10 kV is maintained between edges BI and B2 and the nearest ground by means of resistors, R. The plates are connected to Phase I I identically to those in Phase I. All plates of the conveyor must be made of resistive material such that a current w i l l flow between edges AI and BI and A2 and B2.
This arrangement w i l l
result in a curvilinear AC electric f i e l d as shown schematically in the figure.
The degree of curvature of this f i e l d w i l l be dependent upon the
length of the plates AI-B I and A2-B2 relative to the separation distance, D, to the ground plate. A time lag of (~/2) radian between the two phases w i l l generate a form of t r a v e l l i n g , curvilinear electric f i e l d and resulting centrifugal forces at a l l points on the surface of the conveyor.
T
=.,
CORV,L,NEARAC
,1
I
CONVEYING
~E PHASE I I
"l PHASE I I
i
!~R
, HASEI ACUPLY J
Fig. 1. Schematic diagram of a two-phase t r a v e l l i n g , curvilinear AC e l e c t r i c field.
98 The speed at which the material is conveyed w i l l depend on the length of the d i f f e r e n t plates and the frequency at which the AC p o t e n t i a l is applied a l t e r n a t e l y to one or the other set of plates,
Each phase potential has i t s
own constant frequency which could be anywhere from 10 to 60 Hz.
Certain
materials may be conveyed most e f f i c i e n t l y at predetermined frequencies (ref.4).
I t is worth noting that the frequency of the two sets of plates or
phases need not be the same.
The frequency of alternation of the two phases could be varied widely (e.g. 0.1 to 5 Hz). CONCLUSIONS The proposed novel electrostatic conveyor is believed to offer considerable promise for continuous conveying of particulate materials up to a size which can be l i f t e d and vibrated by an electric f i e l d .
As they advance, the
particles move up and down on curvilinear paths.
It is also envisaged that for
larger particles the system could work well in combination with simple mechanical vibration.
REFERENCES 1 S. Masuda and T. Kamimura, Approximate Methods for Calculating a Non-uniform Travelling Field, Journal of Electrostatics, I (1975), pp. 351-370. 2 S. Masuda, S. Mori and T. Itoh, Applications of Electric Curtain in the Field of Electrostatic Powder Coating, Speech at 1975 Conf. of the Electrostatic Society of America, June 24-26, 1975, Univ. of Michigan. 3 I . I . Inculet and R.K. Rowe, Generation of Curvilinear Electrostatic Fields from Parallel and Inclined Plane Electrodes, Conf. Rec. 1983, 18th Annual Meeting, IEEE Industry Applications Soc., Mexico City, Oct. 3-7, pp. 1122-1124. 4 I . I . Inculet, Y. Murata and G.S.P° Castle, A New Electrostatic Separator and Sizer for Small Particles, IEEE Trans. on Industry applications, IA-19, 1983, pp. 318-323.