Brushless AC Machines Research Articles

Field-oriented control strategy for double-stator single-rotor and double-rotor single-stator permanent magnet machine: Design and operation

Abstract There has been a rapid adoption of electric vehicles (EVs), and the industry has been looking for the most enhanced traction system to improve performance and reliability. Such specifications are mostly dependent on the magnetic materials for brushless DC or AC machines, and typically EVs are manufactured with those machines. In the past two decades, there have also been improved performance ratings of power electronic devices. On one hand, making possible AC motors more suitable for EVs. On the other hand, it allows further advanced topology circuits to fit machines with two rotors or two stators. This paper shows two configurations of electrical machine control for EVs, discuss the pros and cons, and corresponding mathematical models have been developed in order to enhance a Field Oriented Control (FOC) strategy, which has been analyzed and studied through MATLAB based simulations, allowing novel design based on the analytical models developed by the authors.

A Design Technique for Shaping of Back-EMF Using Heuristic Optimization and Winding Factors in Brushless AC and DC Machines

One of the requirements for the electric machine designer is to provide an optimal back-EMF waveform in a high-performance application such as electric vehicles (EV) or hybrid EVs (HEV) systems. This paper presents a design technique for the shaping of back-EMF waveforms in brushless permanent magnet (PM) AC and DC machines. At first, back-EMF waveforms are calculated by an analytical model based on the subdomain field model and the complex permeance function. The finite element and experimental results are used for the verification of the analytical model. Secondarily, the heuristic optimization algorithm is applied to the analytical model in order to search for optimal solutions where the objective function is composed of harmonics in target back-EMF. As a result, the optimal back-EMF waveform can be traced by matching the odd harmonics of the design candidate with those of the target waveform. Also, a detailed investigation of the winding factor is carried out to provide the key design techniques regarding the shape of back-EMF.

Fault-Tolerant Capability and Torque-Speed Measurements of Permanent Magnet Brushless AC Machines

In this paper, the fault-tolerant capability in terms of inductance profile of a dual-stator permanent magnet brushless AC machine is presented. Similarly, its power-speed and torque-speed characteristics are also predicted. Special reference is made to the impact of different rotor pole numbers. A 2D-finite element analysis (FEA) procedure is adopted in this work, and the cross-coupling effect of the machine inductances owing to the influence of direct-axis and quadrature-axis currents is also taken into consideration. The results show that, the investigated machine having odd number of rotor poles would exhibit better fault-tolerant capability compared to their counterparts with even number of rotor poles. Further, the machine having 6-stator slots and 13-rotor poles i.e. 6Slots-13P machine has the greatest field-weakening potential owing to its high full-speed to rated speed relation. The studies also show that, the investigated machines have negligible reluctance torque due to their basically unity saliency ratios obtained from the machines’ almost similar quadrature-axis inductance, L q , and direct-axis inductance, L d , values. Keywords : Direct-axis, Quadrature-axis, Inductance, Power, Speed, Torque, AC machines, Permanent magnet.

A Novel Method to Reduce of Cogging Force in a Linear Flux-Switching Permanent Magnet Brushless AC Machine

The primary linear flux-switching permanent magnet (LFSPM) motor has the merits of high speed, high response, and direct drive. However, the presence of the cogging force in LFSPM motors compromises position and speed control accuracy, which can be particularly troublesome at low speeds. A novel approach of suppressing the slot effect cogging force for LFSPM is proposed according to repetitive control of time-varying periodic signals. The time-varying periodic thrust disturbance in t-domain is transformed to the signals defined in the x-domain, and a repetitive controller is designed. Furthermore, a series of specific harmonic currents are added into q-axis reference current, resulting in additional force components to counteract the end effect cogging force. Finally, experimental evaluations of the control strategy are performed on an AD5435-controlled LFSPM drive platform. The simulations and experiments verify that the proposed method can suppress the thrust ripple effectively.

ANALYTICAL CALCULATIONS OF ELECTROMAGNETIC QUANTITIES FOR SLOTTED BRUSHLESS MACHINES WITH SURFACE-INSET MAGNETS

In this paper, two-dimensional (2-D) analytical magnetic field calculations are used to compute crucial quantities of Brushless permanent-magnet (PM) machines with surface-inset PMs. The analytical magnetic field distribution is based on the subdomain technique in which the slotting and tooth-tip effects have been considered. It includes both different magnetization patterns (e.g., radial, parallel and Halbach) and different spatial distribution of winding (e.g., concentrated and distributed with one or more layers). The saturation effect of the magnetic circuit is neglected. In this investigation, the phase and line induced back electromotive force (EMF) waveforms, electromagnetic/cogging/reluctance torques, self-/mutual-inductances and unbalanced magnetic forces (UMFs) have been analytically calculated. The analytical expressions can be used for Brushless machines having surface-inset PMs with any radius-independent magnetization pattern. In this study, two slotted Brushless PM machines with surface-inset PMs have been selected to evaluate the efficacy of the analytical expressions by comparing the results with those obtained by the finite-element method (FEM). One of the case studies is a 9-slots/8-poles Brushless DC (BLDC) machine with radially magnetized PMs, non-overlapping all teeth wound winding and six-step rectangular armature current waveforms. The other is a 15-slots/4-poles Brushless AC (BLAC) machine with Halbach magnetization PMs, double-layer overlapping winding and sinusoidal armature current waveforms.

Design and Analysis of an Electronic-Geared Magnetless Machine for Electric Vehicles

This paper proposes a new electronic-geared (EG) magnetless machine for electric vehicles. The proposed EG machine can offer the multitooth bipolar-flux (MTBF) operation for the low-gear (high-torque low-speed) situation, and the single-tooth unipolar-flux (STUF) operation for the high-gear (low-torque high-speed) situation. In particular, the balance-position winding arrangement is proposed to enable the machine having balanced flux linkages. Consequently, the proposed machine operates as the brushless ac machine for MTBF operation, leading to offer smoother torque at the low-gear situation. Meanwhile, it operates as the brushless dc machine for STUF operation, leading to produce better torque density at the high-gear situation. Various performances of the proposed EG machine are analyzed, with emphasis on the validity of electronic gearing. The experimental prototype is also built for verification.

Analysis and Comparison of Axial Field Flux-Switching Permanent Magnet Machines With Three Different Stator Cores

Axial field flux-switching permanent magnet machine (AFFSPMM) is a novel brushless AC machine, which incorporates the advantages of flux-switching permanent magnet machine and axial field permanent magnet machine. It is suitable to be applied in the field of direct drive electric vehicles due to short axial size and large torque density. In the paper, the different stator core structures such as U-, C-, and E-shape are researched on basis of a three-phase 600 W conventional AFFSPMM. These parameters including rotor pole numbers, stator tooth shape, and the deflection angle of rotor pole are optimized for obtaining sinusoidal back electromotive force and small cogging torque. The electromagnetic performances are analyzed and compared by 3-D finite element method. It is found that C-core AFFSPMM can get higher output torque in comparison with U-core machine, and E-core machine can improve fault-tolerant capability and reduce the torque ripple.

Design and Analysis of Novel Hybrid-Excited Axial Field Flux-Switching Permanent Magnet Machines

The axial field flux-switching permanent magnet machine (AFFSPMM) is a novel brushless AC machine. The magnets and concentrated windings are both in the stator instead of the rotor. The short axial length is beneficial to save the place of electric vehicles (EVs). Furthermore, the AFFSPMM exhibits the advantages of high power density and torque density. However, it is difficult to regulate the air-gap field since the field is produced by magnets alone. The fault-tolerant capability needs to be enhanced in the application of a direct-drive EV. Hence, a novel hybrid-excited AFFSPMM is proposed, in which the air-gap field can be easily regulated and the mutual inductance is largely reduced. In this paper, the design parameters of the novel machine, such as the split ratio, the rotor pole numbers, the rotor pole width, etc., are optimized in order to attain a sinusoidal back-EMF waveform and low cogging torque. Moreover, the static performance including each phase flux-linkage, inductance, and torque is analyzed on the basis of the 3-D finite-element method (FEM).

Electric Loss Measurement Technique for Synchronous PM Brushless Machines

A novel technique to identify, by direct electric measurements, the total losses at full load in synchronous permanent-magnet brushless ac machines is here presented. The proposed procedure can anyway be extended to anisotropic machines. The procedure adopts a back-to-back electric and mechanic connection capable to use a single three-phase power converter to perform a full power test. The coupled motors are driven in speed as an equivalent electric machine at no load. The motor load can be regulated by the physical angular displacement in between the rotors of the two machines, and the converter will control the speed and equalize the current amplitude in the motors. In this condition, the motors exchange active power at the shaft in mechanical form and at the electric connections in electrical form. The active power supplied by the converter in steady state results in the total power dissipated by the two motors. A simple algorithm to identify the current-to-mechanical angle relationship at a given speed is proposed. The measurement technique is simulated and verified using an experimental test bench that adopts two low-power high-torque electric machines.

Robust Initial Rotor Position Estimation of Permanent-Magnet Brushless AC Machines With Carrier-Signal-Injection-Based Sensorless Control

In sensorless control of permanent-magnet brushless ac machines, it is necessary to identify the initial rotor position prior to start-up of the machine. Although saliency-tracking-based sensorless techniques are proven to be a good candidate to estimate the initial rotor position in sensorless control, magnetic polarity identification is another important issue. Since the machine saliency undergoes two cycles in a single electrical period, the estimated position information based on machine saliency has an angle ambiguity of π. With the aid of magnetic saturation effect, this paper proposes a robust initial rotor position estimation scheme, which utilizes both d- and q-axis carrier currents, instead of only the q-axis carrier current, in conventional pulsating-carrier-signal-injection-based sensorless control algorithm. Integrated with conventional carrier-signal-injection-based sensorless algorithm, the proposed technique could obtain the initial rotor position for both standstill and free-running rotors. Finally, the experimental results confirm the effectiveness and robustness of the proposed method.

Efficiency Comparison between Brushless DC Motor and Brushless AC Motor Considering Driving Method and Machine Design

This paper describes an approach to establish high efficient drive such a Brushless AC (BLAC) drive by a Brushless DC (BLDC) drive. BLDC drive which is excited rectangular voltage for 120 electrical degrees per cycle is commonly used for the low cost applications of Permanent Magnet Synchronous Motors (PMSMs). This drive method contains low-order harmonics in the current waveform. These harmonics cause a mechanical vibration and the electrical losses. Therefore, BLDC drive is not applicable to the sinusoidal back-EMF machines. However, considering about a high frequency loss by the PWM carrier and also about the switching loss, BLDC drive has a potential to have higher efficiency than the BLAC drive. This paper compares the overall efficiency which includes PWM carrier frequency loss by using the coupling analysis between Finite Element Analysis (FEA) and a circuit simulator. The tested machines are two of Surface type PMSMs (SPMSM). One has a sinusoidal back-EMF waveform (BLAC machine) and the other has a trapezoidal back-EMF waveform (BLDC machine). The both driving methods of BLDC drive and BLAC drive are adapted to each machine, their efficiencies are compared by both simulation and experiments. The results show that the BLDC machine with BLDC drive has higher efficiency than BLAC machine with BLAC drive.

Investigation of Torque–Speed Characteristics and Cogging Torque of Fractional-Slot IPM Brushless AC Machines Having Alternate Slot Openings

This paper presents a comparative study of torque-speed characteristics and cogging torque of the fractional-slot interior permanent magnet brushless ac machines having different slot openings, viz. open slot, closed slot, and hybrid slot (sandwiched open and closed slots), and unskewed and step skewed rotors. The influence of such alternate slot openings on the magnetic cross-coupling and cogging torque is particularly emphasized. Furthermore, the repercussion of the manufacturing limitations and tolerances on the cogging torque of machines having these alternate slot openings is also investigated. The magnetic cross-coupling level and the sensitivity of cogging torque to manufacturing limitations and tolerances strongly depend on the slot opening materials. The finite-element analyses are experimentally validated.

A Novel 12/10 Electrical Excitation Flux-Switching Machine

A three phase, electrical excitation flux-switching (EEFS) brushless ac machine is proposed, its electromagnetic performance, viz., the phase flux-linkage and back-EMF waveforms, the torque density, the electromagnetic torque, and the torque-current characteristic, are evaluated based on finite element analysis. An EEFS machine exhibits sinusoidal flux-linkage and back-EMF waveforms. Compared with a hybrid excitation flux-switching (HEFS) machine, when de-excitation at fault is needed, the torque density of EEFS machine is ~20% larger than that of a HEFS machine with the same armature and field current copper loss.

Stator-Flux-Oriented Fault-Tolerant Control of Flux-Switching Permanent-Magnet Motors

Flux-switching permanent-magnet (FSPM) motors are a newly developed brushless AC (BLAC) machine having magnets in the stator. This paper proposes and implements a stator-flux-oriented (SFO) control strategy for fault-tolerant operation of FSPM motors. The key is to set the q-axis component of armature current invariant before and after the fault. In the rotor reference frame, by building a SFO-dq equation of the FSPM motor, the fault-tolerant control strategy is deduced. The finite-element method and the field-circuit cosimulation method are employed to analyze the performance of the FSPM motor drive. Finally, a dSPACE-based FSPM motor drive platform is built for experimental verification. Both the steady-state and dynamic performances at normal and fault-tolerant operations are tested, confirming that the proposed fault-tolerant operation can keep the output torque invariant while offering good dynamic performance during fault.

Reduction of Cogging Force in a Linear Flux-Switching Permanent-Magnet Brushless AC Machine for Direct-Drive Applications

The linear flux-switching permanent-magnet (LFSPM) machine is a novel linear brushless ac machine with magnets and armature windings in the short mover, which is a competitive candidate for direct-drive applications due to the inherent sinusoidal back-electromotive force and high force capability. However, the resulting cogging force is serious because of the double salient structure. In this paper, a one-tooth model for the cogging force mechanism and a method for cogging force analysis are introduced. Moreover, an optimization method for minimizing the cogging force and an optimized design for the LFSPM machine are proposed. Both the finite-element analysis and the test measurement verify that the analytical method and the optimization technique are correct and effective, and these methods can be applied to cogging force analysis in the other double salient permanent-magnet machines.

Influence and Compensation of Inverter Voltage Drop in Direct Torque-Controlled Four-Switch Three-Phase PM Brushless AC Drives

This paper presents direct torque control (DTC) methodology for a four-switch three-phase (FSTP) inverter-fed permanent magnet brushless ac (PM BLAC) machine, with reference to a conventional six-switch three-phase (SSTP) inverter. It has been found that when derived from conventional voltage model flux estimation scheme, the predicted stator flux imbalance may be caused by unbalanced inverter voltage drop in the FSTP inverter, in which one phase winding is directly connected to dc-link midpoint. While this imbalanced problem does not adversely affect the performance of current-model-based DTC, it causes significantly nonsinusoidal current waveforms and considerably unbalanced current magnitudes in voltage-model-based DTC. A new compensation scheme taking into account the different forward voltage-drop values in the switching device and the freewheeling diode is proposed for the voltage-model-based DTC to correct for stator flux imbalance via the addition of corrective voltages to flux equations. The proposed scheme has significantly improved the shape of current waveforms with satisfactory balanced magnitudes, total harmonic distortion, and torque ripple factor, as verified by both simulation and experimental results. It has been shown that it is possible for an FSTP inverter to provide similar performance to an SSTP inverter when driving a PM BLAC machine.

Influence of Slot Opening on Optimal Stator and Rotor Pole Combination and Electromagnetic Performance of Switched-Flux PM Brushless AC Machines

The significant influence of slot opening on the optimal stator and rotor pole combination and on the electromagnetic performance of the switched-flux permanent magnet (SFPM) machine is observed and investigated in this paper. A new SFPM brushless machine with remarkable slot opening relative to the magnet thickness is developed to reduce the magnet usage and to increase the slot area. Its stator slot opening is almost several times of that in the conventional SFPM machine, but its magnet usage is only half. However, the new machine exhibits ~ 40% larger back EMF and electromagnetic torque than those of the conventional machine, while its cogging torque and torque ripple are significantly lower.

A Novel Hybrid-Excited Switched-Flux Brushless AC Machine for EV/HEV Applications

A novel E-core hybrid-excited switched-flux permanent-magnet (SFPM) brushless machine is proposed based on an E-core SFPM machine, which has significantly less magnet and higher torque density than those of a conventional SFPM machine. The proposed machine has a simple structure. The main flux path of dc excitation does not affect the magnet excitation because it is not through magnets. The combination of stator and rotor pole numbers of the proposed machine is optimized, and the flux-enhancing and flux-weakening capabilities are investigated by 2-D finite-element analyses and experimentally validated.

Coil connections and winding factors in flux‐switching PM brushless AC machines

PurposeThe purpose of this paper is to analyze the phase coil connections and winding factors of flux‐switching permanent magnet (FSPM) brushless AC machines with all poles and alternate poles wound, and different combinations of stator and rotor pole numbers.Design/methodology/approachThe coil‐emf vectors, which are widely used for analyzing the conventional fractional‐slot PM machines with non‐overlapping windings, are employed for FSPM machines.FindingsAlthough the coil‐emf vectors have been employed to obtain coil connections in the conventional fractional‐slot PM machines, they are different in FSPM machines. It is mainly due to different polarities in the stator of FSPM machines. In addition, from the coil‐emf vectors it is able to predict whether the back‐emf waveforms are symmetrical or asymmetric.Originality/valueThis is the first time that coil‐emf vectors are used to determine the coil connections and winding factors in FSPM machines with different topologies and combination of stator and rotor pole numbers.

Analytical Prediction of Eddy-Current Loss in Armature Windings of Permanent Magnet Brushless AC Machines

This paper presents an analytical model for prediction of eddy current loss in armature windings of permanent magnet brushless AC machines. The developed model can either be used in the case of internal or external rotor radial-field machines topologies. First, a 2-D exact analytical solution of magnetic field distribution in an actual geometry of slotted surface mounted PM radial flux synchronous machines is established. It involves solution of Maxwell's equations in slots, airgap and PM's region. Then, magnetic vector potential solution in the slots is used for prediction of resistance limited eddy current in armature windings. Finally, results from this analytical model are compared to corresponding finite element analyses.