This paper presents a novel exterior rotor topology of a bearingless brushless synchronous motor with rated power of 300 W. Owing to the large possible magnetic gap and the absence of mechanical bearings, this motor is especially qualified for high-purity and low-shear applications (e.g., bioreactor mixing). Both torque and magnetic bearing forces are created inside this disk-shaped motor using a sophisticated control (proportional-integral-differential vector control) with superimposed drive and bearing currents fed to the concentrated combined stator coils. Optimal design is derived based on an electromagnetic analysis using the three-dimensional finite element method (3D-FEM), and the simulation results are verified with a prototype mixer setup.

Bearingless 300-W PMSM for Bioreactor Mixing

Brief Nonlinear control of a 3-pole active magnetic bearing system

A variety of distance sensors suitable for application to magnetic levitation is examined. The characteristics of the sensors regarding the phase shift between in- and outgoing signals, frequency response, DC-stability, stability to EMF interference, resolution and temperature stability are analyzed. A simple eddy current sensor applicable to levitation is presented, and the effects of its nonlinear signal upon the system's performance are considered. >

Sensors for magnetic bearings

Most magnetic radial bearings adopt the configuration of eight magnetic poles. Such a configuration usually requires four power amplifiers and has high power loss, resulting in high cost. In this study, a 3-pole active magnetic bearing (AMB) is investigated, which requires only two power amplifiers. In addition, it has lower power loss. Thus, the overall cost can be reduced. However, the 3-pole AMB system is strongly nonlinear due to magnetic flux coupling. If the actuator dynamics are also considered, the overall system will be much more complicated. Nonlinear control is thus necessary. By taking magnetic fluxes as part of the system states, the state equations of the voltage-controlled 3-pole AMB system admit a quadratic nonlinear form. It is shown that this quadratic nonlinear system is feedback linearizable. An admissible domain for the initial states such that the resulting system responses will not hit the bearing boundary (or back-up bearing, if exists) is also obtained via a Lyapunov analysis. Numerical simulations are carried out to verify the theoretical results.

Exact linearization of a voltage-controlled 3-pole active magnetic bearing system

We report on an experimental study of a three-pole active magnetic bearing (AMB) system. Two cases (with and without a motor) are investigated. For the system without a motor, we designed and implemented three controllers: a linear state feedback controller (LC), feedback linearization with a linear state feedback controller (FLLC), and feedback linearization with an integral sliding mode controller (ISMC). We found that both LC and FLLC will result in a limit cycle at the steady state, and ISMC yields the best transient and steady-state performance. We also implemented ISMC for the system with a motor. We found that when the rotor speed is zero, the system performance is as good as that of the system without a motor, which shows that the ISMC controller is robust. When the rotor is spinning, a steady-state limit cycle exists. The size of the limit cycle increases with the rotor speed. We discuss the effects that cause the limit cycle and suggest ways to improve the results. The results clearly indicate that the three-pole AMB is indeed feasible. However, delicate calibration and a robust nonlinear controller are necessary to yield good performance.

Experimental validation of a current-controlled three-pole magnetic rotor-bearing system

A magnetically fully levitated rotor system has been designed and built. It consists of stator and rotor bearings for the rotating shaft, an electronic control system, an optical sensing system, and power amplifiers. For an application in which the rotor spins at high speeds in a vacuum, its position is controlled by infrared light sensors. The system has been shown to spin at frequencies of up to 140 Hz, and an extension of the rotational frequency to 1000 Hz is envisaged. The rotor vibrations have been shown to occur at theoretically predicted frequencies. The amplitudes of radial vibrations were kept below +or-22 mu m and mainly occurred at around +or-10 mu m. >

Development of active magnetic bearings for high speed rotors

Two methods for calculating magnetic fields around and ponderomotive forces between arrays of permanently magnetized rectangular prisms are presented and experimentally verified. The methods are then used in the design study of an axial synchronous coupling intended for large power transmission. >

Ponderomotive Interactions Between Arrays Of Permanently Magnetized Rectangular Prisms

The design of electromagnetic bearings for use in a radioactive environment is presented specifically from a control point of view and supported by experimental results. A pump shaft 2 meters long, weighing 110 kg, is levitated by such bearings and driven by an induction motor capable of spinning at variable speeds of up to 3000 rpm. The experimental results demonstrate that this bearing system is stable and robust when the shaft is run through its two critical speeds.

The levitation of the SIXEP pump shaft using active magnetic bearings

Introduction The methods that have previously been applied to analogue systems [ 11 to eliminate mechanical vibrations of shafts, are now being investigated using digital systems [2]. A novel transputer based digitally controlled magnetic bearing system, utilizing a new feedforward control algorithm together with the transputer's parallel computing ability for reducing the shaft vibrations, is presented in this paper. The experimental results demonstrate that not only is this bearing system stable when the shaft is run through its two critical speeds but also that, at the same time, a significant reduction of the shaft vibrations is achieved.

http://ieeexplore.ieee.org/iel3/4676/13086/00597513.pdf

S. Whitley

Papers

Development of active magnetic bearings for high speed rotors

Ponderomotive Interactions Between Arrays Of Permanently Magnetized Rectangular Prisms

The levitation of the SIXEP pump shaft using active magnetic bearings

Vibration Reduction Using Digitally Controlled Magnetic Bearings