Weiguo Liu

Northwestern Polytechnical University, Xi’an, Liaoning, China

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Publications (45)6.2 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: DC microgrid is one feasible and effective solution to integrate renewable energy resources, as well as to supply reliable electricity. The control objective of DC microgrids is to obtain system stability, low voltage regulation and equal load sharing in per unit. The droop control is an effectively method adopted to implement the control of microgrids with multiple distributed energy units. However in the application of low-voltage DC microgrids, the nominal reference mismatch and unequal cable resistances require a trade-off to be made between voltage regulation and load sharing. In this paper, a unified compensation framework is proposed using the common load condition in local controller, to compensate the voltage drop and load sharing errors. The voltage deviation is compensated with a P controller while the load sharing is compensated through a PI controller. An additional low bandwidth communication is introduced to share the output current information, and the average output current in per unit is generated to represent the common load condition. The performance of the proposed method is analyzed and compared with basic droop control and hierarchical structure method. The large signal stability is analyzed to define the margin of compensation coefficients. Simulations and experiments are carried out to verify the performance of the proposed method.
    No preview · Article · Jan 2015 · International Journal of Electrical Power & Energy Systems
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    ABSTRACT: The residential buildings take a large part of the electricity consumption. The building microgrid will play an important role in the future, to improve the energy efficiency, reduce environmental impacts, as well as enhance power system stability. This paper proposes a distributed tariff-driven droop gain-scheduling method, to solve the power distribution between different energy resources as well as the grid connected converter, for residential building microgrid operating in grid-connected mode. Tariff-driven functions are used to adjust droop gains in the voltage control loop. In this way, the power distribution ratio can be changed automatically according to the tariff conditions, as time-of-use electricity tariff. And also near- optimization in economic can be implemented, with properly chosen parameters in tariff functions. A residential building DC microgrid including solid oxide fuel cell, photovoltaic panel and grid connected converter is modeled with MATBLB/Simulink and used to verify the proposed method.
    No preview · Conference Paper · Nov 2013
  • Ling Luo · Bing Luo · En Xie · Weiguo Liu · Manfeng Dou
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    ABSTRACT: The armature field for permanent magnet brushless (PMBL) machine working as DC motor is different from that as AC machine, which causes that the iron losses of PMBL DC motor are not equal with those of PMBL AC machine. An experimental system is designed to separate the iron losses due to armature field from other losses for PMBL DC motor and AC generator. The analysis results of the test data for the prototype PMBL machine of 30kW show that the iron losses due to armature field for the prototype as the DC motor are much more than that as the AC generator, and the iron losses due to armature field in rotor for the PMBL DC motor is more than that for the PMBL AC generator.
    No preview · Conference Paper · Oct 2013
  • Yi Hou · A. Ravey · D. Bouquain · Fei Gao · A. Miraoui · Weiguo Liu
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    ABSTRACT: This paper presents a comparison of different electric motor controls-PI control and sliding mode control (SMC) for a hybrid electric vehicle based on fuel cell. The aim of control is tracking a driving cycle, which represents the vehicle speed by time. Different driving cycles (standard and recorded) are simulated, and the study shows the impact of the different driving patterns on the electric motor control. The simulation results demonstrate that sliding mode control has better dynamic performance than PI control especially when load torque changes at high frequency.
    No preview · Conference Paper · Jan 2013
  • Nanfang Yang · Guangzhao Luo · Weiguo Liu · Kang Wang
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    ABSTRACT: This paper proposes a look-up table based maximum torque control of interior permanent magnet synchronous motor (IPMSM) for traction of electric vehicles (EV). Maximum torque control is preferable for traction requirement of EV due to requirements of high torque ability in lower speed and wide-adjustable operation range. In constant torque region (below rating speed), Maximum Torque per Ampere (MTPA) can achieve perfect performance, while in field-weaking region (over rating speed), the torque control usually is not satisfactory. The main problem is that on-line calculation of current commands is too complex and time-consuming in real control system. We solved the problem through construction of look-up table for d-axis current off-line, and on-line calculation of q-axis current using torque equation. It can not only permit the maximum torque control performance but also easy for micro-controller to implement. Finally, simulation and experimental system were constructed to verify the proposed method, and results have been found in good agreement with theoretic analysis.
    No preview · Conference Paper · Jun 2012
  • Renguo Xi · Shoujun Song · Weiguo Liu
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    ABSTRACT: According to the real structural parameters of the motor and its external circuit, the complete simulation model of a four-phase 8/6 poles switched reluctance motor (SRM) is built based on Maxwell2D of Ansoft Corporation. The basic performances and the starting process of SRM are analyzed based on this model. The simulation results include the distribution of magnetic field at various rotor positions, static electromagnetic characteristics and dynamic performance. These results agree well with the basic running performances, and can benefit for the design and optimization of SRM and its control system.
    No preview · Chapter · Jan 2012
  • Nannan Zhao · Z.Q. Zhu · Weiguo Liu
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    ABSTRACT: In this paper, two identical surface-mounted permanent magnet machines are analyzed, with particular reference to rotor eddy current loss, when they are back-to-back connected and used as a motor and a generator, respectively, the motor being driven by pulsewidth modulation (PWM) inverters and the generator being connected to the 3-phase resistance load. Losses in both motor and generator under three different DC supply voltages and PWM duty ratios, including the eddy current losses in sleeves and magnets, and the iron losses, are predicted using finite element analysis. Thermal fields are analyzed using analytical lumped-circuit method based on the obtained losses. Predicted temperature rises of machines are compared with measured results and good agreement is achieved. The analysis reveals that the mmf time harmonics due to nonsinusoidal phase current and PWM carrier harmonics induce significant eddy current losses in magnets and sleeve of the motor, causing very large difference in temperature rises of the motor and generator. It is also found that the difference becomes more significant with the decrease of PWM duty ratio and increased DC supply voltage.
    No preview · Article · Nov 2011 · IEEE Transactions on Magnetics
  • Nannan Zhao · Z.Q. Zhu · Weiguo Liu
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    ABSTRACT: In this paper, the rotor eddy current loss of surface-mounted permanent magnet (SPM) machine is predicted using a simplified analytical model, which accounts for curvature, time and space mmf harmonics, and is applicable to calculate the eddy current loss in both permanent magnets and sleeve, but neglects the influence of stator slotting and eddy current redistribution effect. The analytical method is employed to research two identical prototype SPM machines when they are back-to-back connected mechanically and used as a motor and a generator, respectively. Finite element analysis (FEA) is employed to validate the model. Thermal fields are analyzed using analytical lumped-circuit method based on the obtained eddy current loss and the predicted temperature rises of machines are compared with the measured results. Good agreement between analysis and measurement is achieved. The analysis in this paper reveals that the time harmonics significantly influence the eddy current loss and temperature rises in magnets and sleeve of motor, which causes large difference in temperature rises of the motor and generator.
    No preview · Conference Paper · Aug 2011
  • Nannan Zhao · Z.Q. Zhu · Weiguo Liu
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    ABSTRACT: In this paper, both analytical lumped-circuit method and finite element analysis are employed to predict the temperature rises of two identical prototype surface-mounted permanent magnet machines when they are back-to-back connected mechanically and used as a motor and a generator, respectively. The predicted results are validated by the experiment. The analysis reveals that the eddy current losses in magnets and sleeve cause higher temperature rise than that in the stator winding in both machines. The analytical lumped-circuit method takes much less time and the accuracy is even higher than the 2D finite element analysis with due account for the duty cycle, cooling effect, and interface gaps between components, although more detailed temperature distribution inside components can be obtained using the finite element analysis.
    No preview · Conference Paper · Jan 2011
  • Minghui Zhang · Weiguo Liu · Nannan Zhao
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    ABSTRACT: This paper investigates the eddy-current loss in a surface mounted PM BLDC motor. The method is based on the 2-D finite element computation and the loss increase due to non-sinusoidal voltage supply is taken into account. Both eddy-current losses in stator core and in rotor permanent magnets are considered. The detailed equations for prediction of those eddy current losses are presented respectively. Using this procedure, the calculation of the eddy-current loss and the analysis of the effects caused by the harmonics are explained in detail.
    No preview · Conference Paper · Jan 2011
  • Yaohua Li · D. Gerling · Weiguo Liu
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    ABSTRACT: In this paper the effect of the voltage vector on the amplitude of stator flux, torque angle and torque of the permanent magnet synchronous motor (PMSM) direct torque control (DTC) drive is studied. A simplified voltage vector selection strategy based on the technology of space vector modulation (SVM) is proposed. Simulation and experimental results show this control strategy can decrease stator current and torque ripples and fix the switching frequency.
    No preview · Conference Paper · Nov 2010
  • Shuanqin Xie · Jianying Gong · Weiguo Liu
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    ABSTRACT: In a power distributed system of More Electric Aircraft with many loads on the dc or ac bus, there are harmonic and reactive currents when a large load is loaded or unloaded from the bus. The bus conditioner can compensate the harmonic and reactive currents on dc and ac bus in power system. This paper presents a sample of a power distributed system of More Electric Aircraft with an ac bus conditioner. The models of three phase dc/ac and ac bus conditioner are established. The controller of the ac bus conditioner is designed. The stability of the power distributed system is anlysised when the ac bus conditioner is loaded. The simulation results are presented. With certain parameters, the ac bus conditioner can reduce the variable frequency ripple current, shape the voltage waveform of the dc and ac bus and increase the system stability margin.
    No preview · Article · Aug 2010
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    ABSTRACT: This paper mainly deals with a high order sliding mode controller for permanent magnet synchronous motor (PMSM). This kind of approach eliminates the chattering phenomenon essentially by comparing with traditional sliding mode control. The simulation and experiment results indicate that effect of synchronous motor chattering problem is avoided.
    No preview · Conference Paper · Jul 2010
  • Qian Yang · Weiguo Liu · Guangzhao Luo
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    ABSTRACT: The adaptive backstepping nonlinear speed controller of permanent magnet synchronous servo motor (PMSSM) with uncertain parameters is designed in this paper. The inertia, viscous friction coefficient and load torque which are vulnerably affected by external disturbances can be estimated in real time. A second-order filter is adopted to reduce the speed overshoot in the starting course of PMSSM. On the basis of cancelling initial rotor position error, position and velocity feedback subsystem based on dSPACE/DS3002 is designed. Permanent magnet synchronous servo motor speed control system model based on adaptive backstepping control algorithm is established in Simulink. With dSPACE system and the external drive circuit, the completed control system hardware-in-loop real-time simulation is achieved successfully. Simulation and experimental results show that the stator flux of adaptive backstepping nonlinear speed controller stator flux is convergence, and the whole control system is global uniform convergence. The whole control system can remain its good speed dynamic tracking performance and strong robustness when system parameters and load torque disturbances appeared.
    No preview · Article · Jul 2010
  • Jun Zhao · Weiguo Liu · Bo Tan
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    ABSTRACT: A modeling approach based on least square support vector machine (LSSVM) had been applied for permanent magnet synchronous motor (PMSM) and inverter with PMSM, which was multi-variable, nonlinear and coupled system. The modeling parameters with RBF kernel function was optimized by using cross validation method. The simulation result show that the tow modeling method is very effective. The maximum root mean square error (RMSE) of modeling of PMSM is 0.3196 and the maximum relative error is 0.2341%. And the maximum RMSE of modeling of inverter with PMSM is 0.4421 and the maximum relative error is 2.4121%. Using LSSVM for modeling of PMSM performs better forecast accuracy and successful modeling of PMSM.
    No preview · Article · Jun 2010
  • Jun Zhao · Weiguo Liu · Bo Tan
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    ABSTRACT: Efficiency optimization control of salient pole permanent magnet synchronous motor (PMSM) based on maximum ratio of torque to current is presented. From basic equations of PMSM in reference frame, the method of maximum ratio of torque to current was derived using maximum principle. Then, based on the Least Squares Support Vector Machine (LSSVM), modeling of LSSVM controller of maximum ratio of torque to current by using simulation data. The simulation result shows that LSSVM has a good generalization ability and capability of modeling nonlinear process, and the original sampling data are very similar to the data predicted by LSSVM model. And using method of LSSVM maximum ratio of torque to current, the simulation of PMSM shows that the PMSM control system has a good dynamic ability and achieves efficiency optimization control.
    No preview · Article · Jun 2010
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    Shoujun Song · Weiguo Liu

    Preview · Article · Jan 2010
  • Bo Tan · Ruiqing Ma · Weiguo Liu · Jun Zhao
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    ABSTRACT: For braking problems of high power BLDCM on aviation, with the characteristics of air power supply, this paper respectively studies the change of phase current in dynamic braking and plug breaking. It compares the speed performance of the different braking methods, and proposes using different phase current detection ways for different braking methods and the phase current control method based on current cut-off negative feedback. Finally, the relevant waveforms are simulated in Matlab environment. The simulation results show that the theory is correct and current detection and control methods are very effective.
    No preview · Article · Jan 2010
  • Ling Luo · Weiguo Liu · Manfeng Dou
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    ABSTRACT: This paper presents two high speed brushless DC (BLDC) prototype motors: one is related to the surface-mounted permanent magnet (SPM) rotor, the other to the interior permanent magnet (IPM) rotor. The IPM rotor is a combination of magnets magnetized in tangential direction and in radial direction. As for the two prototype motors, the magnetic field distribution in the air gap calculated by 2D finite element analysis (FEA), the back electromotive force (EMF) waveforms obtained by 2D FEA and test, and the test speed-torque curves are compared respectively. The comparison results suggest that the leak flux in the IPM rotor reduces the air gap flux per pole and that the no-load speed and the speed-torque curve slope of the IPM motor are much greater than the no-load speed and the speed-torque curve slope of the SPM motor.
    No preview · Article · Jan 2010
  • Bo Tan · Weiguo Liu · Ruiqing Ma · Jun Zhao
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    ABSTRACT: By studying the circuit equation in the process of commutations of brushless DC motor (BLDCM), the reason of phase current ripple is pointed out. The effect of phase current ripple to electromagnetic torque is analyzed. The deficiency of conventional current closed-loop control is studied. On the base of analysis a new current closed-loop control method is proposed. Phase current is inputted to PI controller which generates PWM signal to reduce its ripple. Finally, the relevant wave-forms are simulated in the Matlab environment. The simulation result shows that the new method can control phase current in real time, reduce the ripple of phase current and electromagnetic torque effectively, and improve the system efficiency.
    No preview · Article · Jan 2010