Axis Current Research Articles

Decoupled d–q Axes Current-Sharing Control of Multi-Three-Phase Induction Machines

Multi-three-phase drives are a particular case of multiphase systems, which are often used in high-power applications, such as wind-energy generation, naval propulsion, and railway propulsion. In a multi-three-phase system, the electric machine is fed by more than one three-phase converters. A current-sharing algorithm for multi-three-phase drives allows setting unequal current references among the converters so that each of them differently contributes to the generation of the magnetic torque and flux. Suitable current-sharing control systems already exist and have been presented for multi-three-phase machines. In this article, we illustrate a current-sharing technique where the contributions to the rotor flux for the three-phase inverters, related to the d -axis current, are decoupled from the contributions to the electromagnetic torque, which depends on the q -axis current. Also, the presented algorithm minimizes the joule losses in the stator winding. Finally, the advantages of the proposed method are analyzed and confirmed by experimental tests. The effectiveness of the control strategy is validated on a scaled prototype of a quadruple three-phase starter/generator for More-Electric-Aircraft applications.

A Novel Hybrid Control Method for Single-Phase-Input Variable Frequency Speed Control System With a Small DC-Link Capacitor

A novel hybrid control method, which consists of dual-frequency tracking control (DFTC) and virtual inertia control (VIC), is proposed for single-phase-input variable frequency speed regulation system with a small dc-link capacitor. Because of lacking a large-capacity energy storage device, the dc-link voltage is a pulsating dc, which causes a strong power coupling relationship between the rectifier and inverter side. The instantaneous power and dq -axis currents of the inverter contain both ac and dc components at the same time, which cannot be controlled well using traditional proportional-integrational (PI) controllers. Herein the DFTC that is based on a PI-resonant controller is proposed to accurately track ac and dc components of the instantaneous power and dq -axis currents respectively. With higher open-loop gain at both low frequency and the specified resonant frequency, the proposed DFTC ensures accurate tracking of the ac and dc components of power and dq -axis currents, high-power-factor, and low total harmonic distortion (THD) of input grid current. To maintain the stability of the dc-link voltage under step changes of load, a VIC is proposed. Subsequently, the input power factor and the fluctuation of dc-link voltage can be improved in the system. Simulations and experimental results are given to provide further validation of the proposed DFTC and VIC under different operating scenarios.

A Time-Delay Compensation Method for IPMSM Hybrid Sensorless Drives in Rail Transit Applications

This paper proposes a hybrid sensorless control strategy for rail transit application employing interior permanent magnet synchronous machine (IPMSM), which works under low switching frequency. Due to a long system time delay, the convergence rate of estimated position in the sensorless dynamic region is reduced. As the speed increases, the dq axes currents coupling is more severe, and deteriorates the control performance. In order to solve these problems, the analysis of time-delay effect on the IPMSM sensorless drives is first derived. Then, using q-axis current error, a global time-delay compensation method is designed to eliminate the lag angle of estimated position. Furthermore, the effect of this compensation on audible noise reduction is also analyzed. Through a phase-locked loop, the hybrid estimated position is obtained from a normalized position error. Finally, a 3.7-kW IPMSM is tested to verify the feasibility of the proposed sensorless method.

Vector Current Control Derived from Direct Power Control for Grid-Connected Inverters

We propose a vector current control derived from direct power control (VCC-DPC) for a three-phase voltage source inverter (VSI) in the synchronous rotating frame through instantaneous real and reactive powers. The proposed VCC-DPC method has the same control structure as the conventional VCC except for the coordinate transformation, since we obtain the $d$ – $q$ axes currents model of VSI without using Park transformation and the phase-locked loop (PLL) system. Consequently, the proposed method has the same property as the conventional VCC if the PLL extracts the phase angle of the grid voltage correctly. However, with the consideration of the slow dynamics of the PLL, the proposed method has an enhanced dynamical performance feature compared with the conventional VCC. Moreover, it has another benefit that the reduction of the computational burden could be expected since there is no Park transformation and the PLL in the controller implementation. We can guarantee that the closed-loop system with the proposed method is exponentially stable in the operating range. Finally, both simulation and experimental results using a 15-kW-inverter system match the theoretical expectations closely.

Dual Antiovervoltage Control Scheme for Electrolytic Capacitorless IPMSM Drives With Coefficient Autoregulation

Electrolytic capacitorless vector-controlled interior permanent-magnet synchronous motor (IPMSM) drives have many advantages, such as higher reliability, longer lifetime, and lower cost. However, the dc-link may suffer from the overvoltage during motor transient process due to the use of slim film capacitors in the diode rectifier front end. The conventional antiovervoltage method by controlling the quadrature axis current will lead to energy control error, which causes large dc-link voltage control error. In this paper, a novel antiovervoltage control scheme with coefficient autoregulation is proposed based on the concept of system loss. A dual antiovervoltage controller is designed to enhance the voltage control performance by controlling both the direct and quadrature axis currents to achieve more effective active braking. The coefficients of the dc-link voltage controller are autoregulated to maintain the enough stability margin during the regenerative braking. By using the proposed method, the dc-link overvoltage phenomenon can be avoided while guaranteeing the maximum copper loss during the braking process. The proposed algorithm is ultimately verified on an electrolytic capacitorless IPMSM drive.

Output power quality enhancement of PMSG with fractional order sliding mode control

This paper proposes a fractional order sliding mode control (FOSMC) method for permanent magnet synchronous generator (PMSG) to enhance its output power quality. The control scheme concludes the machine side converter (MSC) control and the grid side converter (GSC) control. In the MSC, the rotor side d-q axis currents are controlled by a novel fractional order sliding mode controller; in the GSC control, the output voltage and the DC-link voltage of the GSC are regulated by another fractional order sliding mode controller. Moreover, a stability analysis based on Lyapunov function is conducted to decide the controller coefficient boundaries. The gravitational search algorithm is used to obtain the optimal parameters. Subsequently, three simulations and three experimental case studies are conducted to show the effectiveness of FOSMC, and the performance of FOSMC is compared with conventional sliding mode control and proportional integral control methods. The simulation and experiment results reveal that FOSMC has faster time response, superior tracking precision in both steady state and variable wind speed conditions, and it shows stronger robustness against parametric disturbances over SMC and PI control.

Maximum Torque Per Ampere (MTPA) Control for IPMSM Drives Based on a Variable-Equivalent-Parameter MTPA Control Law

This paper presents a maximum torque per ampere (MTPA) control method for interior permanent-magnet synchronous motor drives. The proposed method employs a variable-equivalent-parameter MTPA control law to calculate the MTPA current references. Different from conventional MTPA control laws, this novel MTPA control law equivalently considers both the influence of the motor parameter values as well as the influence of the derivatives of motor parameters with respect to d - and q -axis currents on the MTPA control. Therefore, the proposed method can achieve the accurate MTPA control under different load conditions. In addition, this novel MTPA control law avoids the computation of derivatives of motor parameters and the complicated computations that exist in conventional MTPA control laws. Thus, the proposed method can provide the MTPA current references easily and in real time, allowing for fast MTPA control to be achieved. Both simulation and experimental results confirm the effectiveness of the proposed MTPA method.

State Feedback Control for a PM Hub Motor Based on Gray Wolf Optimization Algorithm

This paper presents an optimal control strategy for a permanent-magnet synchronous hub motor (PMSHM) drive using the state feedback control method plus the gray wolf optimization (GWO) algorithm. First, the linearized PMSHM mathematical model is obtained by voltage feedforward compensation. Second, to acquire satisfactory dynamics of speed response and zero d -axis current, the discretized state-space model of the PMSHM is augmented with the integral of rotor speed error and integral of d -axis current error. Then, the GWO algorithm is employed to acquire the weighting matrices Q and R in linear quadratic regulator optimization process. Moreover, a penalty term is introduced to the fitness index to suppress overshoots effectively. Finally, comparisons among the GWO-based state feedback controller (SFC) with and without the penalty term, the conventional SFC, and the genetic algorithm enhanced proportional–integral controllers are conducted in both simulations and experiments. The comparison results show the superiority of the proposed SFC with the penalty term in fast response.

Sizing Optimization Methodology of Tidal Energy Conversion Chain Based on Double Stator Permanent Magnet Generator

This paper presents a variable speed direct drive generator optimization methodology for tidal current energy application based on double stator permanent magnet generator. Maximizing annual energy output and minimizing the generator-converter system cost are the two objectives of the optimization. Generator-converter system cost includes the generator raw material cost, structure cost and converter cost. Particle swarm optimization algorithm is adopted for this multi-objectives optimization. The generator torque-speed operating profile is obtained from the predictable tidal current source and turbine characteristics. 16 parameters including machine geometry and power electronic converter size are optimized over the full operation range rather than only for rated condition. The two stators of the generator are controlled in parallel with dq axis currents which minimize the total power losses of generator and converter. Constraints are introduced to guarantee that optimization solutions are realizable and will not reach electrical limitations, magnetic saturation and thermal limit over the whole operating torque speed profile. Comparing with single stator generator by using the same optimization process, double stator generator is proved more interesting for tidal energy application due to the machine volume is sharply reduced.

Flux-Density Harmonics Analysis of Switched-Flux Permanent Magnet Machines

By developing a simple permeance-magnetomotive force (MMF) model of switched-flux permanent magnet (SFPM) machines, the air-gap flux density produced by both PMs and armature current can be derived, in which harmonics with the same order and rotational speed are called an effective harmonic pair (EHP). By investigating the influences of armature current angle $\delta $ on both the phase and amplitude of each EHP, it is found that the amplitudes of both PM and armature reaction flux-density harmonics maintain fixed, whereas the space phase shift between them changes accordingly with armature current angle. Specifically, the PM and armature reaction flux-density harmonics are orthogonal in space if zero ${d}$ -axis current is fed. Therefore, the maximal torque is realized for each EHP. As the total torque of SFPM machines is the superposition of the contributions by each EHP, the zero ${d}$ -axis current control method turns out to be the optimum for maximal torque per ampere, thus verifying analytically that the ${d}$ - and ${q}$ -axes inductances are equal according to the general torque equation for the investigated machine topology. In addition, the torque adjustment mechanism of each EHP in SFPM machines has also been analytically demonstrated to be resemble that of the surface-mounted PM synchronous machine (PMSM). Finally, the finite-element analysis (FEA) has been performed to validate the previous analytical predictions.

Transient Stability Augmentation of a Wave Energy Conversion System Using a Water Cycle Algorithm-Based Multiobjective Optimal Control Strategy

This paper introduces a novel optimum design of the proportional–integral (PI) controller in the power converter circuits using the water cycle algorithm (WCA) to augment the transient stability of a grid-connected wave energy conversion (WEC) system. The proposed system relies on the Archimedes wave swing device, which is coupled with a linear permanent magnet synchronous generator (LPMSG). The WEC system is interfaced with the power grid via a generator-side converter (GSC) and a grid-side inverter (GSI). The GSC is used to control both of the d -axis and q -axis current of the LPMSG to minimize its power losses and extract its maximum real power, respectively. The GSI is implemented to control the terminal voltage at the point of common coupling and the dc-link voltage through a complete vector control scheme. The proposed optimal WCA-based PI control strategy is applied to both converters. The optimization process depends on the simulation-based optimization approach. In the proposed approach, the criterion of integral squared error is chosen as a multiobjective function. To validate the proposed WEC system model, the simulation results are compared with the practical results, and their error reaches less than 1%. The effectiveness of the proposed WCA-based PI control strategy is tested and compared with that obtained using the genetic-algorithm-based PI control scheme under symmetrical and unsymmetrical grid fault conditions taking into account a successful and unsuccessful reclosure of circuit breakers. The validity of the proposed control strategy is extensively checked based on simulation studies in the PSCAD/EMTDC environment.

Testing a Unit Commitment Based Controller for Grid-Connected PMG-Based WECSs With Generator-Charged Battery Units

This paper presents the performance of a controller for adjusting the power generated by the permanent magnet generator (PMG) in a wind energy conversion system (WECS), which has generated-charged battery units (BUs). The proposed controller is based on the Lagrangian relaxation unit commitment (LRUC) method to determine a command value for the PMG electromagnetic torque ( $T_{e}^{*}$ ) at each wind speed. The determined value of $T_{e}^{*}$ is set to calculate command values for $q$ -axis currents that are required by the controllers operating the generator-side and charging power electronic converters (PECs). The performance of the LRUC-based controller is experimentally evaluated for a grid-connected 7.5-kW PMG-based WECSs with 4.44-kW BUs, when operated under various wind speeds and/or charging and discharging modes of the BUs. Performance results show that the proposed controller is capable of initiating fast and accurate responses to adjust the power generation of the PMG to meet the demands of the generator-side and charging PECs for different wind speeds, and/or charging and discharging of the BUs. These performance features are found to have minor sensitivity to the changes in wind speed, and/or levels of charging the BUs.

A Simple Current-Constrained Controller for Permanent-Magnet Synchronous Motor

Under the noncascade structure, a permanent-magnet synchronous motor (PMSM) regulates the speed and current in one loop. On the one hand, fast dynamic performance requires large transient current to provide a high torque. On the other hand, unlike a cascade control, the q -axis current is no longer governed by a reference current signal. In such a system, the nominal controllers cannot guarantee that the q -axis current is within the required range. But an excessively large transient current may threaten the circuit safety. To solve the overcurrent protection problem, the ordinary solution is to choose conservative control parameters, but the dynamic performance inevitably suffers a certain degree of loss. In order to improve this drawback, a simple and effective control scheme is introduced with a current-constrained technique. By constructing a special nonlinear gain, the punishment mechanism for the q -axis current is established in the control action directly. The proposed control approach does not impose limitations on the initial state. Moreover, it has good robustness to load torque uncertainty and undergoes rigorous theoretical analysis. Besides, the proposed current-constrained controller has a very concise structure. It yields a higher reliability of the PMSM control system. Comparative simulation and experimental results between the classic PID and the current-constrained controller on the PMSM servo system verify the feasibility of the presented control scheme.

Non-Linear Control for Variable Resistive Bridge Type Fault Current Limiter in AC-DC Systems

This paper proposes a non-linear control-based variable resistive bridge type fault current limiter (VR-BFCL) as a prospective solution to ease the effect of disturbances on voltage source converter-based high voltage DC (VSC-HVDC) systems. A non-linear controller for VR-BFCL has been developed to insert a variable optimum resistance during the inception of system disturbances in order to limit the fault current. The non-linear controller takes the amount of DC link voltage deviation as its input and provides variable duty to generate a variable effective resistance during faults. The VSC-HVDC system’s real and reactive power controllers have been developed based on a current control loop where direct axis and quadrature axis currents are used to control the active and reactive power, respectively. The efficacy of the proposed non-linear control-based VR-BFCL solution has been proved with balanced as well as unbalanced faults. The results confirm that the oscillations in active power and DC link voltage have been significantly reduced by limiting the fault current through the insertion of an optimum effective resistance with the proposed control technique. The real time digital simulator (RTDS) has been used to implement the proposed approach. The performance of the proposed non-linear control based VR-BFCL is compared with that of traditional fixed duty control.

Maximum Ratio of Torque to Copper Loss Control for Hybrid Excited Flux-Switching Machine in Whole Speed Range

This paper proposes an optimized control strategy for hybrid excited flux-switching machine (HEFSM) in the whole speed range. First, the machine topology, operating principle, electromagnetic performance, and dynamic models of HEFSM are briefly introduced. Second, in the constant torque range, the optimized positive excitation is utilized to improve the load capacity, and minimize the copper loss, as well as achieve a faster response than the strategy of random combination of excitation and armature current and the zero excitation current control. In constant power range, there are several constraints like torque, current, voltage and speed. The optimized negative excitation current and d -axis current are not only used to achieve higher speed and produce reluctance torque, but also minimize the copper loss for a certain value of torque in the optimized strategy. Then, the maximum ratio of torque to copper loss is realized. Finally, other losses (such as iron loss, friction, and windage losses) and efficiency of HEFSM are analyzed. The feasibility and effectiveness of the proposed control strategy for the HEFSM drive system are confirmed by simulation and experiment.

Estimation of sensor faults and unknown disturbance in current measurement circuits for PMSM drive system

Current sensor faults leads to system degradation of a permanent magnet synchronous motor (PMSM) drive. Accurate estimation of the faults for fault-tolerant control has become an important issue. However, unknown disturbance in measurement circuits greatly aggravates the difficulty of the estimation and seriously influences the performance of the estimation. This paper presents a new estimation method of sensor faults and unknown disturbance in current measurement circuits for an interior PMSM drive system. The system model, which contains faults of virtual sensors for αβ axis currents and the unknown disturbance, is first constructed. Then, the model is transformed into a model with actuator faults by a first-order low-pass filter. Moreover, the faults and disturbance in the model are decoupled by coordinate transformation. And sliding-mode observers are designed based on the new decoupled system. Single and simultaneous faults of current sensors and an unknown disturbance are accurately estimated based on sliding-mode equivalent control methodology. An experiment is carried out to demonstrate the validity and effectiveness of the method.

Using the Stator Current Ripple Model for Real-Time Estimation of Full Parameters of a Permanent Magnet Synchronous Motor

In this paper, a new method is proposed for the real-time estimation of the full parameters of a permanent magnet synchronous motor (PMSM) that is based on the stator current ripple model. By introducing the stator current ripple model together with the known two state equations for the $d$ - and $q$ -axes currents, we resolve the rank deficiency problem, thus enabling real-time full parameter estimation even in the steady state. A signal processing technique for removing adverse effects from noises and uncertainties existing in the stator current ripple measurement is also presented. The efficacy of the proposed method is verified by simulation on a 15-kW PMSM, and both the proposed and conventional methods are compared.

Torque Ripple Suppression Method With Reduced Switching Frequency for Open-Winding PMSM Drives With Common DC Bus

Torque ripple caused by zero-sequence current (ZSC) and relatively high switching frequency severely worsens the operating performance of an open-winding permanent-magnet synchronous machine (OW-PMSM) system with common dc bus. In this paper, a novel q -axis current injection method based on the angle-shift-based voltage distribution is proposed for the OW-PMSM system with common dc bus. With the proposed method, torque ripple caused by ZSC can be counteracted by the torque component generated by the injected q -axis current. Meanwhile, the corresponding modulation range is analyzed to investigate the availability of the proposed method. Moreover, in order to reduce the switching frequency, zero vector (111) is no longer necessary to compensate the zero-sequence component and conventional space vector pulsewidth modulation (PWM) with zero vectors (000) and (111) can be simplified. The switching actions in one PWM period can be reduced from 12 to 8. The comparison of losses shows that this strategy can effectively improve the efficiency under both heavy load and high switching frequencies. Finally, the effectiveness of the proposed method is validated through an OW-PMSM experiment with common dc bus.

Research on Vector Control Strategy of Surface-mounted Permanent Magnet Synchronous Machine Drive System with High-Resistance Connection

High-resistance connection (HRC) is a common electrical fault for the electric machine drive system. This fault can lead to the increased power loss and heat, thus possibly causing the damage of the electric machine drive system due to the excessive temperature. Hence, this paper first proposes a vector control strategy of the surface-mounted permanent magnet synchronous machine (PMSM) drive system with the HRC fault to minimize the copper loss. The mathematical model of the PMSM with the HRC is presented in abc stationary frame. The function relationship between the copper loss and the direct axis current, quadrature axis current, stator resistance, and the additional resistance due to the HRC is established and analyzed. The expression of the direct axis current, which minimizes the copper loss, is deduced under the condition of the HRC in one phase and two phases. Consequently, the proposed vector control strategy for the surface-mounted PMSM driving system is achieved. Both the simulation and experimental results show that the proposed method not only preserves the performance of the conventional vector control method, but also can effectively reduce the copper loss, thus protecting the PMSM from the damage due to the increased temperature possibly caused by the HRC fault.

High Speed PMSM Anti-saturation Regulation Method Based on Hybrid Flux Weakening Technology

For traditional permanent magnet synchronous motor (PMSM) flux weakening method, it is easy to cause voltage command saturation and the system lose control. In order to solve this problem, we propose a high speed PMSM anti-saturation regulation method based on hybrid flux weakening technology. The feedforward channel of this method is designed as a two-dimensional table of torque/flux linkage. The expected value of the current current of dq axis is directly checked through the current flux linkage and torque instruction to ensure the dynamic response of the system. In the feedback channel, the voltage instruction compensation link is proposed in this paper. In combination with the output instruction of the current controller, the speed and bus voltage information, the flux instruction required by the feedforward channel is adjusted. Finally, the experimental results verify that the proposed method meets the requirements of high-speed PMSM fast dynamic response, and improves the reliability of the system when it is running at high speed.