1 000 000 VOLTS

Le générateur électrostatique de Felici et le générateur Felici-Gartner

(Felici's electrostatic generator, circa 1960 )


Entre 1942 et 1970 le chercheur français Noël J. Felici a developpé une série de générateurs électrostatiques de forte puissance, en utilisant une excitation électronique et des cylindres tournant à grande vitesse dans des enceintes pressurisées à l'hydrogène.

Between 1942 and 1970, the French reasearcher Noel J. Felici developed a series of high-power electrostatic generators, based on electronic excitation and using cylinders rotating at high speed and hydrogen in pressurized containers.

Généralités sur les générateurs électrostatiques

Dans toutes les générateurs électrostatiques, la haute tension est engendrée par le déplacement mécanique de charges électriques déposées sur un composant mobile nommé «transporteur». Comme, en électrostatique, les matériaux conducteurs et isolants peuvent être également électrifiés, les générateurs électrostatiques peuvent être regroupés dans les deux catégories suivantes:

• Les générateurs à transporteurs conducteurs
• Les générateurs à transporteurs isolants


1- Les générateurs à transporteurs conducteurs: sont des sortes de condensateurs dont les lames mobiles sont des électrodes transporteuses, chargées et déchargées par un système de commutation à brosse ( ou peigne ) et à collecteur. C'est par exemple:

Capacitor Machine

2- Les générateurs à transporteurs isolants  : ici les charges (ions)  sont déposées et transportées à la surface d'un composant isolant. Le dépôt et l'extraction des charges sont assurés par une ionisation locale du gaz ambiant, sans utilisation du frottement.

In all electrostatic generators, the high voltage is generated by the mechanical displacement of electric charges held on a mobile component named «carrier». Since in electrostatics conducting and insulating materials can be electrified equally, electrostatic generators can be groupe into the following two categories: 

Description de la génératrice Felici-Gartner à cylindre isolant

Cette machine électrostatique compacte diffère du générateur à courroie Van de Graaff.

Page Trommegenerator

La courroie est ici remplacée par un cylindre à parois minces ( quelques millimètres d'épaisseur),  en matière isolante, qui tourne à grande vitesse ( jusqu' à 80 m/s) autour d'un stator cylindrique légèrement conducteur, laissant un interstice très faible ( fraction de mm), et qui est l'équivalent des anneaux équipotentiels des machines Van de Graaff. 

La charge et la décharge du cylindre sont assurée par de minces lames d'acier, disposée à l'intérieur du cylindre parallèlement à son axe, et influencées par des inducteurs métalliques se trouvant à l' intérieur du stator. La machine est fréquemment multipolaire ( jusqu'à 16 pôles), c'est pourquoi elle peut donner des courants relativement intenses. L'hydrogène ( 15 a 25 atmosphères) facilite beaucoup le phénomène de charge et de décharge grâce à la mobilité de ses ions.

La génératrice Felici-Gartner est bien adaptée à la production de tensions de 50 kV à 1000 kV avec des intensités jusqu'à 50 voire 100 mA. Elle a été utilisée dans l'accéleration des neutrons ( Accélérateurs de neutrons SAMES™ modèles T400 et  J15 et J25 ).
Rotor, stator, ionizers.
Fig 1
Fig 2 Generator and its driving motor

La génératrice utilise un transporteur isolant cylindrique (composite époxy) R [fig 1] Les dimensions du tambour sont classiquement de 140 mm de diamètre ( et jusqu'à 300 mm ), 220 mm de longueur, 0.3 mm d'épaisseur. Les électrodes ( ionisateur de charge et ionisateur de débit) sont disposées le long de génératrices diamétralement opposées. L'ensemble est placé dans une enceinte d'hydrogène sous pression. Ni la disposition cylindrique, ni l'hydrogène ne sont très favorables à l'obtention de tensions très élevées; en revanche, la puissance spécifique et le rendement sont supérieurs à ceux de tous les autres générateurs électrostatiques. La forme cylindrique assure une excellente définition geométrique de la machine qui est ainsi très compacte. Les tensions sont de 80 a 1000 kV et les courants de l'ordre de 0.2 à 14 mA.

Principles of operation 

Electrostatic generators of the «insuling carrier» type convert mechanical energy directly into d.c. high voltage electrical energy by utilizing the charge (ion ) transfer effect in an electric field.

An electric charge placed in a field is subjected to a force. If the charge is mechanically displaced in a direction opposite to this force, the potential of the charge will increase and the mechanical work done will be converted into electrical energy. This energy conversion serves as the basis of operation of all electrostatic generators.
Charge in a field

The Felici's generator unit is comprised of three main parts :

Rotor : a hollow cylinder made of insulating material. Electric charges are deposited on the surface of the rotor which is generally driven by an electric motor to effect the transfer of charges in the field. This is the only moving part in the generator.

Ionizers : extremely thin metallic blades placed in dose proximity to the rotor. The charging ionizers generate and deposit the electric charges while the discharging ionizers draw off the charges that have been carried on the surface of the rotor.

Inductors : the purpose of the inductors is to induce a strong electric field on the sharp edge of the ionizers. Excitation inductors hold the electric charges on the rotor whereas the extracting inductors withdraw them. The inductors are, in fact, found « hidden » behind a slightly conductive cylinder (made of special glass) so that the inductors can fulfill their functions without creating local concentrations of the electric field.


The rotor passes successively opposite the charging and discharging ionizers where the rotor is respectively charged and discharged. These operations can take place once or several times per revolution depending upon the number of poles of the generator. The number of poles corresponds to the number of ionizers- or the number of inductors on the other side - around the rotor. Thus, the rotor of a two-pole generator is charged and discharged once per revolution; the rotor of a 2n-pole generator n times per revolution.

ln the operation of electrostatic generator, an auxiliary «excitation generator» is used to apply a sufficient potential difference between inductors and charging ionizers in order that a strong electric field appear on the edge of the ionizer to create local ionization.

The excitation voltage of the generator unit - in order of 20 to 30 kV - is furnished by a small auxiliary generator of very low power that provides pr

actically no current. This auxiliary generator, usually of a standard rectifier type, excites the electrostatic generator either directly or through the intermediary of an another small auxiliary electrostatic generator serving as an amplifier.

It is interesting to note that the mechanism of deposition and extraction of the ionic charges, known as commutation, functions without any material contact. The cylindrical rotor, the only moving part, rotates between the ionizers and the low-conductivity cylinder covering the inductors, all being very close to each other - some few thousands of an inch.

Under the influence of the excitation inductors, some electric charges ( ions) arc deposited on the rotor by the charging ionizers and will thus be driven by the rotor in a tangential electric field.

While the potential of the charges is increasing during the transfer, mechanical work is being converted into electrical energy.

The charges, after traversing a distance equivalent to one «pole pitch», will be removed from the rotor by the discharging ionizers in order to supply the load circuit.

An efficient structure is formed of a highly insulating cylindrical rotor running around a slightly conductive (soft glass, 1012 ohm cm) cylindrical stator, the gap being 0.01/0.02 in.

The insulating gas is 0.9999 pure hydrogen, free from chlorinated compounds, at 10-25 atm. The linear speed v of the rotor is 10-50 m/s. Commutation is performed by 0 - 0012 in. steel blades, facing metallic inductors which are in good electrical contact with the glass cylinder.

Various unit, have been developed in the range of 30-3000 W, voltage 50-600 kV.

Several units can be easily associated in series as well as in parallel.


Efficiency is 0.85-0.95.

For electronic control of such generators is very easy, with high voltage low-power triodes. Standard units have voltage stability 1 per cent and regulation 1 per cent over the whole current range; "stabilized" units exhibit an overall voltage fluctuation less than 1 part in 105 .

Electrical hazard is absolutely negligible, unless a strong capacitor is connected to the terminals.


General description

Current Felici-Gartner's generators, supplied from an a.c. source, contain all the parts necessary for proper functioning of the electrostatic generator, regulation and adjustment of the output voltage and protection of the various circuits.

SAMES™ generators are comprised of t

wo essential and distinct sections: the hermetically sealed unit and the auxiliary systems.

Hermetically sealed unit

The hermetically sealed unit is contained in a perfectly air-tight cylindrical reservoir. pressurized by extremely pure hydrogen under a pressure of 10 to 25 atmospheres. The hermetically sealed unit consists of the following component parts:

All high voltage systems are grouped in the pressurized compartment to make use of the dielectric properties of hydrogen. The hermetically sealed unit forms a compact assembly, sealed from external atmospheric conditions. The unit is penetrated at two ends: by the low or relatively low voltage connections at one end (motor power line, primary excitation. and divider resistor output) and by the high voltage outlet at the other.
It is noted that the pressurization is made once for all. The hermetically sealed unit requires no maintenance
(particularly the pressure of the gas) and should be considered as a standard assembly capable of being replaced as a unit.

Auxiliary systems



The hermetically sealed unit requires for its operation as mentioned above. a certain number of auxiliary systems and circuits. These systems and circuits pertain to electronic, low or relatively low voltage operations; consequently, they are placed outside the hermetically sealed unit. They include, in particular. the following:


All these systems are grouped in the form of special and standard chassis assembled in a package, which, in the case of small or medium power generators, also contains the hermetically sealed unit.

The control desk, on the other hand, can be either incorporated in the package or remotely controlled. The control desk contains all control and measuring circuits: start, stop, high voltage adjustment knobs, kilovoltmeter, milliammeter, etc.

Sizes

The generator is comprised, in general of a hermetically sealed unit selected from the standard units together with the external circuits designed to fulfill the requirements of a particular appli­cation. Similar hermetically sealed units may therefore be found in various generators, for example, in medium or high stability units with the same outputs.

 The standard hermetically sealed units are designated by the « caliber" of the generator, which is a number corresponding to the diameter in millimeters of the charge-carrying rotor. The caliber, to a first approximation, defines the available power of the generator.

 A generator of certain size can be represented, however, by several variables. depending upon the number of poles (usually 2, 4, 6 or 8). The available power is thus broken down into the para­meters of voltage and current.

Notes ( N. J. Felici , 1959)


Only cylindrical machines have been constructed. That kind of structure involves stringent limitations in size, since the clearance between rotor and ionizer is so essential. The elastic expansion due to centrifugal forces and favored by the very low modulus of organic materials is the limiting factor. For the stan­dard running speed of 3.000 r.p.m . the maximum rotor diameter is 15 to 18 in. The length of the cylinder, however, may be much greater, if we choose a "drum" structure (cylinder closed at both ends) rather than a "bell" shape.
Bell
        and drum cylinders

"BELL" AND "DRUM" STRUCTURE. The shaded area outlines the internal stator. Two insulated semi-shafts provide the necessary connections to excitation and output terminals.

The dynamical stability of the drum rotor is, of course, much higher, and it is much less liable to sidewise buckling under centrifugaI forces, even at considerable lengths.On the other hand, however, the drum rotor cannot be convenient for very high voltages, since the shafts must be utilisced as electrical leads for the internal stator, rather than being insulated at a "floating" intermediate potential as in the "bell" machine. As a rule, two-pole machines, i.e., machines giving the highest voltage for a given diameter, must have the "bell" structure, but ail machines with four poles (or more) may be designed with drum rotor, with considerable improvement of the geometrical precision.

The useful length of the rotor (i.e., the length of the ionizing blades) may not overstep 1 1/2 times its diameter for the "bell" geometry; with the "drum" rotor, however, 2.5 diameters is a conservative value. Then, for rotors of 12 to 15 in., a useful length of 3 ft seems very reasonable, that is. a useful surface of. say. 104 cm2.
The maximum rotor surface which may be safely anticipated being 10,000 cm2, the maximum power per unit, for cylin­drical structure and hydrogen in­sulation, turns out to be 20 kW.

Machines of bigger size may, of course, be

contemplated, but a drastic reduction of speed could not possibly be avoided, that is, the power output. instead of being proportional to the cube of linear dimensions as it is at constant angular speed, will be related to a lesser exponent, the square, for instance, if we assume constant linear veloeity. The specifie weight (and price) of the machine will then increase with size, and we thus see that the preceding figure of 20 kW per unit nearly reflects an optimum in specific price and power. As far as cylindrical machines are con­cerned, higher powers would be better achieved by parallel coupling of several generators rather than by an elephantinc unit of poor efficiency in many respects.

The parallel coupling of electro­static generators is a very easy business indeed. One electronic regulator can control as many units as required without any alteration of its structure. That means that an electronically controlled battery of generators has the lowest specific price per kW, since the electronic gear is nearly as expensive as each electrostatic unit. This conclusion holds even more in the case of highly stabilized machines, where the elec­tronics must be very elaborate.

Concerning the current output, it is, of course. proportional to the number of poles, the power remain­ing substantially constant as long as the stator pitch is large enough to accommodate and insulate the in­ductors without lowering the mean tangential component between ionizers.

 


Cylinder size ( mm) 140 140 140 240 240 300 300
Available approx. power 300 W 400 W 385 W 2 kW 2 kW 2.5 kW 2.5 kW
Number of poles
2
4
6
4
8
2
4
Max. voltage ( kV)
300
160
110
250
140
600
300
Max. current ( mA)
1
2.5
3.5
7
14
4
8





Special features

The performance and adaptability to industrial use of Felici generators originate from a special conception based essentially on the following features :

The use of extra-pure hydrogen under pressure (10 to 25 atmospheres) as a dielectric assures excellent insulation while at the same time providing good commutation, due to the high mobility of the ions. The use of hydrogen also has mechanical advantages such as the reduction of losses, the assurance of cooling and the remarkably low rate of wear of the metallic or insulating parts.

- The cylindrical form of the component parts provides an excellent geometrical configuration.
- A rather thick stator cylinder made from slightly conductive material (approximately 1012 ohm cm) avoids local concentrations of the electric field.

Some details

The output current is proportional to the active surface of the rotor, the speed of rotation, the number of poles and the density of the electric charges deposited on its surface. This density varies rapidly with the applied excitation voltage between the inductors and the charging ioni­zers. It is by means of this voltage that regulation is accomplished (see below).

The maximum voltage is approximately proportional to the distance between poles (hence, for a given rotor diameter, inversely proportional to the number of poles) and the allowable tangential electric field.

The efficiency of electrostatic generators is remarkably high. The mechanical losses are negli­gible (operation in hydrogen). The only appreciable electrical losses are those caused by the voltage drop between the ionizers and the rotor. In a motor-generator set, it is the motor that limits the efficiency.

Perspective historique

En France, à Grenoble, entre 1945 et 1960, le laboratoire d'Electrostatique du Centre National de la Recherche Scientifique , CNRS, s'est attaché à une étude systématique des générateurs électrostatiques, afin d'en tirer la maximum au point de vue énergétique, et d'en permettre l'emploi dans les applications courantes ( rayons X , peinture électrostatique, essais électriques, etc.). Ces recherches (celles de  Noël J. Felici, Elie Gartner, Morel ) ont conduit à un modèle assez différent de la machine à courroie ( Van de Graaff), car l'organe mobile est ici un cylindre creux, et le gaz comprimé d'isolement est de l'hydrogène pur. La S.A.M.E.S., Société anonyme de Machines Electrostatiques, créée en 1947, a développé pour le CNRS ( physique des particules , physique nucléaire) ces génératrices électrostatiques de grande puissance, de faible encombrement et de bon rendement . La fabrication de ces unités est terminée.

Plaquette de la S.A.M.E.S.

Patents / Brevets

FR 1051430 US 2675516 US 2523689 US 2702869

Bibliographie / Bibliography

1 Felici N.J. - Elektrostatische Hochapannungs-generatoren-1957 , Karlsruhe: G.Braun.

2 Felici N.J - Ten years of research on electrostatics at the University of Grenoble 1942-1952 - British Journal of Applied Physics - 1953 (PDF)

Felici N.J - Recent Developments and Future Trends in Electrostatic Generation - Direct Current, Dec 1959, Vol 4,n°7

4 Felici N.J- Generators, electrostatic, for d.c. power output- Encyclopaedic Dictionary of Physics - Pergamon Press

Felici N.J- L'avenir de la generation electrostatique - colloques du CNRS , Grenoble , 1960

Dittrich W., Felici N. -Abatement of High-Field Conduction in Liquid Dielectrics by Electrode Conditioning with Non-lonic Cage-Forming Polymers : A Novel Avenue to High-Power Engineering

Biographie sommaire

Noel Felici sur Wikipedia (fr) 


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