Hanxiao Lu

Rotor design considerations for a variable-flux flux-intensifying interior permanent magnet machine with improved torque quality and reduced magnetization current

This paper presents design process of the rotor to obtain a variable-flux flux-intensifying interior permanent magnet machine (VFI-IPM) with improved torque quality and reduced magnetization current. The VFI-IPMs have attracted much attention because the magnetization state of the permanent magnets can be adjusted according to load conditions. By applying a d-axis pulse current to magnetize the PMs to a high state at low speeds and decease the magnetization state of the PMs at high speeds, the VFI-IPM can achieve high efficiency at a wide speed range. However, since low coercive-force magnets, such as Alnico, are always used and the remnant flux density is always lower than rare-earth magnets, methods to achieve high torque density should be explored. Moreover, to re-magnetize the PMs completely when the rotor slows down, the magnetization current is usually several times of the rated current. Therefore, it is essential to reduce the magnetization current and thus reduce the inverter rating. In this paper, tangential magnetization orientated Alnico magnet with suitable magnetic energy (BHmax) and perfect squareness is selected to obtain high torque density. However, torque ripple is serious in tangentially magnetized PM machines, so methods to reduce torque ripple are also proposed in the following study. To further improve the torque density and reduce the magnetization current, four different topologies are built in finite-element (FE) software and then an optimal topology is confirmed by comparing these four models. Based on the optimal topology, prototype has been manufactured.

Comparison of Stator DC Current Excited Vernier Reluctance Machines with Different Field Winding Configurations

Recently, stator dc current excited vernier reluctance machines (dc-VRMs) which have concentrated field and armature windings in stator, are gaining more and more research interest due to their features of robustness rotor structure, low cost, and easy heat dissipation. This paper proposes a novel field winding connection for dc-VRMs. Taking the stator/rotor slots of 12/10 machine as an example, the electromagnetic performance including back electromotive force (EMF), pulsating torque, and capability are analyzed using the theoretical and finite element analysis. The results show that compared with the previous machine having the same stator/rotor slots, machine with the proposed field winding connections exhibits a doubling back EMF, and 60% higher torque density.

Reduction of unbalanced axial magnetic force in post-fault operation of a novel six-phase double-stator axial flux PM machine using model predictrive control

This paper investigated the post-fault operation of a novel six-phase double-stator axial flux permanent magnet machine with detached winding configuration, which was found to be superior to existing winding configuration in previous study. However, the unbalanced magnetic force problem still remains unsolved. In this paper, the axial force balancing control principle is proposed and a group of specific current waveforms are deduced. When applying these currents under post-fault condition, magnetic torque, axial magnetic force and rotor losses of the machine are calculated in finite element analysis. The results are compared with normal condition and commonly-used post-fault current waveforms. It is verified that this method reduced the unbalanced axial magnetic force immensely and the torque ripple was also kept at a low level. In order to achieve the proposed current waveform, finite control set model predictive control (FCS-MPC) is adopted. This paper proposed the post-fault model of dual three-phase permanent magnet machines and designed a cost function to track the desired current waveforms. The model of the machine is used to predict the future behavior of the controlled variables and the cost function decides the next step of the inverter by evaluating all the predictions. At last, it is verified by simulation results that the control strategy performs well in both dynamic and steady-state situations.

Six-Phase Double-Stator Inner-Rotor Axial Flux PM Machines With Novel Detached Winding

This paper proposes a novel detached winding for 6-phase double-stator inner-rotor axial flux PM machines. The connection of detached 6-phase winding is firstly given, followed by a comprehensive comparison with the conventional 6-phase winding in aspect of the winding factor, EMF and inductance, et al. Comparison of performance including rotor eddy current loss, torque ripple and rotor unbalanced axial force under fault tolerance operation is also mentioned in this paper.

Reduction of Unbalanced Axial Magnetic Force in Postfault Operation of a Novel Six-Phase Double-Stator Axial-Flux PM Machine Using Model Predictive Control

This paper investigates the postfault operation of a novel six-phase double-stator axial-flux permanent-magnet machine with detached winding connection. In previous research, this configuration was found to be superior to existing winding connection except that its unbalanced magnetic force in the postfault operation cannot be ignored. In this paper, an axial magnetic force balancing method (AMFBM) is proposed to reduce unbalanced magnetic force by deducing a set of special winding current. By comparing electromagnetic torque and axial magnetic force of the traditional winding current and the proposed method in postfault operations through a finite-element analysis, it is verified that AMFBM can reduce most of the unbalanced axial magnetic force as well as keeping torque ripple at a low level. In order to realize the AMFBM, finite control set model predictive control is adopted. A postfault model of dual-three-phase permanent-magnet machines with modified vector space decomposition method is first brought forward to predict the future behavior of the controlled variables with various voltage inputs. After that, a cost function is designed to track the desired winding current by evaluating all the predictions to decide the next step of the inverter. Experimental results show that the control scheme performs well in both dynamic and steady-state situations.

A new predictive direct torque control for vernier permanent magnet synchronous motor based on duty ratio modulation

This paper proposes a novel predictive duty ratio modulated direct torque control (DTC) for vernier permanent magnet synchronous motor drive to reduce the torque and flux ripples caused by hardware delay. Compared with DTC, conventional duty ratio modulated DTC (DDTC) can significantly reduce the torque and flux ripples, while hardware delay still deteriorates the performance of DDTC. This paper analyzes the influence of hardware delay in detail and solves this problem by the prediction of torque and flux. This predictive DDTC (PDDTC) can maintain the dynamic response of DDTC and eliminate the effect of hardware delay to get better performance. To verify the validity of the proposed method, the simulated and experimental results of DDTC without hardware delay, considering hardware delay and PDDTC are presented.

Finite set model predictive MTPA control with VSD method for asymmetric six-phase PMSM

This paper investigated and compared two types of modified model predictive control methods with finite set for six-phase PMSM MTPA control. The first one has conventional cascaded speed and current controller loops where the PI current loop is replaced with a MPC control loop. The second method uses one MPC controller in place of conventional cascaded loops for both speed and current control. The MPC method is adopted for fast current response and the ability of dealing with multi-variables. Therefore, partial current harmonics in xy sub-plane can be suppressed with the cost function according to vector space decomposition theory. By designing the cost function, the MTPA operation can be implemented.

Post-fault model predictive control of asymmetrical six-phase permanent magnet machine with improved mathematical model

Multiphase machines are superior to three-phase machines with high reliability and large power capability. In the post-fault operation when one phase of an induction machine is open circuit, model predictive control has been proved to have rapid current, torque and speed response. However, the mathematical model in post-fault operations of multiphase permanent magnet (PM) machines is quite different from induction machines and it has rarely been analyzed before, so it still needs further study. This paper looks deeply into the post-fault model of an asymmetrical six-phase PM machine and its drive system, and proposes an improved modeling method in which the phase voltages and inductance matrix are amended. Thus the concision of the prediction process can be maintained the same as in normal operations. Simulation results are given and proves the efficiency of the proposed method in both dynamic and steady states of post-fault operation.

Design Considerations of Stator DC-Winding Excited Vernier Reluctance Machines Based on the Magnetic Gear Effect

Stator dc field winding excited vernier reluctance machines are increasingly attracting research interest recently due to their low cost and highly reliable rotor structure. In this paper, the design of these machines is studied systematically using both theoretical equations and finite element analysis (FEA). First, the general structures of these machines and their operation principles are discussed. Second, based on the operation principle, the back electromotive force, torque, and power factor expressions are deduced in terms of the geometrical variables. Several key design parameters are defined, including stator/rotor slot/pole combinations, air gap, split ratio, field/armature coil turn ratio, and stator/rotor tooth arc combination. Third, the deduced theoretical expressions are verified by the FEA. Finally, a prototype is designed and tested to verify the analysis results.

Improved Overmodulation Technique with Limited Switching Frequency of Integrated Starter-Generator for Hybrid Electric Bus

Axial flux PM machines, featured by compact size and high performance, are suitable for powertrains of electric vehicles. This paper studies the design and control of a totally enclosed axial flux PM integrated- starter-generator system. Torque density and volume benefits from large number of pole pairs when designing PM machines. However, the increased pole pair number decreases the switching-frequency-to-electrical-period ratio, and large power drives are troubled with limited switching frequency. To improve this situation, a control strategy named overmodulation technique with variable sampling frequency is proposed to improve the system performance in the nonlinear region. The heat dissipation potential of the inverter is fully utilized and the DC bus voltage utilization ratio is enhanced with this control strategy. Simulation results are carried out in which the current waveform is also improved.