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TECHNICAL PAPERS: Gas Turbines: Structures and Dynamics

# Experimental Contribution to High-Precision Characterization of Magnetic Forces in Active Magnetic Bearings

[+] Author and Article Information
Klaus Kjølhede

Department of Mechanical Engineering, Section of Solid Mechanics, Technical University of Denmark, DK-2800, Kgs. Lyngby, Denmarkkk@mek.dtu.dk

Ilmar F. Santos

Department of Mechanical Engineering, Section of Solid Mechanics, Technical University of Denmark, DK-2800, Kgs. Lyngby, Denmarkifs@mek.dtu.dk

J. Eng. Gas Turbines Power 129(2), 503-510 (Aug 01, 2006) (8 pages) doi:10.1115/1.2434345 History: Received June 13, 2006; Revised August 01, 2006

## Abstract

Parameter identification procedures and model validation are major steps toward intelligent machines supported by active magnetic bearings (AMB). The ability of measuring the electromagnetic bearing forces, or deriving them from measuring the magnetic flux, strongly contributes to the model validation and leads to novel approaches in identifying crucial rotor parameters. This is the main focus of this paper, where an intelligent AMB is being developed with the aim of aiding the accurate identification of damping and stiffness coefficients of active lubricated journal bearings. The main contribution of the work is the characterization of magnetic forces by using two different experimental approaches. Such approaches are investigated and described in detail. A special test rig is designed where the four pole AMB is able to generate forces up to $1900N$. The high-precision characterization of the magnetic forces is conducted using different experimental tests: (i) by using hall sensors mounted directly on the poles (precise measurements of the magnetic flux) and by an auxiliary system, composed of strain gages and flexible beams attached to the rotor, (ii) by measuring the input current and bearing gap variations, monitoring the bearing input signals. Advantages and drawbacks of the different methodologies are critically discussed. The linearity ranges are experimentally determined and the characterization of magnetic forces with a high accuracy of $<1%$ is achieved (percent error is normalized with respect to the instantaneous measured force obtained from the strain gauges signals).

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## Figures

Figure 1

(a) Active magnetic bearing (AMB) and (b) Schematic view; Stator, rotor, shaft and gap dimensions in millimeters

Figure 2

(a) Hall sensor dimensions: 2×3×0.6mm(L×W×H) and (b) Hall sensor mounted in slot in the pole surface: 1, Pole surface: 2, Hall sensor: 3, Slot (Dimensions: 3.3×0.7mm(W×D))

Figure 3

(a) Experimental setup: 1, Support; 2, Flexible beam; 3, Eddy current sensor and (b) schematic view of the experimental setup without the magnetic bearing

Figure 4

Horizontal external force: 1, Wire for external force; 2, force transducer

Figure 5

Vertical up external force: 1, Wire for external force; 2, force transducer

Figure 10

Strain gage versus external force

Figure 11

Experimentally versus calculated force - Hall sensor

Figure 6

Vertical down external force: 1, Wire for external force; 2, force transducer; 3, payload

Figure 12

Behavior of Hall sensor: Solid line, theoretical; ×, bias 6A (experimental), 엯, bias 8A, (experimental), ⋆, bias 10A (experimental)

Figure 13

Experimentally versus calculated force—reluctance network

Figure 14

(a) AMB and ALB test rig: 1, Motor; 2, belt drive; 3, ball bearings; 4, coupling; 5, shaft; 6, AMB; 7, eddy current sensor; 8, ALB; and (b) schematic view of the test rig

Figure 7

Reluctance network model

Figure 8

Visualization of the magnetic field density around the Hall sensor and slot cross section—numerical investigation

Figure 9

Reduction of bearing load capacity as a function of area ratio Aslot∕Apole and slot depth and gap ratio d∕g0

## Errata

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