Identification and compensation of torque ripple in high-precisionpermanent magnet motor drives

Permanent magnet synchronous machines generate parasitic torque pulsations owing to distortion of the stator flux linkage distribution, variable magnetic reluctance at the stator slots, and secondary phenomena. The consequences are speed oscillations which, although small in magnitude, deteriorate the performance of the drive in demanding applications. The parasitic effects are analyzed and modeled using the complex state-variable approach. A fast current control system is employed to produce high-frequency electromagnetic torque components for compensation. A self-commissioning scheme is described which identifies the machine parameters, particularly the torque ripple functions which depend on the angular position of the rotor. Variations of permanent magnet flux density with temperature are compensated by on-line adaptation. The algorithms for adaptation and control are implemented in a standard microcontroller system without additional hardware. The effectiveness of the adaptive torque ripple compensation is demonstrated by experiments     

六相异步电机的ANSOFT有限元分析

使用最为广泛的交流电机是三相电机,但是由于多相电机可靠性高、谐波成分小、性能好,多相电机将被广泛的应用。利用ANSOFT软件对相同定转子结构的三相电机和六相电机进行有限元分析,得到了三相电机和六相电机的空载磁力线、气息磁密分布曲线及其谐波含量、电机启动时的速度曲线及转矩曲线。分析结果表明,相同结构的三相电机和六相电机的性能基本相同;与三相电机相比,六相电机的5次、7次谐波基本消除,有利于电机效率的提高。     

On-line rotor cage monitoring of inverter-fed induction machines bymeans of an improved method

This paper suggests a fault-detection technique to monitor defects such as cracked rotor bars in induction machines. It has been introduced as the Vienna monitoring method. Rotor bar faults cause an asymmetric magnetic flux pattern in the air gap. Thus, the current phasor (or voltage phasor at current-controlled machines), flux phasor and air-gap torque differ from those of an ideal symmetric machine. The Vienna monitoring method compares the outputs of a reference model, which represents an ideal machine, to a measurement model. Observing the deviations of these two models makes it possible to detect and even locate rotor faults. It can be applied to inverter-fed machines as no frequency analysis is used. The method is verified by online experimental results from a DSP-controlled IGBT inverter drive. The findings match the outcomes of a detailed machine simulation. Air-gap flux density evaluation by a measurement coil system proves both the excellent sensitivity and fault location ability of the proposed scheme     

Decoupling control of induction motor drives

The decoupling control of induction machines is investigated. Three different schemes for decoupling-control methods based on stator flux, airgap flux, and rotor flux field regulation are developed. The control dynamics of each scheme are outlined and studied. Simulation results are presented to verify that these schemes provide decoupling control with excellent dynamic behavior. The transient and steady-state relationships between slip frequency and torque, under constant stator flux, airgap flux, and rotor flux operations, are simulated and compared. The sensitivity characteristics of the three methods of flux-control, machine fed by impressed currents and voltages, are also compared and studied. A prototype torque-drive system is implemented to demonstrate the decoupling control of a squirrel-cage induction machine     

Acoustic noise radiated by PWM-controllel induction machine drives

This paper investigates the acoustic noise radiated from two nominally identical induction motors when fed from sinusoidal, and asymmetric regular sampling subharmonic and space-vector pulsewidth modulation (PWM) converters. The theory for analyzing the noise spectrum is developed further to account for the interaction between the motor and the drive. It is shown that manufacturing tolerances can result in significant differences in the noise level emitted from nominally identical motors, and that mechanical resonances can result in extremely high noise emissions. Such resonances can be induced by stator and rotor slot air-gap field harmonics due to the fundamental component of current, and by the interaction between the airgap field harmonics produced by the fundamental and the PWM harmonic currents. The significance of the effect of PWM strategy on the noise is closely related to the mechanical resonance with vibration mode order zero, while the PWM strategy will be critical only if the dominant cause of the emitted noise is the interaction of the fundamental air-gap field and PWM harmonics     

Design and analysis of permanent magnet-type bearingless motors

Magnetic bearings have been applied to high speed and high power electric machines for machine tools, turbomolecular pumps, etc. Bearingless motors can be expected to realize high speed and high power ratings because magnetic bearing functions are integrated into high-speed motors, which results in a simplified structure with short shaft length. In this paper, permanent magnet type bearingless motors, having built-in capability to achieve high power factor and high efficiency, are proposed. The shaft is suspended and centered by electromagnetic forces produced by currents in the additional radial force windings of the stator slots. At first the relationships of these radial forces, currents and voltages are derived analytically. Moreover, the relationships between radial forces and permanent magnet thickness are found. The optimal permanent magnet thickness is determined. The ratio of radial force over current as well as the peak air-gap flux density are discussed. These relationships are confirmed by prototype machines     

Total Airgap Flux Minimization in Dual Stator Winding Induction Machines

Multiphase machines are gaining attention in specific applications due to their increased reliability and the possibility to split the power between more than one inverter. Dual stator winding machines can be seen under certain conditions as two independent induction machines coupled by the rotor. To ensure control independence of both windings, airgap flux saturation must be avoided. Proper flux synchronization of both windings is required to achieve this goal while maximizing the torque per ampere ratio of the machine. The conditions to obtain proper orientation of the fluxes are discussed in this paper, and the inverter control scheme to perform such orientation is developed.     

A Single Phase Induction Motor Voltage Controller with Improved Performance

Induction motors inherently operate with nearly constant airgap flux and therefore almost constant iron losses. When the load does not require full flux, conventional voltage controllers utilize thyristors in series with the motor to reduce airgap flux by decreasing the applied voltage. Thereby, iron losses decrease and the overall efficiency increases. However, thyristor voltage controllers tend to introduce harmonics into the current waveform which not only reduces motor efficiency but also causes harmonic pollution of the power lines. An improved voltage controller and control strategy for efficiency improvement of single phase induction motors is presented. In particular, thyristor voltage control by dynamic switching of the winding configuration is presented. Laboratory data for a voltage controller, thus enhanced, demonstrates a significant decrease in input motor current distortion and increase in efficiency below one-quarter load.     

High-Temperature Superconducting Homopolar Inductor Alternator for Marine Applications

This paper describes a high power density high-temperature superconducting (HTS) electric machine topology that is scalable for marine propulsion and power generation. The design, currently being pursued for airborne applications, is based on homopolar inductor alternator (HIA) technology, which is new within HTS applications. The basic machine design configuration of the HTS HIA is based on a stationary HTS field excitation coil, a solid rotor, and an advanced but conventional stator comprising liquid-cooled air-gap armature winding and an advanced iron core. High power density is obtained by the enhanced magneto-motive force capability of the HTS coil, the increased airgap flux density and armature current loading, and the high tip velocity of the rotor. Preliminary scaled up designs look attractive for three marine applications: propulsion drive, primary ship power generation, and power generation modules. The generators are driven directly by the turbines without the additional complexity of a clutch and gear system. A conceptual design study of a 36-MW 3600-r/min generator, a 4-MW 7000-r/min auxiliary generator, and a 36-MW 120-r/min and 4-MW 132-r/min propulsion motor are summarized.     

基于感应电机电流谐波频谱分析的无速度传感器的速度辨识

基于电流谐波频谱分析的无速度传感器速度辨识方法提高了速度辨识性能,尤其是在低频情况下,这种速度辨识方法优点更为突出。由转子斜槽和转子偏心率产生的谐波和速度信息相关,此信号可以通过数字信号处理获得。这些谐波存在于任何非零转速情况下,且与随时间变化的参数(如定子绕组电阻)无关。频谱估计可以对决定转子速度的多个电流谐波进行分析,这与滤波分析方法或快速傅里叶变化的方法相比可以使速度检测对噪音不敏感、检测精度更高。在线的初始化程序可以求出槽谐波计算所需的电机的特定参数。实验证明在频率低于1 Hz时,此速度辨识方法仍可以为参数调整或为磁场定向驱动的无传感器的磁链观测器提供鲁棒的、参数独立的速度信息,此外这种算法的性能在宽范围的变频和整个负载状态下均已做了验证。     

Sensorless speed measurement using current harmonic spectralestimation in induction machine drives

This paper proposes a sensorless speed measurement scheme that improves the performance of transducerless induction machine drives, especially for low-frequency operation. Speed-related harmonics that arise from rotor slotting and eccentricity are analyzed using digital signal processing. These current harmonics exist at any nonzero speed and are independent of time-varying parameters, such as stator winding resistance. A spectral estimation technique combines multiple current harmonics to determine the rotor speed with more accuracy and less sensitivity to noise than analog filtering methods or the fast Fourier transform. An on-line initialization routine determines machine-specific parameters required for slot harmonic calculations. This speed detector, which has been verified at frequencies as low as 1 Hz, can provide robust, parameter-independent information for parameter tuning or as an input to a sensorless flux observer for a field-oriented drive. The performance of the algorithm is demonstrated over a wide range of inverter frequencies and load conditions     

Sensorless position control of induction motors-an emergingtechnology

Concepts for the sensorless position control of induction motor drives rely on anisotropic properties of the machine rotor. Such anisotropies can be incorporated as periodic variations of magnetic saliencies in various ways. The built-in spatial anisotropy is detected by injecting a high-frequency flux wave into the stator. The resulting stator current harmonics contain frequency components that depend on the rotor position. Models of the rotor saliency serve to extract the rotor position signal using phase-locked loop techniques. A different approach makes use of the parasitic effects that originate from the discrete winding structure of a cage rotor. It has the merit of providing high spatial resolution for incremental positioning without sensor. The practical implementation of sensorless position identification and of a high-accuracy position control system are reported     

Fault diagnosis of active magnetic bearings

Active magnetic bearings (AMBs) are intrinsically unstable systems and require feedback control to ensure stable operation. Further, sensors, actuators, and the rotor need to operate under normal conditions, and a fault detection and diagnostics system is necessary to ensure a safe and reliable operation. Accordingly, several studies have developed methods to detect failures associated with the rotor or the electrical system (i.e., AMB). However, prior identification of the dynamic system parameters or the magnetic forces is usually desired, which can be impractical for real machines. To overcome this problem, this study proposes a failure detection method based on a mathematical model and the correlation between the measured states related to the rotor and the control. Artificial neural networks are used to map the states that cannot be measured, and faults are determined by comparing the output correlations of neural networks. Faults in the AMB/rotor system are identified considering various rotor unbalance configurations (mechanical failures) and failures in the position sensor gain and in the magnetic actuator current (electrical failures). Various fault configurations were explored for each case cited. A comparison of the theoretical and experimental results showed good agreement, which demonstrates the adequacy of the method in detecting mechanical and electrical failures in industrial machines.     

A high-efficiency magnetic suspension actuator with reluctance-balanced permanent magnet biasing and flux-based control

To reduce power consumption of active magnetic bearings (AMBs), permanent magnets (PMs) may be employed to generate a bias flux through the magnetic circuits connecting the actuators with the rotor. In this way, control algorithms based on zero-current operating points and linearized models may be used. This paper describes a new modular design of PM-biased AMB actuator involving a balanced-reluctance topology where the bias flux is set by an auxiliary air gap within the magnetic circuit. A theoretical model and design methodology for achieving a specified range of actuation forces are also defined. Each actuator unit within an AMB suspension system may by positioned freely and operated independently because, unlike many previous PM-biased designs, there is no flux linkage between actuators. To improve the robustness of the actuator control to uncertainty in material properties and air gap sizes, a feedback control scheme is proposed based on inductive sensing of the mean flux density at the actuator pole faces. Testing of the actuator with a current-cancelling flux-based control scheme shows that stable suspension with very low power consumption can be achieved over a range of operating conditions involving both static loading and unbalance excitation.     

Characteristics and control of a bidirectional axial gap combinedmotor-bearing

Introduces the design characteristics of a bidirectional axial gap combined motor-bearing where the flat disc motor has both rotation and axial position control capability. This motor consists of a disc rotor with a stator on each side of the rotor. The axial motion of the rotor is actively controlled while the other axes are constrained by additional passive or active radial magnetic bearings. Each stator produces a rotating magnetic flux in the air gap, to generate the motor torque. The axial force is controlled by changing the amplitude of the rotating flux. Both permanent-magnet motor and induction motor versions were analyzed theoretically and tested experimentally. The results demonstrated the capability of providing both the functions of a motor and a magnetic bearing     

Time-optimal single-step velocity response control scheme forfield-oriented induction machines considering saturation level

A time-optimal single-step velocity response control scheme of field-oriented induction machines, taking magnetic saturation of rotor flux into account, is implemented based on the analytical solutions of the time-optimal control problem. High velocity response is achieved in spite of field-weakening for low acoustic noise or high-efficiency drives. The equation of the optimal control law is derived, and the look-up table is obtained. The optimal controls scheme is realized by simply adding the look-up table to the flux controller of the variable flux field-oriented control system. Simulation is carried out to verify feasibility of the proposed control algorithm. Problems of the overshoot of the rotor speed and the constraint on rate of change of acceleration or deceleration at the instant of switching from the time-optimal control to the conventional field-oriented control are discussed. The experimental results agree well with the simulation results, and show satisfactory dynamic and steady-state performance on both start-up and acceleration/deceleration     

Corrosion Model of a Rotor-Bar-Under-Fault Progress in Induction Motors

A corrosion model of a rotor-bar-under-fault progress in induction motors is presented for simulations of induction machines with a rotor-bar fault. A rotor-bar model is derived from the electromagnetic theory. A leakage inductance of the corrosion model of a rotor bar is calculated from the relations of magnetic energy, inductance, current, and magnetic-field intensity by Ampere's law. The leakage inductance and resistance of a rotor bar varies when the rotor bar rusts. In addition, the skin effect is considered to establish the practical model of a rotor bar. Consequently, the variation of resistance and leakage inductance has an effect on the results of motor dynamic simulations and experiments, since a corrosive rotor bar is one model of a rotor bar in fault progress. The results of simulations and experiments are shown to be in good agreement with the spectral analysis of stator-current harmonics. From the proposed corrosion model, motor current signature analysis can detect the fault of a corrosive rotor bar as the progress of a rotor-bar fault. Computer simulations were achieved using the MATLAB Simulink with an electrical model of a 3.7-kW, three-phase, and squirrel-cage induction motor. Also, experimental results were obtained by real induction motors, which had the same specification as the electrical model used in the simulation     

Design and control of active magnetic bearing system with Lorentz force-type axial actuator

As the size of five-axes active magnetic bearing (AMB) system gets smaller, the space limitation for installation of axial magnetic bearing unit and the eddy current induced braking of radial magnetic bearing unit become a stringent design concern. In this paper, a new type of compact, high-performance five-axes AMB with solid cores and rotor is proposed, which consists of four permanent magnets, four U-shaped cores and 16 control coils. It features that the radial and axial magnetic bearing units are integrated for compact design and that the homo-polar type configuration of poles with optimized pitch length is adopted to minimize the eddy current induced braking force. The proposed homo-polar AMB system is levitated by the Lorentz-type axial as well as Maxwell-type radial forces. Based on the magnetic flux distribution analysis, the control algorithm is designed to account for the coupled effect between the radial and axial control fluxes. Experiments are also carried out with a prototype AMB system to validate the new design concept.     

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

The 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 of induction motor with free-rotating magnets insideits rotor

This paper presents a new induction motor that has free-rotating magnets inside a rotor. The magnets can revolve freely against the rotor with the shaft. In this motor, the airgap flux is provided by both rotating permanent magnets and the stator coil current. A prototype motor was fabricated by modifying the rotor of a conventional three-phase four-pole 400 W squirrel-cage induction motor. The experimental results of a prototype motor showed superior performance in comparison to conventional motors in terms of the power factor, efficiency and torque characteristics. The power factor can be controlled to be unity, leading or lagging by changing the supply frequency and/or source voltage. The efficiency of the motor, over a wide output power range, is remarkably higher than that of the same size conventional induction motor. A high torque can be obtained in a high speed area