electrostatic generators, circa 1960
and 1970, the French reasearcher Noel J. Felici developed
a series of high-power electrostatic generators, based on
electronic excitation and using cylinders rotating ( Fig 1 ) at high
speed and hydrogen in pressurized containers.
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
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 ( Fig 2 ) 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 practically 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.
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 two 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.
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.
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.
|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||
|Max. voltage ( kV)||
|Max. current ( mA)||
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.
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.
Bibliographie / Bibliography
1 Felici N. J. - Elektrostatische Hochapannungs-generatoren-1957 , Karlsruhe: G.Braun.
Ten years of research on electrostatics at the University of
Grenoble 1942-1952 - British Journal of Applied Physics - 1953
3 Felici N.J - Recent Developments and Future Trends in Electrostatic Generation - Direct Current, Dec 1959, Vol 4,n°7