Stator Current Harmonics Research Articles

Control and Performance Evaluation of Multiphase FSPM Motor in Low-Speed Region for Hybrid Electric Vehicles

The flux-switching permanent-magnet (FSPM) motor has been viewed as a highly reliable machine with both armature windings and magnets on the stator. Owing to the high torque-production capability with low torque ripple, FSPM motors with a higher number of phases are potential candidates for traction applications in hybrid electric vehicles (HEVs). However, existing research has mostly focused on the principles and static performance of multiphase FSPM motors, and little attention has been paid to advanced control strategies. In this paper, the fully decoupled current control of a 36/34-pole nine-phase FSPM (NP-FSPM) motor is developed and the performance under different operating conditions is investigated. The aim of the design is to alleviate cross coupling effects and unwanted low-order stator harmonic currents, to guarantee fast transient response and small steady-state error. In addition, its fault-tolerance is further elaborated. These features are very important in automotive applications where low torque pulsation, high fault-tolerant capability and high dynamic performance are of major importance. Firstly, the research status of multiphase FSPM motors is briefly reviewed. Secondly, the mathematical model in the dq reference frames and control strategies are presented. Then, the control and performance of the NP-FSPM motor are evaluated by using MATLAB/Simulink. Finally, experiments on an NP-FSPM motor prototype are carried out to validate the study.

Advanced model of IM including rotor slot harmonics

Purpose – The purpose of this paper is to present a detailed advanced dynamical model of induction machine (IM) with unskewed rotor bars, including rotor slot harmonics. Design/methodology/approach – Procedure of IM modeling using results from finite element analysis (FEA). Series of magneto-static FEA simulations are used to obtain matrix of IM inductances as a function of rotor angular position and geometry. Each element in this matrix is represented by Fourier series (FS) and incorporated in proposed dynamical model. Using or neglecting various elements in FS of inductance matrix may be useful for determining which component of the series has dominant influence on harmonic content of stator currents, torque ripple or speed variation. The usefulness of application of presented model is verified comparing with time-stepping FEA simulations. Findings – Although the model is not suitable for usage in on-line regulation of IM drives, but the results of simulations may be used to thoroughly explain origins of higher order harmonics in stator currents of IM and help improve sensorless speed estimation algorithms and fault diagnostics. Originality/value – This paper shows an approach to the modeling of IM which includes effects of non-uniform air gap and non-sinusoidal distributions of magneto-motive forces. Inductance matrix elements are complex functions of rotor position, geometry and winding distributions and it gives an opportunity for detail analysis of IM behavior in numerous applications.

Modified switching‐table strategy for reduction of current harmonics in direct torque controlled dual‐three‐phase permanent magnet synchronous machine drives

The switching-table-based direct torque control (ST-DTC) strategies of dual-three-phase permanent magnet synchronous machine (PMSM) drives are investigated in this study. Owing to its inherited advantage of suppression of sixth harmonic torque pulsation, dual-three-phase machine has attracted more and more interests among various sorts of multiphase machines. However, unexpected stator harmonic currents are often observed in the classical ST-DTC strategy for dual-three-phase PMSM drives, which cause large losses and decrease the efficiency of the drive system. An optimised ST-DTC strategy which consists a two-step process to determinate the most appropriate inverter voltage vector is proposed to reduce the stator harmonic currents. The merits of the classical DTC, that is, simple structure and good dynamic performance, are still preserved. The experimental results verify that the proposed method can significantly reduce the current harmonics.

DFIG Harmonic Current Controlling with the Grid Low Harmonic Voltage

This study presents a vector control strategy based on stator harmonic current closed-loop, it adds individually the control loop about of each stator harmonic current to restrain the stator harmonic current, in order to meet the THD criteria. The control strategy of restraining the harmonic current presents the design of the stator harmonic current restrains the current controller. It influences the rotor voltage of the stator harmonic current restraining strategies.

Broken bar indicators for cage induction motors and their relationship with the number of consecutive broken bars

The study presents analysis of the broken bar induction machine, through numerical, analytical and experimental investigations. Numerical finite element analysis is carried out with a commercial software in which coupling between the windings circuit equations and equations of the magnetic field has been implemented and verified through intensive measurements. An analytical model has also been constructed to understand and explain the origin of different frequencies detected in the case of broken bars. The analytical model enabled prediction of frequencies, in a new manner, when the number of consecutive broken bars increases. The novelty of this study arises from derivation of an expression linking stator current harmonics (using the wave number of a broken cage), slip harmonic and saturation harmonic, which is closely related to the number of consecutive broken bars. The model also takes into account the speed ripple. This expression has been validated by both finite element model (FEM) and measurements on a 22 kW induction machine with healthy and faulty rotors.

Sliding‐mode control of a wind turbine‐driven double‐fed induction generator under non‐ideal grid voltages

Control algorithms for the rotor- and grid-side power converters of a double-fed induction generator (DFIG)-based wind turbine under non-ideal grid voltage conditions are proposed, and guidelines for tuning the controller parameters are presented. The control schemes are based on sliding-mode control (SMC) theory. Apart from directly controlling the DFIG's average active and reactive powers, the proposed methods also fulfil two additional control targets during voltage unbalance and harmonic distortion, that is, the rotor-side converter (RSC) eliminating electromagnetic torque fluctuations and the grid-side converter (GSC) compensating for the stator current harmonics to ensure a sinusoidal total current from the overall generating unit. The described control strategies are proved to be robust against parameter deviations and of fast dynamic response. In spite of the discontinuous nature of the standard SMC, constant converter switching frequency is achieved. Besides, the RSC control algorithm does not require a phase-locked loop and, furthermore, there is no need for decomposing the grid voltage and different currents into symmetrical sequences or harmonic components in any of the converters’ control systems. Finally, the excellent performance of the system, as well as its robustness, is verified by means of simulation results under different grid voltage conditions.

High-Efficiency Current Excitation Strategy for Variable-Speed Nonsinusoidal Back-EMF PMSM Machines

Opportunities for energy efficiency improvements in motor drives remain a forefront topic of interest in recent times due to their extensive use and applications. Then, it is the right juncture to revisit various stator current excitation strategies and their impact on the efficiency of brushless dc (BLDC) machines favored in high volume variable speed applications. BLDC machines with nonsinusoidal back electromotive force (EMF) are considered in this paper, and the influence of sine, square, and nonsinusoidal harmonic current on their performance is investigated. To facilitate such a study, analytical expressions for the generalized back EMF of all the three excitation schemes have been derived to highlight their influence on torque ripple and motor harmonic losses, thus leading to efficiency evaluation. An analytical approach on the estimation of iron losses under loaded operation has been elaborated to highlight the impact of stator current harmonics on the efficiency of the motor. Analytical as well as experimental results show that it is advantageous to use the nonsinusoidal harmonic injection (NSHI) scheme in the speed range until the sum of the drive conduction and stator resistive losses exceeds the iron losses of the machine. When iron losses become dominant, the algorithm is modified to sinusoidal current control, thus maintaining higher efficiency operation. To achieve such an operation, a hybrid control scheme encompassing sinusoidal to NSHI field-oriented sensorless controller has been implemented.

Stator Current Harmonic Control With Resonant Controller for Doubly Fed Induction Generator

Voltage harmonics in the grid can introduce stator current harmonics in a doubly fed induction generator (DFIG) wind turbine system, which may potentially impact the generated power quality. Therefore, wind turbine current controllers need to be designed to eliminate the impact of grid voltage harmonics, especially low-order harmonics. This paper proposes a stator current harmonic suppression method using a sixth-order resonant controller to eliminate negative sequence fifth- and positive sequence seventh-order current harmonics. A stator current harmonic control loop is added to the conventional rotor current control loop for harmonic suppression. The overall control scheme is implemented in dq frame. Based on a mathematical model of the DFIG control system, the effects on system stability using the resonant controller, an analysis of the steady-state error, and the dynamic performance, are discussed in this paper. Taking these effects into account, the parameters of the resonant controller can be designed and effectively damp the influence from the grid voltage harmonics. As a result, the impacts of the negative sequence fifth- and positive sequence seventh-order voltage harmonics on the stator current as well as the electromagnetic torque are effectively removed. Simulations and experiments are presented to validate the theoretical analysis.

Dual-Rotor Multiphase Permanent Magnet Machine With Harmonic Injection to Enhance Torque Density

Torque density of a novel dual-rotor, radial-flux multiphase permanent magnet (PM) machine is enhanced by injecting stator current harmonics without increasing stator yoke thickness. In order to enhance a regular PM machine torque capability by injecting harmonic currents, the no-load air-gap flux density should be designed to be trapezoidal, which increases the flux per pole, and results in a thicker stator yoke. However, this drawback can be avoided in a dual-rotor, radial-flux PM machine. In such a dual-rotor PM machine, the main flux can be designed to go directly from the inner rotor poles, to stator inner and outer teeth, then to the outer rotor poles, or reverse, without through the stator yoke. Therefore, the stator yoke thickness is not affected by air-gap flux at all and can actually be reduced to minimum for mechanical reasons. This paper applies stator current harmonic injection technique to a dual-rotor, radial-flux PM machine. Sizing equations of the multiphase PM machines are deduced and used for initial design and theoretical comparison. The torque performance of the proposed dual-rotor, radial-flux multiphase PM machine with stator harmonic current injection is evaluated using finite element simulation, and compared to a traditional three-phase dual-rotor, radial-flux PM machine. Simulation results validate the feasibility of torque density enhancement. The same technique is proven that it is also suitable in a dual-rotor axial-flux PM machine topology.

Torque Ripple Reduction of the Torque Predictive Control Scheme for Permanent-Magnet Synchronous Motors

The direct torque control (DTC) technique of permanent-magnet synchronous motors (PMSMs) receives increasing attention due to its advantages in eliminating the current controllers and quicker dynamic response, compared with other motor control algorithms. However, high torque and stator flux ripples remain in the system when using DTC technologies. This means large stator voltage and current harmonic contents exist in the PM motors. Since the variation of motor electromagnetic torque is related to the voltages that are applied to the motor, by analyzing the relationships between stator flux, torque, and voltages, a PMSM torque predictive control scheme is proposed in this paper. In each digital signal processor cycle, the optimized voltage is utilized to reduce torque ripple, and the voltage vector angle is determined by the output of torque and flux hysteresis controllers. The proposed scheme is simulated and experimentally verified. Both simulation and experimental results have shown that low torque ripple and reduced stator current harmonics are achieved by using the proposed scheme.

A Novel Direct Torque Control Permanent Magnet Synchronous Motor Drive used in Electrical Vehicle

In this paper, a modified direct torque control (DTC) scheme for permanent magnet synchronous motor (PMSM) is investigated, which enables low torque ripple by using an improved voltage vector selection strategy instead of switching table used in conventional DTC. Based on the control of stator flux, torque angle and torque, voltage vector selection strategy of PMSM DTC drive is proposed. In the proposed voltage vector selection strategy, the applied voltage vector is determined according to outputs of hysteresis comparators for stator flux and torque, angular position of stator flux and torque angle, which is finally synthesized by space vector modulation (SVM). Modeling and experimental results for an interior PMSM used in Honda Civic 06My Hybrid electrical vehicle are given. Simulation and experimental results show torque ripple is reduced and the total harmonics of stator current is decreased when compared those of conventional DTC. And a fixed switching frequency is obtained with the help of SVM. In addition, the proposed DTC doesn’t need any additional PI controller, which maintains the simplicity in conventional DTC. DOI: http://dx.doi.org/10.11591/ijpeds.v1i2.141 Keywords : direct torque control, permanent magnet synchronous motor, electrical vehicle, torque ripple, switching frequency

Eddy-Current Loss Prediction in the Rotor Magnets of a Permanent Magnet Synchronous Generator With Modular Winding Feeding a Rectifier Load

In a permanent magnet synchronous generator (PMSG) with modular winding, significant eddy current may be induced in the rotor magnets due to asynchronous rotating stator magneto-motive forces (MMFs), and a rectifier load may signify the situation further. The eddy-current loss prediction in the rotor magnets of a PMSG with modular winding feeding a rectifier load is described. An analytical method considering the stator current harmonics and stator MMF spatial harmonics and a time-stepping, coupled-circuit, 2-D finite-element method (FEM) are adopted. The predicted losses obtained from these two methods are compared and investigated.

Research on an Improved Control Strategy for Harmonics Restraint of DFIG System

An improved control strategy for harmonics restraint of doubly fed wind power system is studied in this paper. Considered low order current harmonics (5th, 7th and so on) caused by the distortion or unbalanced grid voltage conditions and the variations of generator itself parameters, an improved control strategy is proposed. In this control strategy, a proportional integral (PI) regulator paralleled with a harmonic resonant (R) controller in the synchronous reference frame is proposed for restraining the stator current harmonics. The control strategy presented in this paper has not effect on the control of fundamental current, and need not the harmonic separation filter and the sequence decomposition in multi reference frames. To verify the effectiveness of this control strategy, simulation model of DFIG system is developed in Matlab/Simulink, and experiments are implemented on the doubly fed experiment platform. The results of both simulations and experiments show that the control strategy proposed in this paper could restrain harmonic currents effectively and it is easy to implement digital control.

Active Filtering of DFIG Stator and Rotor Current Harmonics Caused by Distorted Stator Voltages

Power quality requirements related to renewable energy systems and distributed generation are becoming more restrictive, especially in the total harmonic distortion of the injected grid current. The doubly-fed induction generator is particularly appropriate to be used in wind power generation systems. However, in weak grids its stator connection to unbalanced or distorted voltages not only causes harmonic currents in the stator but also imposes low frequency and high amplitude rotor current harmonics. Also, the grid-side converter must deal with the same voltage conditions. The paper addresses the first issue presenting a voltage harmonics controller in the d-q reference frame to be included in the high dynamics rotor current controller. Simulation and experimental results demonstrate the performance of the proposed controller imposing a sinusoidal input current

Stator Harmonic Currents in VSI-Fed Synchronous Motors With Multiple Three-Phase Armature Windings

Electric motors equipped with multiple three-phase windings are often used for the advantages they offer in terms of reliability, performance, and inverter power segmentation. When the windings are fed by independent voltage-source inverters (VSIs), circulation harmonic currents can occur in stator phases. If the motor is an induction machine, circulation currents are known to originate form transient imbalances in the applied voltages due to inverter switching. This paper investigates the problem when a synchronous machine is used and shows how additional (and possibly major) sources for circulation currents can arise in this case (even with a round-rotor design) due to the nonsinusoidal air-gap flux distribution. The phenomenon is illustrated for a 45-MW quadruple three-phase synchronous motor supplied by four medium-voltage (MV) multilevel VSIs. Its circulation currents are predicted with two alternative methods, i.e., analytically and from time-stepping finite-element simulation. The results obtained in both ways are shown to well match measurement results collected on the actual motor during full-load system testing.

Analytical Determination of DC-Bus Utilization Limits in Multiphase VSI Supplied AC Drives

Two-level multiphase voltage source inverters (VSIs) are typically used as the supply for multiphase machines. Such machines are often with concentrated winding in which case, certain low-order stator current harmonics are injected to provide torque enhancement. Also, a multimotor drive system can be realized by connecting a number of multiphase machines in series, using phase transposition while supplying the whole system from a single multiphase VSI. In both situations, it is important to know the limits of the inverter operation in the linear modulation region. This paper develops a simple method that enables analytical determination of the boundaries of the linear modulation region for all multiphase inverters with a prime number of phases. The limits are independent of the actual pulse width modulation (PWM) method utilized, and are equally applicable to both carrier-based and space vector PWM techniques. Theoretical considerations are verified experimentally using the five-phase and seven-phase VSI rigs.

A New PWM Strategy Based on a 24-Sector Vector Space Decomposition for a Six-Phase VSI-Fed Dual Stator Induction Motor

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents a new space vector pulsewidth modulation (SVPWM) technique for the control of a six-phase voltage source inverter (VSI)-fed dual stator induction machines (DSIM). A DSIM is an induction machine which has two sets of three-phase stator windings spatially shifted by 30 electrical degrees and fed by two three-phase VSIs. Despite their advantage of power segmentation, these machines are characterized by large zero sequence harmonic currents, and in particular those of order <formula formulatype="inline"> <tex>$6k\pm 1$</tex></formula>, which are due to the mutual cancellation between the two stator windings. The proposed SVPWM scheme, while easy to implement digitally, reduces significantly these extra stator harmonic currents. Experimental results, collected from a 15 kW prototype machine controlled by a digital signal processor, are presented and discussed. </para>

Analytical Prediction of Eddy-Current Loss in Modular Tubular Permanent-Magnet Machines

The paper describes an analytical technique for predicting the eddy-current loss in the moving armature of a tubular permanent magnet machine. This loss component is usually neglected in conventional tubular permanent magnet machines since high-order time harmonics in the stator current waveform and space harmonics in the winding magnetomotive force (MMF) distribution are generally considered to be insignificant. However, a relatively new topology of tubular permanent magnet machine, sometimes referred to as "modular", has emerged in which the fundamental component of the stator MMF has fewer poles than that of the permanent-magnet armature, the thrust force being developed by the interaction between a higher order MMF harmonic and the permanent magnet field. Thus, the presence of lower and higher order space harmonics in the winding MMF distribution of a modular machine may gives rise to a significant eddy-current loss in the moving-magnet armature. An analytical model is developed to predict the eddy currents which are induced in the magnets, as well as in any electrically conducting supporting tube which may be employed, and to quantify the effectiveness of axially segmenting the magnets in reducing the eddy-current loss. The validity of the developed model, which is also applicable to conventional designs of tubular permanent-magnet machine, is verified by time-stepped transient finite-element analysis (FEA).

Very low speed sensorless control of induction motor drives using high‐frequency signal injection

This paper addresses the control issues of speed sensorless induction motor drives at very low speeds, 1 r/min to 30 r/min. Low voltage high‐frequency signals superimposed on the two‐axis voltage commands in the synchronously rotating reference frame are injected into the motor stator. Due to inherent parasitic saliencies of the cage rotor, magnetic saturation in particular, the resultant stator current harmonics contain frequency components that depend on the synchronous frequency. Band‐pass and low‐pass filtering techniques combined with coordinate transformations are used to extract the synchronous angular position embedded in the stator current harmonics. The rotor speed can be computed by subtracting the estimated slip frequency from the extracted synchronous frequency. The experimental results further show that effective speed sensorless control can be achieved at very low speed operating range.

The influence of the harmonics on the temperature of electrical Machines

In this paper, we study the temperature rise in an asynchronous motor with nonsinusoidal voltage feeding. The stator and rotor current harmonics are calculated using a time step method. Inverse problem techniques are used to evaluate the homogenized electromagnetic and thermal characteristics of the stator windings. Temperatures in sinusoidal approximation and harmonic currents are compared with experimental measurements.