GB2133936A

GB2133936A - Electrical apparatus

Info

Publication number: GB2133936A
Application number: GB08301803A
Authority: GB
Inventors: Raymond Basil Jarn
Original assignee: PRE FORMED WINDINGS Co LI
Current assignee: PRE FORMED WINDINGS Co LI
Priority date: 1983-01-22
Filing date: 1983-01-22
Publication date: 1984-08-01
Also published as: GB8301803D0; GB2133936B

Abstract

An electrical apparatus e.g. a stator or rotor, has first (15) and second (10) conductors positioned adjacent one another, and being insulated from one another, at least one of the conductors (15) in use, carrying a current, wherein a conductive element (20) is interposed between the first (15) and second (10) conductors, which element (20) is separated from the first (15) and second (10) conductors by insulating means (17, 21), sensing means (23) being provided to sense current flowing from the first conductor (15) to the conductive element (20). <IMAGE>

Description

SPECIFICATION Electrical apparatus Description of invention This invention relates to an electrical apparatus having first and second conductors positioned adjacent one another, and being insulated from one another, at least one of the conductors in use carrying a current. Such an electrical apparatus will hereinafter be referred to as "of the kind specified".

One such apparatus is an electrical motor comprising a stator through windings of which a current is, in use, to cause a rotor of the motor to rotate. In such apparatus, the stator commonly comprises an earthed core in which coils of the winding are placed and are insulated from one another and from the core, which coils carry the current.

Although coils are insulated from the core, the insulation, in use, tends to deteriorate and allow a discharge current to pass thus allowing current to discharge from the windings to the earthed core.

It is an object of the present invention to provide a new or improved electrical apparatus of the kind specified having means to facilitate maintenance of the apparatus.

According to one aspect of the invention, we provide an electrical apparatus of the kind specified wherein a conductive element is interposed between the first and second conductors, which element is separated from the first and second conductors by insulating means, sensing means being provided to sense current flowing from the first conductor to the conductive element.

It will be appreciated that although it is desirable for the first and second conductors to be completely electrically insulated from one another, and from the conductive element, as the insulation deteriorates, current flows through the insulation from the first conductor to the conductive element, which discharge the sensing means senses. Thus the term "insulating means" does not imply that current is completely prevented from flowing therethrough.

Preferably the interposed conductive element comprises a corona discharge shield such as a foil element, which completely surrounds the first conductor, at least where the first conductor is close to the second conductor where discharge through the insulating means is more likely to occur.

Alternatively, the shield could comprise a gauze, preferably of fine mesh size, or further alternatively the shield could comprise a plurality of spaced conductors and although it is preferred, it is not essential for the conductive element to completely surround the first conductor.

In a preferred arrangement, where the conductive element comprises a foil element, the foil element is separated from the first conductor by a first insulating layer, and from the second conductor by a further insulating layer.

The conductive element may be connected to earth and the sensing means may comprise an earth current leakage detector or another other current detector between the conductive element and earth.

The invention has particular application in an electrical apparatus having a large number of first conductors each carrying a current, separated and insulating from a common second conductor, which may be earthed or merely at a lower potential, each first conductor having its own individual conductive element, and sensing means being provided so that any discharge from a first conductor through its associated insulation can be easily located whereby it is possible, by ascertaining which sensing means of which conductor/conductive element detects the discharge, to determine from which conductor discharge has occurred.

One such apparatus is the stator or rotor of an electrical motor or generator, which stator or rotor comprises a plurality of coils, i.e. first conductors in an earthed core, i.e. second conductor, the coils each being insulated from the core by insulating means.

Each coil may have an associated conductive element or elements and sensing means so that faulty insulating means between a coil and the core can be quickly detected, located, and repaired or replaced.

The invention is more suited to the stator of such an electrical apparatus because where applied to the rotor, it would be necessary to provide a plurality of connections from the conductive elements of each of the coils of the rotor to the sensing means.

In stators, sides of the coils are commonly received in slots of a core, with ends of the coils extending from the ends of the core. Preferably, each coil side has an associated conductive element and sensing means. Thus it is possible not only to locate from which coil, but from which side of that coil a fault, i.e. leakage of current through the insulating means to earth, has occured. Each coil may comprise a plurality of windings of a single or muiti-cored conductor around which a first insulating layer may be provided, a foil element or elements surrounding at least the sides of the coil, a further insulating layer surrounding the foil element or elements.

The first layer is normally much thicker than the second layer because more insulation is needed between the higher potential first conductor, and foil than between the lower potential second conductor and foil.

Although a separate sensing means could be provided for each coil side, this is impractical where there are a large number of coils because of the large number of sensing means which would be required. Preferably therefore, a scanning apparatus is provided to which each of the conductive elements for each of the coils or coil sides, may be connected, the scanning apparatus scanning each of the coil sides in turn to detect any earth discharge. When any discharge is detected, the apparatus may provide a signal containing information as to the size and location, i.e. coil side, of the discharge. If desired the apparatus may be arranged only to provide a warning signal when the size of the discharge is greater than a predetermined size.

The scanning apparatus may comprise a microprocessor controlled apparatus which may comprise an integral control part of the electrical motor or generator.

According to a second aspect of the invention, we provide a coil for use in an electrical apparatus according to the first aspect of the invention, the coil comprising a plurality of windings, the windings at least on the sides of the coils, being surrounded by a first insulating layer, a conductive element surrounding the first insulating layer and being adapted to be connected to a sensing means which senses current flowing from the first conductor to the conductive element, and a further insulating layer surrounding the conductive element to separate the conductive element from a second conductor of the apparatus.

The invention will now be described with the aid of the accompanying drawings in which: FIGURE 1 shows a diagrammatic sectional representation through part of a core of the stator of an electrical apparatus according to the first aspect of the invention; FIGURE 2 is a section through one of the sides of the coil of Figure 1; FIGURE 3 is a circuit diagram showing the electrical connections made to the stator of Figure 1.

Referring to the drawings, there is shown part of a core 10 of a stator of an electrical motor, which stator in use completely surrounds a rotor (not shown), the stator being arranged to have an alternating electrical current passed through coils thereof, to produce a varying magnetic flux, as is well known, to cause the rotor to rotate.

The invention is equally applicable to electrical generators where a current is induced in the coils of the stator, by the rotating rotor.

Referring to Figure 1, the section of the core 10 shown is arcuate, the stator being of cylindrical configuration overall, the axis of the cylinder being contained in a plane P. The core 10 comprises a plurality of packs 12 of magnetic material separated from one another by spacers 11. The packs 11 each comprise a plurality of laminations; the laminations being separated from one another by insulation.

The packs 12 and spacers 11 of the core 10 each have a plurality of internal slots cut therein around the inner circumference of the cylindrical stator, which slots together provide channels extending axially.

Pairs of slots, such as slots 1 a and 1 b each receive a coil C, one side 1 2a of the coil C being received in one slot 1 a and the other side 1 2b of the coil C in the other slot 11 b. The ends 13 of the coil C which connect the sides extend outwardly of the ends of the core 10. The winding comprises a plurality of such coils C such that sides of adjacent coils are recessed in adjacent slots. For example, a side of a next adjacent coil (shown in dotted lines at C') may be received in a slot S adjacent the slot containing coil side 1 2b, the other coil side of the adjacent coil C' being received in a slot (not shown) adjacent the slot containing coil side 1 2b of coil C so that the coil C overlaps with the adjacent coil C'.

Alternatively, the coils may be arranged concentrically so that the coil sides of another coil are received in slot S and a slot S' inside coil C.

Many alternative winding arrangements are possible, an arrangement being selected with regard to the application in which the stator is to be used.

The slots, or at least some of the slots, could be deep enough to accommodate two coil sides or more from different coils.

Each coil C comprises a plurality of windings 15 of conductor, the windings 1 5 being separated from one another by insulation 16 and from one of the projecting ends 13, e.g. end 1 3a of the coil C, there are leads 14 to which either an electrical supply is connected in a motor, or from which leads the induced current is fed, in a generator.

Where the apparatus is a motor, the motor may be a three-phase operational motor in which case some of the leads from some of the coils C may be connected to one phase of the supply, leads of others of the coils to another phase of the supply, and the remaining leads to the other phase of the supply.

Of course if required, the motor may be single phase in which case all of the leads may be connected to a common supply, or mutli-phase, i.e. two phase or more as required.

Surrounding the windings 15 of each of the coils C is a first, thick, insulating layer 17 of insulation. Problems can arise in use as the insulation 17 deteriorates, when current can leak through the insulation 1 7 and discharge to the core 10 which is earthed. As the deterioration progresses, the extent of discharge from the coils.

C to the earthed core increases and further damage is thus done to the insulating layer 17. This damage impedes the efficiency of the stator and is dangerous, particularly in a high tension motor or generator, i.e. of IIKV or more, when the discharge could be very large. Of course the discharge causes further damage to the insulating layer 1 7 and eventually there is a complete short circuit direct from the windings of the coil to earth.

Presently, it is necessary to carry out tests at regular maintenance intervals on the windings of the entire stator, or at least 6n all of the windings of the coils connected to one phase of the supply, to determine the state of the insulation of the coils.

One such test is the Tan Delta test It is not economically practical orfeasbile to test each of the coils independently, because of the large number of interconnections of the windings which would need to be disconnected in order to isolate each coil.

Further, it is not at present possible when carrying out such a test, to ascertain on which coil side the discharge is occurring.

In the present invention, this problem is overcome by providing around the first insulating layer 1 7 of each side 12a,12b of each coil C, a Corona discharge shield 20, which in the present example comprises a foil element and then embedding the windings 15, first insulating layer 17 and foil element 20 in a further thinner layer 21 of insulation, at least in the region of the coil C within the slots of the core.

It can be seen that the foil 20 surrounds the layer 17 but that the edges of the foil 20 are overlapped at 0, with insulation 1 between the overlapped edges, so that the foil 20 is not electrically continuous around the windings 1 5.

Thus there is no risk of a current being induced in the foil 20 by current flowing in the windings 15.

The foil 20 is connected via a lead 22 to the core 10, which is normally earthed, via an earth leakage current detector 23 which is shown diagrammatically in Figure 3. Thus any current leaking from the windings 1 5 which passes through the first insulating layer 17 is discharged to earth via the foil 20, lead 22 and detector 23, and finally core 10, rather than directly to the core 10 as happens in known constructions.

Of course, rather than being earthed, the core 10 could be merely maintained at a potential lower than that of the windings, in which case an alternative sensing means able to detect the flow of current from foil 20 to the core would need to be provided.

It is thus possible to detect the current leakage in each of the individual coil sides 12a,12b so that the performance of the winding can be continually monitored without having to isolate the coils and perform tests at intervals.

In practice, rather than provide an individual earth leakage current detector 23 or other sensing means for each coil side 12a,12b, (although such a construction could be used) the leads 22 of all the coils C would be connected to a micro-processor controlled screeing apparatus which would sequentially and continually scan the coil sides 1 2a and 1 2b of each coil C, and when a fault is detected, give an output or warning signal giving the magnitude of the discharge and locating the faulty coil and coil side.

Where as previously it would have been necessary either to rewind the entire stator when a fault has occurred, or at least one phase of the stator, because of the difficulty in locating the faulty coil, in the arrangement of the present invention, faults can be repaired and corrected as they occur and in many cases it is possible merely to make local repairs to the faulty coil sides. Hence large cost savings can be made.

Another advantage is that any fault can be detected well before the insulating layer 1 7 completely breaks down and thus the risk of damage to other coils 12 of the winding or other parts of the apparatus, for example if the faulty coil short-circuits, is minimised.

The earth leakage detector 23 or other sensing/scanning apparatus, can be arranged to signal that a fault has occurred only when the discharge detected exceeds a predetermined value. Thus, any small discharges as are normally likely to occur from the windings 1 5 to the core 1 0 due to the imperfection of the insulation even when new, can be ignored and/or repairs need not be made until absolutely necessary.

Although as described, the invention is particularly applicable to the coils of a stator, of course the invention could be applied to the coils of the rotor, although in such a case it would be necessary to provide brushes and/or slip rings or the like, or other connectors to provide the necessary connections from the foils 20 of the coils to the detection apparatus 23.

Although the Corona discharge shield has been described as being a foil element, of course if required this could comprise any other conductive element interposed between the windings 1 5 of the associated coil C and the core 10. For example, a close mesh gauze or a series of closely spaced conductors could be arranged around the windings 1 5.

In some designs of stator/rotor it may be usual for any discharge which is likely to occur to take place preferably at a particular location, for example where the core material is closest to the windings. In this case, the Corona discharge shield could only be provided at that location on the coils although the arrangement described above-where the shield completely surrounds the windings is obviously preferred, where costs allow.

The invention of course has applications other than in stators or rotors of motors'dynamos or other electrical rotational machines. The invention could be used in any application where it is required to detect a discharge through insulation being a first to a second conductor, particularly where there are a large number of separate insulated first conductors and it is required to discover when and in which insulation associated with which conductor dielectric failure has occurred, e.g. in transformers, or high tension electrical switchgear where bus bar are provided.

Claims

1. An electrical apparatus of the kind specified wherein a conductive element is interposed between the first and second conductors, which element is separated from first and second conductors by insulating means, sensing means being provided to sense current flowing from the first conductor to the conductive element.

2. Electrical apparatus according to Claim 1 wherein the interposed conductive element comprises a corona discharge shield which completely surrounds the first conductor, at least where the first conductor is close to the second conductor.

3. Electrical apparatus according to Claim 2 wherein said shield comprises a foil element.

4. Electrical apparatus according to Claim 2 wherein the shield comprises a gauze of fine mesh size.

5. Electrical apparatus according to Claim 2 wherein the shield comprises a plurality of spaced conductors.

6. Electrical apparatus according to Claim 3 wherein the foil element is separated from the first conductor by a first insulating layer, and from the second conductor by a further insulating layer.

7. Electrical apparatus according to any one of the preceding claims wherein the conductive element is connected to earth and the sensing means comprises a current detector between the conductive element and earth.

8. Electrical apparatus according to Claim 7 wherein the sensor is an earth current leakage detector.

9. Electrical apparatus according to any one of the preceding claims having a large number of first conductors each carrying a current, separated and insulated from a common second conductor, which is earthed or at a lower potential, each first conductor having its own individual conductive element, and the sensing means being arranged so that any discharge from a first conductor through its associated insulation can be easily located.

10. Electrical apparatus according to Claim 9 comprising the stator or rotor of an electrical motor or generator, which stator or rotor comprises a plurality of first conductors comprising coils, a second conductor comprising an earthed core, the coils each being insulated form the core by insulating means.

11. Electrical apparatus according to Claim 10 wherein each coil has an associated conductive element or elements and sensing means.

12. Electrical apparatus according to Claim 10 or Claim 11 wherein the apparatus comprises a stator, sides of the coils being received in slots of a core, with ends of the coils extending from the ends of the core.

13. Electrical apparatus according to Claim 12 wherein each coil side has an associated conductive element and sensing means.

14. Electrical apparatus according to any one of Claims 10 to 1 3 wherein each coil comprises a plurality of windings of a single or multi-cored conductor around which a first insulating layer is provided, a foil element or elements surrounding at least the sides of the coil, a further insulating layer surrounding the foil element or elements.

15. Electrical apparatus according to Claim 14 wherein the first layer is much thicker than the second layer.

1 6. Electrical apparatus according to any one of Claims 9 to 1 5 wherein the sensing means comprises a scanning apparatus to which each of the conductive elements are connected, the scanning apparatus scanning each of the conductive elements in turn to detect any earth discharge.

1 7. Electrical apparatus according to Claim 1 6 wherein when any discharge is detected, the apparatus is arranged to provide a signal containing information as to the size and location of the discharge.

18. Electrical apparatus according to Claim 1 7 wherein the apparatus is arranged only to provide a warning signal when the size of the discharge is greater than a predetermied size.

19. Electrical apparatus according to any one of Claims 1 6 to 1 9 wherein the scanning apparatus comprises a micro-processor controlled apparatus which may comprise an integral control part of the electrical apparatus.

20. Electrical apparatus substantially hereinbefore described with reference to and as shown in the accompanying drawings.

21. A coil for use in an electrical apparatus according to any one of Claims 1 to 20, the coil comprising a plurality of windings, the windings at least on sides of the coils, being surrounded by a first insulating layer, a conductive element surrounding the first insulating layer and being adapted to be connected to a sensing means which senses current flowing from the first conductor to the conductive element, and a further insulating layer surrounding the conductive element to separate the conductive element from a second conductor of the apparatus.

22. A coil substantially as hereinbefore described with reference to the accompanying drawings.

23. Any novel feature or novel combination of features disclosed herein and/or shown in the accompanying drawings.