Wound Rotor Synchronous Machine Research Articles

A Novel Rotor Harmonic Winding Configuration for the Brushless Wound Rotor Synchronous Machine

In the last decade, permanent magnet (PM)-free or hybrid PM machines have been extensively researched to find an alternative for high cost rare-earth PM machines. Brushless wound rotor synchronous machines (BL-WRSMs) are one of the alternatives to these PM machines. BL-WRSMs have a lower torque density compared to PM machines. In this paper, a new topology is introduced to improve the torque producing capability of the existing BL-WRSM by utilizing the vacant spaces in the rotor slots. The new topology has two harmonic windings placed on the rotor which induce separate currents. A capacitor is used between the two harmonic windings to bring the currents in phase with each other. The harmonic winding currents are fed to the rectifier which is also placed on the rotor. Due to additional harmonic winding, the overall field current fed to the rotor field winding has been increased and hence the average torque has also increased. Finite element analysis (FEA)-based simulations are performed using ANSYS Maxwell to validate the proposed topology. The results show that the average torque of the machine has been significantly increased compared to the reference model. The detailed comparison results are provided in this paper.

A Sinusoidal Doubly Salient Electromagnetic Machine Drive Fed by Third-Harmonic Injection Two-Stage Matrix Converter With Integrated Injection Inductor

This manuscript studies a sinusoidal doubly salient electromagnetic machine (SDSEM) drive fed by third-harmonic injection two-stage matrix converter (3TSMC) with integrated injection inductor to improve the power density of the whole system. First, compared with the conventional matrix converter (MC), the 3TSMC suffers from larger volume due to the inductor in the harmonic current injection circuit. In the proposed system, the harmonic injection current is synthesized by the field current of SDSEM and the injection inductor is removed. Then, the control strategy is proposed to decouple the current injection control and field current control, which means that the harmonic current and the field current can be regulated flexibly. Finally, a prototype is built to evaluate the proposed system and control strategy. Furthermore, the proposed 3TSMC-fed drive with the corresponding control strategy is also suitable for the other DC-excited three-phase sinusoidal AC machines, such as wound rotor synchronous machines.

Electromagnetic analysis of a wound rotor synchronous machine using an improved subdomain technique considering finite permeability of the core

This study proposed an improved analytical method for electromagnetic field analysis of a wound rotor synchronous machine (WRSM) considering the permeability of soft magnetic materials. A simplified analytical model considering the magnetic permeability of each region is presented. The governing equations of each region are derived from Maxwell’s equations and the electromagnetic field theory, and a general solution is derived by using mathematical techniques. The analytical solutions of all domains are derived by calculating the boundary conditions. To validate the proposed analytical method, the radial and circumferential magnetic flux densities are compared with finite element analysis (FEA) results. In addition, electromagnetic performance parameters such as flux linkage, back-electromotive force, and torque are determined using electromagnetic theories. In particular, the magnetic saturation of the soft magnetic material is due to field and armature current, and the superiority of the proposed method is demonstrated by comparing the improved analytical method considering the global saturation of each region with the nonlinear FEA result considering local saturation. The proposed analytical method can be widely used in the initial and optimal design of WRSM because it can consider saturation of the core due to changes in field and armature current.

Sub-harmonic-based Cost-effective Brushless Wound Rotor Synchronous Machine Topology

This paper proposes a sub-harmonic-based brushless wound rotor synchronous machine (WRSM) topology. The proposed topology involves two three-phase stator windings with a different number of turns. Both windings are linked in parallel and are provided with current from a single inverter. One of these windings is a four-pole winding while the second winding is a two-pole winding. This arrangement generates a magneto-motive force (MMF) in the air-gap of the machine comprising of two components: a regular fundamental MMF and a sub-harmonic MMF. The fundamental component produces the main stator field whereas the sub-harmonic component generates a sub-harmonic field that is utilized to produce a harmonic current in a two-pole rotor harmonic winding. The induced harmonic current is rectified to inject direct current (DC) to the field winding and produce a four-pole rotor magnetic field. The four-pole rotor magnetic field when magnetically interacts with the same number of the main stator field poles producing torque. Finite element analysis (FEA) is carried out to confirm the operation and achieve the electromagnetic behavior of theproposed topology.

Piecewise Affine Maximum Torque per Ampere for the Wound Rotor Synchronous Machine

Power-efficient torque control of the Wound Rotor Synchronous Machine (WRSM) below base speed requires a minimization of electrical losses, namely copper losses. In this paper, the Maximum Torque per Ampere (MTPA) optimization problem is presented and solved using a convex pareto frontier of simulated or measured data points. Three filtered solution sets are mapped to current space using piecewise affine functions, which approximate the current using a piecewise linear function for a given torque. This set of piecewise linear functions enables a machine controller to implement MTPA online or as an offline lookup table. Simulated and experimental results are presented for a 65 kW, FEA-sampled WRSM. Compared to linear MTPA, the PWA MTPA functions are shown to reduce torque error by > 25%, reduce average copper loss by > 20%.

Novel Double Mode Dual-Stator Wound Rotor Synchronous Machine for Variable Speed Applications

This paper offers a novel dual-mode double stator wound rotor synchronous machine for variable speed applications. The proposed motor integrates the benefits of both the traditional wound rotor synchronous machine (WRSM) and brushless wound rotor synchronous machine (BL-WRSM). A constant torque can be attained in the maximum torque per ampere region by operating the proposed machine as a traditional WRSM in Mode I, and a constant power can be attained in the field-weakening region by operating it as a BL-WRSM in Mode II. Moreover, due to the dual-stator structure, the proposed machine exhibits improved performance in terms of high torque density as compared to the existing single stator BL-WRSM. By using a special stator winding arrangement to achieve the sub-harmonic component of the stator magnetomotive force, the brushless operation of the proposed machine is achieved. The additional sub-harmonic component induces a voltage in the harmonic winding placed on the rotor, which is then rectified and provided a DC current to field winding for brushless excitation. In order to validate the effectiveness of the proposed machine, a two-dimensional finite element analysis (FEA) is carried out.

Piecewise Affine Modeling of Wound-Rotor Synchronous Machines for Real-Time Motor Control

High-performance control of Wound Rotor Synchronous Machines (WRSM) motor drives requires a nonlinear magnetic model that captures magnetic saturation and cross saturation. This research proposes a generalized Piece-wise Affine (PWA) magnetic and state-space model that divides the current and flux space into simplices. PWA functions are obtained from experimental or Finite Element Analysis (FEA) data leveraging the Delaunay triangulation. Error margins and memory requirements can be optimized for given applications by varying the number of PWA functions. The PWA domains can be irregular and an algorithm to optimize accuracy for a given number of PWA domains is proposed. The PWA performance is compared with a high-fidelity magnetic model. Irregular PWA functions have lower error than regularly gridded tables at reduced size: average error is improved from 15% to 1%; peak error is improved from 20% to 9%. Finally, the PWA state-space model is validated on an experimental testbench with a real-time Micro-Controller Unit (MCU). The state space equations are verified using steady-state and dynamic tests.

A Parameter-Free Method for Estimating the Stator Resistance of a Wound Rotor Synchronous Machine

This paper presents a new online method based on low frequency signal injection to estimate the stator resistance of a Wound Rotor Synchronous Machine (WRSM). The proposed estimator provides a parameter-free method for estimating the stator resistance, in which there is no need to know the values of the parameters of the machine model, such as the stator and rotor inductances or the rotor flux linkage. In this method, a low frequency sinusoidal current is injected in the d axis of the stator current to produce a sinusoidal flux in the stator. In this paper, it is shown that the phase difference between the generated sinusoidal flux and the injected sinusoidal current is related to the stator resistance mismatch. Using this phase difference, the stator resistance is estimated. To validate the proposed model-free estimator, simulations were performed with Matlab Simulink and the results were compared with the extended Kalman filter observer. Finally, experimental tests, under different conditions, were performed to estimate the stator resistance of a WRSM.

Investigation of Nonlinear PI Multi-loop Control Strategy for Aircraft HVDC Generator System with Wound Rotor Synchronous Machine

In order to enhance the transient performance of aircraft high voltage DC (HVDC) generation system with wound rotor synchronous machine (WRSM) under a wide speed range, the nonlinear PI multi-loop control strategy is proposed in this paper. Traditional voltage control method is hard to achieve the dynamic performance requirements of the HVDC generation system under a wide speed range, so the nonlinear PI parameter adjustment, load current feedback and speed feedback are added to the voltage and excitation current double loop control. The transfer function of the HVDC generation system is derived, and the relationship between speed, load current and PI parameters is obtained. The PI parameters corresponding to the load at certain speed are used to shorten the adjusting time when the load suddenly changes. The dynamic responses in transient processes are analyzed by experiment. The results illustrate that the WRSM HVDC generator system with this method has better dynamic performance.

Brushless Operation of Wound-Rotor Synchronous Machine Based on Sub-Harmonic Excitation Technique Using Multi-Pole Stator Windings

This paper presents a topology for the brushless operation of a wound-rotor synchronous machine based on the subharmonic excitation technique by using two sets of multi-pole windings on the armature as well as on the rotor. The armature windings consist of a four-pole three-phase main winding and a two-pole single-phase additional winding, responsible for the generation of fundamental and subharmonic components of magnetomotive force (MMF), respectively. The rotor contains four-pole field winding and two-pole excitation winding. From the generated air gap MMF, the additional winding is responsible for induction in excitation winding, which feeds DC to the field winding through a rotating rectifier without the need of brushes. Then, the interaction of the magnetic field from the main and the field windings produces torque. The proposed topology is analyzed using 2D finite element analysis (FEM). From the analysis, the generation of the subharmonic component of MMF is verified, which helps in achieving the brushless operation of the wound-rotor synchronous machine. Furthermore, the performance of the proposed brushless multi-pole topology is compared with the existing dual three-phase winding multi-pole topology in terms of current due to induction, output torque, torque ripples, and efficiency.

Analysis of DC Winding Induced Voltage in Wound-Rotor Synchronous Machines by Using the Air-Gap Field Modulation Principle

In order to analyze the DC winding induced voltage in the wound-rotor synchronous machine, this paper uses the air-gap field modulation principle to investigate its operation mechanism and harmonic order. By establishing the analytical magneto-motive force (MMF)-permeance model, the DC winding induced voltage per electrical cycle under open-circuit condition, armature reaction condition and on-load condition are deduced. Analytical analysis shows that the MMF function, stator and rotor permeance function are critical factors that influence the harmonic order of the DC winding induced voltage. The analysis results are compared with those predicted by the finite element analysis (FEA). Both non-linear steel and linear steel conditions are accounted in the FEA analysis, and the results show that the analytical deduction result agrees well with the FEA analysis result.

Comparative Study of Electrically Excited Conventional and Homopolar Synchronous Motors for the Traction Drive of a Mining Dump Truck Operating in a Wide Speed Range in Field-Weakening Region

A synchronous homopolar motor (SHM) has a salient pole passive rotor, an excitation winding located on the stator, and no permanent magnets, which ensures high reliability and makes this type of motor a good alternative to motors traditionally used in traction drives. However, there is no comparison between SHMs and conventional brushed synchronous machines for traction applications in the literature. In this paper, the performances of a wound rotor synchronous machine (WRSM) and SHM are theoretically compared at the operating points of a 370 kW dump mining truck drive traction curve that has a 10:1 constant power range in the field weakening region. The nine-phase motors under comparison have the same outer diameter of the stator lamination. Before comparison, both motor designs are optimized using the Nelder–Mead method to minimize the semiconductor inverter rated current and the operating cycle power loss. The main advantages of the WRSM, which was designed, are reduction in stator length, smaller losses, and smaller inverter. The reduction in the total stator length was by 1.23 times taking into account the winding end parts as well. Losses were reduced by 1.21 times for the same radius of the stator lamination. Finally, the cost of power modules of the inverter was decreased by 1.4 times. SHM is more reliable since its rotor does not have an excitation winding and a diode rectifier, as in a WRSM with a brushless exciter. In addition, SHM provides lower consumption of copper, which reduces the total mass and cost of active materials. This article also introduces a new term, “inverter utilization factor”, which can be useful, more informative than motor power factor, when comparing traction drives with different types of motors.

Comparative Study of Biased Flux PM Machines Having Different Stator Core Segments and Armature Winding Configurations

This article proposes a comparative study of biased flux permanent magnet (BFPM) machines having different stator core segments and armature winding configurations. The topologies of the four investigated machines and their operating principles are introduced first. Then, in the case of same envelope size and copper loss, all four BFPM machines are designed based on the single variable optimization method to obtain higher average torque, along with their comparison of electromagnetic characteristics such as phase back-electromotive force (back EMF), cogging torque, and torque capability. It is shown that by exploiting both flux focusing (FC) and dual armature (DA) design, the average torque of the BFPM machine has a 132% sharp increase. Furthermore, to reveal the real performance of the DA-FC-BFPM machine, its comparison with machines such as DA doubly salient permanent magnet (DA-DSPM) machine, DA flux reversal permanent magnet (DA-FRPM) machine, DA flux switching permanent magnet (DA-FSPM) machine, wound rotor synchronous (WRS) machine, and surface-mounted permanent magnet (SPM) machine is performed. Finally, a prototype machine is built and tested to verify the finite-element predictions.

Current Sensorless Control for a Wound Rotor Synchronous Machine Based on Flux Linkage Model

Reducing manufacturing costs and increasing reliability are two major issues for the automotive industry. The development of sensorless control methods and estimation techniques for synchronous machines are ways to reduce the impact of these constraints. In this article, a control method is applied to a wound rotor synchronous machine (WRSM), often used in HEV applications, to estimate stator currents and control the machine without using any current sensor on the stator side. In this estimator, instead of the electric state model based on the currents, a flux-based model is used, which increases the precision of the estimator, improves the dynamics of stator currents, and has a better behavior in the case of magnetic saturation. Using this model and applying a Luenberger observer, the stator currents are estimated and then used in the current control loop. The performances of the proposed method are studied in a simulation part. Experimental tests are also performed to verify the effectiveness of the proposed control method.

Brushless Wound Rotor Synchronous Machine Based on a Consequent-Pole Rotor Structure with Better Torque Attributes

This paper presents a subharmonic-based brushless wound rotor synchronous machine (BL-WRSM) topology with permanent magnet-assisted (PMA) and consequent-pole (CP) rotor structures to achieve better torque characteristics as compared to the conventional subharmonic-based brushless WRSM. The proposed topologies use the conventional high-efficient subharmonic field excitation technique to achieve a brushless operation and a lower volume of permanent magnet (PM) to achieve better average, starting, maximum, and minimum torques and lower torque ripple. A four-pole, twenty-four-slot (4-pole/24-slot) machine with conventional, PM-assisted (PMA), and consequent-pole (CP) rotor structures are developed in a JMAG-Designer 20.1 environment to carry out two-dimensional finite element analysis (2D-FEA). The developed machines are used to validate the operation and achieve electromagnetic and electromechanical performances of the proposed subharmonic-based BL-WRSM topologies with better torque attributes.

A novel two-layer winding topology for sub-harmonic synchronous machines

With optimized design and modern brushless operation, wound rotor synchronous machines are resurrecting as a strong contender in many applications presently dominated by permanent magnet machines. Considering the notion, this paper introduces a novel brushless synchronous machine topology that utilizes sub-harmonic magnetomotive force (MMF) of its stator winding for desirable brushless operation. It is significant to state that the sub-harmonic MMF component that is used in this novel topology is one-fourth of the fundamental MMF component, whereas, in previous practices, it was half. Moreover, the stator of the machine uses a new winding arrangement of two sets of balanced three-phase windings wound in two layers to produce the fundamental and the sub-harmonic MMF. To achieve the brushless excitation, the rotor utilizes an additional winding that is used to induce the electromotive force (EMF) by the sub-harmonic MMF component of the stator. This novel two-layer stator winding topology permits the utilization of maximum allowable space in the stator to house conductors in all of its 48 slots, which was not the case in previous papers. To validate the performance, and feasibility, an 8-pole 48-slot brushless wound rotor synchronous motor is designed, and a 2-D finite element analysis simulation is conducted, where the topology shows immense potential in terms of better torque performance.

A Review about Flux-Weakening Operating Limits and Control Techniques for Synchronous Motor Drives

This paper deals with motor design aspects and control strategies for the flux-weakening (FW) operation of synchronous motors. The theory of FW is described by taking into account different control schemes. The advantages and drawbacks of each one are discussed, as well. Moreover, some motor design considerations for achieving an effective FW operation are illustrated for permanent magnet (PM), wound rotor (WR) and reluctance (REL) synchronous machines, using the per unit approach. The distinguishing characteristic of this review provides two-fold attention on both machine design and control strategies obtained by several collaborations with industrial and commercial companies.

Optimization and Performance Evaluation of Non-Overlap Wound-Field Converter-Fed and Direct-Grid Wind Generators

Non-overlap winding technology continues to remain relevant in the design of wound-field machines as an alternative for high torque density permanent magnet machines. In this paper, the finite element analyses-optimisation of two variants of the non-overlap wound-field machines viz., wound-field flux switching machine (WF-FSM) and phase shifting wound rotor synchronous machine (WRSM), are compared, first in terms of their performance for large-scale converter-fed wind generator drives, then experimentally as sub-scaled converter-fed versus direct grid-connected wind generators, respectively. This study is unique because there are no prior attempts to design, optimize and analyse on these machines for medium-speed wind power generation, as well as experiment on sub-scale prototypes for direct-grid and converter-fed operation. All investigations are contemplated in the medium-speed wind generator drivetrain which provides a tradeoff for generator efficiency and size. From the global optimisation of both machines at large-scale power levels, the torque per mass of the WF-FSM is found to be 50 % lesser compared to the WRSM. This is due to approximate volume, with closely matched optimal split and aspect ratios. In terms of the sub-scaled experimentation, both generators can easily vary their generated output power to match with varying wind resource, but direct grid-connected WRSM generator yields better efficiency performance compared to the WF-FSM converter-fed operating mode, given that the generator terminal voltage of the former is highly regulated.

Novel Sub-Harmonic-Based Self-Excited Brushless Wound Rotor Synchronous Machine

This article aims to realize a self-excited wound rotor synchronous machine (WRSM) topology established while considering the subharmonic field excitation scheme. Unlike the conventional subharmonic-based brushless WRSMs that require a dual-inverter configuration, the proposed topology uses a single inverter and a dual-armature winding pattern. The employed dual-armature winding configuration involves a four-pole main armature winding (ABC) and a two-pole open winding (X). The ABC winding is supplied with a three-phase current from a single customary current source inverter (CSI), whereas the X winding carries no current due to its open winding pattern and is responsible for generating subharmonic magnetomotive force (MMF) in the air gap along with the fundamental-harmonic MMF. The fundamental-harmonic MMF is utilized to create a four-pole stator field, while the subharmonic MMF is used to induce the harmonic current in the two-pole harmonic winding of the rotor. The generated harmonic current is rectified to energize the rotor field winding and develop a four-pole rotor field. The electromagnetic interaction of the four-pole stator and rotor fields generates torque. As the proposed subharmonic-based self-excited brushless WRSM employs a single inverter for its brushless operation, this makes it cost-effective compared to the conventional dual-inverter subharmonic-based brushless WRSM topologies. The proposed self-excited brushless WRSM topology is validated through the finite-element analysis (FEA). JMAG-Designer tool is employed to carry out FEA for a four-pole, 24-slot (4p24s) machine. The quantitative relative performance evaluation of the proposed self-excited WRSM topology with the recently developed dual-inverter-controlled subharmonic-based brushless WRSM topology is presented to show its better performance in terms of average, maximum, and minimum torques and torque ripple.

Dual-Mode Brushless Wound Rotor Synchronous Machine for High Starting Torque

This paper proposes a dual-mode brushless wound rotor synchronous machine (DBL-WRSM) for high starting torque. The proposed DBL-WRSM consists of two modes: induction and brushless (BL)-synchronous modes. First, the induction mode operates as a wound rotor induction machine (WRIM) to achieve a high starting torque. Second, the BL-synchronous mode operates as a brushless wound rotor synchronous machine (BL-WRSM) using the subharmonic of the stator magnetomotive force (MMF) in the steady state. To realize the dual-mode topology of the proposed machine, the stator is composed of two divided armature windings and two inverters that can produce subharmonics. The rotor has five switches that convert the rotor winding configuration between the two modes. To verify the proposed topology of DBL-WRSM, finite element analysis (FEA) was performed. Through the FEA results of the transient electromagnetic torque for the voltage source, it was confirmed that the proposed DBL-WRSM not only operates as a BL-WRSM in a steady state but also has a sufficiently high starting torque even under a full-load condition.