The present invention relates to method and a device for testing the tightness of an electric machine stator core.
BACKGROUND OF THE INVENTION
Electric machines are generally known to comprise an annular stator and an internal rotor, however different topologies have been already adopted and are actually manufactured.
The stator comprises an iron core provided with slots housing the stator winding.
The stator core is made of packets of electrically insulated iron sheets, joined together by thin spacers, which define the cooling channels between the packets for the relevant cooling gas flow.
All stator packets and the spacers are tightened together under pressure by means of press plates at both core ends and additional key bars, generally welded to the core back and to both press plates.
During operation, the stator core can loose its tightness, due to electromagnetic, mechanical and thermal stresses and ageing. In particular the iron sheets can start to separate each other and to vibrate, finally leading to localized hot spots due to short circuits of the sheets and/or to breakdown in the stator winding, i.e. to electric machine failures.
In addition, in case an upgrade (to increase its rated power) or a rewind of the electric machine is foreseen, the stator core conditions must be checked to assess whether it is capable of withstanding the new operating conditions or respectively bearing the expected lifetime extension. The tightness of the stator core is one of the required assessments of the electric machine conditions, which are to be performed before any renewal.
Traditionally, in order to test the stator core tightness, the rotor must be extracted so as to allow enough space within the stator to perform the required tests.
Nevertheless rotor extraction is very time consuming and both rewinds and upgrades have strict time constraints for the full implementation.
In addition, rotor extraction causes the risk of stator and/or rotor damaging.
SUMMARY OF THE INVENTION
The technical aim of the present invention is therefore to provide a method and a device by which the said problems of the known art are eliminated.
Within the scope of this technical aim, an aspect of the invention is to provide a method and a device that permit tests for ascertaining the tightness of the stator core to be carried out without the need of rotor extraction.
Another aspect of the invention is to provide a method and a device that allow tests to be carried out in an easy and fast manner.
A further aspect of the invention is to provide a method and a device that reduce the risks that the stator core and/or the rotor are damaged because of the tightness tests.
The technical aim, together with these and further aspects, are attained according to the invention by providing a method and a device in accordance with the accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Further characteristics and advantages of the invention will be more clear from the description of a preferred but non-exclusive embodiment of the method and device, illustrated by way of non-limiting example in the accompanying drawings, in which:
Figure 1 is a schematic view of a device associated to a stator core and rotor (in dashed lines) of an electric machine such as an electric generator;
Figures 2 and 3 show a particular of a first embodiment of the invention; and
Figures 4 through 6 show further embodiments of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
With reference to the figures, reference number 1 generally indicates an electric machine such as an electric generator having a stator core 2 and a rotor 3.
The stator core 2 is made of a plurality of packets 4 of iron sheets 5 that are spaced apart by means of ventilation spacers (such as ribs, not shown) to determine the stator core cooling channels 6.
The device for testing the tightness of the stator core 2 comprises a movable support 10, which can be introduced into the air gap 11 between the stator core 2 and the rotor 3.
The support 10 carries a test instrument 12 to locally place it within the gap 11 and locally test defined zones of the generator stator core 2.
Preferably the support 10 is arranged to place the test instruments 12 over at least half of the air gap axial length (i.e. the length of the air gap 11 along the longitudinal axis 13 of the electric machine such as generator).
In particular, the support 10 may be able to place the test instrument 12 over the whole air gap axial length, such that it is possible to test the whole stator core 2 by mounting the device 1 only once, or over half the gap axial length, such that it is possible to test the whole stator core by mounting the device 1 twice (i.e. at both stator core ends).
The support 10 comprises a guide 15 that can be circumferentially connected to the electric machine such as generator 1 along the air gap 11 (for example it can be connected to the retaining ring of the generator); preferably the guide 15 extends over the whole air gap circumferential length, such that the whole stator core 2 may be tested by mounting the device 1 only once, it is anyhow possible that in different embodiments the guide 15 circumferentially extends over only a part of the air gap 11.
The guide 15 carries a cart 16, movable along it to reach different circumferential positions of the stator core 2.
The cart 16 carries an extendable arm 17 that carries the test instrument 12.
In addition, the extendable arm may be provided with wheels 18 to guarantee a secure connection to the stator core 2 and/or rotor 3 during test operations.
In the following particular embodiments of the invention with different test instruments are described.
Figures 2 and 3 show an embodiment in which the test instrument 12 comprises a mechanical sensor.
As shown in Figures 2 and 3, the arm 17 has a detecting head 20 that has hinged a mechanical sensor such as an elliptical plate 21 that can rotate, around an axis G, between an inserting position (as shown in figures 2 and 3) and a testing position, rotated as indicated by arrow F. Testing with this mechanical sensor is achieved by placing the plate 21 in the zone of the stator core 2 to be tested, and then making the plate 21 rotate as indicated by the arrow F; the force to be applied to the plate 21 to make it enter into a packet 4 between the iron sheets 5 is proportional to the remaining stiffness of the packet 4 to be measured.
Figures 4 and 5 show two different embodiments in which the testing instrument comprises an electric sensor.
In these embodiments a detecting head 20 connected to the arm 17 carries a sensor such as a coil 23 arranged to inject a high frequency magnetic flux into a packet 4, so as to induce a proper vibration of it.
A signal generated by the vibrating packet may be detected using the same coil 23 (as shown in figure 4), or using a different sensor 24 (embodiment shown in figure 5).
In particular the sensor 24 may be an electric sensor, such as a second coil, or an acoustic sensor, such as a microphone, or a mechanical sensor, such as an accelerometer to be placed onto the packet 4 under testing (for example in this case the sensor 24 may be supported by an auxiliary arm movable towards the stator core and vice versa).
Alternatively, the sensor 23 may be an acoustic sensor that generates an acoustic signal that makes the packet 4 vibrate and also detects the signal generated by the vibrating packet.
In addition, also in this case a second sensor 24 may be provided and, as already described, it may be an electric sensor such as a coil, an acoustic sensor such as a microphone or a mechanical sensor such as an accelerometer to be placed on the packet 4 to be tested.
Fig 6 represents another embodiment, wherein the test instrument 12 comprises a mechanical sensor.
In this embodiment a detecting head 20 connected to the arm 17 carries an electric driven mechanical device such as a hammer 25 (micro-hammer), which hits the packet 4 so as to make it vibrate. The signal generated by the vibrating packet can be detected by another sensor 26. In particular the sensor 26 may be a mechanical sensor such as an accelerometer or an acoustic sensor such as a microphone.
Combination of the embodiments shown in Figures 2 through 6 is also possible.
The method for testing the tightness of an electric machine stator core with the rotor 3 inserted in the stator core 2 comprises:
- introducing the test instrument 12 that is connected to a movable support 10 into the air gap 11 between the stator core 2 and the rotor 3,
- locally placing the test instrument 12, i.e. placing the test instrument 12 in correspondence of the zone of the stator core 2 to be tested. This can be done by regulating the axial and circumferential position of the test instrument 12 within the gap 11;
- locally testing defined zones of the generator stator core 2.
Since testing is carried out on defined zones of the stator core 2 and since the testing instrument 12 may be brought in correspondence of any zone of the stator core 2, it is possible to test only the packets 4 that are more subject to become loose.
In addition, since tests are carried out locally, the exact position of the loose packets 4 is automatically known (because it is known the axial and the circumferential position where the tests are carried out).
Advantageously, tests are repeated a number of times at different axial and/or angular positions.
In different embodiments, tests are carried out by introducing the plate 21 between the stator core iron sheets 5, or by stressing the stator core iron sheets 5 to make them vibrate and detecting the vibrations. Preferably vibrations are detected by measuring a signal generated by the vibrating iron sheets 5.
Naturally the features described may be independently provided from one another.
In practice the materials used and the dimensions can be chosen at will according to requirements and to the state of the art.
- electric machine (electric generator)
- stator core
- packets of 5
- iron sheets
- cooling channels
- air gap
- test instrument
- longitudinal axis of 1
- extendable arm
- detecting head
- rotation axis
1. Method for testing the tightness of an electric machine stator core, wherein the electric machine comprises a stator core (2) and a rotor (3) defining an air gap (5) inbetween, the method comprising introducing a test instrument (12) that is connected to a movable support (10) into the air gap (11), locally placing the test instrument (12) and locally testing defined zones of the generator stator core (2).
2. Method as claimed in claim 1, characterised in that tests are repeated a number of times at different axial and/or angular positions.
3. Method as claimed in claim 1, characterised in that tests comprise introducing a plate (21) between the stator core iron sheets (5).
4. Method as claimed in claim 1, characterised in that tests comprise stressing the stator core iron sheets (5) to make them vibrate and detecting the vibrations.
5. Method as claimed in claim 4, characterised in that vibrations are detected by detecting a signal generated by the vibrating stator core iron sheets (5).
6. Device for testing the tightness of an electric machine stator core comprising a movable support (10), insertable in an air gap (11) between an electric generator stator core (2) and a rotor (3), and a test instrument (12) carried by the movable support (10) and locally placeable to locally test defined zones of the generator core (2).
7. Device as claimed in claim 6, characterised in that said support (10) is arranged to place the test instrument (12) over at least half of the gap axial length.
8. Device as claimed in claim 7, characterised in that said support (10) comprises a guide (15), circumferentially connectable to the generator along at least a portion of its gap (11), a cart (16) movable along the guide (15), and an arm (17) that is connected to the cart (16) and carries the test instrument (12).
9. Device as claimed in claim 8, characterised in that said arm (17) is an extendable arm.