Method and device for balancing a rotor of an electrical machine

A balancing device is provided that is able to balance a rotor of an electrical machine. The machine includes a mounted rotor in a central bore of a stator and coaxially thereto. Between the rotor and the stator there is an air gap. The rotor preferably comprises a plurality of axially extending ferromagnetic teeth and non-magnetic wedges being arranged alternately around the circumference of the surface of the rotor. The teeth and/or the wedges preferably include balancing openings for balancing weights. The balancing device includes a mounting device and/or a locking device and/or a confirmation device. The device has a radial height that is smaller than the minimal clearance of the air gap and an axial extension such that it can be inserted into the air gap. With such a device it is possible to balance a rotor in an efficient way, when it is located in the bore of the stator.

FIELD OF INVENTION

The invention relates to a device and a method to balance a rotor of an electrical machine.

BACKGROUND

Electrical machines transform mechanical energy into electrical energy (generator) and vice versa (motor). Electrical machines usually comprise a rotor and a stator. The rotor rotates within a bore that is provided in the stator. When used in generators, such a rotor may have a length from 1 meter up to 6 meters or even more. Also the diameter may vary from 250 mm up to 1200 mm or even more. Due to high rotational speeds and weight of the rotor, careful balancing is compulsory.

A lack of balance will lead to vibration. Vibrations are detrimental in view of lifetime of the mechanical components such as bearings etc. Furthermore it is not possible to pass a critical rotational speed with a rotor that is not balanced. If the balancing state of the rotor does not reach a certain degree, it is possible that the electrical machine will be damaged. Especially if the electrical machine will be operated over longer time period.

Balancing of a new rotor during construction of the same is not an issue, because this step is usually done in a high speed balancing pit. After planned or unplanned maintenance work during operation however, the step of balancing the rotor is a problem. Rewinding work is one maintenance operation that serves as an example. During such a procedure the rotor is removed from the bore of the stator. It is then shipped or transported to a workshop of the manufacturer. In the workshop of the manufacturer the rewinding work, replacing step or any other maintenance work will be carried out. However it has to be noted that it is possible to carry out such a replacement and many other maintenance steps in the power plant itself, meaning on site. After the replacing step is finished, the rotor is often out of balance and the step of balancing the rotor becomes necessary. This step is nowadays conducted on a balancing pit located at the workshop of the manufacturer. Usually a power plant, in which the rotor is used, does not have an own balancing pit and therefore transporting the rotor from the power plant to the workshop of the manufacturer becomes compulsory for the balancing step. The step of transporting the rotor to the workshop of its manufacturer is usually time consuming, costly and leads to an even longer interruption of operation.

SUMMARY

The present disclosure is directed to a balancing device for balancing a rotor of an electrical machine. The electrical machine includes a mounted rotor in a central bore of a stator and coaxially thereto and between the rotor and the stator there is an air gap. The rotor includes a plurality of axially extending ferromagnetic teeth and non-magnetic wedges arranged alternately around the circumference of the surface of the rotor. At least one of the teeth or the wedges has balancing openings for balancing weights. The balancing device includes a mounting device and/or a locking device and/or a confirmation device. The device has a radial height that is smaller than the minimal clearance of the air gap and an axial extension such that it can be inserted into the air gap.

The present disclosure is also directed to a method of balancing a rotor of an electrical machine. The rotor being located in a bore of a stator with an air gap defined between the rotor and the bore. The method includes mounting a balancing weight and/or a test weight to the rotor through said air gap.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Introduction to the Embodiments

An object of the present invention is to provide a method and a device to balance a rotor of an electrical machine in an efficient way, preferably with the rotor located in the bore of the stator.

A balancing device according to the present invention is able to balance a rotor of an electrical machine in an efficient manner. The machine comprises a mounted rotor in a central bore of a stator and coaxially thereto. Between the rotor and the stator there is an air gap. The rotor preferably comprises a plurality of axially extending ferromagnetic teeth and non-magnetic wedges being arranged alternately around the circumference of the surface of the rotor. The teeth and/or the wedges, or generally the rotor surface, comprise balancing openings for balancing weights. The balancing device comprises a mounting device and/or a locking device and/or a confirmation device. The device has a radial height that is smaller than the minimal clearance of the air gap and an axial extension such that it can be inserted into the air gap.

It is possible to introduce such a balancing device into the air gap of an electrical machine for balancing the same. This is particularly advantageous, because demounting of the rotor is not necessary any more. Thus the balancing procedure is more efficient, repair time can be limited and costs can be saved.

The air gap has a substantially constant clearance in the middle area of the rotor due to cylindrical shape of rotor and bore, and a varying clearance in the terminal regions due to varying diameters of rotor and bore. The device therefore preferably comprises at least a first section and a second section, which first section and second section are moveable in respect of each other, in order to pass through the air gap in the terminal regions.

This is advantageous since a device according to the present invention can be used or adapted for/to several electrical machines which terminal regions of which have different shapes.

The balancing device preferably further comprises a third section, which third section is moveable in respect of the first section and/or in respect of the second section in order to pass through an air gap with varying clearance.

Therefore a preferably chain-like structure is being built by the first section and/or the second section and/or the third section.

Such a chain-like structure may be handled from the operating personnel with particular ease.

The first section and/or the second section and/or the third section preferably comprise at least a guiding element for circumferential guidance, a contact element for establishing a contact between the surface of the rotor and the device, and an actuation element, for axial movement of the device.

The guiding element preferably comprises at least a first magnet and a second magnet, wherein the magnetic field of the first magnet and of the second magnet interacts with the field of the ferromagnetic teeth of the rotor, causing the balancing device to be centrally arranged over at least one of the ferromagnetic teeth or over at least one of the non-magnetic wedges.

The contact element is preferably a roller bearing. The actuation element is a flexible rod that is pivotably connected to the balancing device.

The balancing device preferably comprises an adjustable camera as a positioning aid and a light emitting device for providing light in the focus area of the camera. The image captured by the camera may be transmitted to a display, such as computer monitor or video glasses.

This allows an easy positioning of the balancing device in the air gap.

The mounting device preferably comprises in one of its sections a holder for holding at least one balancing weight and a mounting tool for mounting the balancing weight.

The locking device comprises on its side facing the surface of the rotor a locking tool for locking the balancing weight.

The locking tool for locking the balancing weight is preferably a center punch that is driven by a hydraulic piston in order to provide a punchmark for securing the balancing weight against loosening due to vibrations etc.

The invention also relates to a method of balancing a rotor of an electrical machine while the rotor is located in the bore a stator. There is an air gap between rotor and bore. A balancing weight and/or a test weight are mounted onto and/or into the rotor through the air gap.

Preferably the mounting is conducted by using a device according to the present invention.

Preferably the method further comprises: a step of determining a position, where a balancing weight has to be placed, whereby said step comprises substeps of introducing at least one test weight into the air gap, mounting said at least one test weight onto the rotor, running the rotor in order to measure vibrations, and removing said at least one test weight; and/or a step of introducing a balancing device into the air gap between the rotor and the stator; and/or a step of positioning the balancing device at the determined position; and/or a step of mounting the balancing weight.

Additionally the method preferably comprises a further step of locking the balancing weight.

Additionally the method preferably comprises a further step of checking if all the balancing weights have been locked.

DETAILED DESCRIPTION

FIG. 1shows an electrical machine in a cross sectional view. The electrical machine comprises a rotor5and a stator6, wherein the rotor5is mounted in a central bore8of the stator6and coaxially thereto. Between the rotor5and the stator6there is an air gap7.

The rotor5comprises a plurality of axially extending ferromagnetic teeth and axially extending wedges made of a nonmagnetic material, such as for example copper or aluminum. The teeth and the wedges are arranged alternately around the circumference of the surface of the rotor5. Balancing openings for receiving balancing weights are arranged in the teeth and/or in the wedges. The balancing openings are preferably distributed with an equal distance between one and the other over the whole length of the teeth and/or the wedge. Such an arrangement may be designated as row or balancing opening row. Other arrangements are also possible. Such a balancing opening may be a simple radial hole, a threaded hole or a hole equipped with a bayonet catch. Other balancing openings are also conceivable, if they are able to hinder a balancing weight from being detached from the rotor5, if the rotor5is rotating. This means that the balancing weights need to be secured in radial direction.

The air gap7has a substantially constant clearance71in the middle area73of the rotor5. This is due to the cylindrical shape of the rotor5and the bore8. The constant clearance71has a actually the shape of an annular gap, if viewed axially. Since the rotor5and the stator6have varying diameters in a terminal region74of the rotor5, usually in front or in the region of the retaining rings, the clearance between rotor5and stator6is varying. This is designated as varying clearance72. An electrical machine comprises usually two terminal regions.

In the present embodiment the balancing weight has the shape of a setscrew or headless pin. Such a headless pin has a cylindrical shape, wherein the cylindrical surface is provided with a threaded structure. On the surface of the pin there is arranged an opening that is able to engage with a corresponding tool in a form closure. Usually a hexagon socket is used for such a purpose. The balancing weight will be introduced into one of the balancing openings. Depending on the depth of the balancing opening and the length of the pin, it is possible to introduce more than one balancing weight into one balancing opening.

FIGS. 2a-cshow the balancing device1according to the present invention. The balancing device1comprises a mounting device2and/or locking device3and/or a confirmation device4. The balancing device1has a radial height that is smaller than the minimal clearance of the air gap7. This means that the balancing device1is able to be introduced into the air gap7. Furthermore the balancing device1has an axial extension that has a dimension which enables the balancing device1to be inserted through the terminal region74with the varying clearance72without being blocked by the surface of the rotor5or the bore8of the stator6. Therefore the axial length of the balancing device1also a limiting factor for introducing the balancing device1through the terminal region74into the middle area73. However the dimension of the limiting axial length depends on the physical design of the terminal region.

In the preferred embodiment as shown inFIGS. 2a-cit may be seen that the balancing device1comprises at least two, here three sections. Namely a first section11, a second section12and a third section13. The first section11is connected to the second section12, both of which are moveable with respect of each other. In the present embodiment the first section11is connected to the second section by a hinge100. The axis of the hinge100is preferably orientated tangentially and/or axially to the rotor5. Other suitable devices may also be used for that purpose. The third section13is connected to the second section12in the same manner. Therefore the balancing device1of the present invention is a structure having three sections11,12,13which are moveable or pivotable in respect of each other. Preferably the sections11,12,13are arranged one behind the other, so that a chain-like structure results.

It is clear that if the overall length of the chain is chosen to be smaller than the limiting axial length, i.e. having a suitable length to pass the terminal section without being blocked by the surfaces of the rotor5or bore8, it is possible that the balancing device1may comprise only one section. The overall length may also be designated as axial length. If for some reasons the overall axial length has to be longer, more sections have to be connected together. The number of sections of a balancing device1with a given length depends therefore on the physical characteristics of the terminal region72.

At least one of the sections, in the preferred embodiment the first section11and the last section13comprise at least a guiding element. The guiding element is responsible for circumferential guidance of the balancing device1on the surface of the rotor5. In the preferred embodiment the guiding elements are magnets101,102of cylindrical shape having a central axis. The magnets101,102are arranged between a first yoke103and a second yoke104in a manner that their central axes are parallel. When the balancing device1is placed on the surface of the rotor5, the magnetic field of the two magnets101,102interact with the field of the ferromagnetic teeth. Thereby the balancing device1may be positioned over such a teeth and due to the interaction between the magnetic field of the magnets101,102and the field of the ferromagnetic teeth the balancing device1will remain in a self-centering and stable position. For that reason it is possible that the balancing device1may be moved in the direction of the teeth, i.e. axially, in a manner without shifting tangentially. The actual position above the teeth or above the wedges depends on the distance between the magnets101,102and on the arrangements of the same. It is possible that the balancing device1will be positioned over a single tooth, over a single tooth and a single wedge or over several teeth and several wedges.

At least one of the sections, in the preferred embodiment the first section11and the third section13comprise at least a contact element. The contact element is responsible for establishing a contact between the surface of the rotor5and the balancing device1. As it may be seen inFIG. 4the contact element used in the preferred embodiment is a roller bearing105. In the present embodiment four roller bearings105are arranged in the first section11and four roller bearings105are arranged in the second section12. Thereby the two roller bearings105are incorporated in the yokes103,104. Roller bearings105are particularly suitable for such a purpose, since the rolling friction between the balancing device1and the surface of the rotor5is reduced to a minimum. Other suitable elements may also be used for such a purpose.

With reference toFIG. 2a-cit can be seen that at least one of the sections comprises an actuation element. Such an actuation element is used to move the balancing device1in the air gap7along the wedges or teeth back and forth. In the present embodiment the actuation element is a rod106that is, with its first end106′, pivotably connected to the first section11of the balancing device1. The rod will be operated by an operator who has to push the rod106in order to move the balancing device1as described. The rod106has elastic properties. This is necessary, because the rod106has to enable to pass through the terminal region74, where the air gap7has a varying clearance72. A second end106″ of the rod106is equipped with a connector or a fitting107that allows a connection to a further rod. If the rod106is hollow, it is possible to supply the balancing device1with at least one of the following supplies: electrical supply, hydraulic fluids, and/or control signals. If electrical energy is to be supplied through the rod106that suitable elements such as cables have to be provided. Such a cable can be arranged inside the rod106or it may be mounted for example with clamps on its outer side. Hydraulic fluids, such as oil, air or water, may be provided in a hose that is arranged in the same manner as the cable as described above. It is also possible to use the hollow rod itself as a hose for supplying a fluid to the balancing device1. If this is the case, the fittings or connectors need to leakproof for the fluid in question.

Recapitulating it may be seen fromFIG. 2a-cthat the balancing device1according to the preferred embodiment comprises the first section11, the middle section12and the third section13. The first section11and the third section13show more or less the same features and the same design for guiding the balancing device1in axial direction of the rotor5, for establishing a contact between the surface of the rotor5and the balancing device1, and for moving the balancing device1axially back and forth.

Reference is now made to the second section12according to the present embodiment and toFIG. 2a. In the second section12, there is arranged a mounting device2. The mounting device2is responsible for providing the balancing opening with at least one balancing weight. The mounting device2comprises at least a holder200for receiving and holding the balancing weight and a mounting tool201for mounting the balancing weight. The mounting device2will be seen inFIGS. 3,4,5and6, which the following description refers to.

The holder200and the mounting tool201are arranged in a housing202having a base203and a cover204.

The holder200is shown inFIG. 6. The holder200comprises an opening210for receiving the balancing weight. Preferably the shape of the opening corresponds to shape of the balancing weight. If the balancing is used as it is described above, the opening210is preferably circular. On the bottom211of the opening210there is arranged a protruding element212that is able to hold a balancing weight. Preferably by a magnetic force, hence the protruding element212or the balancing must be magnetic or ferromagnetic. The protruding element212has a shape that is able to engage in a form closure with the balancing element. It is also possible to designate the protruding element as screwtool212. In the preferred embodiment and in accordance with the above described balancing weight a hexagonal shape for the screwtool212has been chosen. Other shapes such as a groove or any other shape are also possible.

The mounting tool201comprises an element that is able to move the balancing weight in a longitudinal movement towards the surface of the rotor5. Such an element may be a hydraulic piston, or a pneumatic piston. Preferably a pneumatic piston will be used that will be driven by introducing air or by removing air, i.e. generating a vacuum. Said element moves the balancing weight to the balancing opening, preferably partly in the balancing opening. After being moved towards the balancing opening the balancing weight may be introduced into the balancing opening. For that reason, the mounting tool201comprises furthermore a drive that transmits a rotational movement form the mounting device2to the balancing weight. An example is seen inFIG. 5. An electric motor213is connected via a worm gear214to the screwtool212. Since the balancing weight is already introduced partly in the balancing opening, the balancing weight has already a contact with the thread that is arranged in the opening. Upon a rotational movement, the balancing weight will be introduced into the balancing opening.

Additionally the mounting device2comprises an adjustable camera205and light emitting devices. The camera205focuses the area of the balancing opening and the surface of the rotor5. The images, which may be single images or a series of images or a nonstop running image, e.g. a film, are provided via a suitable mean to a display. Such a mean may be a cable or a wireless transmitter. The person that operates the balancing device1is able to position the balancing device inside the air gap7at a correct position.

Reference is now made to the second section12according to the present embodiment and toFIG. 2b. A locking device3is now arranged in the second section12. The locking device3is responsible for locking the balancing weight against becoming loose due to vibrations during operation of the electrical machine.

FIGS. 7 and 8show the locking device3in detail. The locking device3comprises a locking tool300and a counter plate301. Thereby the locking tool300is arranged on that side of the balancing device which faces the rotor5. The counter plate301is arranged on the side which faces the bore8of the stator6. The locking tool300used in the preferred embodiment comprises a hydraulic piston303and center punch302that is connected to a part of the hydraulic piston303. The hydraulic piston303is provided with a hydraulic fluid via a hydraulic tube304in order to be actuated. If the piston303is a two-way piston, two hydraulic tubes, as it is shown inFIGS. 6 and 7have to be present. One of the hoses is for moving the piston out, whereas the other is for moving the piston. If the piston is a one-way piston only one hydraulic hose is necessary. Preferably a two-way hydraulic telescopic piston is used for moving the punch. The center punch302will form a punchmark in the area of the edge of the balancing weight and therefore also in the edge of the balancing balancing opening. Thereby the center punch302is moved by the hydraulic piston303. During that procedure of forming the punch, i.e. when the hydraulic piston is actuated, the counter plate301will touch surface of the bore8of the stator6in order to provide a counter force.

The surface of the counter plate301is preferably adapted to the radius or the curvature of the bore8in the stator6. Additionally the surface is also fully or at least partly covered by a resilient layer. A rubber sheet is suitable for such a purpose. The curvature and covering is necessary to prevent any damages to the bore8. A camera205having the same characteristics as the camera explained with the mounting device2is also arranged. The operator is able to position the locking device3at a correct position by using the camera.

Reference is now made to the second section12according to the present embodiment and toFIG. 2c. A confirmation device4is now arranged in the second section12. The confirmation device4is responsible for the confirmation if all the balancing weights are correctly introduced and all the balancing weights are securely locked.

In further not shown embodiments it is clear that the characteristics applying to the first section or to the second section or to the third section as described above can also be applied to any other section.

With the device according to the present invention it is also possible to place at least one test weight in a balancing opening. Such test weights are used to provide to rotor with an imbalance in order to measure vibrations during a test run or measurement run. Preferably only one of the openings or only one of the rows of balancing openings are provided with a test weight during the procedure of measuring the vibrations. Thereby the balancing device according to the present invention is also able to remove the test weights after a test run. The measured vibrations serve as input data for the influence coefficient matrix. By such a matrix it is possible to determine or calculate the correct position of the balancing weights.

With reference to the drawings one possible embodiment is described. The drawings and the description illustrate a preferred embodiment rather than limit the invention.

LIST OF REFERENCE NUMERALS