Balancing method for balancing at high speed a rotor of a rotary machine

A balancing method for balancing at high speed a flexible rotor of a rotary machine, the rotary machine having a stator, and the rotor being supported in the stator by at least two radial magnetic bearings. The balancing method including a step of placing the rotor inside the stator, a step of performing at least one first run in order to identify amplitude and angular location of the unbalance in a first speed range below critical speed, a step of placing first balancing masses inside the rotor on predefined first balancing planes, a step of performing at least one second run in order to identify amplitude and angular location of the unbalance in a second speed range above critical speed, and a step of placing second balancing masses inside the rotor on predefined second balancing planes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to European patent application no. 15306970.3 filed on Dec. 10, 2015, the contents of which are fully incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of rotary machines comprising magnetic bearings for supporting the weight and load of a rotor of the rotary machine by active magnetic bearings thanks to magnetic fields. In particular, the invention relates to a balancing method for balancing a magnetically suspended rotor system.

BACKGROUND OF THE INVENTION

It is known to provide axial and radial magnetic bearings in rotary machines having a vertical or horizontal rotor arrangement and to provide auxiliary mechanical bearings supporting the rotor in case of failure of the magnetic bearings, for example if the magnetic bearings are overloaded or if the electrical or electronic control system fails.

It is necessary to correctly balance the rotor of a rotary machine. Indeed, without correct balancing of the rotor, the rotary machine will not pass critical rotational speeds without contacting the auxiliary bearings.

It is known to balance the rotor of a rotary machine on a balancing facility.

In the case of a “rigid” rotor, there would be no rotor deformation due to unbalance forces increasing with speed. For a rigid rotor, the balancing can be performed at low speed, using a classical balancing facility.

The invention relates more to rotors having a “flexible” structure. In case of a rotor with a flexible structure and according to the rotor structure, there will be a deformation due to the unbalance forces increasing with speed. A flexible rotor which is operated above close to critical speeds must be rotated and balanced close to these critical speeds and above these critical speeds, close to the final speed.

High speed balancing facilities for such “critical”, flexible rotors are particularly expensive and necessitate many trial runs and the use of plurality of sensors. Furthermore, rotating a rotor of large diameter at very high speed on known balancing facilities can be particularly dangerous if the rotor is not correctly balanced.

Finally, vacuum is needed for such balancing facilities, which increases the costs of the balancing.

One aim of the present invention is to provide a balancing method adapted to balance at high speed a flexible rotor of a rotary machine directly when the rotor is mounted inside the rotary machine, without using a specific balancing facility.

BRIEF SUMMARY OF THE INVENTION

It is a particular object of the present invention to provide a balancing method for balancing at high speed a rotor of a rotary machine comprising a stator, a rotor having a rotational axis and supported in the stator by at least two radial magnetic bearings, and an energy storage cylinder secured to the rotor shafts. The stator provides a casing having an opened end and a top cover adapted to close the opened end of the casing.

The balancing method provides a step of placing the rotor inside the stator, a step of performing at least one first run in order to identify amplitude and angular location of the unbalance in a first speed range, a step of placing a first set of balancing masses inside the rotor on predefined first balancing planes, a step of performing at least one second run in order to pass critical speeds and identify amplitude and angular location of the unbalance in a second speed range, and a step of placing second balancing masses inside the rotor on predefined second balancing planes.

In one embodiment, the step of performing at least one first run provides a step of rotating the rotor until a first threshold, a step of switching the radial magnetic bearings to a synchronous force rejection mode, a step of rotating the rotor until a second threshold in synchronous force rejection mode, and a step of identifying amplitude and angular location of the unbalance in a second speed range provided between the first and second threshold, according to information received from an electronic control device controlling the magnetic bearings.

By “switching the radial magnetic bearings to a synchronous force rejection mode”, it should be understood that the radial magnetic bearings are switched in a way that the rotor rotates without force around its inertia axis.

The balancing method further provides a step of removing the top cover and of placing first balancing masses inside the rotor against the inner cylindrical surface of the rotor. The first balancing masses are disposed on predefined first balancing planes given by the rotor model and a step of closing the upper part of the stator casing by the top cover.

The step of performing at least one first run provides a step of rotating the rotor until a first threshold, a step of switching the radial magnetic bearings to the synchronous force rejection mode, a step of rotating the rotor until a second threshold in synchronous force rejection mode, a step of activating an active resonance damping mode of the radial magnetic bearings, a step of rotating the rotor until a third threshold in active resonance damping mode, a step of switching the radial magnetic bearings to synchronous force rejection mode, a step of rotating the rotor until a fourth threshold in synchronous force rejection mode, and a step of identifying amplitude and angular location of the unbalance in a fourth speed range R4provided between the third and fourth thresholds, according to information received from an electronic control device controlling the magnetic bearings.

By “switching the radial magnetic bearings to an active resonance damping mode”, it should be understood that the radial magnetic bearings are switched in a way that the rotor rotates with the force generated by the radial magnetic bearings.

The balancing method further provides a step of removing the top cover and of placing second balancing masses inside the rotor against the inner cylindrical surface of the rotor, the second balancing masses being disposed on predefined second balancing planes determined by the flexible rotor model.

In one embodiment, each balancing mass is made of two individual masses located in opposition position on the inner circumference of the rotor on one balancing plane.

In one embodiment, each balancing mass has an annular shape.

In one embodiment, the balancing masses are made from metal material.

In one embodiment, the balancing masses are fixed on the inner surface of the rotor by gluing.

In one embodiment, the balancing masses are made in magnetic material.

The first threshold is, for example, provided between 80 Hz and 120 Hz, such as for example 100 Hz, the second threshold is, for example, provided between 150 Hz and 200 Hz, such as for example 160 Hz, the third threshold is, for example, provided between 250 Hz and 350 Hz, such as for example 300 Hz and the fourth threshold is, for example provided between 500 Hz and 1000 Hz, such as for example 750 Hz.

In one embodiment, the rotor provides an upper and a lower shaft.

The first run is, for example, configured to balance the upper shaft of the rotor and the second run is, for example, configured to balance the lower shaft of the rotor.

As an example, two balancing planes are associated with each rotor shaft.

DETAILED DESCRIPTION OF THE INVENTION

A rotary machine10is illustrated onFIG. 1; the rotary machine10may for example be a high speed flywheel for energy storage, or any high speed rotary machine having a vertical rotor arrangement.

The rotary machine10provides a stator12and a rotor14having an upper shaft16and a lower shaft18rotating around a vertical axis X-X. An energy storage cylinder20is secured in a flexible way to the shafts16,18of the rotor14.

The energy storage cylinder20is adapted to rotate at very high speed in vacuum, such as up to 50 000 rpm.

The upper and lower shafts16,18of the rotor14are supported rotatably with respect to the stator12by an active magnetic bearing system comprising two radial magnetic bearings22,24, respectively an upper radial magnetic bearing22and a lower radial magnetic bearing24, and by an axial actuator26secured to the stator12and configured to produce an axial attractive force on the upper shaft16of the rotor14.

The two radial magnetic bearings22,24may be identical and arranged at opposite ends of the rotor14. The two radial magnetic bearings22,24provide a plurality of sensors (not shown) and are controlled by an electronic control unit (not shown) adapted to receive information from the sensors.

The upper and lower shafts16,18of the rotor14are further supported rotatably with respect to the stator12by an upper radial touch down bearing28and by lower radial and axial touch down bearings30,32. The touch down bearings are, for example, mechanical auxiliary bearings adapted to support the rotor in case of failure of the magnetic bearings.

Each radial magnetic bearing22,24provides an annular armature22a,24amade of ferromagnetic material mounted on an outer cylindrical surface16a,18aof the rotor shafts16,18and a stator armature22b,24bsecured to the stator12. The stator armatures22b,24beach provides, in a conventional manner, a stator magnetic circuit having one or more annular coils and ferromagnetic body and are placed facing the rotor armature22a,24aso as to define a radial airgap. The details of the stator armatures are not shown on the Figures. Thanks to the active magnetic bearing system, the rotor14rotates without mechanical contact within the stator12.

As illustrated onFIG. 1, each rotor shafts16,18are hollowed and provided at one end with a shoulder16b,18bprojecting radially towards the stator12.

The stator12provides a casing34surrounding the rotor14formed by the energy storage cylinder20, the upper shaft16and the lower shaft18. As illustrated onFIG. 1, the casing34provides a lower part34ahousing the lower shaft18of the rotor14and an upper part34bhousing the upper shaft16of the rotor14. The lower part34ais provided with a lower opening34cadapted to receive a lower holder36for the lower radial and axial touch down bearings30,32. The upper part34bis axially opened in order to mount the rotor shafts16,18with the energy storage cylinder20inside the stator12. The stator further provides a top cover38adapted to axially close the opened end34dof the upper part34bof the stator12. The top cover38is provided with an upper opening38aadapted to receive an upper holder40for the upper radial touch down bearing28. The top cover38, the upper holder40and the lower holder36are mounted removable on the casing12.

As illustrated, the axial actuator26is secured to the upper part34bof the casing and is configured to produce an axial attractive force on the shoulder16bof the upper shaft16of the rotor14.

The upper magnetic bearing22, the axial actuator26and the upper holder40are secured to the top cover38, so that after removing the top cover38, the rotor14can be pulled out easily.

As illustrated, the stator of a motor/generator42is secured to the lower part32aof the stator, facing the lower shaft18of the rotor14.

The upper and lower shafts16,18of the rotor14are made from magnetic steel. The storage cylinder20can be made from carbon fibers or metal material, such as for example steel.

The energy storage cylinder20is flexibly connected to the shaft shoulders16b,18b. Thanks to the flexible connection between the storage cylinder20and the rotor shafts16,18, the shafts bending frequencies and the critical speeds are low, which reduces the necessary magnetic bearing power to pass critical speeds.

The Campbell diagram shown onFIG. 2illustrates the resonance frequencies F versus the rotational speed S of the rotor14of the rotary machine10. The evolution of the natural frequencies corresponding to a mode is drawn in function of the rotational speed of the rotor.

As illustrated onFIG. 2, the rotary machine is operated at an operation nominal speed Sn above the upper and lower shafts bending mode frequencies Bm1, Bm2and below but close to the storage cylinder bending mode frequency Bm3. The upper and lower shafts bending mode frequencies Bm1, Bm2are around 180 Hz, while the maximum operation speed Sn of the rotary machine can be, for example, of 750 Hz. The storage cylinder bending mode frequency Bm3is around 1000 Hz.

The critical speeds S1, S2are crossed when the operation speed line OS of the rotary machine crosses the line of the upper and lower shafts bending mode frequencies Bm1, Bm2, in the region between 180 Hz and 10 800 rpm. Once the critical speeds S1, S2are crossed, the rotary machine10can operate without crossing any more critical speeds. However, at high speed, when the speed is approaching the storage cylinder bending mode, the shaft runout could increase again. This runout increase can for example be caused by a not perfect attachment between shaft and cylinder. Such runout increase can be minimized by placing balancing mass at a predefined balancing plane close to the shaft attachment area.

The correct balancing of such rotary machine is thus mandatory to pass the critical speeds of the rotor shafts with low runout and vibration level. The correct balancing of such rotary machine is also mandatory to rotate the rotary machine at an operation nominal speed Sn close to the cylinder bending mode frequency Bm3.

The balancing method according to the present invention will be described in reference to the flow diagram illustrated onFIG. 3and to the diagrams illustrated onFIGS. 4 and 5.

In a first step50, the rotor14, comprising the shafts16,18and the energy storage cylinder20, is placed inside the rotary machine10, through the aperture of the upper part34bof the stator casing34. In a non-limiting way, the rotor14can be previously pre-balanced at low speed in a common balancing facility. The aim of the balancing method according to the present invention is to identify and compensate possible unbalance at different locations of the flexible rotor structure and which can only be identified when rotating at high speed.

In a second step51, one first run of the rotor is performed.

The first run provides a step52of rotation the rotor in levitation on the magnetic bearings22,24until a first threshold F1, for example 100 Hz. In a first speed range R1, for example provides between 0 Hz and 100 Hz, the magnetic bearings22,24control the rotation of the rotor shafts around the rotational axis X-X.

The first run further provides a step53of switching the radial magnetic bearings to synchronous force rejection mode the rotor thus rotates without force around its inertia axis until a second threshold F2, for example 160 Hz. In the second speed range R2, for example between F1=100 Hz and F2=160 Hz, the magnetic bearings22,24are active but do not generate any synchronous force and the rotor rotates around its inertia axis.

As a typical example, the runout vector trajectory in rotor coordinates in speed range R2is shown by curve1inFIG. 4.FIG. 5shows the corresponding runout amplitude evolution. In the speed range R2the runout trajectory is a straight line between F1and F2. The angular location and the amplitude of the unbalance responsible for the runout increase in speed range R2can be deduced from curve1inFIG. 4. Curve1shows the runout behavior with unbalanced rotor.FIGS. 4 and 5show the typical runout evolution for the upper or lower radial bearing. However, the described unbalance identification method is about the same for the upper or the lower bearing.

The amplitude and the angular location of the unbalance responsible for the runout increase in the second and third speed range R2and R3are identified, in step54, by using information received from the electronic control device controlling the magnetic bearings.

In step55, the top cover38is removed and balancing masses are placed inside the rotor shafts.

A set of upper balancing masses72,74are placed inside the rotor upper shaft16, against the inner cylindrical surface16cof the rotor upper shaft16. The upper balancing masses72,74are disposed on predefined upper balancing planes82,84depending on the type of rotor. In a similar way, a set of lower balancing masses76,78are placed inside the rotor lower shaft18, against the inner cylindrical surface18cof the rotor lower shaft18. The lower balancing masses are disposed on predefined lower balancing planes84,86depending on the type of rotor.

As illustrated onFIG. 1, there are two upper predefined balancing planes82,84associated with the upper shaft16. However, the number and the location of balancing planes are predefined in a cartography as a function of the rotor type used. A first predefined upper balancing plane82is located radially between the upper magnetic bearing22. The first upper balancing mass72located on the first upper predefined balancing plane82allows compensating the unbalance at the end of the upper shaft16. A second predefined upper balancing plane84is located radially between the shoulders16bof the upper shaft16. The second upper balancing mass74located on the second predefined upper balancing plane84allows compensating unbalance and eventually concentricity error between the upper rotor shaft16and the energy storage cylinder20.

After the upper and lower balancing masses72,74,76and78have been placed inside the rotor, the upper part of the stator casing is closed by the top cover38and a second run of the rotor can be performed at step56.

The second run provides a step57of rotating the rotor in levitation on the magnetic bearings22,24until the first threshold F1, for example 100 Hz. In the first speed range R1, for example provided between 0 Hz and 100 Hz, the magnetic bearings22,24control the rotation of the rotor shafts around the rotational axis X-X.

The second run further provides a step58of switching the active magnetic bearing to synchronous force rejection mode the rotor thus rotates around its inertia axis until the second threshold F2, for example 160 Hz. In a second speed range R2, for example provided between 100 Hz and 160 Hz, the magnetic bearings are active but do not generate any synchronous force and the rotor rotates around its inertia axis.

At step59, the magnetic bearings22,24operate in active synchronous damping mode in order to pass the critical speeds. The rotor14is rotated in active synchronous damping mode until a third threshold F3, for example 300 Hz. In a third speed range R3, for example provided between 160 Hz and 300 Hz, the magnetic bearings control the rotation of the rotor shafts around the rotational axis X-X and provide active damping of the rotor modes.

At step60, above F3, the magnetic bearings22,24are switched from active synchronous damping mode to synchronous force rejection mode, the rotor thus rotates around its inertia axis until a fourth threshold F4, for example 750 Hz. In a fourth speed range R4, for example provides between 300 Hz and 750 Hz, the magnetic bearings are active but do not generate any synchronous force and the rotor rotates without force around its inertia axis.

The typical runout vector trajectory in rotor coordinates for compensated unbalance is shown by curve2inFIGS. 4 and 5. Curve2shows the runout behavior with compensated unbalance.

The amplitude and the angular location of the unbalance responsible for the runout increase in the fourth speed range R4are identified, in step61, by using information received from the electronic control device controlling the magnetic bearings. If necessary, the unbalance can be compensated by placing mass at the balancing planes84,86.

In step62, if necessary, the unbalance responsible for the runout increase in speed range R4can be compensated. The top cover38is removed and balancing masses74,76are placed inside the rotor shafts.

As illustrated onFIG. 1, there are two predefined lower balancing planes86,88associated with the lower shaft18and two predefined upper balancing planes82,84associated with the upper shaft16. However, the number and the location of lower and upper balancing planes are defined in a cartography as a function of the rotor type used.

Each balancing mass72,74,76and78can be made of one or more individual masses located on the inner circumference of the rotor shafts, such as for example two masses disposed in opposition position on one balancing plane. Each balancing mass72,74,76and78can have for example a parallelepiped shape, and can weight, for example several g, such as2g. As an alternative, each balancing mass72,74,76and78can have an annular shape. The balancing mass72,74,76and78are made from metal material and are fixed on the inner surface16c,18cof the rotor shafts by any way, such as for example gluing. The balancing mass72,74,76and78can be magnets made from magnetic steel. In case balancing mass72,74,76and78are magnets, no fixing means, such as for example glue, are necessary. Furthermore, when rotating the rotor shafts, the centrifugal force press the balancing masses72,74,76and78against the inner surface16c,18cof the rotor shafts16,18.

The balancing masses can be introduced from the top of the stator using an appropriate tool.

The first threshold F1is, for example, provided between 80 Hz and 120 Hz, such as for example 100 Hz. The second threshold F2is, for example, provided between 150 Hz and 200 Hz, such as for example 160 Hz. The third threshold F3is, for example, provided between 250 Hz and 350 Hz, such as for example 300 Hz and the fourth threshold F4is, for example provided between 600 Hz and 1000 Hz, such as for example 750 Hz.

The balancing method is described as comprising a first run and a second run. However, the balancing method could provide a plurality of first run and a plurality of second run in order to have a rotor almost perfectly balanced.

The balancing method according to the invention uses the rotor unbalance information generated by the magnetic bearings, given in rotor coordinates. The number of balancing masses needed and the place where to place the balancing masses inside the rotor is then calculated according to the model of the flexible rotor structure.

Thanks to the present invention, the rotor is balanced directly inside the rotary machine. The balancing method described above is able to determine the exact location of the unbalance of the rotor by using information generated by the magnetic bearings given in rotor coordinates, and to correct the unbalance by simply opening the stator and placing balancing masses inside the rotor.

There is thus no need to use expensive high speed balancing facilities under vacuum, and no need to pull out the rotor for or during balancing.

The balancing method according to the present invention allows fast and accurate balancing, and decreases balancing time and costs.