Abstract:
The invention relates to an arrangement for indicating a fault in an electrical machine having a stator and a rotor with an air gap between it and the stator. In order to detect impurities or similar objects entering the air gap, a means of supervision is arranged on the stator surface facing the rotor. The means of supervision is at least partially protruding from the stator surface, and the integrity of the included detection element, such as a copper wire, is established using a supervision circuit connected to the detection element.

Description:
RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. §119 Finish Application 20041098 filed in Finland on Aug. 19, 2004 and is a US national phase application of PCT/FI2005/000348 filed as an International Application on Jul. 29, 2005 designating the U.S., the entire contents of which are hereby incorporated by reference in their entireties. 
   TECHNICAL FIELD 
   An arrangement for indicating failure in an electrical machine is disclosed. 
   BACKGROUND INFORMATION 
   In an electrical machine the rotor rotates with an air gap between it and the stator. The rotor is most often fitted inside the stator, but solutions also exist in which the rotor is external to the stator or fitted with an air gap between it and the stator in the axial direction. The present invention is applicable to many different types of machines. 
   To the greatest extent possible, the air gap in the machine must be free of any dust and dirt coming from outside the machine and any impurities created in the machine during operation. Extra particles may affect the operating characteristics of the machine, such as noise and cooling, and may also cause direct or indirect damage to the machine&#39;s components. In particular, any impurities and particles of various sizes that have entered the air gap between the stator and rotor must be detected and removed before they cause damage to the stator or rotor. 
   The rotor of an electrical machine is supported on the stator body or end plate by bearings. These are also used to align the rotor with the centre of the stator so that the air gap is essentially equal in all positions. The rotor position may deviate from the centre line of the machine due to wear and tear on the bearings or potential bearing failure. In an extreme case this may lead to the rotor and stator coming into contact with each other. 
   SUMMARY 
   The purpose of the present invention is to create an arrangement that would detect and indicate any extraordinary situation, inappropriate object or particle in the air gap of an electrical machine well in advance so as to prevent damage to the electrical machine. 
   A solution according to the invention is able to reliably detect and react to any objects or particles found in the air gap of the machine that may damage the structure of the stator and/or rotor during operation. A means of supervision is arranged on the surface of the stator facing the rotor and protrudes from the stator surface towards the air gap, and a supervising circuit will react if a foreign object breaks the means of supervision. The supervision covers the entire length of the air gap in the axial direction of the machine. 
   According to a preferred embodiment, the breakage of a supervision conductor is monitored by means of an electric current, so that the number of conductor loops is even. This prevents a current from being induced into the supervision conductor due to variations of magnetic flux within the air gap. 
   According to a preferred embodiment of the invention, the means of supervision is attached to a stator tooth. The means of supervision is directly attached to the stator sheet pack and its connecting conductors can be located between the ends of the stator windings on the edge of the tooth. 
   According to another preferred embodiment of the invention, the means of supervision is attached to a wedge key in the stator groove. The means of supervision may also be integrated into a single entity with the wedge. 
   According to yet another preferred embodiment, the detection element within the means of supervision is a conductive wire coiled onto a core so that the wire is closest to the rotor in one or more positions in the longitudinal direction of the machine. According to another embodiment, the detection element within the means of supervision is a fibre optic conductor. 
   According to a preferred embodiment of the invention, the means of supervision is fitted at least onto the ends of the stator pack. Such a solution is particularly sufficient to indicate a bearing failure as the air gap will be reduced most at the ends of the machine. 
   An embodiment in which the means of supervision extends over the entire length of the stator is able to detect any foreign particle in the air gap. 
   According to a preferred embodiment of the invention, a supervision circuit will react to a break in the means of supervision and provide a fault indication signal. The supervision circuit can simultaneously disconnect the power supply to the machine or just provide an alarm signal. 
   According to a preferred embodiment, the means of supervision is connected in series with another supervision circuit. This eliminates the need to expand or modify the construction of the machine&#39;s supervision circuits. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following the invention will be described in more detail with the help of certain embodiments by referring to the enclosed drawings, where 
       FIG. 1  illustrates the adaptation of a device according to the invention into an electrical machine, 
       FIG. 2  illustrates a detail of a stator tooth end, 
       FIG. 3  illustrates an alternative position for the device, and 
       FIG. 4  illustrates a device according to the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a cross-section of the lower half of an electrical machine. The stator  2  is supported in a horizontal position, and the rotor  3  is installed with an air gap  5  between the stator and rotor. The stator sheet pack  6  is attached to the stator body  4 . The stator sheet pack forms stator teeth  8  protruding towards the machine&#39;s air gap and stator grooves  10  between the teeth, opening towards the air gap. With regard to the implementation of the invention, the grooves can also be half-open or completely closed. The windings to be fitted into the grooves are not illustrated and can be implemented by generally known methods upon application of the invention. 
   When the machine is installed horizontally, a means of supervision  12  is fitted to the positions of the two teeth  8  in the lower section of the stator  2  and, preferably, extends over the entire length of the stator in the axial direction of the machine. The surface of the means of supervision  12  slightly protrudes from the surface of the stator teeth facing the air gap, and the included detection element is connected by a conductor  14  to a supervision circuit not included in the illustration, which is used to disconnect the machine from the power supply if necessary. The means of supervision  12  are preferably installed onto two teeth  8  located approximately 15 to 45 degrees to each side of the centre line  16  of the stator—that is, the lowest point. The exact location of the installation is determined on the basis of the machine&#39;s structure and manufacturing technology, for example the degree of support needed when inserting the rotor into the stator. A means of supervision mounted at the lower section of the stator will detect any objects detaching from and/or adhering to the rotor, as well as any other heavy particles and objects that have entered the machine. 
   Enlargement A in  FIG. 2  illustrates a detail of the attachment of the means of supervision  12 . A strip of elastic material, such as felt, is glued onto the surface of the stator tooth  8  facing the air gap, and a sensor strip  20  is glued onto the elastic material using a suitable glue, such as Loctite® Hysol. Adhesion of the glue is ensured by suitably compressing the means of supervision over the entire length of the stator. The connecting conductor attached to the sensor strip  20  is to be protected and supported on the stator using known methods. 
   The means of supervision  12  can alternatively be located in the stator groove  10 .  FIG. 3  illustrates such a solution in which the means of supervision means is attached to the wedge  22  closing the stator groove  10 . As the wedge is slightly deeper than the stator teeth, the felt strip  24  must be sufficiently thick to make the sensor strip  26  for the means of supervision protrude enough from the inner surface  28  of the stator. The attachment of the means of supervision  12  and its connection to the supervision circuit is essentially similar to the embodiment of  FIG. 2 . Furthermore, the means of supervision means can be integrated with the wedge. 
   Instead of placement on both sides of the lowest point of the stator&#39;s inner circumference, the means of supervision can be equally distributed across the entire inner circumference of the stator or just placed at certain positions on the stator. There may also be only one means of supervision, in which case it is preferably located at the lowest point of the stator&#39;s inner circumference or in its immediate vicinity. However, it should be noted that as the breakage of a supervision conductor is monitored using an electric current, the number of conductor loops thus formed is preferably even, as this will prevent current from being induced into the supervision conductor due to variations in magnetic flux within the air gap. 
     FIG. 4  illustrates a sensor strip structure according to the invention. The sensor strip  20  is formed of glass laminate that is approximately 0.5 mm thick and at least slightly narrower than the tooth. The first end  30  of the strip includes connection points for attaching the sensor to a connecting conductor going to the supervision circuit. A copper wire  32  sparsely coiled around the sensor strip acts as the detection element, extending from the connection end  30  of the strip to the other end  31  so that the side of the sensor strip facing the machine&#39;s air gap has copper wire across the entire length of the strip. In the example of  FIG. 4 , copper wires on the top and bottom sides jointly form a diamond pattern, which means that the top of the sensor strip has copper wire across the entire length of the strip. The edges of the sensor strip preferably have slots  35  at each location where the copper wire goes from one side of the strip to another. The other end  31  of the sensor strip has an opening  34  through which the copper wire goes. This also serves as attachment for the copper wire in the longitudinal direction. At the first end of the strip  30  the copper wire  32  is attached to the connecting conductor  36  at the connection point  38  fitted with a cable clamp  40 . The connection point is protected with heat-shrink tubing and the connecting conductor is protected with material such as Teflon sheathing. 
   A stand-by current monitored by the supervision circuit is conducted into the copper wire  32  operating as the detection element. If a foreign object or particle enters the internals of the electrical machine, or if the machine is damaged so that a part becomes loose and ends up in the air gap between the stator and rotor, the copper wire coiled around the sensor strip will be broken by abrasion and the detection element&#39;s electric circuit will open. Correspondingly, should a bearing failure cause eccentricity in the rotor, the rotor will contact a means of supervision fitted on the inner surface of the stator. This will result in either a fault indication signal provided by the machine supervision equipment or automatic disconnection of the power supply to the machine. The supervision circuit may form its own electric circuit or it can be connected in series with another circuit that supervises the machine&#39;s condition, such as a stator winding temperature sensor. 
   As an alternative to electrically conductive wire, the detection element can be an optical fibre such as a fibre optic conductor. A light signal transmitted into the conductor is monitored by a component such as a photosensitive transistor that will provide a fault signal to the supervision circuit in the event of conductor breakage. 
   In the above, the invention has been described with the help of certain embodiments. However, the description should not be considered as limiting the scope of patent protection; the embodiments of the invention may vary within the scope of the following claims.