Flux-weakening Region Research Articles

Comparison of Flux-Weakening Control Strategies of Novel Hybrid-Excited Doubly Salient Synchronous Machines

For hybrid-excited doubly salient synchronous machine, both the field excitation current and the d-axis current can be utilized to adjust the flux-linkage, which provides more flexible control parameters for flux-weakening operation. In this paper, three flux-weakening control methods, i.e., utilizing field excitation current alone (Method I), utilizing armature current alone (Method II), and optimal method (Method III), are proposed and compared. All three methods can achieve similar torque performance in the constant-torque region. In the flux-weakening region, Method I exhibits low torque and limited operating speed range. The operating speed range can be further extended by Methods II and III. In addition, Method III can provide a higher efficiency in flux-weakening region than Method II since the copper loss of field winding can be decreased in proportion to the reduction of field excitation current. Those flux-weakening control methods are verified by experimental results.

Challenges of the Optimization of a High-Speed Induction Machine for Naval Applications

In several industrial sectors, induction machines are being replaced with permanent magnet based alternatives, owing to the potential for higher power density and efficiency. However, high-speed applications feature a wide flux-weakening region, where advanced induction machines could bring benefits in terms of system-level optimization. This paper gives an overview the technological challenges for high-speed drives with induction machines, materials, simulations and future challenges for the power electronics in these applications. The target application is a high-speed induction machine for a naval turbocharging system. The comparison with permanent magnet synchronous machines will demonstrate how the extended flux weakening operation effectively allows for a weight reduction of the overall system.

Analytical maximum torque per volt control strategy of an interior permanent magnet synchronous motor with very low battery voltage

The operation of an interior permanent magnet synchronous motor included in an electric power steering system is related to two demanding requirements: (i) the low-voltage DC source pushes the motor to a deep flux weakening region, and (ii) the motor is so optimised that it can withstand only few cycles at nominal torque. Owing to that, maximum torque per ampere (MTPA) and maximum torque per volt (MTPV) current reference generation strategies are common in this type of application. Most of the published MTPA or MTPV strategies are applied to standard voltage motors, so stator resistance is typically neglected, leading to simpler equations. Other works consider the stator resistance but, as the resulting equations are complex, look-up tables or numerically adjusted polynomials are employed in current generation tasks. This work presents analytical expressions allowing the exact computing of current references. These expressions include stator resistances. The battery voltage is considered as an input variable, together with motor speed and reference torque, and direct and quadrature current references are the output variables. Contrary to look-up tables or numerically adjusted polynomials, the proposed expressions can take into account any parameter variation during real-time operation. Simulation and experimental results validate the proposed approach.

Stepwise Magnetization Control Strategy for DC-Magnetized Memory Machine

A memory machine (MM) equipped with hybrid permanent magnets (PMs), i.e., NdFeB and low coercive force PMs, exhibits acceptable torque capability at low speeds and high efficiency at high speeds. Previous literatures have addressed that the constant power speed range (CPSR) of MMs can be further extended by online PM flux control and the requirement of the flux-weakening (FW) inverter current can be reduced as well. Nevertheless, how to coordinate the d -axis vector FW and PM magnetization control over a whole operating range in a reasonable manner remains unreported. Therefore, this paper proposes and implements a stepwise magnetization control strategy on a dc-magnetized MM based on the operating characteristics under various PM magnetization states and speed ranges. The configuration, principle, and mathematical model of the investigated machine are introduced first. Then, the proposed control strategy is established by dividing the operating envelop into several FW regions and an appropriate FW control scheme is utilized at each stage. It demonstrates that the CPSR can be effectively extended by simply applying the demagnetizing current pulses in several steps. This simplifies the control efforts considerably without resorting to continuous PM flux control and frequent actions of the switching devices. The effectiveness of the proposed control strategy is verified by experimental results.

Current closed‐loop control and field orientation analysis of an induction motor in six‐step operation for railway applications

In railway traction drive systems, six-step operation is widely used for motors in a flux-weakening region. Traditional vector control algorithms in six-step operation cannot work effectively due to the limitation of a single degree of freedom. This study analyses the dq current coupling relationship when the voltage amplitude is limited and applies a current closed-loop control strategy in six-step operation. This study proposes a proper switching control strategy to achieve a dual-mode control for induction motors in a full-speed region. The accuracy of field orientation is affected by changes in motor parameters and plays a key role in current control precision. This study analyses the effect of field orientation error on motor six-step operation. It is found that the proposed current closed-loop control strategy can correct the field orientation error and guarantee the motor current to track the reference precisely. A case study of a 5.5 kW experimental platform is presented to validate the control schemes.

An Effective Flux Weakening Control of a SyRM Drive Including MTPV Operation

A synchronous reluctance motor drive exhibits an inherent unlimited speed range capability; in fact, the maximum speed of this kind of machine is bounded only by mechanical limits. In order to exploit this peculiar characteristic of the drive while assuring the lowest power losses, a proper control algorithm has to be used. In particular, while the working speed is increasing, the control must be able to command the reluctance motor under the maximum torque per ampere condition at first, into the flux-weakening region, and then on the maximum torque per voltage trajectory. To this purpose, a novel voltage control loop is presented in this paper. It is combined with a field-oriented current control and profitably exploits a polar coordinate representation of the current reference to operate the reluctance machine in compliance with current and voltage rating constraints in all the different operating conditions. Additional merits of the proposed scheme are the ease of implementation and the simple design. The stability of the proposed control algorithm is proved through a small signal analysis and simulations and experimental tests confirm its effectiveness.

Optimal Three-Dimensional Current Computation Flux Weakening Control Strategy for DC-Biased Vernier Reluctance Machines Considering Inductance Nonlinearity

An optimal three-dimensional current computation flux weakening control strategy for dc-biased Vernier reluctance machines (VRMs) is proposed in this paper. Compared with permanent magnet synchronous machines, dc-biased VRMs have an additional degree of freedom to regulate the rotor flux through variable dc-biased armature current. The conventional flux weakening control strategy does not utilize the adjustable dc field current, and the output capacity in the flux weakening region is limited. In this paper, the optimal three-dimensional current distribution is calculated at the intersection of current and voltage constraint. Meanwhile, in order to maintain the armature voltage within the voltage constraint, the inductance nonlinearity is reflected by constructing the inductance table. The algorithm provides maximum output capability and high efficiency for dc-biased VRMs despite the changes in inductance parameters in the whole flux weakening region. Finally, the effectiveness of the proposed control strategy is validated by experimental results for a prototype machine.

Torque Ripple Reduction in Five-Phase IPM Motors by Lowering Interactional MMF

This paper proposes a torque ripple reduction method for a five-phase interior permanent magnet (PM) motor by shifting asymmetrical rotor poles. The key of this method is to consider the PM torque ripple, reluctance torque ripple, and cogging torque simultaneously from two aspects. First, the main interactional harmonic, contributing to PM torque ripple, between stator magnetic motive force (MMF) and rotor MMF is developed. Then, the appropriate combination of unequal pole arcs is adopted to eliminate the main interactional harmonic. Second, the asymmetrical rotor pole-pair is selected as a repeating unit which produces cogging torque and reluctance torque with consistency in waveforms and phases. Then, three methods for shifting asymmetrical rotor pole-pair are proposed showing that the torque ripple can be reduced greatly and the lower-order harmonics of radial force is acceptable when the shifting angle of the repeating unit is 4.5 mechanical degrees. Through theoretical analysis and experimental test, it is verified that the proposed motor can offer lower torque ripple even under deep flux-weakening region.

Operating Envelopes of the Variable-Flux Machine With Positive Reluctance Torque

Variable-flux interior permanent magnet synchronous motors (VFIPMSMs) find growing attention for electrified transportation applications, especially in the area of electric vehicles. While a prior focus was on optimizing the magnetization requirement to not oversize the inverter, and improving the machine power over a wide operating range, this paper aims to investigate and compare different possible operating envelopes of the VFIPMSM from the drive point of view. Demagnetizing the low-coercive magnets via only a short $d$ -axis current pulse eliminates the need of continuously applying a negative $d$ -axis current in the flux-weakening region; hence, lower copper loss and improved motor efficiency are expected. In this paper, this has been investigated and compared with the utilization of continuous negative $d$ -axis current in the flux-weakening region considering the nonlinear demagnetization characteristics of the low-coercive magnets. The latter scheme has been seen to improve the high-speed output characteristics and to extend the speed range. Although a constant-power–speed range with VFIPMSMs is not feasible due to the irreversible demagnetization of low-coercivity magnets, an improvement of high-speed output power is shown to be feasible via saliency manipulation with the latter scheme. A VFIPMSM with a positive-reluctance torque (inverted saliency $L_{q} ) is used for experimental validation.

Dynamic DC-link Voltage Adjustment for Electric Vehicles Considering the Cross Saturation Effects

The demands of remarkable reliability and high power density of traction systems are becoming more and more rigorous. The conflicting requirements imposed on the control strategy are higher accuracy and higher efficiency over the whole speed range. However, parameter variations caused by the cross coupling and magnetic saturation effect (omitted from the cross saturation effects in the following) are usually neglected in conventional control strategies, which could reduce the control precision. In order to fully consider the influence of parameter changes on the motor control and derive an approach that could realize the maximum efficiency during the whole speed range, this paper proposes a dynamic DC-link voltage adjustment strategy considering the cross coupling and magnetic saturation effects. The strategy can be categorized into three parts. Firstly, the torque request is transformed to the optimal current reference signal. Secondly, the differences between the setpoint and the real-time feedback signals of torque and voltage can be applied in the linearized function in the did,q coordinate. The solution guides the current vector into the optimal direction under the current and voltage limits to ensure the safety and reliability of the motor. Finally, last, the bus voltage can be modified according to the asked terminal voltage. A 10 kW prototype which instrumented a bidirectional DC-DC converter to regulating the bus voltage has been studied. The simulation and experiment results verify that the proposed control strategy can reduce the inverter losses in low speed region by offering the low bus voltage and track the actual maximum torque control trace more accurately, meanwhile, the flux weakening region can be delayed in high speed region by applying a high bus voltage. It helps the motor realize the high utilization rate of the DC-link voltage and guarantees the system reliability and robustness.

Flux-Weakening Control for Induction Motor in Voltage Extension Region: Torque Analysis and Dynamic Performance Improvement

Flux-weakening control for induction motor (IM) in voltage extension region (outside the inscribed circle but in the hexagon) is meaningful to yield a further maximum torque. However, as the voltage-limit trajectory migrates out of the inscribed circle, torque ripple becomes more severe. Meanwhile, the insufficient voltage margin results in the degradation of current dynamic performance in transition period (from base speed region to flux-weakening region), especially in harsh conditions, e.g., a step speed command. To address the problems above, this paper gives a quantitative analysis of the torque ripple and an explicit discussion on the current dynamic performance. A novel “Voltage Reference Adjustable” flux-weakening controller with “Self-Locking Limit Block” (SLLB) is proposed. There are two advantages. The first is the capability to operate in any voltage extension regions, offering a tradeoff between obtaining the maximum torque and suppressing the torque ripple. The second is an optimized voltage distribution, achieving a better track characteristic of d - and q -axis currents with the help of triggered SLLB in transition period. Experimental results on a commercial IM control system verify the validity of the proposed scheme.

Electromagnetic Performances Analysis of IPMSM According to the Current Control Method Under Flux-Weakening Control Region

In this paper, we dealt with the design and characteristics analysis of a 130 kW interior permanent magnet synchronous motor (IPMSM) through the finite element analysis for propulsion system of an electric vehicle. In flux-weakening control region of IPMSM, six-step operation is considered to widen a high-speed region. However, electromagnetic performance of the motor depends on the input current condition, known as time harmonics. To verify its effect, first, we implement two types of current control method by simulink: one is space vector pulse width modulation, and the other is six-step. Then, we conduct the finite element analysis to verify a difference of electromagnetic performance of IPMSM such as force, loss, permanent magnet irreversible demagnetization, especially under flux-weakening control region according to these current control methods.

Variable Time Step Control for Six-Step Operation in Surface-Mounted Permanent Magnet Machine Drives

The six-step operation of surface-mounted permanent magnet machine drives in a flux weakening region has many advantages compared to the pulse width modulation mode, such as the reduced switching loss and fully utilized inverter output voltage. However, if the ratio of the sampling frequency to the fundamental frequency is low in fixed sampling system, the low-frequency oscillation in the current can be incurred in the six-step operation. The low-frequency current causes a system stability problem and reduces system efficiency due to an excessive heat and high power loss. Therefore, this paper proposes the variable time step controller for six-step operation. By updating an output voltage, sampling phase currents, and executing the digital controller synchronized with the variable sampling time, the turn on and off switch signals for six-step operation can be generated at the exact moment. As a result, the low-frequency oscillation in the phase current can be eliminated. In addition, the system transfer function of the proposed control method is discussed for the system stability and system dynamic analysis. The effectiveness of the proposed method is verified by the comparative simulation and experimental results.

Design of external rotor permanent magnet synchronous motor based on genetic algorithm and differential evolution algorithm

Permanent magnet synchronous motors have been preferred in industrial fields for a few decades. It is reason for that the permanent magnet synchronous motors have high torque/volume ratio, large flux weakening region, and especially highly efficient. The main factor to obtain these advantages is the selection of suitable geometric parameters in their design optimizations. As a design optimization this study investigates external rotor permanent magnet synchronous motor with fractional slot windings. Pre-analytical designs and subsequently design optimizations by genetic algorithm and differential evolution algorithm have been studied. The better results obtained were tested by the finite element method. Thus, so much more compact and efficient motor model was to be achieved based on the design geometries. The results are very reasonable and useful.

An Improved Continuous-Time Model Predictive Control of Permanent Magnetic Synchronous Motors for a Wide-Speed Range

This paper proposes an improved continuous-time model predictive control (CTMPC) of permanent magnetic synchronous motors (PMSMs) for a wide-speed range, including the constant torque region and the flux-weakening (FW) region. In the constant torque region, the mathematic models of PMSMs in dq-axes are decoupled without the limitation of DC-link voltage. However, in the FW region, the mathematic models of PMSMs in dq-axes are cross-coupled together with the limitation of DC-link voltage. A nonlinear PMSMs mathematic model in the FW region is presented based on the voltage angle. The solving of the nonlinear mathematic model of PMSMs in FW region will lead to heavy computation load for digital signal processing (DSP). To overcome such a problem, a linearization method of the voltage angle is also proposed to reduce the computation load. The selection of transiting points between the constant torque region and FW regions is researched to improve the performance of the driven system. Compared with the proportional integral (PI) controller, the proposed CTMPC has obvious advantages in dealing with systems’ nonlinear constraints and improving system performance by restraining overshoot current under step torque changing. Both simulation and experimental results confirm the effectiveness of the proposed method in achieving good steady-state performance and smooth switching between the constant torque and FW regions.

A General Analytical Three-Phase Induction Machine Core Loss Model in the Arbitrary Reference Frame

An analytical three-phase induction machine model is proposed, derived, and validated in this paper. This model is capable for arbitrary qd0-frame analysis and core loss estimation at both line-fed and inverter-fed situations. Detailed model-based machine copper and core loss estimations are presented. A simulation verification of the model consistency is given under a changing load profile in MATLAB/Simulink. Then, the model is verified comprehensively using three induction machines (1.5 HP, 3 HP, and 10 HP), where the model is proved to be scalable, and to provide excellent machine loss estimation in line-fed situation and inverter-fed situation with machine input line filters, as well as in the flux-weakening region. Finally, a series of sensitivity tests of the model parameters are performed and the effects of the parameters on the machine losses are discussed. It is believed that the proposed model will be beneficial for various qd0-frame model-based research works of three-phase induction machines.

Characterization of Iron Loss for Integral-Slot Interior Permanent Magnet Synchronous Machine During Flux Weakening

Iron loss resistance is widely used for rapid motor design and maximum efficiency control. However, the harmonic loss obviously increases during flux weakening, and even dominates in total iron loss, especially in deep flux weakening region. This paper presents an analytical calculation method of iron loss for an integral-slot interior permanent magnet synchronous machine (IPMSM) by iron loss resistance, considering harmonic effect during flux weakening. To predict the harmonic loss, both a harmonic loss voltage and a harmonic loss coefficient are employed into an iron loss resistance model, and the relation of harmonic loss to the harmonic voltage and coefficient are studied. The iron loss and efficiency of an integral-slot IPMSM are, respectively, obtained by the proposed model, the conventional model, the experiment, and finite-element analysis (FEA). The proposed model is validated by the comparison with FEA and experimental results.

Investigation and implementation of a new hybrid excitation synchronous machine drive system

Hybrid excitation synchronous machines (HESMs) inherit the high-torque density of permanent magnet synchronous machines and flux regulation capability of wound rotor synchronous machines. This makes HESMs attractive candidates for vehicle traction applications. To further improve torque density, an optimised HESM with magnet shunting rotor is proposed in this study. This study presents a maximum torque control strategy with zero d-axis current. Furthermore, an optimised control strategy that utilises the coordinated operation between the field current and d-axis current is proposed. The optimised control strategy exhibits advantages of low-speed high-torque, wide constant power range, and high efficiency in flux-weakening (FW) region. A 100 kW drive system based on the optimised HESM with the proposed control strategy is developed, experimentally realised, and validated. The experimental results show that the FW range is 3.5–1, and the efficiency of the drive system is >92% in most operating region, which verify the superiority and effectiveness of the proposed HESM drive system.

Finite Set Model Predictive Control of Interior PM Synchronous Motor Drives With an External Disturbance Rejection Technique

This paper designs a predictive speed controller of interior permanent-magnet synchronous motor (IPMSM) based on finite control set (FCS)–model predictive control (MPC). The proposed predictive speed controller has a cascade-free structure that comprises a single predictive control function for the speed and dq- axis stator currents. The future rotor speed and stator currents are predicted at each sampling interval. Then, the proposed predictive control function is evaluated to determine the optimal switching states for the IPMSM drive. In high-performance speed-controlled drive, the unknown load torque disturbance has to be suppressed. Therefore, the load torque disturbance estimator using the two-stage extended Kalman filter (TSEKF) is proposed to enhance the dynamic performance of the IPMSM drive. The proposed TSEKF simultaneously estimates the system states, which greatly improve the model prediction process. The proposed predictive speed control is simple and compact, which demonstrates a faster transient response and a robust feature to mechanical system parameter variations when compared with the conventional cascaded speed control under a wide speed range and load torque variations. The experimental results on a prototype IPMSM drive built on a Texas Instruments TMS320F28335 floating point digital signal processing (DSP) board are presented considering the constant torque and flux-weakening regions to confirm the feasibility of the proposed FCS-MPC scheme.

Stand-Still Self-Identification of Flux Characteristics for Synchronous Reluctance Machines Using Novel Saturation Approximating Function and Multiple Linear Regression

Motor characterization has a fundamental role in dynamics, torque accuracy, and efficiency of vector controlled Synchronous Reluctance Machine (SynRM) drives. Control performances and robustness in the whole speed/torque range, including the flux-weakening region, and in sensorless operation strongly rely on the knowledge of machine flux versus current characteristics. A convenient flux saturation approximating function is proposed in this paper, together with an efficient parameters self-identification procedure. The adopted strategy is very simple and can be performed at stand-still by injecting a proper voltage stimulus (current control is not involved), and does not require any additional hardware (motor can be either connected or disconnected from mechanical load). Nevertheless, an excellent fitting for the flux curves on both axes is obtained, using reasonable memory and computational resources. These features make the technique very suitable to motor self-identification in industrial drives. Experimental results based on a commercial drive and two SynRMs are reported to demonstrate the effectiveness of the proposal. Extensions of the method to the evaluation of the whole flux map (including cross-saturation effects) or to interior permanent-magnet machines is also investigated and verified.