Modelling & Simulation of Magnetic Fields in the Radial Magnetic Bearing

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1 R. Nirmalkumar, B.K. Sreedhar, Vijay Sharma, G. Padmakumar and K.K. Rajan Fast Reactor Technology Group, Indira Gandhi Center for Atomic Research, Kapakkam, India Abstract Centrifugal sodium pumps are employed in the primary and secondary circuits of Fast Breeder Reactors (FBR s). The pump rotor is supported within sodium by a hydrostatic bearing and above sodium by Kingsbury thrust bearings. The free sodium level in the pump is topped by argon cover gas which is sealed from the atmosphere using mechanical seals. The mechanical seals and top bearing are cooled by oil. Although engineered features are provided to ensure that no oil leaks into sodium, there is instance of contamination of primary sodium with oil in the case of the UK Prototype Fast reactor (PFR). Any oil leak into sodium can cause reactivity increase and also lead to blockage of close clearances necessitating extensive cleaning and leading to prolonged shutdown. The oil circuit can be eliminated by replacing the oil cooled top bearings with Active Magnetic Bearings (AMB) that do not require oil cooling as they are contactless. Active Magnetic radial and thrust bearings to suit a small centrifugal sodium pump has been developed and tested. This paper discusses the modeling of magnetic flux and radial force of the radial active magnetic bearing using COMSOL 4.1 Multiphysics and compares the measured stiffness of radial bearing with the values obtained by modeling and theory. Keywords: Active magnetic bearings, magnetic field, stiffness check INTRODUCTION to a micropro Fig. 1: The Basic Magnetic Bearing Control Loop for the Vertical Displacement DEVELOPMENT OF AMB 3 /h PRINCIPLE OF OPERATION Description of Test Pump 3 /hr and 22.5 m at the rated arrangement. 12 International Journal of Surface Engineering & Materials Technology, Vol. 2 No. 2 July-December 2012, ISSN:

2 Nirmalkumar, Sreedhar, Sharma, Padmakumar and Rajan Modelling & Simulation of Magnetic Fields in the Radial Magnetic Bearing RADIAL MAGNETIC BEARING the rotor. Simple Case: Force Acting on the Rotor by a Single Electromagnet To Table 1: Details of the Radial Magnetic Bearing S. No Description Dimension 1 Rotor outer diameter 165mm 2 Rotor Inner diameter 45mm 3 75mm 4 100mm 5 116mm 6 Slot width 18mm 7 Slot depth mm Nominal air gap 1mm mm Fig. 3: Geometry of a Radial Magnetic Bearing Fig. 4: Dimensions of Radial Magnetic Bearing Flux Density and Attractive Force International Journal of Surface Engineering & Materials Technology, Vol. 2 No. 2 July-December 2012, ISSN:

3 Nirmalkumar, Sreedhar, Sharma, Padmakumar and Rajan (1) NI B= 2s B F 2 C a o (2) Fig. 6: Magnetic Flux Density (Plot 1) μ 0 C a N I S Table 2: Values for Equation 5 Ampere 1mm FINITE ELEMENT MODEL OF THE MAGNETIC BEARING Geometry of Radial Magnetic Bearing bearing. Table 3: Comparing Results of Analytical and Simulated Value S. No. Parameter Analytical COMSOL (in air gap) 2 x-direction in N Fig. 7: Fabricated Rotor Fig. 8: Fabricated Stator Static Analysis Forces on the Magnetic Bearing Fig. 5: Magnetic Flux Density Plot 14 International Journal of Surface Engineering & Materials Technology, Vol. 2 No. 2 July-December 2012, ISSN:

4 Nirmalkumar, Sreedhar, Sharma, Padmakumar and Rajan Modelling & Simulation of Magnetic Fields in the Radial Magnetic Bearing Table 6: Total Current through Each Coil (Obtained From the AMB System) S. No. Coil Total Current I in Amp No. of Turns 1 Coil Coil Coil Coil Outputs Fig.10, 11, 12. Fig. 9: Forces in a Radial Magnetic Bearing Table 4: Variable Values for Force Calculation C a m 2 N I c 4 E 7 H / m Specifying Material Properties material Table 5: Material Properties of Each Region S. No. Component Material Relative Permeability 1 Stator Silicon Steel 2 Gap-medium Air 1 3 Rotor Silicon Steel Excitation Load at Static Equilibrium Position other around the rotor Fig N( I b I c ) B T 2s Table 7: Force and Flux Density Calculated using COMSOL m [3 PID controller. Hence it wa Table 8: Comparison of the Displacement Values Calculated from Analysis with the Actual System Sensor Output Displacement Values S. No. Calculated, (μm) Actual System Output (from Sensor ), (μm) μm x-direction 2 y-direction 177.9μm 133 International Journal of Surface Engineering & Materials Technology, Vol. 2 No. 2 July-December 2012, ISSN:

5 Nirmalkumar, Sreedhar, Sharma, Padmakumar and Rajan Experimental Measurement on Radial Stiffness of Bearing Fig. 10: Magnetic Flux Density Fig. 11: Magnetic Flux Density (Plot 2) Fig. 12: Magnetic Flux Density (Plot 3) Fig. 13: Levitated Component with Radial Loading Procedure for Measuring Radial Stiffness Table 9: Radial Stiffness Measurement Values S. No Load (N) Radial Stiffness (kn/m) International Journal of Surface Engineering & Materials Technology, Vol. 2 No. 2 July-December 2012, ISSN:

6 Nirmalkumar, Sreedhar, Sharma, Padmakumar and Rajan Modelling & Simulation of Magnetic Fields in the Radial Magnetic Bearing Stiffness Calculation of Radial Magnetic Bearing Based on Experimental Displacement Value of Rotor DYNAMIC CONDITION Calculation of Stiffness of Radial Magnetic Bearing using Centrifugal Force and Displacement of Rotor F C = mr 2 plane (Uper) = g mm per plane [8] Cen 2 = N = / 2.817e- 4 = kn/m CONCLUSION and theory. REFERENCES [1] Sreedharan, K.V. et al. (Internal Report-PFBR/32111/DN/1014/R-A) [2] Proceeding of a Technical Committee Meeting on Unusual Occurrences during LMFR Operation, IAEA-TECDOC-1180, Vienna, pp [3] Soumendu Jana et al. NAL, Bangalore. [4] Soumendu Jana et al. 3 /h [5] [6] Elsevier, pp [7] Springer, pp [8] [9] [10] [11] Int. J. Appl. Math. Comput.Sci., Vol. 14, No. 4, pp International Journal of Surface Engineering & Materials Technology, Vol. 2 No. 2 July-December 2012, ISSN:

7 Nirmalkumar, Sreedhar, Sharma, Padmakumar and Rajan NOMENCLATURE B : μ 0 : 2 N : I b : I c : Control current, Amp I : (I b + I c ), Total current, Amp S : Nominal air gap between the rotor and the electromagnet, mm Ca : 2 m : r : F : Force, N 18 International Journal of Surface Engineering & Materials Technology, Vol. 2 No. 2 July-December 2012, ISSN:

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