Hybrid Excited Machine Research Articles

Partitioned Stator Hybrid Excitation Doubly Salient Machine With Slot Halbach PM Arrays

A new partitioned stator (PS) hybrid excitation (HE) machine with slot Halbach permanent magnet (PM) arrays is proposed in this paper. Due to the adoption of PS structure, its armature winding (AW) is located in the outer stator, and the PMs and the field winding (FW) are housed in the inner stator, thus the space utilization is significantly enhanced. The key difference of proposed machine is that Halbach PM arrays are employed in the inner stator slot openings, so that the parallel magnetic circuit between PM field and wound field is performed and the ratio of PM field short-circuited in the inner stator is effectively reduced. Thus a wider speed regulation region and a higher torque/power density can be achieved. Firstly, the configuration of proposed machine is described, based on which its operating principle is discussed. Then the electromagnetic performances are calculated and compared based on 2-dimensional finite element analysis (FEA). The results show that the torque can be improved by 76% with a similar magnetic field regulation ratio compared with an existing PS-HE machine with slot PMs. Finally, a prototype machine is built and tested to verify the analysis and FEA results.

Partitioned Stator Hybrid Excited Machine With DC-Biased Sinusoidal Current

This article proposes a new partitioned stator hybrid excited (PS-HE) machine with dc-biased sinusoidal current. On the inner stator, the consequent pole permanent magnets (PMs) are employed as the PM excitation. On the outer stator, a set of integrated dual three-phase winding is adopted to produce the wound and armature fields at the same time by injecting the sinusoidal current with dc bias. So, the flexible flux regulation can be realized by controlling the dc bias component. The rotor composed of segmented iron cores is sandwiched between two stators. By employing the air-gap field modulation theory, the air-gap magnetic field harmonics are analyzed to explain the operating and flux regulation principle. Furthermore, a design optimization and the electromagnetic performances of the proposed machine are investigated by finite element analysis. When compared with the existing series and parallel PS-HE machines, the proposed machine can avoid the disadvantage of low torque density of PARALLEL HE machine, and it can solve the problem of the poor flux regulation ability of series HE machine as well, which is mainly contributed by the reasonable space allocation of PMs and field winding excitations. Finally, a prototype is built and tested to verify the theoretical analyses.

Investigation of Novel Doubly Salient Hybrid Excited Machine With Non-Overlapped Field Winding

Hybrid excited machines, with synergies of permanent magnet (PM) and wound field (WF) excitations, exhibit high torque density as well as controllable flux and are perceived as promising candidate for variable speed application. A novel doubly salient hybrid excited (DSHE) machine with non-overlapped field winding is presented in this paper. It inherits simple structure of conventional doubly salient PM (DSPM) machine and facilitates the regulation of magnetic field by field winding. The electromagnetic performances of the proposed machine with different stator and rotor pole number combinations are investigated by finite element method (FEM), confirming these advantages. Furthermore, the effect of iron bridge on the hybridization between PM and WF excitations is analyzed to obtain a trade-off between high torque density and wide flux regulation range with the frozen permeability method (FPM). It is revealed that the proposed DSHE machine possesses higher power and fault tolerance at flux weakening region compared with the DSPM machine, albeit with sacrificed torque density and efficiency with introduction of field winding. Finally, a prototype is fabricated and tested to validate the theoretical analyses.

Investigation of Novel Fractional Slot Nonoverlapping Winding Hybrid Excited Machines With Different Rotor Topologies

This article investigates novel fractional slot nonoverlapping winding hybrid excited (HE) machines with consequent-pole permanent magnet (PM) rotors. The proposed machines accommodate the rotor PM and stator field winding separately, which has solved the spatial confliction of double excitations. Different consequent-pole interior PM rotor configurations are extended to retain the PMs and utilize the flux-concentrating effect. Then, the relationship of field and armature current ratio with the PM and wound field fluxes is identified theoretically, which provides design and optimization guidelines for the HE machines. Furthermore, the HE machines are optimized and compared with conventional PM machines. It is revealed that the HE machines exhibit enhanced torque density with flux-enhancing field excitation, and extended operation range with flux-weakening field excitation. Besides, the HE machine utilizing flux-concentrating effect exhibits the highest PM torque but sacrificed flux regulation capability due to magnetic saturation. Finally, the HE prototypes with different rotor configurations are fabricated and tested to validate the analyses.

Influence of Stator Slot and Rotor Pole Number Combination on Field Winding Induced Voltage Ripple in Hybrid Excitation Switched Flux Machine

The induced voltage ripple in DC field windings can cause many problems in hybrid excitation machines (HEMs). It can generate ripple current in field winding, thus causing unsteady magnetic field excitation and hence additional torque ripple and losses. Besides, the torque and power density of hybrid excitation machines may be deteriorated as a result of the influence of the induced voltage ripple on field winding's excitation circuit, particularly under high-speed conditions. Based on a hybrid excitation switched flux machine (HESFM), the influence of stator slot and rotor pole number combination on no-load and on-load field winding induced voltage ripples are studied and compared by the finite element method (FEM). It reveals that the harmonic content and thus the peak-to-peak value of the field winding induced voltage ripple are closely related to the stator slot and rotor pole number combination at both no-load and on-load operating conditions. The FEM calculations are verified by experiments on a prototype HESFM.

Comparative Study of Series and Parallel Hybrid Excited Machines

Hybrid excited (HE) machines have the synergies of high torque density and high efficiency of permanent magnet (PM) machines and excellent flux controllability of wound-field machines. The PMs and field coils (FCs) coexist in HE machines to provide excitations simultaneously, and can be coupled either in series or parallel. The topologies of HE machines are very diverse, which bring challenges in the selection and design of HE machines. Based on the partitioned stator (PS) platform, a pair of series and parallel HE machines having similar structures and PM usage are evaluated and compared to identify their merits and demerits. With the help of finite element analysis, the flux regulation, energy conversion capability, parasitic effects and irreversible demagnetization of the two kinds of the HE machines are comprehensively investigated and the guidance to the machine design is provided. A pair of prototypes are fabricated and tested to validate the analyses.

Investigation of DC Winding Induced Voltage in Hybrid-Excited Switched-Flux Permanent Magnet Machine

The dc winding induced voltage in hybrid-excited machines can lead to dc winding current ripple and hence, fluctuating field excitation. It may also challenge the dc power source and increase the difficulty of machine control, especially at high speed. The dc winding induced voltage in a hybrid-excited switched-flux permanent magnet (HESFPM) machine is investigated in this article. The phenomenon and mechanism of the dc winding induced voltages under both open-circuit and on-load conditions are analyzed, with particular emphasis on on-load operation which consists of open-circuit induced voltage and armature current induced voltage, respectively. Two techniques, i.e., rotor step skewing and unequal rotor teeth, are proposed and comparatively investigated by the finite-element (FE) method to suppress the dc winding induced voltage under on-load condition. FE results show that the peak-to-peak value of on-load dc winding induced voltage can be effectively suppressed by 96.15% or 89.05% by rotor step skewing or unequal rotor teeth, respectively, while the average on-load electromagnetic torque can be maintained at 89.4% or 87.8%. A prototype HESFPM machine is built and tested to verify the FE results.

A Novel Fractional Slot Non-Overlapping Winding Hybrid Excited Machine With Consequent-Pole PM Rotor

A novel fractional slot non-overlapping winding (FSNW) hybrid excited (HE) machine with consequent-pole permanent magnet (PM) rotor is proposed in this article. The abundant harmonics in the stator magnetomotive force (MMF) of FSNW, which are harmful and tend to cause parasitic effects in conventional PM machines, are utilized to regulate the magnetic field and produce torque through field modulation. The DC excitation current can be injected by open-winding drive circuit and a biased current excited HE machine without field winding can be achieved, which solves the spatial confliction of field and armature windings. The electromagnetic performances of the proposed HE machine are evaluated with finite element method (FEM). It is revealed that the DC excitation can effectively regulate the magnetic field, and consequently, the back-EMF as well as electromagnetic torque. Finally, a prototype is fabricated and tested to validate the theoretical analyses and FE calculations.

Analytical Model of Air-Gap Field in Hybrid Excitation and Interior Permanent Magnet Machine for Electric Logistics Vehicles

With the increasing energy shortages and environmental degradation, electric logistics vehicle (ELVs) with energy conservation and environmental protection has become a research hotspot. The design of machine is the key to develop ELVs. Magnetic field analysis is the most critical issue since its accuracy affects the calculation of motor torque, loss, and other characteristics. To provide a calculation method for the field and performance analysis of the machine for ELVs, this paper presents an analytical model of air-gap field for hybrid excitation and interior permanent magnet machine. In the proposed model, it is taken into account for the shape of the stator and rotor teeth. The flux density on the rotor side is derived by equivalent magnetic circuit (EMC) with leakage magnetic flux. Taking the calculated flux density as one of the second boundary conditions, the air-gap field distribution is calculated by magnetic potential model with the eccentricity of the rotor. To verify the analytical method, we adopted the finite element method. The simulation results of the air-gap flux, back electromotive force, and cogging torque are in good agreement with the analytical results. Besides, applying the analytical model, the machine can be optimized for obtaining the optimal air-gap flux density distribution. The hybrid excitation machine with salient pole and interior magnets can provide a good flux density waveform. The study offers a helpful analytical method for design and optimization of the type of machine for ELVs.

Comparative Study of Relieving-DC-Saturation Hybrid Excited Vernier Machine With Different Rotor Pole Designs for Wind Power Generation

Hybrid excited (HE) machines are a potential solution for wind power generation for their high efficiency from permanent magnet (PM) and good flux regulation ability from flexible field current. However, the principle and machine performance of different structures are diverse, which brings challenges in selection of machine topologies and in the dimensional parameter design. Hence, in this article, two machines with different topologies are presented and analyzed to show their merits and demerits. Both machines have relieving-DC-saturation PMs located in the slot openings, while only one of them has consequent rotor PMs to provide an auxiliary flux and to enhance torque production. In this article, the machine structure and working principle are presented, including the relieving-DC-saturation-relieving effect and air-gap field harmonic analysis. For a fair comparison, under the same outer diameter, rated current density, the dimensional parameters of two machines are optimized with genetic algorithm (GA). Further, the machine performance of the optimal models is compared in terms of torque production, back EMF, voltage regulation ability and speed range. The merits and demerits of both machines are summarized in the conclusion.

Equivalent Magnetic Circuit Model for Consequent Pole Hybrid Excitation Synchronous Machine with AC Field Control

At present, almost all hybrid excitation machines (HEMs) apply DC field current to control the air-gap flux. However, the DC field control mode results in a problem that the capability of field-weakening is not equal to that of field-strengthening. This paper briefly explains the mechanism of asymmetric bidirectional field control capability in present HEMs, proposes a consequent pole hybrid excitation synchronous (CPHES) machine with AC field control mode to solve the asymmetric problem. The structure and principles of CPHES machine are presented. Additionally, the equivalent magnetic circuit model of CPHES machine is derived to reduce the computational complexity of analyzing CPHES structure by using three-dimensional finite element analysis (3DFEA). The proposed equivalent model and 3D-FEA are compared, the results verify the proposed model is basically consistent with the 3D-FEA, and the proposed model is able to reduce the computational complexity greatly.

A Novel Parallel Hybrid Excited Machine With Enhanced Flux Regulation Capability

A novel parallel hybrid excited (HE) machine with wide flux regulation range is proposed in this paper. The flux paths of permanent magnet (PM) and field winding are separated to avoid irreversible demagnetization and achieve improved flux regulation capability. Since the field winding and armature winding are located in the same stator slots, open-winding drive circuit can be employed to feed biased ac excitation in one set of winding and solve the spatial confliction of conventional hybrid excited machine. Finite element (FE) method is utilized to analyze the electromagnetic performances of the proposed HE machine, in comparison with the original HE machine. It is revealed that the proposed HE machine exhibits lower PM excited back-electromotive force (EMF), which is beneficial to high speed fault tolerant operation. Besides, the torque density can be higher with flux enhancing, albeit with reduced PM usage. Finally, a prototype is manufactured and tested to validate the FE predictions.

A New Relieving-DC-Saturation Hybrid Excitation Vernier Machine for HEV Starter Generator Application

Benefiting from flexible flux control and short-circuit withstand ability, the hybrid excitation machine is an emerging starter generator solution for hybrid electric vehicle propulsion. However, to realize a brushless hybrid design in conventional permanent magnet machines, a three-dimensional magnetic circuit is usually adopted, leading to complicated mechanical structure and torque density sacrifice. To address this issue, a new relieving-dc-saturation hybrid excitation Vernier machine is proposed in this article, which integrates good torque density and bidirectional flux control within a simple and brushless structure. The key is to artificially construct reluctance effect in a consequent-pole Vernier permanent magnet machine (CP-VPMM) by introducing extra dc field excitation equipped with the relieving-dc-saturation ability. In this way, the advantages of bidirectional flux control in stator-dc-excited reluctance machine and good torque density in CP-VPMM are well combined in this topology, making it especially suitable for multimode starter generator application. In this article, the machine structure and its design mechanism are introduced, with its electromagnetic performance evaluated by the finite-element simulation. A prototype is manufactured and tested. Experiment results verify the feasibility of this new topology.

Design of Dual-Electrical-Port DC-Coil-Free Hybrid-Excited Machines

This paper incorporates the idea of integrated field and armature windings into the design of novel hybrid excited machines with bi-directional flux modulating effect, which results in a new hybrid-excited machine concept that is referred as dual-electrical-port dc-coil-free hybrid-excited machine (DEP-DCF-HEM). In the proposed design, dc field windings are eliminated and dc bias currents are injected into the windings to generate dc excitation field. In addition, the armature windings are divided into dual electrical ports, and zero-sequence current flows between these two ports can adjust the air-gap field effectively. Compared with existing HEMs with additional dc coils, the proposed machines can achieve both higher torque density and wider flux weakening range. The presented design principle can be utilized in a variety of configurations of DEP-DCF-HEMs. Topologies with stator permanent magnet (PM), rotor PM, and dual PM are possible designs, which are all investigated in this paper. A dual PM DEP-DCF-HEM prototype is manufactured, and experimental measurements are conducted to verify its performances.

Hybrid excited permanent magnet machines for electric and hybrid electric vehicles

This paper reviews various hybrid excited (HE) machines from the perspective of location of PM and DC excitation, series/parallel connection of PM and DC excited magnetic fields, and 2D/3D magnetic fields, respectively. The advantages as well as drawbacks of each category are analyzed. Since an additional control degree, i.e. DC excitation, is introduced in the HE machine, the flux weakening control strategies are more complex. The flux weakening performance as well as efficiency are compared with different control strategies. Then, the potential to mitigate the risk of uncontrolled overvoltage fault at high speed operation is highlighted by controlling the field excitation. Since additional DC coils are usually required for HE machines compared with pure PM excitation, the spatial confliction inevitably results in electromagnetic performance reduction. Finally, the technique to integrate the field and armature windings with open-winding drive circuit is introduced, and novel HE machines without a DC coil are summarized.

Uncontrolled Generator Fault Protection of Novel Hybrid-Excited Doubly Salient Synchronous Machines With Field Excitation Current Control

In the safe-critical applications such as electric/hybrid electric vehicles, the reliability of the machine drive system is vital. Among the various machine fault types, the uncontrolled generator fault (UCGF) at high speed is one of the most serious faults that could damage the machine drive system. In this paper, the fault protection capability of a novel hybrid-excited doubly salient synchronous machine (HEDSSM) is examined to illustrate its great capability to prevent the dangerous overvoltage issue from the UCGF. Permanent magnet flux of the novel HEDSSM is inherently short-circuited when the field excitation current is not fed into the excitation windings. Therefore, when the UCGF is detected in the high-speed region, the dc-link overvoltage issue can be effectively reduced by forcing the field excitation current to zero. Since only the field excitation current is utilized to protect the machine and power inverter from the UCGF, it is easy to implement and the concept of proposed control strategy can also be applied to other hybrid-excited machines. Finally, the proposed control strategy is verified by both simulation and experimental results on the HEDSSM.

A Parallel Hybrid Excited Machine Using Consequent Pole Rotor and AC Field Winding

Hybrid excited (HE) machines with both permanent magnet (PM) and field winding excitations, inherit the benefits of PM machines and improve the flux regulation capability. This paper proposes a novel consequent pole HE (CPHE) machine. The PM pole pair and the iron pole are alternately placed on the rotor, while both the armature winding and ac field winding are located on the stator slots, which achieve the brushless excitation. Since the flux of the field winding goes through the iron poles, the PM and field winding exhibit parallel magnetic paths, which is favorable for the flux regulation and avoids the irreversible demagnetization of the PMs. In this paper, the operating principle of the proposed CPHE machine is illustrated. Moreover, the electromagnetic performance, including back electromotive force, torque, and flux regulation range considering different coil pitch of field winding is analyzed by finite-element analysis. It is demonstrated that the proposed machine exhibits flexible flux regulation capability, which is suitable for applications requiring wide speed range operation or voltage regulation.

Influence of Stator and Rotor Pole Number Combinations on the Electromagnetic Performance of Stator Slot-Opening PM Hybrid-Excited Machine

A new type of three-phase stator hybrid excited machine with permanent magnets (PMs) located at the slot openings of field winding slots is presented in this paper. It has the advantage of good flux regulation capability due to the field excitation. The machines with different stator/rotor pole combinations, i.e., 6-stator pole and 7-/8-/10-/11-/13-/14-rotor poles, have been comparatively investigated in terms of the open-circuit and on-load characteristics, as well as the influence of dc current and the unbalanced magnetic forces. The 2-D finite element analysis (FEA) has been employed for analysis and optimization of the machines together with experimental validation.

Design and Analysis of a New Parallel-Hybrid-Excited Machine With Harmonic-Shift Structure

This paper proposes a new hybrid-excited machine, termed parallel-hybrid-excited machine (PHEM) with harmonic-shift (HS) structure, or HS-PHEM. The key is to alternately place the dc-field pole pair and permanent magnet (PM) pole pair in the stator, thereby achieving the dc-field HS from the PM-field harmonics. Consequently, the dc-field harmonics interact with the PM harmonics to regulate the resultant effective harmonics in the airgap while the parallel flux paths of PM flux and dc-field flux can also be obtained, leading to more effective flux regulation ability. The operation principle and harmonics analysis are discussed, with emphasis on the validity of HS concept. Moreover, the electromagnetic performances of the proposed machine are evaluated by finite-element analysis and compared with other existing topologies in terms of torque, core loss, and efficiency. Finally, the experimental prototype is manufactured and tested for verifications.

Study of a Hybrid Excitation Synchronous Machine: Modeling and Experimental Validation

This paper deals with a parallel hybrid excitation synchronous machine (HESM). First, an expanded literature review of hybrid/double excitation machines is provided. Then, the structural topology and principles of operation of the hybrid excitation machine are examined. With the aim of validating the double excitation principle of the topology studied in this paper, the construction of a prototype is presented. In addition, both the 3D finite element method (FEM) and 3D magnetic equivalent circuit (MEC) model are used to model the machine. The flux control capability in the open-circuit condition and results of the developed models are validated by comparison with experimental measurements. The reluctance network model is created from a mesh of the studied domain. The meshing technique aims to combine advantages of finite element modeling, i.e., genericity and expert magnetic equivalent circuit models, i.e., reduced computation time. It also allows taking the non-linear characteristics of ferromagnetic materials into consideration. The machine prototype is tested to validate the predicted results. By confronting results from both modeling techniques and measurements, it is shown that the magnetic equivalent circuit model exhibits fairly accurate results when compared to the 3D finite element method with a gain in computation time.