Jtam.dvi JOURNAL OF THEORETICAL AND APPLIED MECHANICS 45, 1, pp. 53-60, Warsaw 2007 APPLICATION OF ACTIVE MAGNETIC BEARINGS FOR IDENTIFICATION OF THE FORCE GENERATED IN THE LABIRYNTH SEAL Dorota Kozanecka Zbigniew Kozanecki Tomasz Lech Andrzej Kaczmarek Insitute of Turbomachinery, Technical University of Łódź e-mail: dkozan@p.lodz.pl; zkozan@p.lodz.pl; tlech@p.lodz.pl; akaczm@p.lodz.pl A magnetic bearing system of a rotating shaft is characterized by a unique feature that consists in the possibility of monitoring static and dynamic loads (reactions) of eachbearing under normal operation of the machine. In the paper, an identification procedure of bearing response forces,whichallows for simultaneousmeasurementof the journalposition with respect to the bush and the control currents that flow in magnetic bearing bush windings, is presented. The proposed procedure ensures effective protection of the rotating shaft supported in active magnetic bearings against too high loads and displacements that can occur in the rotating system. Experimental investigations were carried out on test rigs with the shaft supported in active magnetic bearings. Key words: dynamics, magnetic bearing, load, identification, control 1. Introduction The search for new solutions of bearing systems in turbomachinery that have to satisfy special performance demands has resulted in the interest in rotor active magnetic suspension systems. New solutions in bearing systems ha- ve been more and more frequently applied in modern rotating machines. An active magnetic bearing system is a qualitatively different technology in com- parison with classical solutions and requires co-operation of specialists from two branches of technology as it is a combination of amechanical systemwith an electronic automatic control systemwhich controls this mechanical system (Schweitzer et al., 1993; Kozanecka, 2000, 2001). 54 D. Kozanecka et al. Application of this advanced technology to the design of machines with special performance requirements leads to the search for new, optimal desi- gningmethodsofbearingmeasurementandcontrol systems.Uniqueproperties of active magnetic bearings allow us to consider their application tomachines that are particularly liable to damage due to external excitations (Kozanecka et al., 2003). 2. Bearing load identification Themagnetic bearing response vector is a sum of forces generated by bearing electromagnets. It alters in each control cycle. The value of each component force Fm[N] is connected with themean va- lue of the control current I [A]measured in a given control cycle and the value of the magnetic gap S [m], whose value is found by measuring the instanta- neous journal position with respect to the center of the bush, whose clearance is known. The value of each response force component is also a function of the bearing constant K [Nm2/A2], which depends on bearing design parame- ters and can be calculated theoretically. In order to increase the accuracy of the proposed measurement method of the magnetic force, the constant K is verified experimentally for each bearing (Kozanecka et al., 2001, 2002, 2003). In Fig.1, some exemplary results of the identification of themagnetic bearing dynamic response, connected with an occurrence of synchronous excitation due to unbalancing, are shown. Fig. 1. Changes in the gap (a), currents (b) and the bearing response force (c) versus time for the control axis X Figures 1a and 1b present changes in the gap S and the currents I, in the top (SXT ,IXT) and bottom (SXB,IXB) pair of electromagnets of the journal Application of active magnetic bearings... 55 bearing, respectively, for the control axis X as a function of time. These chan- ges result in the calculation of themagnetic force response of the bearing along the axis X (Fig.1c). The identification procedure of the bearing response for- ces comprises various configurations of themeasurement system, which allows for simultaneousmeasurement of the journal positionwith respect to the bush and the control currents that flow in magnetic bearing bush windings. 3. Test rig The investigations were aimed at the identification of the external load related to the forces generated by the labyrinth seal through the examinations of the response forces of the bearing in which the shaft was supported. The test rig (Fig.2) consisted of a vertical rotor levitating in themagnetic field and controlled in five control axes (two magnetic journal bearings – 2, 3 and one magnetic thrust bearing – 4) with a system of two-stream labyrinth seal 1 of the diameter ∅359mm.Application of the vertical systemof the rotor allowed for the identification of unsteady forces generated by the seal through indirectmeasurement of the forces in one, uppermagnetic journal bearing (the control axes X-Y ), whichwas located in a direct vicinity of themodel seal. In that rotor system, the bottom journal bearing did not exhibit any influence of the labyrinth seal and was characterized by a low level of relative vibrations as well as static and dynamic loads. 4. Identification of unsteady forces generated by the labyrinth seal In the first stage of the investigations, an experimental verification of thema- gnetic bearingparameters X-Y employed in the indirectmeasurementmethod of the bearing response was carried out. In the indirect measurement of the magnetic force, the knowledge of the so-called bearing constant K and instan- taneous values of currents and air gaps for all 4 electromagnets is required. In practice, a relationship for the magnetic force along one axis assumes the form as follows FXm =KT I2 XT (ST0 −x) 2 −KB I2 XB (SB0 +x) 2 −Fm0 (4.1) where 56 D. Kozanecka et al. Fig. 2. A schematic view of the test rig and the magnetic journal bearing IXT ,IXB – currents in the top and bottom electromagnet [A], ST0,SB0 – magnetic gaps of the top and bottom electromagnet for the journal located in the center of the displace- ment measurement system [µm], x – journal displacement [µm], KT ,KB – top and bottom electromagnet constants [Nµm 2/A2], Fm0 – correction constant [N]. After the verification procedure of themagnetic bearing parameters, expe- rimental investigations of the seal were carried out. In order to conduct a reliable identification of the external load vector connected with the forces generated by the labyrinth seal, a comparison of the magnetic response force of the upper journal bearing for two states of the system operation, namely: • when the seal is not supplied with the working medium, • when the seal is supplied with the workingmedium, was required. The first state of operation of the investigated system is the background for the analysis of its state with the operating seal. A comparison of both these states provides a possibility of identification of additional forces that occur in the bearing under analysis while the seal is operating. It allows for the determination of dynamic properties of the seal and its influence on the rotating system dynamics. Application of active magnetic bearings... 57 The basic quantity assumed in the experiment was a step change in the pressure of the working medium in the seal under consideration from the va- lue of p=1bar up to p=4bar. The pressure was measured with a pressure transducer manufactured by Aplisens, of the range of p= 10bar, and recor- ded in each measurement cycle. The measurements were taken at a constant angular frequency equal to n=35Hz. The parameter for each measurement series was the given seal eccentri- city eV = 0.13,0.26,0.39,0.52, which was obtained through introduction of the defined, constant value of displacement for the control axis of the journal bearing. The driving system of the test rig (Fig.2) consisted of engine 5 integra- ted with the shaft and driven by a frequency converter. In order to eliminate possible disturbances caused by the driving system and themeasurement and recording procedure of the results for the given eccentricity eV , the rotating systemwas startedup to reach the required frequency n=35Hz, and then the driving systemwas switched off. Under such conditions, themeasurement and recording of the following parameters of the journal bearing which are indi- spensable todetermine the response forces according to themethoddeveloped, namely: • instantaneous changes of the displacement [µm] along the axes X and Y of the bearing, • instantaneous changes of currents [A] in the windings of each pair of bearing electromagnets, were carried out. During the recording procedure, the operating conditions of the seal were altered, that is to say, the pressure of the working medium feeding the seal of the value p=4bar was provided in a step-like manner. The measurement and recording of displacements and currents comprised the whole sequence of the seal operation with and without feeding by a workingmedium. The next stage consisted in the processing of the results recorded, and its final effect was the identification of instantaneous changes in themagnetic re- sponse force for individual axes of the bearing FXm, FYm, the determination of their average values, and the determination of the resultant magnetic force and its average value (Fm)av. In order to determine instantaneous changes in themagnetic response forces under the known operating conditions of the seal p=1bar and p=4bar, the known eccentricity eV and the given frequency n, the averaged values of the displacement Xav, Yav and the bearing electroma- gnet currents (IXT)av, (IYT)av, (IXB)av, (IYB)av were required. The obtained values of the displacement made it possible to generate the X-Y orbits. 58 D. Kozanecka et al. 5. Exemplary results of the investigations Some sample results of the analysis of static forces acting on the rotor in relation to the operation of the labyrinth seal for threemeasurement series are presented in the Table 1. For the known operating conditions (the frequency of shaft rotations n, the labyrinth seal eccentricity eV ), the static components of the magnetic response forces Fm were measured, and then the increments of these responses related to the operation of the labyrinth seal ∆Fm were calculated, which allowed for finding the components of the force Fseal with which the seal acts on the rotor. The results were recorded in a form of plots presenting changes in the static components of themagnetic bearing response FXm, FYm and the static components of the labyrinth seal response FXseal , FYseal as a function of the seal eccentricity eV for both the axes X and Y . Tabela 1. Static forces acting on the rotor under different operating conditions Series n eV p FXm FYm ∆FXm ∆FYm FXseal FYseal No. [Hz] – [bar] [N] [N] [N] [N] [N] [N] 1 0 0 1 −3.2 −1.2 0 0 0 0 4 −70.9 98.7 −67.7 99.9 −53.5 78.9 2 35 0 1 0.8 −0.7 0 0 0 0 4 −34 119.5 −33.2 120.2 −27.5 94.9 3 35 0.13 1 0 −0.4 0 0 0 0 4 −40.3 44.5 −40.3 44.9 −31.8 35.5 6. Conclusions A unique test rig of the model rotor with a five-axis active magnetic bearing system has been built. The test rig allows for investigations of the rotor dyna- mics under assigned operating conditions, such as angular velocity, labyrinth seal eccentricity, pressure in the seal. The developed method of indirect measurement of the bearing response makes it possible to identify the external forces that act on the rotor. The measurement of instantaneous values of journal positions and intensities of the currentflowing inbearing electromagnetwindingsduringthe systemoperation enables one to calculate components of the vector of the magnetic bearing response at any moment. This provides a diagnostic capability that is not to bemet in any other bearing system. Application of active magnetic bearings... 59 Themethod is used to measure the forces with which the model seal acts on the rotor-bearing system through a comparison of the forces of the bearing magnetic response in two states of the systemoperation.Themeasurable chan- ges in the static and dynamic load of themagnetic bearingmake it possible to determine the forces that act on the shaft in relation to the operation of the seal under analysis. Thus, the identification of the external forces generated by the seal operation and, consequently, the determination of the dynamic properties of the investigated seal and its influence on the machine rotating system dynamics are possible. References 1. Kozanecka D., 2000, Diagnostic capabilities of active magnetic bearing ac- tuators with digital control,Proceedings of International Conference MECHA- TRONICS2000, Warsaw, Vol. II, 314-317 2. KozaneckaD., 2001,Dynamics of the flexible rotorwith an additional active magnetic bearing,Machine Dynamics Problems, 25, 2, 21-38 3. Kozanecka D., Kozanecki Z., Lech T., 2001, Modelling the dynamics of active magnetic bearing actuators, Proc. World Multiconference on Syste- mics, Cybernetics and Informatics, SCI 2001, USA,Vol. IX, Industrial Parts I, 232-235 4. KozaneckaD.,KozaneckiZ., LechT., 2002,Theoretical andexperimental investigation of dynamics of the flexible rotor with active magnetic bearings, Advances in Vibration Engineering, 1, 4, 412-422 5. Kozanecka D., Kozanecki Z., Lech T., Świder P., 2003, New concept of the spin test system with active magnetic bearings, Proc. of the 2nd Int. Symp. on Stability Control of RotatingMachinery, BentlyNevadaCorporation, 199-208 6. Schweitzer G., Traxler A., Bleuler H., 1993, Magnetlager, Springer- Verlag, Berlin [in German] Zastosowanie aktywnych łożysk magnetycznych do identyfikacji sił generowanych w uszczelnieniu labiryntowym Streszczenie System łożyskowania magnetycznego wirującego wału charakteryzuje unikatowa możliwość pozwalająca na monitorowanie obciążeń statycznych i dynamicznych (re- akcji) w czasie pracy maszyny.W artykule przedstawiono procedurę identyfikacji sił 60 D. Kozanecka et al. reakcji łożyska, pochodzących od działającego uszczelnienia labiryntowego, która po- lega na jednoczesnympomiarze pozycji czopawpanwi oraz prądu płynącegowuzwo- jeniach elektromagnesów panwi. Eksperymentalną weryfikacją proceedury przepro- wadzono na stanowisku badawczymmaszyny z wałempodpartymw aktywnych łoży- skachmagnetycznych. Manuscript received August 3, 2006; accepted for print October 18, 2006