Research Papers

Reluctance Force Magnetic Suspension Characteristics and Control for Cylindrical Rotor Bearingless Motors

[+] Author and Article Information
Lei Zhou

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: leizhou@mit.edu

David L. Trumper

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: trumper@mit.edu

Contributed by the Dynamic Systems Division of ASME for publication in the JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT, AND CONTROL. Manuscript received April 18, 2015; final manuscript received October 11, 2016; published online January 10, 2017. Assoc. Editor: Azim Eskandarian.

J. Dyn. Sys., Meas., Control 139(3), 031003 (Jan 10, 2017) (8 pages) Paper No: DS-15-1186; doi: 10.1115/1.4035007 History: Received April 18, 2015; Revised October 11, 2016

In this paper, the modeling and control of reluctance-force-based magnetic suspension in cylindrical rotor, smooth air-gap bearingless motors are presented. The full suspension system dynamics, including both the destabilizing forces due to the motor field and the active magnetic suspension control forces, are modeled, and a transfer function of the bearingless motor suspension plant is derived. It is shown that the suspension system dynamics in a bearingless motor depend on the motor winding current amplitude. This requires the magnetic suspension controllers to address the changing system dynamics and to stabilize the suspension under different driving conditions. A controller design with its gains changing with the motor winding current amplitude is proposed. The derived model and the proposed controller design are verified by experiments with a hybrid hysteresis–induction type bearingless motor. It is shown that the derived mathematical model provides an effective basis for loop-shaping control design for the reluctance-force-based magnetic suspension systems in bearingless motors, and the proposed controller design can stabilize the rotor's suspension under varying excitation conditions.

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Fig. 1

Winding arrangement in a bearingless motor assuming a two-phase configuration. Here, the four-pole windings 4a, 4b are the motor windings, and the two-pole windings 2a, 2b are the suspension windings.

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Fig. 2

The MMF distribution generated by the winding 2a and its associated fundamental component. The horizontal axis is the spatial angle ϕs. The black line shows the MMF of a concentrated winding, and the dark green line shows the fundamental component.

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Fig. 3

Air-gap length variation with a rotor eccentric displacement to the θ-direction

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Fig. 4

Unstable radial force generation when only the four-pole motor windings are excited. Attractive forces are generated in the air gaps 1, 2, 3, and 4. The rotor has an eccentric displacement s in the air gap 1 direction, which induces a radial force in this direction.

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Fig. 5

Photograph of the experimental hardware for the bearingless motor 1D-MSRS: (a) structure and (b) stator and rotor

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Fig. 6

Modeled plant frequency response for the x-directional suspension of bearingless motor in 1D-MSRS under different motor winding current amplitudes Im (zero-to-peak)

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Fig. 7

Experimentally measured plant frequency response for x-direction rotor suspension from the equivalent two-phase suspension control current i2a (A) to the x-directional rotor displacement x (m) under different three-phase four-pole excitation amplitudes Im (zero-to-peak)

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Fig. 8

Modeled and measured break frequencies of plant Bode plot with varying driving current amplitudes

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Fig. 9

Modeled and measured DC gain of plant Bode plot with different driving current amplitudes

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Fig. 10

Block diagram of the reluctance force magnetic suspension control system for the bearingless motor

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Fig. 11

Measured loop return ratio of the radial suspension in the testing bearingless motor




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