Suspension Winding Research Articles

Rotor Vibration Control of a Bearingless Induction Motor Based on Unbalanced Force Feed-Forward Compensation and Current Compensation

To handle the unbalanced vibration caused by uneven rotor mass distribution in a bearingless induction motor (BIM), a control strategy that integrates unbalanced feed-forward compensation and current compensation is presented. Firstly, based on the analysis of the BIM rotor vibration mechanism and the unbalanced vibration influence on the performance of the BIM, the dynamic model of the rotor is derived. Secondly, an unbalanced force feed-forward compensation controller is designed to extract the synchronous vibration signals through the synchronous signal detection unit, and then the compensation force components are generated by unbalanced feed-forward compensation controller. In addition, considering the influence of current in the rotor induced by suspension winding, a current compensation link is applied to the system to enhance the suspension performance. Finally, a rotor unbalanced vibration compensation control system of the BIM is established. The simulation and experimental results show that the proposed compensation control strategy not only can effectively reduce the rotor radial displacement and suppress the unbalanced vibration, but also can improve the suspension performance.

Investigation of Operational Characteristics and Efficiency Enhancement of an Ultra-High-Speed Bearingless Motor at 100,000 r/min

This article introduces the operational characteristics of an ultra-high-speed bearingless motor with 2-pole concentrated motor windings and 4-pole concentrated suspension windings. Experiments with a prototype machine indicate that iron loss is the largest component of losses in the proposed bearingless motor. Therefore, in order to reduce the iron loss, comparison of the experimental results and the 3D-FEM results is discussed. As a result, because it is turned out that iron loss of the switching frequency component accounts for a large proportion in total iron loss, a method of enhancing efficiency of the proposed bearingless motor by means of high switching frequency using SiC-MOSFET is examined. The experimental results with the examined method show that the prototype machine can attain the high efficiency of 94.3% at rated operation of 1.70 kW and 100,000 r/min in spite of including suspension input power for magnetic levitation. In addition, operational characteristics comparison of the bearingless motor at switching frequency of 40 kHz, 80 kHz and 120 kHz is discussed in detail.

Active Disturbance Rejection Control for Bearingless Induction Motor Based on Hyperbolic Tangent Tracking Differentiator

To meet the operation requirements of a bearingless induction motor (BIM) control system, such as fast speed response, stable suspension, and high disturbance rejection ability, an active disturbance rejection control (ADRC) strategy based on hyperbolic tangent tracking differentiator (HTTD) is proposed. First, based on the air-gap magnetic field decoupling control of the BIM, the uncertainties of the system itself and the load disturbance are regarded as a total disturbance. The extended state observer (ESO) is used to observe the total disturbance in real time. The speed ADRC and suspension ADRC are designed for the torque winding and suspension winding, respectively. Second, the hyperbolic tangent function is applied to the tracking differentiator (TD) of ADRC to simplify its structure. The frequency characteristic curves are analyzed through the sweep-frequency test, and the design parameters are further determined. Finally, this strategy is analyzed with simulation in MATLAB/Simulink and verified on an experimental prototype. Both simulation and experimental results show that the strategy not only has good speed and current characteristics but also reduces the effect of load disturbance on the system and improves the suspension characteristics of the BIM.

Design and analysis of a novel wound rotor for a bearingless induction motor

ABSTRACTThe traditional squirrel-type bearingless induction motor (BIM) suspension winding generates induced current in its squirrel-cage rotor and affects the phase and amplitude of the suspension force. Based on the analysis of the spatial distribution of torque winding magnetic field and suspension winding magnetic field, a new type of wound rotor BIM is designed. Different from the squirrel-cage rotor, the wound rotor uses a special method of embedding a set of coils at any symmetrical four rotor slot positions, so as to only induce the torque winding magnetic field. The induced current, air-gap magnetic density, magnetic field line distribution, suspension force as well as electromagnetic torque of the traditional squirrel-cage motor and the new wound motor are analysed by Maxwell finite element calculation. The results show that the designed new wound BIM can not only effectively suppress the induced current of suspension winding, eliminating its influence on the suspension force, but also has a better starting performance.

Performance comparison of 12/12 pole with 8/10 and 12/14 pole bearingless switched reluctance machine

In this Letter, three types of bearingless switched reluctance machines (BSRMs) with different hybrid stator structures are designed and analysed. First of all, topologies of three BSRMs, the conventional 8/10 pole, 12/14 pole BSRMs and a novel 12/12 pole BSRM with axial split phase inner stator permanent magnet structure, are given. Then, the electromagnetic performances, including the distribution of magnetic fields, suspension force and electromagnetic torque are comprehensively compared by means of 2D and 3D finite-element method (FEM). The results based on theoretical analysis and FEM reveal that the proposed 12/12 pole BSRM would exhibit complete decoupling, shorter magnetic circuit, higher torque (power) density and suspension force than the conventional ones. Further, it is found that four degree-of-freedom suspension of the rotor can be realised by two-phase suspension windings in the proposed 12/12 pole BSRM. Finally, a prototype machine is designed and manufactured to verify the validity of the topology design and electromagnetic performance.

A High-Precision Rotor Position and Displacement Prediction Method Specially for Bearingless Permanent Magnet Synchronous Motor

The high performance sensorless performance of the bearingless permanent magnet synchronous motor is the main direction to improve the reliability of the drive system and reduce the cost of the system, and the high-precision rotor position and displacement prediction method is the key technology to realize the high performance sensorless operation. In view of the above problems, a rotor displacement and position prediction method based on kernel extreme learning machine is studied in this paper. On the basis of the mathematical model of BPMSM, this method predicted the position and displacement of the rotor according to the current and flux linkage of suspension windings and torque windings by KELM. The construction method of rotor position and displacement prediction model was described; meanwhile the implementation steps of offline training and online prediction were given. Finally, the error between the method and the actual value was compared by simulation and experiment. The results showed that the proposed method had high accuracy and could achieve real-time rotor position and displacement and then provides the basis for realizing sensorless operation control of BPMSM.

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.

Operational Characteristics of an IPM-Type Bearingless Motor With 2-Pole Motor Windings and 4-Pole Suspension Windings

A bearingless motor (BelM) integrates the characteristics of an electric motor and a magnetic bearing. Typically, a surface-mounted permanent magnet (SPM)-type BelM with a ring magnet magnetized in parallel is employed for high-speed operation. It is noteworthy that a carbon fiber bandage is needed to fix the ring magnet. However, when an SPM-type BelM is applied to a high-output motor, it becomes difficult to generate sufficient suspension force because of the wide magnetic gap arising from the carbon fiber bandage and the ring magnet. In addition, the manufacturing cost for the large-scale ring magnet and the carbon fiber bandage is high. Therefore, to realize a high-output and high-speed BelM at a low cost, we have focused on an interior permanent magnet (IPM)-type BelM with 2-pole motor windings and 4-pole suspension windings. Moreover, a rotor structure for suppressing suspension force ripple arising in the IPM-type BelM has been proposed based on a three-dimensional finite element analysis. In this study, a prototype of the IPM-type BelM with the proposed rotor structure is produced. Experiments with this prototype show that the proposed rotor structure is effective in reducing suspension-force ripple. Additionally, the operational characteristics of the prototype are shown in detail.

Radial Displacement Extraction Method of BLIM based on High Frequency Signal Injection

To achieve the radial displacement observation of bearingless induction motor (BLIM), a displacement extraction method based on high frequency signal injection is presented. According to the variation principle of the mutual inductance between torque and suspension windings, by injecting a high frequency voltage to torque winding, the induced high frequency current signal in suspension winding is detected; then the radial displacement is extracted by demodulation. Simulation results show that the radial displacement can be effectively extracted, the BLIM can operate stably under the conditions of none displacement sensor.

Design of Dual Purpose No-Voltage Combined Windings for Bearingless Motors

A winding design approach is proposed to create a single motor winding, which is able to produce both radial force and torque. This approach can be used to design new bearingless motors, as well as to transform conventional motor designs into bearingless motors by simply modifying the winding end connections. The resulting winding has two sets of terminal connections, one for torque and one for suspension. The suspension terminals experience no motional-electromotive force when the rotor is centered, which means that the suspension drive can have a low voltage rating and that rotor vibrations can be passively dampened by simply short-circuiting the suspension terminals. Bearingless motors that use these so-called dual purpose no voltage windings can be designed to have higher torque density and lower losses associated with the magnetic suspension operation than traditional bearingless motors, which utilize separate torque and suspension windings. It will be shown that many popular winding designs, including fractional-slot and concentrated windings, can be realized as dual purpose no-voltage windings. The proposed approach applies to traditional $p\pm 1$ pole-pair bearingless motors, as well as the bearingless consequent-pole and ac homopolar motors. Fractional-slot motor winding theory is used to derive the new winding requirements and a generalized design procedure; example designs are explored through finite element analysis and experimental results from a hardware prototype of a bearingless ac homopolar motor.

Equivalent two-phase mutual inductance model and measurement algorithm of three-phase bearingless motor

For a bearingless motor that has two sets of stator windings, i.e. torque windings and suspension windings, the mutual inductance between the two sets of stator windings is a critical parameter; it is the basis of displacement sensorless control technology. Aiming at a three-phase bearingless motor, the equivalent two-phase mutual inductance model between two sets of stator windings is deduced. Then, based on the mutual inductance measurement method of a simple two-phase bearingless motor, a novel measurement algorithm of the equivalent two-phase mutual inductance is proposed, and the proposed measurement algorithm is applicable for a three-phase bearingless motor. Finally, based on a three-phase bearingless induction prototype motor, the experimental measurement of the equivalent two-phase mutual inductance is carried out. From the experimental results, it is clear that within the limited radial eccentricity of the rotor, the measured value of the equivalent two-phase mutual inductance is approximately proportional to the radial displacement of the rotor, and the correction coefficient kc of the equivalent two-phase mutual inductance model is derived; the consistency between measured and calculated values has verified the validities of the equivalent two-phase mutual inductance model and the proposed measurement algorithm from one instance. Thus, the theory foundation has been laid for research on the displacement sensorless vector control technology of a three-phase bearingless motor.

Investigation of a New Ironless-Stator Self-Bearing Axial Flux Permanent Magnet Motor

In this paper, a new type of self-bearing axial flux permanent magnet (SBAFPM) motor with ironless stator is proposed. The torque windings and the suspension windings are integrated into a cohesive whole with the same pole number for both torque and radial force generation. The fundamental structure and the working principle of the presented ironless-stator SBAFPM motor with ironless stator are illustrated. The control method and the implementation for the integrated windings system to produce force and torque independently are presented, as well as an FEA model for calculating the achievable drive torque and suspension force. Results indicate that the bearing force and the drive torque depend linearly on the respective current. The presented ironless-stator SBAFPM motor is free from tradeoff between force and torque, which will simplify the control of the torque and the levitation force.

A mathematical model of a synchronous reluctance bearingless machine as a control object

In this article, one type of bearingless electrical machine, a synchronous reluctance bearingless machine (SRBM), is considered. A simplified analytical description of the static characteristics of the SRBM nodule has been derived. A control law for the suspension current phase has been proposed. The validity of the derived equations was verified by numerical calculations by the finite element method. The radial electromagnetic forces of the module are described by linear dependences on the magnitude of the suspension currents and the displacement of the rotor in the corresponding radial direction. The dependence of the forces on the displacement in the perpendicular direction is simplified. The analytical model displays high accuracy of calculations in the operating ranges of the currents and displacements. The principal component of the SRBM torque is a square-law function of the drive current magnitude. The error of the torque prediction according to the analytical model was 1.8% with the rotor in the central position and no current in the suspension winding within the entire range of the current values at any torque angles. The graphs presented in the article show the impact of the suspension current and the rotor displacement on the torque. The control law proposed for the suspension current phase is a linear function of the rotation angle of the rotor, the radial displacement direction, and the torque angle of the synchronous machine drive. The proportionality ratio between the torque angle and the suspension current phase has been determined approximately. The error of calculation at different rotor displacements has been assessed and plotted.

The Mechanism Study of Alternating Arc(AC)Magnetic Levitation Induction Motor

Magnetic levitation (no bearings) motor by using magnetic force to make rotor suspend and drive realize its high or ultra-high speed rotating. The stator’s structure of traditional no bearing magnetic levitation motor is double winding which is polar logarithmic difference 1 of 2 sets of winding (torque winding and suspension winding) and embedded in the stator. Using two inverter respectively for the two sets of winding to go into the same frequency current in order to realize the suspension of the rotor and motor’s driven, small carrying capacity of motor’s structure, controlling complex system. This paper based on the traditional motor technology puts forward a kind of arc principle and respectively decorates two arc motors in horizontal and vertical direction symmetric to rotor according to the electromagnetic bearing suspension technology that is constituted the arc magnetic levitation induction motor. Establishing air-gap transformation regular between rotor and stator (air-gap length) motor is under the effect of interference. Based on the electromagnetic theory establishing distribution regular of the air-gap magnetic induction intensity. Virtual displacement principle is used to establish electromagnetism mathematical model and motor electromagnetism levitation. By the finite element analysis carrying on simulation research to the magnetic induction intensity, electric magnetic levitation force and distribution features of electromagnetic torque and so on.

Radial Force Analytic Modeling for a Novel Bearingless Switched Reluctance Motor When Considering Rotor Eccentricity

—Analytic modeling of radial forces is proposed for the novel bearingless switched reluctance motor, where the rotor eccentricity is taken into account. The novel bearingless switched reluctance motor's model is never disclosed, even though there are many advantages, such as fewer suspension windings and simpler control circuit and algorithm, when compared to the conventional bearingless switched reluctance motor. The analytic model of radial forces is very important to achieve this new bearingless switched reluctance motor's suspension control. The rotor eccentricity is a key cause to affect the radial forces. This article calculates the air-gap permeances through considering rotor eccentricity. The self-inductance and mutual-inductance expressions of the motor torque windings and the suspension windings are derived by using the magnetic equivalent circuit method. The derived radial force model discloses the effects of winding currents, rotor position angle, and rotor eccentricity displacement. The finite-element analysis based results verify the built model.

Design of a High-speed Superconducting Bearingless Machine for Flywheel Energy Storage Systems

In this paper, an 8-pole/12-slot high-speed superconducting bearingless machine is proposed for flywheel energy storage systems. The proposed machine adopts a homopolar configuration: the rotor only consists of iron lamination with eight salient iron poles and the 12-slot stator accommodates all three groups of windings: the high temperature superconducting (HTS) field winding; the armature winding; and the suspension winding. With the HTS field winding, eight iron poles in the rotor are magnetized as 4-pole-pair electromagnets. Also, by adjusting the dc current in the HTS field winding, the air-gap flux can be controlled flexibly. By deploying the suspension winding which shares the same slot with the armature winding, a two-degree-of-freedom suspension force can be generated for bearingless operation.

Mathematical modeling of a novel bearingless switched reluctance motor

Purpose – The novel bearingless switched reluctance motor (BSRM) is proposed recently, which is different from the conventional BSRM in the stator structure and suspension winding arrangement. The reduced number of suspension windings makes the novel BSRM much simpler, so that the control circuit and algorithm are greatly simplified when compared to those of the conventional BSRM. This paper for the first time proposes the novel BSRM's analytic model, including the mathematical relationships among the winding currents, rotor angle, radial forces, and motor torque, to further achieve the suspending forces and torque control. The paper aims to discuss these issues. Design/methodology/approach – The magnetic equivalent circuit method is employed to obtain the self-inductances and mutual-inductances of the motor torque windings (main windings) and suspension windings (control windings). The straight flux paths are combined with the elliptical fringing flux paths to calculate the air-gap permeances, and the stored magnetic energy. Then, the mathematical expressions of radial forces and torque are derived. A novel BSRM prototype is analyzed through using the proposed analytical model and the finite element model. The results of both methods are compared to verify the proposed mathematical model. Findings – The proposed mathematical model of the novel BSRM considering unsaturated magnetic circuits is verified by finite-element analysis results. Research limitations/implications – The mathematical model represents the situation of magnetic circuit unsaturated and is not suitable for the magnetic circuit saturation. It cannot be used to control the motor which is working in the deep magnetic circuit saturation region. Practical implications – Building mathematical model is a necessary step for the motor's suspension and rotating control. The built model provides the fundamental for the preliminary control algorithm and experimental study of this novel BSRM. Originality/value – For the first time, the novel BSRM's mathematical model is proposed. It provides necessary fundamental for the motor's further analysis, design, and suspending and rotating controls.

A Radial Position Control Method of Bearingless Motor Based on d - Q -Axis Current Control

This paper presents a radial position control method of bearingless motor (BELM) based on a d-q-axis current control and an integrated winding arrangement. The BELM performs the functions of a motor as well as magnetic bearing. Rotor is suspended with noncontact by suspension force. In a conventional BELM, there are two kinds of stator windings such as motor and suspension windings. The motor winding outputs the rotational torque, and the suspension winding outputs the suspension force. To simplify the structure of conventional BELM, integration of two windings is proposed in this paper. By integrating the windings, the structure of BELM becomes simple, and also, the torque and force productions are increased. The d-axis stator current controls the suspension force, and the q-axis current controls the developed torque of the motor. In the case when there is less number of slots in the machine, the actual suspension force cannot follow the command. In order to make the actual suspension force follow the command, a compensation method is also proposed in this paper. A simulation study is done for the proposed radial position control based integrated winding BELM using finite-element analysis software. Furthermore, a prototype machine of the proposed BELM based on d-q-axis current control is built to confirm the performance of the motor and the magnetic suspension in real time. Rotor stabilization with magnetic levitation is confirmed using the prototype machine.

A Novel Parallel Motor Winding Structure for Bearingless Motors

A novel winding structure without separated suspension winding has been proposed for bearingless motors. The proposed winding structure basically has two parallel paths in three-phase motor windings. One three-phase winding path has a neutral point in general, but another three-phase winding path is connected to a suspension inverter, which injects currents for suspension force generation. Additional suspension windings are not needed in the stator slots, because the two parallel path windings are used for both torque and suspension force generation. In the proposed winding structure, speed induced voltages are cancelled each other so that no speed induced voltage is induced at suspension terminals.

Optimal Suspension Winding Configuration in a Homo-Polar Bearingless Motor

The authors have proposed a six-pole homo-polar type bearingless motor intended for a use as a small power and ultra high-speed motor. In a six-pole homo-polar rotor, crucial suspension force interference is caused. Thus, the suspension winding configuration should be carefully designed. In this paper, theoretical calculation is carried out with several structures to find the optimal winding configuration for reduction of the force interference. To verify the theory, finite-element-method (FEM) analysis is carried out. A test machine is also fabricated. FEM analysis and experimental result have demonstrated that the proposed optimal winding configuration can successfully reduce the force interference.