Arrangement for Suppressing High Frequency Currents in Bearings of Shafts

This invention relates to an arrangement for suppressing high-frequency currents in bearings of shafts in treatment apparatus having operating frequencies above 10 kHz.

It is known to short circuit the bearings of shafts in order to avoid radio and television interferences (see, for instance, German Pat. No. 146 636). In connection with electric d-c and a-c machines it is further known to avoid detrimental bearing currents by an insulated construction of the bearing (see, for instance, German Auslegeschrift No. 1 161 992). The known arrangements, however, are ineffective for high frequency currents above 10 kHz, since, due to the geometric dimensions of the insulation structure of the bearing, especially for larger shaft diameters from 20 mm up, a capacity is obtained, the reactance of which is not high enough in relation to the capacity of the bearing. This brings about an excessively high bearing voltage due to the capacitive voltage division. It has been found that the bearing voltage must not exceed approximately 0.7 V, since, otherwise, fritting takes place in the bearing, which leads to the electroerosive destruction of the bearing. It is furthermore necessary, on the basis of interference elimination principles as well as for reasons of insulation, to bring the shaft ends of rotating parts in high frequency current carrying apparatus to ground or chassis potential as far as possible.

It is an object to the present invention to bring the shaft ends to a potential close to ground and to reduce detrimental bearing currents to a tolerable magnitude by simple means, or to avoid them altogether. According to the present invention, this is achieved, in an arrangement of the kind mentioned at the outset, by connecting impedances in series with the resulting capacitive bearing impedance in such a manner that the series branch has high impedance and by shunting a low resistance electric resistor across this series branch in such a manner that an electric quadrupole, which causes the high frequency voltage at the bearing impedance to reach a minimum, results. With this specially designed pi filter, a potential close to ground is obtained and, in addition, the high frequency currents are conducted away from the bearing.

The low impedance of the pi quadrupole is advantageously realized as a sliding contact. As the high impedance, a capacitor or a parallel resonant circuit tuned to the operating frequency is used. For reducing the bearing currents further, it may be advantageous to also provide a further sliding contact directly parallel to the bearing capacity.

For checking and monitoring the operational reliability of the quadrupole, the voltage at the series circuit of the high impedance and the bearing capacity can advantageously be monitored and utilized for enabling alarm signals.

FIG. 1 shows an equivalent electrical circuit of the high frequency arrangement of the present invention.

FIG. 2 illustrates the mechanical design of a bearing support of a shaft in a high frequency apparatus.

In the equivalent electric circuit shown in FIG. 1, the high frequency source 21 which induces voltages in the shaft is shown. Assume that the frequency of these voltages is, say, around 30 MHz. The internal impedance of this voltage source is represented by resistor 22. In an ordinary bearing, a bearing capacity 26 through which the high frequency current would then flow off the housing also exists. To prevent this from happening, and in order to control the potential of the shaft so as to be as near to ground as possible, the bearing capacity 26 is arranged in the configuration of an electrical pi quadrupole 28. On the input side, this pi quadrupole has a shunt arm with a resistor 23 which has an impedance much lower than the bearing impedance. Into the series arm is connected a relatively high impedance 24, which can be realized, for instance, as a capacitor or as an inductance, or is represented by a section of the shaft. In the next shunt arm, which contains the bearing capacity 26, a relatively high impedance 27 is additionally provided in series with the bearing capacity which determines the bearing impedance. Shunted across the bearing capacity 26 is a further resistor 25 having a resistance low in relation to the bearing impedance.

The low impedance 23 can be, for instance, a sliding contact 30 sliding on the shaft, a capacitor 32 or a sliding contact 30 in series with a series resonant circuit 31. As the series impedance 27 either a capacitor 35 which is small relative to the bearing capacity 26, or a parallel resonant circuit in the form of a capacitor 34 and inductance 33, tuned to the operating frequency can be used. The low resistance resistor 25 is advantageously also a sliding contact 36. The arrangement described here has the property that it lowers the potential after the impedance 22 to a great extent and keeps the residual currents or residual voltages that remain largely away from the bearing.

The voltage at the midpoint between the bearing capacity 26 and the high impedance 27 can be utilized for monitoring the quadrupole for proper functioning; more specifically, by rectifying the high frequency voltage prevailing there by means of the diode 40 and making it available as a measuring signal to a monitoring device 41.

FIG. 2 shows the mechanical design of the equivalent circuit. A shaft 2 leading through the housing wall 1 of a high frequency treatment chamber is supported in a bearing 4 outside the HF chamber. This part corresponds to the voltage source 21, the internal impedance 22, the series resistance 24 (if applicable) and the bearing capacity 26. As the low impedance 23, in the context of the quadrupole 28 shown in FIG. 1, sliding contact 10, which is fastened to the housing 1 by means of compression springs 12 and a mounting arm 11, is used.

The bearing 4 is enclosed by a bearing housing 3, which is fastened on a support plate 14. Support plate 14 rests on an insulating plate 5 and is fastened insulated via screws 7, insulating washers 6 and nuts 8. In analogy to the equivalent electric circuit, the insulating plate 5 corresponds to the capacitor 35 or the capacitor 34 of the parallel resonant circuit, if holes in which a tunable inductance 13 is accommodated are further located in the insulating plate 5. Inductance 13 in the insulating plate 5 corresponds to the inductance 33 in the equivalent circuit according to FIG. 1.

The impedance 25 of FIG. 1, which is shunted across the bearing capacity 26 is represented in FIG. 2 by the sliding contact 61 and the compression spring 62; the sliding contact slides on the shaft 2 and the compression spring is connected to the bearing housing 3 in an electrically conducting manner.

This arrangement described here has the property of bringing the shaft to a potential close to that of the housing and of preventing bearing currents from flowing via the bearing 4.