Dc Field Current Research Articles

Influence of DC-Field Current on Short-Circuit Current in Variable Flux Reluctance Machine

DC-field windings of variable flux reluctance machine are wound on the same teeth with armature winding, which increases the short-circuit probability among the armature and field windings. Therefore, in this article, the various short-circuit faults of single phase, phase-to-phase, single field and phase-to-field are illustrated, and the influence of dc-field current on short-circuit current (SCC) and braking torque (BT) with consideration of initial rotor position are investigated. The SCC with dc-field current is analytically established, where the SCC and BT in transient and steady states are analyzed. The maximum SCC and BT are comprehensively compared by finite element method. It is found that the influence of dc-field current on the SCC and BT under transient conditions can be neglected. However, when the machine under un-saturated conditions, the SCC in steady states is associated with the saturation of iron core, i.e., the more saturated of the core excited by dc-field current in prefault, the smaller of the SSC in post fault. Both the SCC and BT are accompanied by the 12/14-pole VFRM prototype experimental verification.

Open-Delta Flux-Modulating Consequent Pole Motors

This paper proposes a new type of variable-flux permanent magnet motor, named “open-delta flux-modulating consequent pole motor” (OFCM). The armature winding ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">W_a</i> ) of the proposed OFCM is connected in an open delta, and it also serves as a field winding ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">W_f</i> ) by simultaneously supplying it with three-phase armature current and DC field current. The flux linkage of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">W_a</i> can be adjusted by changing the field current. The flux-regulation capability and torque characteristics of the OFCM were investigated through finite element analysis (FEA) and experiments on a prototype machine. Using FEA, efficiency maps of the OFCM were also compared to those of a flux-modulating consequent pole motor (FCM) with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">W_f</i> . The results show that the OFCM without <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">W_f</i> has significantly higher efficiency and maximum torque than the FCM.

A Novel Field Current Estimation Method for Brushless Wound-Field Synchronous Machine

Compared with other electric machines, wound-field synchronous machines (WFSMs) have the advantage of having three control variables (stator <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d$ </tex-math></inline-formula> - and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$q$ </tex-math></inline-formula> -axis currents and rotor current). Determining the optimum combination of field and armature currents is necessary for ensuring the high-performance control of the WFSM. Two methods are usually employed to supply the field winding of the WFSM. The first one uses brushes and slip rings, while the second one uses a brushless excitation system. Concerning the WFSM with a brushless excitation system, a dc field current is provided by a rotating diode bridge rectifier fed by an exciter machine. Therefore, direct measurement of the field current is not possible. As a solution, field current needs to be accurately estimated. Previously, field current estimation methods proposed in the literature have been based on modeling the brushless exciter machine and the rotating rectifier. Using the brushless exciter machine model, these methods estimate electric quantities of the exciter rotor (currents and voltages); then, through the rotating rectifier model, the rectified field current is estimated. Accordingly, any inaccuracy in the models leads to estimation errors. The effectiveness of the techniques relying on the brushless excitation system model is not clearly justified, especially because the nonlinearity of the brushless system has been intensified by the existence of the rotating uncontrolled diode bridge rectifier. This article proposes an accurate online field current estimation method for WFSMs without the need neither to estimate exciter rotor voltages or currents nor to use a rotating rectifier model. The accuracy and effectiveness of the proposed field current estimation method are demonstrated by experimental validation tests.

A new dual-ferrite–assisted hybrid reluctance machine with two-stage excitation for starter generator application

Reluctance machines with DC field coil in stator are a competitive candidate for starter generation application, due to the elimination of rare-earth permanent magnet (PM), robust structures, and controllable excitation. However, due to the poor excitation ability of DC field coils, the torque density is disadvantageous. Moreover, with the increase in the DC field current, it is exposed to the risk of extra DC saturation in stator teeth. As a consequence, the torque density and efficiency are both constrained. To solve the aforementioned problems, based on the comprehensive consideration of production cost and torque performance, this study proposed a novel type of hybrid reluctance machine with dual-ferrite–assisted in stator slots. The inner-layer ferrite PM is magnetized tangentially, which can effectively achieve the DC-saturation–relieving effect, while the outer-layer ferrite PM is magnetized radially to increase the machine torque density through the flux modulation effect. Based on finite element analysis, the machine torque density and efficiency can be improved by 20 and 5%, respectively. Furthermore, to simplify the excitation system of the DC terminal, a two-stage excitation method is proposed by splitting some turns of armature winding to feed DC field winding with passive rectifier. No power switching devices are needed for the excitation system in this way, making the system highly robust. The effectiveness of the proposed design is verified by time-stepping finite element analysis.

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.

MW-Class Stator Wound Field Flux-Switching Motor for Semidirect Drive Wind Power Generation System

Stator wound field flux-switching (SWFFS) motors with all excitation sources placed on a stator are suitable for wind power generation systems (WPGSs) because of their simple and robust structure, nonusage of rare-earth magnets, wide speed, controllable magnetic field, and easy heat dissipation. However, the claimed advantages of SWFFS motors have not been fully investigated based on megawatt (MW)-class WPGS. Hence, an SWFFS machine is designed and investigated for semidirect drive WPGS in this paper. First, the topology and operation principle of the SWFFS machine are investigated. Second, the air-gap flux density excited by the dc field and armature currents is analyzed separately based on the Fourier analysis method. Third, electromagnetic performances, including air-gap magnetic field, flux linkage, and electromagnetic torque, are calculated by using the theoretical field model and two-dimensional finite element analysis (FEA). Fourth, the output characteristics of the SWFFS generator, such as output voltage, output power, and efficiency, under different operating conditions are obtained. Finally, the analytical model and FEA results are validated through experiments based on an MW-class prototype.

Design and Optimization of a New Magnetic-Geared Pole-Changing Hybrid Excitation Machine

This paper presents a new magnetic-gear pole-changing hybrid excitation machine (MG-PCHEM), which can provide high torque density and improved flux-weakening ability. The key is to flexibly change the pole-pair number of inner excitation sources by injecting variable dc field currents, and, hence, regulate the dominant pole-pair flux components to realize an effective magnetic field adjustment. In the following paper, the machine configuration and its operation principle of flux control are introduced first. Then, the influences of some leading design parameters are analyzed including the slot-pole combination and other dimension parameters, and after which a comparative study is carried out with traditional single-stator topology. Based on the optimal design, electromagnetic performance of the MG-PCHEM is further evaluated by using the finite-element method. Finally, a prototype is designed, manufactured, and fully tested. Relevant experimental results verify the feasibility of this new hybrid solution as well as the finite-element predictions.

Torque Analysis on a Double Rotor Electrical Variable Transmission With Hybrid Excitation

An electrical variable transmission (EVT) can be used as a power splitting device in hybrid electrical vehicles. The EVT analyzed in this paper is a rotating field electrical machine having two concentric rotors. On the outer rotor, permanent magnets (PMs) are combined with a dc-field winding, being the first implementation of its kind. The magnetic field in the machine as well as the electromagnetic torque on both rotors are a function of the q - and d -axis currents of the stator and inner rotor, as well as the dc-field current. To describe and fully understand this multiple-input multiple-output machine, this paper gives an overview of the influence of the different current inputs on the flux linkage and torque on both rotors. Focus is given to the hybrid excitation in the d -axis by combining the dc-field current and the alternating currents. This has the advantage compared to other EVT topologies that unwanted stator torque can be avoided without stator d -axis current flux weakening. Results of the analysis are presented by means of the torque to current characteristics of a double rotor PM-assisted EVT, as well as the torque to current ratios. The machine characteristics are finally experimentally verified on a prototype machine.

Finite-Element Analysis and Experiment of Current Superimposition Variable Flux Machine Using Permanent Magnet

We have proposed a current superimposition variable flux reluctance machine that can control its torque constant. This machine can be operated by using a six-phase superimposition current composed of ac-armature and dc-field currents. In this paper, we propose a current superimposition variable flux machine using permanent magnets. The permanent magnets increase the dc magnetic flux. First, the structure and operational principle of this machine are described. Next, the torque characteristics are computed under vector control. Finally, the computed characteristics are verified by carrying out measurements on a prototype.

Novel Fault-Tolerant Design of Axial Field Flux-Switching Permanent Magnet Machine

Axial field flux-switching permanent magnet machine (AFFSPMM) has significant advantages, such as the short axial size and high torque density. It is suitable for the direct drive in-wheel applications for the electric and hybrid electric vehicles. For improving the reliability, a novel fault-tolerant structure is applied in the AFFSPMM. The stator is composed of E-shaped laminated segments. The phase windings are physically isolated to each other by the middle teeth of E-shaped cores. The field windings wounded around the middle teeth are used to regulate air-gap flux in the proposed machine. The electromagnetic performances are investigated and analyzed by three-dimensional finite element method (FEM), including the inductance, the back-EMF, and the torque capability, etc. It is shown that the fault tolerance is improved in the novel AFFSPMM, and a wide speed range of operation can be attained by a small DC field current.

Design and performance evaluation of compound permanent magnet generator with controllable air-gap flux

A novel compound permanent magnet (PM) generator with adjustable air-gap flux is investigated. The proposed structure and control principle are described in detail and design equations are completely derived. By using three-dimensional (3-D) finite-element analysis, the electromagnetic distribution and the performance evaluation are carried out. A 7.5 kW prototype generator is manufactured and tested. The calculated data are compared with the experimental data. It is obvious that the proposed structure generator has good regulation characteristic by means of variation of dc field current.

BEGA Starter/Alternator—Vector Control Implementation and Performance for Wide Speed Range at Unity Power Factor Operation

The Biaxial Excitation Generator for Automobiles (BEGA) is proposed as a solution for integrated starter/alternator systems used in hybrid electric vehicles. This paper demonstrates through experiments and simulations that BEGA has a very large constant power speed range. A vector control structure is proposed for BEGA operation during motoring and generating, at unity power factor with zero d-axis current (i d) and zero q-axis flux (?q) control. In such conditions, BEGA behaves like a separately excited dc brush(commutator) machine, in the sense that no stator inductance voltage drop occurs in such constraint control conditions. A high i q current is required in order to cancel the q-axis flux, during unity power factor operation. This engages higher copper losses in the machine under light load. In order to minimize the copper losses, for lower load levels, a current referencer is proposed. Due to higher dc field excitation time constant, the dc field current response is not very fast, particularly for high-current excursion. In order to increase the torque response quickness, the d-axis current i d is controlled with a nonzero reference value only during transients, when there is a difference between the reference and measured dc field currents. This way, high dynamic performance is secured. Implementation, digital simulation, and experimental results validate the proposed solutions.

Efficiency Optimization of a Permanent-Magnet Hybrid Brushless Machine Using DC Field Current Control

This paper proposes a new efficiency optimization approach for a new kind of permanent-magnet hybrid brushless (PMHB) machine. The key is to propose the self-searching control (SSC) for the PMHB machine in such a way that the input power at the given load and speed can be progressively minimized by tuning the dc field current. A loss analysis of the machine is given to justify the use of dc field current as the control parameter. Detailed experimental results confirm that the proposed approach can effectively accomplish the desired efficiency optimization.

Design and control of a flux-controllable stator-permanent magnet brushless motor drive

In this paper, a new flux-controllable stator permanent magnet (PM) brushless motor drive is proposed which is particularly attractive for electric vehicles. The key of the proposed motor is to incorporate both PM materials and dc field windings in the stator, hence offering a compact arrangement of hybrid-field excitations, while the rotor is simply composed of salient poles without windings or PMs. Thus, the flux control, including both strengthening and weakening, can be easily achieved by tuning the dc field current. Both finite element analysis and experimental results confirm that the proposed motor drive can offer effective flux control, fast speed response, wide constant-power operating range, and high efficiency over wide-speed operation.

Consequent-pole permanent-magnet machine with extended field-weakening capability

In this paper a description and operating principles of a consequent-pole permanent-magnet machine are presented. In addition, a sizing analysis, finite-element analysis, and experimental results for a prototype machine are addressed. Due to its particular configuration, this machine allows for a wide range of control of the air-gap flux with minimum field ampere-turn requirements and without brushes or slip rings. Two components of the field flux are produced. One, which is almost constant, is produced by the permanent magnet located on the rotor surface. The other, which is variable, is produced by a field winding positioned circumferentially in the center of the stator. These two flux components converge in the air gap. The excitation level of the machine is manipulated by controlling the DC field current. Three-dimensional finite-element analysis and experimental results demonstrate that it is possible to vary the flux over a wide range to keep the terminal voltage constant as the speed increases. A 3 kW 1000-3000 r/min eight-pole and 32 VAC generator using this configuration is tested to verify the flux control capability of this structure.

Energy Saver Power Factor Controller for Synchronous Motors

A technique for maintaining a synchronous motor at unity power factor (or minimum line current) from no-load to full-load conditions, assuring peak efficiency, is described. This concept stems from an adaptation of the Energy Saver Power Factor Controller for induction motors recently developed and patented by NASA Marshall Space Flight Center. The method constantly and automatically adjusts the dc field current of a 3-phase synchronous machine such that the ac line current will always be operating at the minimal point of the well known "V" curves. This is accomplished as the load varies over its full operating range.

Excitation Requirement for Rectified Output AC Generators

This paper develops a method for calculating the required dc field current of a three-phase synchronous ac generator for known dc load conditions and machine characteristics. The method is simple to apply and has shown good agreement with test data. The method is of utility for preliminary design and analysis of rectified output alternators, such as may be used in excitation systems of the largest ac generators, space vehicle power applications, or high-power industrial rectification.