Research Issues of a Small-Sized Self-Excited Electrostatic Generator

Yu. G. Leleko

(Presented by the scientific seminar of the Department of Theoretical Foundations of Electrical Engineering and the laboratory of rotary ESGs of the Scientific Research Institute of Nuclear Physics)

The creation of an industrial prototype of a small-sized self-excited ESG is associated with solving a whole range of issues of both a theoretical and a technological nature. The latter include issues related to ensuring the dielectric strength between the high-voltage elements of the generator and the grounded parts, and to selecting the structural design of the system for stabilizing the generator output voltage. The solution of these problems for the design of a self-excited ESG in a small-sized version is the most urgent task. The fundamental possibility of creating a small-sized self-excited ESG and the principal characteristics for each of the rotors separately are described in detail in [1]. Tests of experimental samples in the self-excitation mode, both in air and in an atmosphere of gas compressed to 6 atm, showed that the magnitude of the maximum output voltage is limited by the dielectric strength of the gap: carriers of the additional rotor—high-voltage inductor. An electric discharge in this gap is most probable at those instants when the carriers of the additional rotor are connected to the grounded brush in the zone of the high-voltage inductor. It is precisely at this instant that the full load voltage of the generator is applied to the gap consisting of a layer of epoxy compound approximately \( \approx 2 \) mm thick and a rotor–stator gas gap of approximately 0.5 mm. With considerable nonuniformity of the electric field along the end faces of the carriers of the additional rotor in the compound, and with the comparatively small magnitude of the gas gap, the dielectric strength of the gap during operation of the generator in an environment of gas compressed to 6 atm proves insufficient, which leads to the appearance of electric discharges limiting the output voltage of the generator.

Eliminating discharges in the gap carrier of the additional rotor—high-voltage inductor at nominal voltage on the inductor is possible either by improving the uniformity of the electric field between the electrodes, or by means of structural changes increasing the overall length of the gap, or by reducing the potential difference applied to the gap.

It is practically impossible to substantially improve the uniformity of the electric field in the epoxy compound of the rotor. Increasing the gap between the rotor and stator substantially redu-

the overall and specific energy indices of the generator [2] are reduced and, in addition, as the gap is increased, the time required to establish the rated operating voltage increases, which is highly undesirable for certain types of consumers.

The potential difference in the gap between the transporters of the additional rotor and the high-voltage inductor, without changing the geometrical dimensions of the generator elements, can be reduced by inserting an ohmic resistance into the circuit of the grounded brush of the additional rotor. During generator operation this resistance raises the potential of the brush and, consequently, of the transporters. During the generator start-up period, when the voltage on the high-voltage inductor is insignificant, the value of the induced current in the grounded-brush circuit and the voltage across the ohmic resistance are correspondingly small. As the voltage on the high-voltage inductor rises, the current in the grounded-brush circuit will increase, increasing the voltage drop across the resistance and thereby the potential of the brush. The sign of the potential of the brush grounded through the ohmic resistance always coincides with the sign of the potential of the high-voltage inductor and, consequently, the higher the brush potential, the smaller the potential difference in the gap.

The introduction of an ohmic resistance also affects the conditions of commutation of the transporters with the brush, increasing the time constant of the transient process in the circuit formed by the approaching transporter and the brush [3]. This time constant must be greater than the time interval required for the next transporter to commute with the brush. Under these conditions, at the brush of the additional rotor grounded through a resistance, there will always be a certain potential of the same sign as the potential of the high-voltage inductor, the value of which reaches 30% of the rated operating voltage of the generator. Figure 1 shows the dependences of the voltage on the brush on the voltage on the high-voltage inductor for several values of the ohmic resistance. Tests of the generator with a resistance in the brush circuit of the additional rotor showed that the voltage on the high-voltage inductor can exceed the optimum value by 20–25% without the occurrence of electrical discharges in the gap between the transporters of the additional rotor and the high-voltage inductor. However, at such high values of the output voltage, generator operation is also unsatisfactory because of the occurrence of discharges along the collector of the main rotor. The cause of these discharges is evidently the following. From the principle of operation of a self-excited ESG it follows that its output voltage is stabilized by leakage currents

Fig. 1. Dependences \(U_{\text{g.br}} = f(U_{\text{h.v.i}})\)
1 — \(R_{\text{g.br}} = 3.3\ \text{G}\Omega\)
2 — \(R_{\text{g.br}} = 2.5\ \text{G}\Omega\)
3 — \(R_{\text{g.br}} = 1.5\ \text{G}\Omega\)

from the generator elements that are at high potential. In the generator design under consideration, especially when an ohmic resistance is used in the circuit of the grounded brush of the auxiliary rotor, the magnitude of the leakage currents is very small, which leads to an increase in the potentials of the high-voltage inductor and the excitation inductor to values significantly exceeding the optimum ones [4]. In this case the main rotor operates in a regime of deep saturation, determined by intense ionization processes in the rotor–stator gap, which is the main cause of discharges along the collector of the main rotor.

To limit the excitation voltage within the optimum range, while leaving the geometry of the generator elements unchanged, it is necessary to increase artificially the magnitude of the leakage currents in the generator, i.e., to stabilize the load voltage within certain limits.

This can be done by connecting a corona gap to one of the high-voltage elements of the generator; its current–voltage characteristic is stabilized at a certain value of the voltage on the electrodes, depending on the geometry of the latter and on the working medium. However, as the experiments performed showed, it is expedient to connect the corona gap only to the elements of the auxiliary rotor, since when it is connected into the system of the main rotor the stabilizing effect of the gap appears at a current through it of the order of 10–15 μA, which is several times greater than the load current.

The possibility of stabilizing the output voltage of a self-excited electrostatic generator by including a corona gap in the circuit of the auxiliary rotor follows from the operating principle of this type of electrostatic generator. By stabilizing the voltage of the excitation inductor by means of a corona gap, we thereby create, in the self-excited electrostatic generator, conditions for the main rotor similar to those under its operation with an external excitation source, i.e., we stabilize the value of the output voltage. However, as the investigations showed, the inclusion of a corona gap directly in the circuit of the excitation inductor is also undesirable, because this reduces the efficiency of the generator and requires an increase in the dimensions of the auxiliary rotor.

The problem can be solved more simply if an additional grounded brush is installed at some distance from the collector plates, opposite those conveyors that move from the grounded brush toward the brush connected to the excitation inductor. In this way we obtain a corona gap: the additional grounded brush—the collector of the auxiliary rotor. This structural embodiment of the corona gap is advantageously distinguished from those considered earlier in that it does not require increasing the dimensions of the other generator elements and has only a slight effect on the efficiency of the generator, since only a small current flows through the additional brush. The operation of the generator in the presence of an additional brush proceeds as follows. During the starting period, when the load voltage is considerably below the rated value, the potential of the conveyors of the auxiliary rotor is so small that leakage of charge from the conveyors through the additional brush is practically absent, and the potential of the excitation inductor increases. When the potential of the conveyors reaches a certain value, depending on the geometry of the brush, the distance to the collector, and the electric strength of the surrounding medium, a corona discharge arises in the gap between the collector plates and the brush, by means of which the excess charge is removed from the conveyors. The magnitude of the charge transferred by the conveyors to the excitation inductor

is thus limited by charge leakage through the additional brush, stabilizing the value of the potential of the excitation inductor and of the generator output voltage within certain limits.

Fig. 2 shows the load characteristics of the additional rotor when it operates with an external power source and when an additional brush with two gaps of different lengths is introduced. These characteristics show that, at a voltage on the excitation inductor of approximately 10 kV, the linear relationship between current and voltage is disrupted, i.e., leakage currents arise.

The voltage at which corona discharge begins remains, for the given gap geometry, a constant value independent of the load voltage. At the same time, the limit of the stabilized voltage changes in accordance with the volt–ampere characteristic of the corona discharge.

Fig. 3 shows the external characteristics of the generator in the self-excitation mode, with characteristic 2 taken with the additional brush switched on from the side of the additional rotor. In this case the output voltage of the generator fluctuated within ±2 kV of the voltage of 40 kV, taken as nominal, during continuous operation of the generator for several hours.

Fig. 2. Dependences \(U_{\mathrm{в}} = f(I_{\mathrm{н}})\) for the additional rotor: \(1,1' — U_{\mathrm{в}} = 18\) kV; \(2,2' — U_{\mathrm{в}} = 24\) kV; \(3,3' — U_{\mathrm{в}} = 30\) kV; \(4,4' — U_{\mathrm{в}} = 34\) kV; \(5,5' — U_{\mathrm{в}} = 40\) kV.

Fig. 3. Characteristics of the generator in the self-excitation mode:
1 — without stabilization;
2 — generator stabilized at a voltage of 40 kV.

Thus, tests carried out on an experimental model of the generator made it possible to draw the following conclusions:

1. To increase the electric strength of the gap between the transporters of the additional rotor and the high-voltage inductor, an ohmic resistance connected into the circuit of the grounded brush may be used.

2. For operation in the optimal mode, it is necessary to increase the charge leakage from the transporters of the additional rotor.

1. The inclusion of an additional corona gap in the circuit of the additional rotor stabilizes the generator output voltage within certain limits, which can be established in advance.

2. The inclusion of an additional corona gap has only a slight effect on the generator efficiency and its output power.

References

1. V. V. Pashevich, Yu. G. Deleko. Small-sized self-excited ESG, Izv. TPI, vol. 210 (in press).

2. V. I. Levitov, A. G. Lyapin. Electrostatic Generators with a Rigid Rotor, part I. TsINTIelektroprom, Moscow, 1963.

3. A. M. Kuptsov. Physical processes in an ESG with conductor carriers embedded in a solid dielectric. Dissertation, Tomsk, 1967.

4. Proceedings of the III Interuniversity Conference on Electron Accelerators, section 1, Tomsk, 1961.