Analysis and Design of a Brushless WRSM with Harmonic Excitation Based on Electromagnetic Induction Power Transfer Optimization

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.

This paper introduces a novel topology for generating a spatial third-harmonic current component to excite the rotor field winding of a wound rotor synchronous machine to achieve brushless operation. In this topology, each of the three-phase windings on the stator is connected with a switch in parallel. In particular, two antiparallel thyristors are used to switch during positive and negative half cycles. Two windings are mounted on the rotor of the machine: 1) the harmonic winding and 2) the field winding. Zero-sequence currents are generated, when the switches are closed near zero crossing for a short time interval. Consecutive operation of the switches creates an additional spatial third-harmonic current pulsating in the stator winding. The number of poles of the stator winding and the field winding is the same (four-poles in this case) to intercept the torque generation component of the air-gap flux, whereas, the number of poles of the harmonic windings is adjusted (12-poles in this case) to intercept the harmonic component of the air-gap flux and develop voltage across the harmonic windings. Harmonic voltage is rectified through a rotating rectifier to feed dc current to the field windings. Results verify the proposed topology simulated by a 2-D finite-element method.

A sub-harmonically excited novel brushless wound rotor synchronous machine using a new two-layer winding topology.

This paper proposes a new topology for a brushless wound rotor synchronous machine (BL-WRSM) using a specially designed stator winding supplied through a single three-phase inverter. In this topology, a special stator winding, which comprises of two sets of series-connected windings with an unequal number of turns, is used to generate an additional spatial sub-harmonic component in the stator magneto-motive force (MMF). This additional sub-harmonic MMF (SH-MMF) component is utilized for exciting the field winding of the BL-WRSM. The advantage of the proposed brushless topology is the use of a single inverter compared to a dual inverter used in the existing dual inverter BL-WRSM. For the rotor, there are two separate windings: (1) harmonic winding and (2) field winding. Both the harmonic and field windings are connected in parallel with each other through a rotating rectifier. The additional SH-MMF component generates a rotating air-gap magnetic field, which induces the voltage in the harmonic winding. This induced voltage is then rectified and used to supply a dc current to the field winding. A 2-D finite element analysis is performed to analyze and verify the operating principle of this new BL-WRSM.

This paper proposes a new scheme for a brushless wound rotor synchronous machine (WRSM) by generating an additional, two-pole component of magneto-motive force (MMF) with a series-connected additional three-phase winding with the armature three-phase winding. Unlike existing brushless excitation schemes, which use the inverter to inject harmonic currents in the stator windings, the proposed scheme uses series-connected additional winding on the stator with the armature winding in a two-pole configuration. Consequently, as the current flows in the armature winding, it creates a fundamental rotating air gap flux to interact with the field flux. At the same time, additional rotating flux is created from the additional three-phase winding, which cannot synchronize with the field winding. This additional flux can cause the induction of a voltage in a winding with exactly the same number of poles. For this purpose, a harmonic winding is installed in the rotor along with the field winding connected through a diode bridge rectifier, in order to feed the direct current (DC) to the field winding for rotor excitation without an input current from the brush-slip-ring assembly. The 2D finite-element analysis (FEA) was performed to validate the brushless operation of the proposed machine system.

This manuscript presents a novel consequent pole hybrid brushless wound rotor synchronous machine (CPH-BLWRSM) topology. Compared to existing BLWRSM, the proposed topology mainly exploits the inherent subharmonic component (SHC) field excitation to attain brushless operation with limited usage of rare permanent magnet materials and assists in achieving improved torque attributes. The proposed CPH-BLWRSM topology is validated on a 12-slot, 8-pole machine with a 3-phase single-layer armature winding. The rotor of the proposed machine contains 4 poles of harmonic winding (HW), 8 poles of field winding, and permanent magnets on alternate poles. The armature winding on the stator is supplied by a single 3-phase inverter to generate stator MMF, which comprises two dominant components: (1) the fundamental component with supply frequency and (2) the subharmonic component (SHC) with half of the supply frequency. The fundamental component of the stator MMF produces the main stator field, while the SHC induces AC voltages in the HW. A rotating rectifier mounted between the HW and the field winding converts this AC to direct current (DC), and thus the field winding on the rotor receives DC from the rectifier without brushes. The JMAG-designer software is employed to perform 2-dimensional finite element analysis of 12-slot and 8-pole CPH-BLWRSM. The proposed machine exhibits high starting, maximum, and average torques as compared to existing BLWRSM with less input stator current magnitude, making it suitable for loads that require high starting torque.

ABSTRACTSynchronous machines are electric machines characterised by a rotating speed proportional to the AC supply and are independent of load. Traditional permanent magnet synchronous machines utilise rare earth minerals for rotor flux generation, posing challenges due to rising costs. In such conditions, wound rotor synchronous machines with electromagnets may offer several economic and operational benefits, particularly for applications requiring a wide range of speeds and better torque density. However, rotor excitation in these machines requires brushes and slip rings, which lead to higher initial and maintenance costs, increased sensitivity to fluctuations, reduced speed regulation, and lower power factor. Existing designs of brushless wound rotor synchronous machines use additional rotor windings, leading to increased complexity and reduced efficiency. Techniques such as third harmonic excitation have been employed to eliminate brushes, but these solutions often suffer from low starting torque and inefficient stator utilisation. To overcome the issues associated with existing brushless wound rotor machines, this article proposes a novel winding scheme that incorporates an additional stator winding to enhance the slot fill factor. Furthermore, the benefits of single and dual inverter‐fed designs are analysed, highlighting the reduced cost, size, and simplified circuitry of the single inverter design, along with the improved control and performance offered by the dual inverter design. Finally, an advanced harmonic excitation method is integrated to optimise torque production. A thorough 2D finite element analysis has been conducted to validate the proposed designs and operational principles of the brushless wound rotor synchronous machine, demonstrating its potential for enhanced performance in various electric machine applications.

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.

Several brushless topologies for wound rotor synchronous machines (WRSMs) have been presented in recent years to replace the brushes and slip-ring assembly in conventional WRSMs [1–4]. The brushless operation of the WRSM presents these machines as an alternative to the permanent magnet synchronous machines (PMSMs) because of their low cost and competitive performance as compared to the PMSMs. However, the drawback of the brushless wound rotor synchronous machines (BL-WRSMs) is their low starting torque because of the absence of the excitation source on their rotor. Although, the BL-WRSM achieves the rated torque at the base speed but low starting torque under the base speed makes these machines less suitable for variable speed applications. A PM assisted BL-WRSM was presented in [5] to overcome the problem of the low starting torque and to provide better performance under full-load conditions. However, the presence of the PM on each pole increases the cost of the machine. Moreover, the leakage flux is increased in PM assisted machines. These issues can be resolved by the consequent pole (CP) machines [6]. In this paper, a consequent pole hybrid brushless wound rotor synchronous machine (CPH-BLWRSM) is proposed to overcome the problem of low starting torque, and to reduce the PM usage as compared to the PM assisted machines. A 2-D finite element analysis is performed to validate the proposed CPH-BL-WRSM and its performance is compared with the existing BL-WRSM and PM assisted BL-WRSM.

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.

This paper presents an optimal design of a recently introduced brushless wound-rotor synchronous machine (BL-WRSM). The BL-WRSM, with a special stator winding arrangement, utilizes a single three-phase inverter for generating an additional spatial subharmonic component in the stator magnetomotive force (MMF). This subharmonic component of the stator MMF is used for the brushless excitation of the rotor. The pole span and pole shoe height were the optimized parameters, with the goals of improving the quality of output power and reducing torque ripple. Moreover, the average torque of the machine was improved by optimizing the harmonic winding placed on the rotor. The optimized BL-WRSM was further analyzed for the flux weakening operation. Finite element analysis (FEA) was carried out to analyze the performance of the BL-WRSM. Finally, the performance of the optimal BL-WRSM model was verified through an experimental test, which demonstrated good agreement with the simulation results.

This paper presents a novel dual-mode wound rotor synchronous machine (DWRSM) for variable speed applications. The proposed machine combines the advantages of both the conventional wound rotor synchronous machine (CWRSM) and the brushless wound rotor synchronous machine (BWRSM). Unlike the existing BWRSM, through the dual-mode operation of the proposed machine, constant torque is achieved in the constant torque region by operating the machine in mode-I, i.e., as a CWRSM, and constant power is achieved in the field weakening region by operating the machine in mode-II, i.e., as a BWRSM. The mode change is performed through an additional thyristor drive circuit. The airgap magnetomotive force (MMF) in both modes is derived analytically. To verify this principle, finite element analysis (FEA) and an experiment on a 1- horsepower prototype machine was performed, and key influential factors were verified. The transients in the stator currents and torque during the mode change was analyzed. The test results validated the correctness of the theory and the FEA results.

This manuscript discusses a novel harmonic current excitation technique for a wound rotor synchronous machine based on control of the time harmonic magneto-motive force (MMF) for brushless operation. Two inverters are used to feed the armature winding in such a way that at a time, one inverter supplies the current to armature winding. The two inverters are 180° phase-shifted. The inverter assignment is based on time-division multiplexing technique to generate continuous armature currents containing reasonable third-harmonic component beside fundamental component. This third-harmonic current component generates pulsating MMF and is induced in the rotor harmonic winding; the induced current is rectified and applied to rotor field winding to generate rotor field. The rotor field interacts with armature fundamental current component for torque generation. Finite element analysis is performed to verify the idea.

Permanent magnet synchronous machines (PMSMs) utilize permanent magnets to generate the rotor flux needed to produce torque. They are ideal for high-accuracy fixed-speed drives. But rise in the price of rare earth magnet caused by its scarcity is a key consideration while developing such kind of machines. In this situation, wound rotor synchronous machines (WRSMs) have benefits compared to PMSMs as it does not need magnetic material and can be operated in a wide range of speeds [1]. In a WRSMs field windings are used, which is supplied with a DC current that generates the rotor flux. DC current can be applied to the rotor winding either using the brush and slip-ring structure or brushless excitation method. The brushes and slip rings have losses and maintenance issues, so they are used for small synchronous machines. For brushless excitation, additional (auxiliary) stator windings are used to excite the rotor field winding. Several patents are granted according to this excitation method [2]-[4]. The conventional method faces issues, since both the stator windings are located in the same stator core and require a large stator volume for making place for the both windings. Using a single winding in the stator to perform the brushless operation for the WRSMs is presented in [5]. But in that case concentrated windings are used to develop the higher harmonics for the excitation which will result in more losses. Thus a new idea for brushless WRSMs is proposed in which single stator winding is used to excite the rotor as well as generate the fundamental air gap harmonic for electromagnetic torque. A 2D finite element analysis (FEA) have been done to analyze and verify the proposed brushless WRSMs.

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.