OVERVOLTAGE PROTECTOR FOR A DEVICE HAVING TWO PARTS MOVABLE RELATIVELY TO EACH OTHER

Abstract: An overvoltage protector in an apparatus having two moving parts (1, 5). The parts are in mutual communication via a bearing (2, 3, 4), one part being stationary in relation to a substructure (13) at an electrical earth potential. The moving part (5) can be subjected to a voltage and the stationary part (1) is electrically connected to the earthed substructure. For preventing a current passing through the bearing between the parts, an electrical insulator (6, 7) is provided in the current path which can occur through the bearing as a result of the different potentials of the parts. The moving part (5) has a projection (9) at a distance from a means (48) connected to electrical earth such that a spark discharge gap (10) is formed between mutually moving parts. The bearings (2, 3, 4) are protected against current by the electrical insulator (6, 7) up to a given voltage threshold. In order that this voltage threshold will not be exceeded, the spark discharge gap (10) comes into operation at a lower value than this voltage threshold and keeps the voltage difference between the parts at this lower value.

Description: OVERVOLTAGE PROTECTOR FOR A DEVICE HAVING TWO PARTS MOVABLE RELATIVELY TO EACH OTHER TECHNICAL FIELD The invention relates to an overvoltage proctector for a device having two parts movable relatively to each other, one of which is stationary in relation to a frame at earth potential, said parts being in connec¬ tion with each other via at least one bearing, the movable part being subjected to an electrical voltage and the stationary part being elec¬ trically connected to the frame. BACKGROUND ART In apparatus with mutually moving parts which are in connection with each other via a bearing and are subjected to different electrical potentials, there is the risk that a current path can occur through the bearing. The current passing the bearing surfaces may be a magnitude such that the bearing is damaged by the current pitting the bearing surfaces. It is known to protect the bearings by insulating the bearing seatings in the cases where the voltage difference is small, as is the case for asynchronous machines, for example. This is not satisfactory when there are large voltage differences, since there is the risk of flashover. Examples of this are to be found in wind turbines and radar aerials which are subjected to lightning. DISCLOSURE OF INVENTION The problems mentioned above are solved by the bearing being provided with insulation while the moving part is provided with a spark discharge gap at the same time. The bearing is protected by the insulation against electric current up to a given voltage treshold. In order that this voltage threshold will not be exceeded, the spark discharge gap comes into operation and lowers the voltage. 01 OMPI PREFERRED EMBODIMENT - • The invention will now be described in detail below with the aid of an embodiment and with reference to the appended drawing figure. The figure illustrates a bearing housing 1 with ball bearings 2, 3 and 4, which car¬ ry a shaft 5, e.g. the shaft of a wind turbine 15. The shaft is insulated from the ball bearings by bearing seatingsό and 7, consisting of material with high electrical insulation power. The housing is connected to electric earth potential by a frame 13, and carries at one end a ring 8 having a bevelled outer edge. The shaft supports a ring 9 coacting with, and surrounding the ring 8, and having an inner surface with a bevel corre- sponding to that of the outer edge of the ring 8, such that a circular gap 10 is formed between the two rings. This gap constitutes an elec¬ trical flashover zone during a spark discharge. The shaft 11 of a ma¬ chine 14 such as a generator, which is driven by the shaft 5, is con¬ nected to the shaft via an insulating part 12. The thickness and ma- terial of the bering seatings 6 and 7, which determine the dielectric strength, and the axial extension of said seatings, which determines the flashover resistance, are such in relation to the insulation power of the gap 10 that the flashover voltage is lower across the gap than the dielectrical breakdown voltage and the flashover voltage over the bearing seatings. If the shaft is subjected to a voltage lower than the flashover voltage, the insulating bearing seatings prevent a current from going through the bearings. For a voltage exceeding the flashover voltage of the air gap, e.g. lightning, there is a spark discharge in the gap which limits the voltage which can occur at the bearing seatings. As an alternative to the described embodiement, instead of insulating bearing seatings the shaft may be provided with an insulating layer or consist entirely of insulating material at the portion in contact with the bearings. If the shaft is mounted in journal bearings, the bushes themselves may consist of insulating material. The arrangement in accor- dance with the invention thus protects the bearings from conducting current and thus being destroyed. Bearing damage in a wind turbine or radar antenna results in great inconvenience, since the whole machine must be lifted down from its tower and transported to a workshop for repairs. Other inconveniences are also avoided. The gap 10 works without friction and is only worn during spark discharges. These parts can easily be dimensioned for long life and can be made easily exchange¬ able. The invention is not limited to the described embodiment and can also be applied to bearings for linear movement^ e.g. where a bearing carries a slide on a path along which the slide can move. The bearing is electrically insulated from the slide. The zone for spark discharge consists in this case of a projection on the slide and an electrically conductive rail running alongside the path at a distance from the projection. OMPl

Claims: CLAIMS 1-Overvoltage protector for a device having two parts movable relatively to each other, one of which is stationary in relation to a frame at earth poten¬ tial, said parts being in connection with each other via at least one bearing, the movable part being subjected to an electrical voltage and the sta- tionary part being electrically connected to the frame, characterized in that, for preventing a current from passing through the bearing bet¬ ween the parts, an insulator is arranged in the current path which can occur through the bearing, due to the electrical field to which the bearing is subjected as a result of the parts having potentials, the moving part having a projection passing during the movement at a distan¬ ce from an electrically earthed means with which it forms a spark gap. 2. Overvoltage protector as claimed in claim 1, characterized in that the projection and the earthed means each consists of a ring, the rings being mutually concentric. 3. Overvoltage protector as claimed in claim 1, characterized in that the earthed means consists of a rail along which the projection is moved. 4. Overvoltage protector according to claim 1 or 2, characterized in that the electrical insulator is a bearing seating for a ball bearing. 5. Overvoltage protector as claimed in claim 1 or 2, characterized in that the electrical insulator consists of a bearing bush in a journal bearing.

Inventors: AOKERLUND JOHN, SCUKA VIKTOR, AOKERLUND, JOHN, SCUKA, VIKTOR
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1983
Main classification: H01T7
IPC codes: F03D11, H01Q1, H01T7