Bearingless Permanent Magnet Motor Research Articles

Development of Slotless Single-Drive Bearingless Permanent Magnet Motor With Aerostatic Bearings for High-Speed Applications

For electrical motors, three translational ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x–y–z</i> ) and two tilting ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ_x–θ_y</i> ) rotor motions must be restricted. Ball bearings are commonly used to restrict motion; however, they cannot be applied to high-speed motors with large <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DN</i> (product of bearing diameter and speed) values. The rotor in this study has an aerostatic bearing support in radial ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x–y</i> ) and tilting ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ_x–θ_y</i> ) motions. However, if an additional magnetic bearing or actuator is applied to the motor for axial motion control ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</i> ), the motor size and shaft length increases. To avoid this problem, a novel bearingless motor structure with a magnetically integrated bearing function is proposed in this paper. The structure combines the functions of both torque production ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ_z</i> ) and axial force generation ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</i> ) in one unit. It uses only a set of three-phase slotless windings and three-phase voltage source inverter. Such bearingless motors are referred to as single-drive bearingless motors. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> -axis and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis currents regulate the axial suspension force and torque, respectively. This paper presents the structure, operation principle, 3D-finite element calculation results, and experimental results. The fabricated prototype bearingless motor with a shaft diameter of 30 mm can actively control the axial motion of the rotor at 60,000 rpm, that is, with a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DN</i> value of 1,800,000.

Complementarity analysis of consequent-pole bearingless flux reversal motor windings with different pitch matchings

First, a topology of a consequent-pole bearingless flux reversal (CPBFR) motor suitable for flywheel battery magnetic suspension support systems is proposed, and the specific forms of different winding pitches are analyzed. Then, the phenomenon of winding complementarity, which is unique to certain permanent magnet bearingless motors, is explained theoretically. The magnetic density distribution characteristics of the no-load air gap of the consequent-pole permanent magnet rotor are given, and the influence of different winding pitches on the no-load induced potential as well as the influence of different winding pitches on no-load induced potential harmonics are revealed. Finally, the working feasibility of the CPBFR motor is verified from the perspective of magnetic field distribution and the magnetic line of force.

Three-Coil Combined Winding Configuration for a 2-DOF Actively Controlled Bearingless Permanent Magnet Motor

This study describes a simple combined winding configuration for a two-degrees-of-freedom (DOF) controlled bearingless motor with a six-slot/six-pole permanent magnet (PM) structure. This bearingless PM motor has only three concentrated coils that are wound alternately on six teeth of the stator and employs only one three-phase inverter for both the two-DOF radial magnetic suspension and single-phase motor drive. Theoretical calculation and finite element method (FEM) analysis demonstrated that the proposed three-coil combined winding configuration generates a four-pole rotating field and six-pole alternating field for generating radial suspension force and torque, respectively. To verify the proposed configuration, we built and tested an outer rotor bearingless PM motor with a diameter of 58 mm and a height of 5 mm. The rotor was successfully levitated and driven up to 7500 rpm. The calculated and measured results were compared for both the proposed combined winding configuration and the previously developed separated winding configuration. The measured motor efficiency of the prototype with the combined winding at 5000 rpm with 20 mNm output is improved by 5% due to the copper loss reduction when compared to that with the separated winding.

Development of Axial-Flux Single-Drive Bearingless Motor With One-Axis Active Positioning

This study proposes a novel axial-flux bearingless permanent magnet motor with one-axis active positioning. This bearingless motor consists of an axial-flux surface permanent magnet motor with coreless coils and a repulsive permanent magnet coupling located at its center. Therefore, it has a single three-phase winding and needs only one three-phase inverter for both rotation and magnetic suspension; it is, thus, referred to as a single-drive bearingless motor. The axial position of the rotor is actively controlled, whereas the radial and tilting motions are passively stabilized. The suspension force and torque are regulated by the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> -axis and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis currents, respectively. The structure of the proposed axial-flux single-drive bearingless motor is presented, and its torque and suspension force are obtained by both theoretical derivation and numerical modeling (using the finite-element method). A prototype machine is fabricated, and the obtained experimental results are presented and discussed.

Optimum Pole Number Combination of a Buried Permanent Magnet Bearingless Motor and Test Results at an Output of 60 kW With a Speed of 37000 r/min

An ideal pole number combination of motor and suspension windings is essential for bearingless motors. In this study, the performance of a high-speed, high-power buried permanent magnet bearingless motor using several pole combinations is compared. The torque, power factor, efficiency, suspension force amplitude and fluctuation, and eddy current loss are compared for a 37000 r/min bearingless motor with an output of 60 kW. The basic experimental results of the test bearingless motor with the optimum pole number combination are presented. The test machine has an output of 61.9 kW with an efficiency of 94.9% at a speed of 37000 r/min and a static radial force of 530 N. The radial force and torque density are as high as 1.01 kgf/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 64 Nm/L, respectively. The output of 61.9 kW is the highest experimentally obtained for bearingless motors.

Design and Analysis of a Bearingless Permanent-Magnet Motor for Axial Blood Pump Applications

Because of their high power density and compact size, permanent-magnet (PM) motors have been commonly used to drive rotary blood pumps (RBPs), which are focused on the treatment of end-stage heart failure or as the bridge to a heart transplant. In this paper, a bearingless PM motor has been proposed for axial blood pump applications. The finite-element method (FEM) is used to predict the electromagnetic characteristics of the designed motor with improved performance. Two topologies are investigated, namely the integral-slot and distributed-windings method and the fractional-slot and double-layer concentrated windings method. Both motors are analyzed and optimized. FEM reveals that, compared with the integral-slot motor, the fractional-slot motor offers significantly enhanced performance, including reduced cogging torque, improved back electromotive force (back EMF), and decreased magnetic flux leakage. Finally, hydraulic experiments have been conducted in a mock-circulation loop to validate the feasibility of the designed motor for an axial blood pump. The results show that the fractional-slot bearingless PM motor can drive the RBP to produce physiological blood flow with reasonable efficiency.

Simple Driving Method for a 2-DOF Controlled Bearingless Motor Using One Three-Phase Inverter

In this paper, we propose a simple driving method that uses only one three-phase inverter for an actively controlled 2-DOF bearingless motor. This motor has a six-slot/six-pole structure, including two separated windings for the single-phase six-pole motor and the three-phase four-pole suspension. The stator core has a staggered-tooth structure to produce starting torque. The single-phase motor winding is connected between the neutral point of the three-phase suspension winding and the midpoint of the dc power sources. The radial suspension force is calculated analytically and verified using the finite-element method. In addition, we introduce a ripple compensation method for the suspension force. A prototype of a bearingless permanent magnet motor with 2-DOF active suspension was built and tested. The results of experiments demonstrated that the rotor, which was driven at speeds up to 8000 r/min, could be levitated successfully, using only one three-phase inverter.

Design and Analysis of a Novel Bearingless Motor for a Miniature Axial Flow Blood Pump

In this paper, a novel design for the axial flow blood pump is proposed, based on the bearingless permanent magnet (PM) motor concept. The design of the impeller, the rotor, and the stator is explained. The axial length of the impeller is optimized based on the finite-element analysis (FEA), and because of reduced axial length of the rotor, only one radial active magnetic bearing is used, whose functionality is embedded in the same stator, which is used for generating motor torque. A novel design of the rotor and the stator is proposed for measuring the radial position of the rotor using four linear Hall effect sensors. This sensor assembly can measure the radial position of the rotor with good accuracy. The angular position of the rotor is measured with two Hall effect sensors. The 3-D FEA of the complete system is performed, and suspension forces, stiffness, and torques are evaluated under various control conditions. The control strategy for the bearingless PM motor based on the Hall effect position sensors is explained. The experimental setup is built in order to guide the process of design. The experimental results of the system are evaluated under different operating conditions and compared with FEA results.

Axial Vibration Suppression by Field Flux Regulation in Two-Axis Actively Positioned Permanent Magnet Bearingless Motors With Axial Position Estimation

This paper presents a novel axial vibration suppression method in two-axis actively positioned permanent magnet bearingless motors. Only radial directions are actively positioned. The axial and tilting directions are passively stabilized by the magnetic attraction force between the stator core and the rotor permanent magnet. Generally, a damping force is considerably low in the axial and tilting directions. Therefore, when an axial disturbance force is applied to the rotor, the axial vibration is generated, and then, the vibration cannot be suppressed at a critical speed. In this paper, a novel axial vibration suppression method by d -axis current is proposed. The damping force is generated by d -axis current regulation. In this proposed method, the rotor axial position is estimated from the flux linkage variation in the motor winding. Therefore, an additional displacement sensor is not necessary. In experiments, it is confirmed that the rotor axial vibration is successfully suppressed by the proposed method.

Analysis of Winding Forms for Bearingless Flux-Switching PM Motor with U-core Stator Laminations

Due to low-cost and magnet-free rotor structure, bearingless flux-switching permanent magnet motors (BFSPMs) are more applicable to high-temperature and/or disposable applications. A BFSPM with U-core stator laminations is investigated in this paper. First, the operational principle is analyzed in accordance with the influence of winding connection manners on torque and radial forces, and then the basic winding arrangement for torque windings and radial force windings is determined. The mathematical models of torque and radial forces are derived based on the basic winding arrangement. Followed by that, a method to select winding terminal connection manners is proposed by taking into account control scheme and power inverter topologies. A promising terminal connection is found from three possible candidates and a detailed current model for torque and radial force control are given for the promising winding forms. Finally, relevant electromagnetic properties of the motor are verified and analyzed by finite element analysis.

Design and Analysis of a Novel Bearingless Flux-Switching Permanent Magnet Motor

Conventional bearingless permanent magnet motors have magnets on their rotor, and their maximum electric loading performance may be restricted by thermal instability and lack of mechanical integrity. To overcome such drawbacks, a novel bearingless motor with magnets, suspension force windings, and torque windings in the stator is proposed; the proposed motor is called bearingless flux-switching permanent magnet motor. First, the structure and winding configuration of the bearingless flux-switching permanent magnet motor are designed and introduced. Second, the operation principle of radial suspension force is illustrated based on the principle of reverse direction magnetization for symmetrical rotor teeth. Third, the corresponding electromagnetic characteristics, including flux field distributions, torque, and controllability of radial suspension forces, are investigated. Finally, the decoupling characteristics between suspension force windings and torque windings are discussed and analyzed comprehensively. High power per permanent magnet volume, good controllability of radial suspension forces, and excellent decoupling characteristics are achieved with the proposed bearingless flux-switching permanent magnet motor. Analytical results are validated through finite-element analysis and experimentation.

Spectral modelling of force ripples in permanent magnet bearingless motors

Purpose The purpose of this paper is to propose an analytical model for the calculation of forces ripples in bearingless permanent magnet motor. Design/methodology/approach The model is based on a spectral modeling of the forces based on a spectral analysis of the air-gap flux density. Findings The proposed methodology allows the prediction of the ripples of the levitation according the design of the machine and the topology of the winding. It can be used in the design procedure of bearingless motor in order to avoid mechanical resonance of the rotor shaft. Research limitations/implications This model cannot be used for high saturated machines. Originality/value The main originality of this paper consists in the evaluation of the field harmonics according to their sources and the contribution of the different harmonics to the force ripples. This can help the designer to focus on reducing or eliminating only the harmonics of flux density which create the force ripples.

Radial force modeling for bearingless motors based on analyze of field origins

This article proposes a global harmonic analysis of normal and tangential field components in air gap of multiphase bearingless permanent magnet motor topology to determine the radial force. This allows establishing the semi-analytical expressions of the radial force considering the location of field sources. The accuracy of the model is verified by full finite element method model using a statistic approach. An important result of this approach of modeling is that it permits to choose the most convenient topology for the bearingless permanent magnet motors by taking into account the location of the field sources in stator and rotor. In addition, the semi-analytic model allows predicting the force waveforms according to the stator and rotor topologies. The analysis of two different rotor topologies is carried out and it highlights the influence of the tangential component of the air-gap magnetic field on the motor performances. Moreover, the influence of the radial or parallel magnetization of the rotor magnet on the force ripples is discussed. The reduction of CPU time using the semi-analytical model makes it suitable to be implemented in a design optimization process.

New Model of Radial Force Determination in Bearingless Motor

This paper proposes to determine the radial forces in bearingless permanent magnet motor topology. We consider normal and tangential components of the magnetic field in the air gap. Their harmonic components and their evolution versus current and rotor position are considered. This allows establishing the semianalytical expressions of the radial forces considering different field sources. Their precision has been verified using a full finite element model and statistic analysis. The advantage of this semianalytical model lies in the swiftness of the different force components evaluation and consequently the reduction of CPU time.

Practical Implementation of Dual Purpose No Voltage Drives for Bearingless Motors

Practical implementations of the power electronics required to operate bearingless machines with the so-called “dual-purpose no-voltage” (DPNV) windings are proposed. These windings enable a bearingless machine to use the same coils to produce both radial suspension forces and torque, which has been shown in the literature to enable better machine performance. In this paper, the use of two-level inverters is investigated for the two configurations of DPNV windings: the “parallel” and “bridge” configurations. The investigation is valid for conventional p ± 1 pole-pair bearingless motors (i.e., bearingless permanent magnet and induction motors) and p = 1 bearingless motors (bearingless ac homopolar and consequent-pole motors). The advantages and disadvantages of each configuration are explored in terms of control complexity and required hardware, potential paths for circulating currents are identified along with mitigation strategies, and experimental results are presented to validate the proposed power electronic implementation.

Analysis and Control of Multiphase Permanent-Magnet Bearingless Motor With a Single Set of Half-Coiled Winding

In this paper, a multiphase bearingless motor (MBLM) with a single set of half-coiled winding is proposed, with advantages such as simpler construction and capability of fault-tolerant operation, comparing with the traditional BLM with dual groups of three-phase windings. The magnetomotive force distribution of the half-coiled winding is analyzed. Then, the winding configuration of a five-phase permanent-magnet bearingless motor is introduced, and the inductance of the MBLM is deduced by the modified winding function approach and verified by finite-element analysis. Furthermore, the control system is proposed based on the derived voltage model, torque model, and suspension force model, considering rotor eccentricity. Finally, static and dynamic experiments are provided on a prototype motor to verify the validity of the analysis.

Analysis of Half-coiled Short-pitch Windings with Different Phase Belt for Multiphase Bearingless Motor

The analysis and comparation of the half-coiled short-pitch windings with different phase belt are presented in the paper. The half-coiled short-pitch windings can supply the odd and even harmonics simultaneously, which can be applied in multiphase bearingless motor (MBLM). The space harmonic distribution of the half-coiled short-pitch windings with two kinds of phase belt is studied wi th respect to different coil pitch, and the suitable coil pitch can be selected from the analysis results to reduce the additional radial force and torque pulse. The two kinds of half-coiled short-pitch windings are applied to the five- and six-phase bearingless motor, and the comparation from the Finite Element Method (FEM) results shows that the winding with 2π/m phase belt is fit for the five phase bearingless motor and the winding with π/m phase belt is suitable for the six phase bearingless motor. Finally, a five phase surface-mounted permanent magnet (PM) bearingless motor is built and the experimental results are presented to verify the validity and feasibility of the analysis. The results presented in this paper will give useful guidelines for design optimization of the MBLM.

Current-Controlled Multiphase Slice Permanent Magnetic Bearingless Motors With Open-Circuited Phases: Fault-Tolerant Controllability and Its Verification

The fault-tolerant control of bearingless motors is vital for their safe and robust operation. In this paper, the operation of current-controlled multiphase slice permanent-magnet bearingless motors (PMBMs) with different open-circuited faulty phases is analyzed, and their fault-tolerant controllability in the general case is investigated. As an example, the feasibility of fault-tolerant control with arbitrary single, double, or triple open-circuited faulty phase(s) for a six-phase slice PMBM is discussed in detail. Simulation results from finite-element analysis are presented to demonstrate the operation of the motor under the proposed fault-tolerant control strategy. Moreover, experimental results are also provided to further verify the theoretical development.

Evaluation of a Bearingless PM Motor With Wide Magnetic Gaps

A new noncontact drive system has been developed employing a consequent-pole-type bearingless permanent magnet (PM) motor and a thrust magnetic bearing. This bearingless motor has a wide magnetic gap. The wide magnetic gap may cause a decrease in radial magnetic suspension force because of an increase in magnetic reluctance. A thrust magnetic bearing is installed at the end of the shaft because strong thrust force exists in centrifugal pump applications. In this paper, the magnetic suspension forces and rotation characteristics of the experimental prototype are evaluated while the rotor is magnetically suspended. Experimental results are also compared to the calculated ones using the finite-element method. It is demonstrated that the developed consequent-pole-type bearingless PM motor possesses sufficient magnetic suspension force for noncontact operation.

Voltage Characteristics of a Consequent-Pole Bearingless PM Motor With Concentrated Windings

A bearingless permanent magnet (PM) motor combines functions of non-contact magnetic suspension and torque generation together, thus it can drive without mechanical contact. Unlike conventional bearingless PM motors, such as a surface-mounted PM motor, a consequent-pole type of bearingless PM motor has inherent advantages. It can regulate the radial rotor positions independently of the motor drive and has no tradeoff between radial suspension force and torque. In the consequent-pole bearingless PM motors, distributed or concentrated windings can be employed to the motor winding structure. However, relationship between voltage characteristics and the winding design has not yet been revealed. In this paper, the induced electromotive force (EMF) of the consequent-pole bearingless PM motor is numerically calculated on the basis of the magnetic flux distribution in the airgap around the rotor. It is found that the EMF is distorted by the second harmonic at a specific slot number in a consequent-pole rotor. The calculated EMFs are confirmed with the experimental results in the fabricated test machines.