A repulsive-type magnetic bearing system has been fabricated in which the rotor of a vertical-shaft-type motor is levitated due to the repulsive force between two sets of permanent magnets. A novel arrangement of permanent magnets has been reported here, which has made the suspension of the rotor possible. The system is planned to be applied for pumping milks and other related products in the New Zealand dairy industry

Chakraborty, C.

Donaldson, J.

Kacprzak, D.

Mukhopadhyay, S. C.

Sen Gupta, G.

Yamada, S.

IEEE Transactions on Magnetics

English

Massey Research Online

3220 IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 5, SEPTEMBER 2003Fabrication of a Repulsive-Type Magnetic BearingUsing a Novel Arrangement of Permanent Magnetsfor Vertical-Rotor SuspensionS. C. Mukhopadhyay, Senior Member, IEEE, J. Donaldson, G. Sengupta, Member, IEEE, S. Yamada, Member, IEEE,C. Chakraborty, Senior Member, IEEE, and D. Kacprzak, Member, IEEEAbstract—A repulsive-type magnetic bearing system has beenfabricated in which the rotor of a vertical-shaft-type motor is lev-itated due to the repulsive force between two sets of permanentmagnets. A novel arrangement of permanent magnets has been re-ported here, which has made the suspension of the rotor possible.The system is planned to be applied for pumping milks and otherrelated products in the New Zealand dairy industry.Index Terms—Circular disk, dairy industry, magnetic bearing,permanent magnet, repulsive type, rotor suspension.I. INTRODUCTIONI N A repulsive-type magnetic bearing system, usually therotor is levitated by the repulsive forces between stator androtor permanent magnets. The system is unstable in nature.The controlled electromagnet is used to keep the rotor in thedesired position. The repulsive magnetic bearing system hasthe advantages of using a smaller number of electromagnetsand simplified control scheme compared to an active magneticbearing system. The advantages of using a magnetic bearingsystem compared to mechanical bearing in high-speed motorsare long life, frictionless and lubrication free operation, feasibleoperation at high speed, etc. Many research papers have beenpublished on magnetic bearings using permanent magnets[1]–[2]. But the satisfactory performance of this type of mag-netic bearing is strongly dependent on the characteristics of thepermanent magnet and its configuration in the bearing system.Two repulsive magnetic bearing systems using permanent mag-nets for the levitation were fabricated a few years back [3]–[4],and their levitation and control performance were compared.Due to aging and/or as both the magnets are repelling eachother, there will be demagnetization of the permanent magnet,resulting in the field distribution along the magnet’s peripherynonuniform, and this will affect the performance of the bearingsystem [5].Manuscript received December 16, 2002.S. C. Mukhopadhyay, J. Donaldson, and G. Sengupta are with the Institute ofInformation Sciences and Technology, Massey University, Palmerston North,New Zealand (e-mail: S.C.Mukhopadhyay@massey.ac.nz).S. Yamada is with Kanazawa University, Kanazawa 920-1192, Japan (e-mail:yamada@magstar.ec.t.kanazawa-u.ac.jp).C. Chakraborty is with the Indian Institute of Technology Kharagpur,Kharagpur 721302, India (e-mail: chandan@ee.iitkgp.ernet.in).D. Kacprzak is with the University of Auckland, Auckland 1020, NewZealand (e-mail: D.Kacprzak@auckland.ac.nz).Digital Object Identifier 10.1109/TMAG.2003.816727Fig. 1. Magnetic bearing for vertical shaft machine.II. BACKGROUND OF THE PROJECTThe repulsive-type magnetic bearing system for the verticalshaft machine is shown in Fig. 1, and it was fabricated a fewyears back. The magnet used for the system was of circular con-figuration. This type of bearing system is stable along the radialdirection but is unstable along the vertical axis.After around five years of operation, the flux density distribu-tion of the magnets for this system became nonuniform, whichhas affected its performance. The different remedial measureswere reported in [5], but the best option is to replace the perma-nent magnets, which is a costly solution.III. SYSTEM CONFIGURATIONIn this project, we aim to fabricate a repulsive-type magneticbearing system for a vertical shaft machine using a novelarrangement of permanent magnets and also a cheaper optioncompared to our earlier model. Instead of using big circularmagnets, many small circular permanent magnets are usedwhich are arranged along the periphery of a circular disk madeup of aluminum. The schematic arrangement of the permanentmagnets in two dimension and the forces are shown in Fig. 2.Two such disks are used as shown in Fig. 3, one of which isfixed to the stator and the other fixed to the rotor shaft. Theyform a magnetic bearing. Two such bearings are used to levitatethe rotor of the motor; the top one is shown in Fig. 4.The thickness, number of magnets, and their arrangements isa matter of interest from stability consideration. An analyticalmodel has been done to characterize the forces along three axes.0018-9464/03$17.00 © 2003 IEEEMUKHOPADHYAY et al.: FABRICATION OF A REPULSIVE-TYPE MAGNETIC BEARING 3221Fig. 2. Schematic representation of magnet arrangement and forces.Fig. 3. Fabricated magnetic bearing.Fig. 4. Upper magnetic bearing.The analytical expression of force between two magnet pole ofintensity and is given by (1)(1)in which the intensity is given by ; isthe coercive force and is the pole face area. It is assumedhere that the pole intensity of the magnet is concentrated at thecenter of the magnet. The unit vector is directed along theline joining and . There are four forces acting betweentwo magnets as shown in Fig. 2. and are the at-tractive forces and and are the repulsive forces.The resultant force is given by the vector sum of all the forces.Table I shows the specifications of the magnet used to fabricatethe system.Fig. 5 shows the force characteristic as a function of gap dis-tance between the two disks for different magnet thickness inTABLE ISPECIFICATIONS OF THE MAGNETFig. 5. Variation of repulsive force with gap distance for different PMthickness.Fig. 6. Variation of repulsive force with radial displacement.which 24 magnets are used in each disk. It is seen that the re-pulsive force between the disks appears when the gap betweenthem is becoming larger than that of a certain value. So the se-lection of magnet thickness and the gap between the disks to beselected based on some criterion.The distance of the center of the magnets from the disk centeris 50 mm for stator disk and that of rotor disk is 47.5 mm. Theselection of this parameter is explained in Fig. 6. If there is adisplacement of 1 mm along the radial axis, the rotor will comeback to the original central position for 47.5 mm while the othercases it will go to one extreme side.Based on the above studies, a system has been fabricated inwhich 24 magnets are arranged in a circular disk separated by15 from each other as shown in Fig. 3.IV. ROTATIONAL CHARACTERISTICThe repulsive force varies considerably when the rotor rotatesaround its axis. Fig. 7 shows the variation of repulsive force asa function of rotor movement for two different radii of rotordisk. Even though the radius of rotor permanent magnet of thefabricated system is 47.5 mm, it is seen from Fig. 7 that there3222 IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 5, SEPTEMBER 2003Fig. 7. Variation of repulsive force with rotor movement.Fig. 8. Fabricated bearing with two sets of PMs in stator disk.Fig. 9. Variation of repulsive force for the system shown in Fig. 8.is a considerable change of force. This will behave as a rippletorque during the normal running condition of the rotor. In orderto reduce the gravity of this problem, another set of permanentmagnets is decided to be placed on the stator disk at a radius of40 mm and is displaced by half pitch (7.5 ) to that of other. Theradius of the rotor disk is changed to 45 mm and the fabricatedsystem is shown in Fig. 8. The simulated force characteristicsis shown in Fig. 9. It is seen from Fig. 9 that the ripples in theforce reduces to a negligible value.V. FABRICATION AND EXPERIMENTBased on the magnetic bearing as shown in Fig. 8, the com-plete system has been fabricated and is shown in Fig. 10. Thesystem is unstable along the vertical direction. A controlledelectromagnet has been used for controlling the rotor positionalong this axis. The repulsive force due to one set of magneticbearings has been measured using available strain-gauge basedforce measurement rig. Fig. 11 shows the experimental repul-sive force characteristics for one set of bearings and it is seenFig. 10. Fabricated magnetic bearing system.Fig. 11. Measured repulsive force characteristic.that a total weight 1.4 kg to be levitated by both the bearings.The electromagnet is used to share a part of the weight of therotor.VI. CONCLUSIONA prototype model of a repulsive-type magnetic bearingsystem using a novel arrangement of permanent magnets hasbeen fabricated. Our aim is to make this magnetic bearingsystem useful to dairy industries. Since New Zealand is verystrong in the dairy industry, this system will find a suitableapplication for pumping milk and other such products in whicha dirt-free, clean-room atmosphere is required. By providingmagnetic bearing, it is possible to achieve that, and to get ridof the sensors, a sensorless scheme has been planned to beimplemented in near future.REFERENCES[1] J. P. Yonnet, “Passive magnetic bearings with permanent magnets,”IEEE Trans. Magn., vol. 14, pp. 803–805, Sept. 1978.[2] J. Delamare, E. Rulliere, and J. P. Onnet, “Classification and synthesisof permanent magnet bearing configuration,” IEEE Trans. Magn., vol.31, pp. 4190–4192, Nov. 1995.[3] S. C. Mukhopadhyay, T. Ohji, M. Iwahara, and S. Yamada, “Design,analysis and control of a new repulsive type magnetic bearing,” IEEProc. Elect. Power Appl., vol. 146, pp. 33–40, Jan. 1999.[4] , “Modeling and control of a new horizontal shaft hybrid type mag-netic bearing,” IEEE Trans. Ind. Electron., vol. 47, pp. 100–108, Feb.2000.[5] T. Ohji, S. C. Mukhopadhyay, M. Iwahara, and S. Yamada, “Perfor-mance of repulsive type magnetic bearing system under nonuniformmagnetization of permanent magnet,” IEEE Trans. Magn., vol. 36, pp.3696–3698, Sept. 2000.MASSEY UNIVERSITYMASSEY RESEARCH ONLINE http://mro.massey.ac.nz/Massey Documents by Type Journal ArticlesFabrication of a repulsive-type magneticbearing using a novel arrangement ofpermanent magnets for vertical-rotor suspensionMukhopadhyay, S. C.2003-09http://hdl.handle.net/10179/961920/01/2020 - Downloaded from MASSEY RESEARCH ONLINE

Fabrication of a repulsive-type magnetic bearing using a novel arrangement of permanent magnets for vertical-rotor suspension

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Fabrication of a repulsive-type magnetic bearing using a novel arrangement of permanent magnets for vertical-rotor suspension

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