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.
Recently, the stable supply of rare-earth resources has become a challenge due to their rapid price fluctuations and environmental concerns associated with their mining and extraction [1–5]. Accordingly, researchers, these days, are actively investigating the nonrare-earth motors that can reduce or eliminate the usage of rare-earth permanent magnets (PMs) [6–8]. Among the nonrare-earth motors, wound rotor synchronous machines (WRSMs) have gotten much consideration due to their high-peak power density [6, 9, 10]. In addition, WRSMs offer the advantage of the ease in field-weakening operation as the rotor field current can easily be controlled from the external direct-current (DC) power source [11, 12]. Therefore, WRSMs have been widely adopted in the market and employed for various products. For example, Renault adopts the WRSM by using it for electric vehicles such as Twizy and ZOE .
However, the conventional WRSMs have a limitation that is mainly related to their excitation method. To excite the rotor field of WRSMs, injecting the direct-current (DC) into the rotor field winding is required. A method suitable for small and medium-sized equipment is to use brushes and sliprings. These brushes wear out due to friction, so regular maintenance is required, and when the field current is large, a voltage drop occurs which increases the power loss and decreases the efficiency of the machine. Exciters and pilot exciters can be adopted as excitation methods suitable for large devices. However, in these methods, since additional devices must be added to the rotor shaft, the overall volume of the motor increases, which further raises the manufacturing cost of the machine. Currently, the latter is adopted as the excitation method of many WRSMs [14, 15].
Researchers have made various efforts to develop simpler, cost-effective, and more convenient excitation methods for the rotor field of WRSMs as compared to the traditional field excitation techniques which involve brushes, sliprings, exciters, and pilot exciters regardless of the power rating of the machine [16–21].
Harmonic current induction-based brushless WRSM topologies have recently received much attention . The structural features of these topologies include harmonic winding, rectifier, and field winding installed on the rotor. The stator is fitted with the armature windings that can produce a magnetomotive force (MMF), which consists of fundamental and harmonic components in the air gap . The harmonic MMF generated by the armature winding induces a current in the harmonic winding of the rotor. The field winding is supplied with a DC from the harmonic winding through a rectifier. Finally, the field winding is excited to produce the main rotor field, and its interaction with the main stator rotating field allows the BL-WRSM to operate and develop torque .
Based on the generation of the harmonic component in the stator MMF of the machine, harmonic current induction-based brushless techniques are classified into two categories . In the first category, the stator MMF with space harmonic characteristics is produced. In this method, a subharmonic component of MMF is commonly generated and employed for the brushless operation of WRSMs through a specific armature winding arrangement [19, 20]. The second category uses a time harmonic-based method to generate stator MMF . In this method, a third harmonic MMF is usually produced and engaged for the brushless operation of WRSMs.
In general, nonrare-earth motors such as WRSMs have lower power density than the rare-earth PM motors such as interior permanent magnet (IPM) motors and surface permanent magnet (SPM) motors . However, the nonrare-earth motors must produce a comparable performance to replace the rare-earth PM motors which are frequently used in various applications. To overcome this problem, researchers have proposed various topologies that can improve the performance of the conventional WRSMs. For example, in [22, 23], topologies that improve the torque characteristics of WRSMs by employing a single air barrier to their rotor pole structures are proposed. The torque profiles of these topologies are further improved by utilizing an optimal design method in . In , the performance of WRSMs was improved by changing the tip structure of the rotor poles. In , a WRSM topology was proposed to enhance its performance by placing a rare-earth magnet between the rotor poles. In , a topology of WRSM is proposed in which a single air barrier and a ferrite magnet are placed in the rotor, and by employing the optimal design process, the performance of the machine was maximized.
In , PMABL-WRSM and CPBL-WRSM topologies are proposed. These topologies used rare-earth PM and are based on the subharmonic field excitation technique presented in [19, 20]. However, this subharmonic field excitation technique involves the armature winding split into two halves, each having a separate star connection. Both halves of the armature winding are powered by a separate inverter and a unique magnitude of input current. This arrangement generated a subharmonic component in the air gap of the machine which was used to induce a harmonic current in the harmonic winding of the rotor to achieve brushless operation. Despite achieving a brushless operation, this topology suffers from several disadvantages such as high torque ripple and high unbalanced radial forces for the rotor structure due to the uneven magnitude of MMF for two halves of the machine. The machine topology had an 8-pole/48-slot structure. It was verified that the average torques of the proposed PMABL-WRSM and CPBL-WRSM topologies were improved by 1.5% and 12.2%, respectively, through FEA simulations.
In this paper, we employed a high-efficient subharmonic-based BL-WRSM that involves the circumferential distribution of four-pole and two-pole armature windings. Both windings are supplied with a different magnitude of input current from different inverters. This results in a subharmonic MMF component which is used for the brushless operation of WRSM . This topology performed exceptionally well in terms of efficiency, torque characteristics, and unbalanced radial forces as compared to the subharmonic-based BL-WRSM topologies presented in [19, 20]. However, this topology is having a low starting torque which makes it unsuitable to be adopted in applications where a high starting torque is required, especially in EV applications. To improve the torque characteristics of this topology, we propose PMABL-WRSM and CPBL-WRSM topologies in which rare-earth permanent magnets are employed to be embedded into the poles of the rotor structures. For the comparative performance analysis, the performances of the conventional BL-WRSM and proposed PMABL-WRSM and CPBL-WRSM topologies in terms of starting, average, maximum, and minimum torques, and torque ripple were compared and analyzed using FEA in JMAG-Designer 20.1, a verified commercial tool.
2. High-Efficient Subharmonic-Based BL-WRSM Topology
We proposed a high-efficient subharmonic BL-WRSM topology as shown in Figure 1(a) in . The stator and rotor winding configurations of this topology are shown in Figure 1(b). It consists of a four-pole main armature winding (ABC) and a two-pole additional armature winding (XYZ). The operating principle of this topology is presented in Figure 2 and is described in detail as follows:
First, the main armature winding (ABC) receives current (IABC) from inverter-1 to create a four-pole fundamental rotating field. At the same time, the additional armature winding (XYZ) receives a different magnitude of current (IXYZ) from inverter-2 to create a two-pole harmonic MMF. These two types of armature windings generate the intended MMF including the subharmonic component. The values of current injected by each inverter are shown in Table 1 and are represented by the following equations:where IA, IB, and IC are the currents generated from inverter-1 and are injected into the main armature winding (ABC). I1 is the maximum magnitude of the current generated by inverter-1. IX, IY, and IZ are the three-phase currents generated by the inverter-2 and are injected into the additional armature winding (XYZ). I2 is the maximum magnitude of the current generated by inverter-2. M means the ratio of I2 and I1. Here, and.
When IABC and IXYZ currents are injected into the armature windings, an MMF with the subharmonic component is created in the air gap. This MMF is defined as follows:where NA and NX denote the ABC and XYZ windings number of turns, respectively. IA and IX are the ABC and XYZ windings currents, P represents the pole number, θ is used to denote the spatial angle, and ω represents the electrical angular frequency.
This MMF produces a current in the equal number of harmonic winding poles installed on the rotor. The induced current is rectified through the rectifier and transferred to the rotor field winding to excite the rotor field. Finally, when the excited rotor and stator fields interact, the high-efficient BL-WRSM operates and generates a torque.
3. Machine Topology
In this study, we employed the PM-assisted and consequent-pole rotor structures to the high-efficient BL-WRSM proposed in  to improve its performance in terms of starting, average, maximum, and minimum torques, and torque ripple. The proposed topology is validated using a 4-pole/24-slot machine. Figure 3(a) displays the stator of the machine used for all the investigated BL-WRSM topologies. The rotor structures of the conventional BL-WRSM, PMABL-WRSM, and CPBL-WRSM are presented in Figures 3(b), 3(c), and 3(d). The conventional BL-WRSM rotor structure is entirely made up of iron, whereas the PMABL-WRSM rotor is having a NdFeB PM embedded on the rotor poles.
On the other hand, in the rotor structure of the CPBL-WRSM topology, PMs are inserted into the alternate rotor poles. The difference between the PMABL-WRSM and CPBL-WRSM rotor structures is only the arrangement of PMs and their volume.
Figure 4 illustrates the flux line loops of the investigated models. Figure 4(a) shows the flux line loop of the basic i.e., high-efficient sub-harmonic-based BL-WRSM topology. Since there are no additional design elements in the basic topology, the d-axis and q-axis flux loops are the same as the conventional WRSM. Figure 4(b) shows the flux line loop of the PMABL-WRSM. It is confirmed that PMs are arranged at each pole of the rotor structure to strengthen the magnetic flux of the rotor poles excited by the field winding. At startup, a torque is generated by the embedded PMs in the rotor poles. As the time passes, the current induced in the harmonic winding is rectified and transmitted to the field winding, and a torque is generated by the PMs and the current in the rotor field winding. Figure 4(c) displays the flux line loop of CPBL-WRSM. In this topology, the PMs are fitted into the alternative rotor poles, and the total volume of the PM is reduced as compared to the PMABL-WRSM topology. The usage of PM in the proposed PMABL-WRSM and CPBL-WRSM topologies increases the starting torque of the machines.
4. Magnetomotive Force (MMF)
Figure 5 shows the input armature currents for the high-efficient subharmonic-based BL-WRSM used in this study. Figure 5(a) shows the waveform of the current injected into the main armature winding (ABC). Figure 5(b) shows the current waveform injected into the additional armature winding (XYZ). The MMF generated by the three-phase currents IABC, and IXYZ injected to the stator windings of the employed BL-WRSM topology are presented in Figure 6(a). Figure 6(b) shows the harmonic components of the MMF using a fast Fourier transform (FFT) plot. This FFT plot shows that a considerable magnitude of subharmonic component is produced along with the fundamental. The waveforms of these prominent harmonic components are presented in Figure 7.
5. Electromagnetic Performance Analysis
In this study, FEA simulations were performed using JMAG-Designer 20.1, a proven method to substantiate the operation and performance improvement of the proposed BL-WRSM topologies with conventional, PM-assisted, and consequent-pole rotor structures. Table 2 specifies the detailed parameters of the investigated models used for the verification. The simulations are carried out for 1s. However, the machines are operated at 1800 rpm. The basic model employs the conventional rotor pole structure entirely made up of iron. In contrast, the rotors of the proposed PMABL-WRSM and CPBL-WRSM topologies are embedded with the PM made up of NdFeB into the rotor poles. The volume of these PMs is specified. In the CPBL-WRSM model, PMs are put on the alternative rotor poles, so that the volume of the PM may be decreased as compared to the PMABL-WRSM model.
Figures 8–11 present the electromagnetic performance analysis results of the investigated machine models using FEA. These results are achieved to compare the basic model and the proposed PMABL-WRSM and CPBL-WRSM topologies. Figure 8(a)–8(c) presents the flux linkages of the conventional, PMABL-WRSM, and CPBL-WRSM models under loaded conditions. Figure 9(a) reveals the magnetic flux density for the basic model, whereas Figure 9(b) and 9(c) presents the magnetic flux density of the proposed PMABL-WRSM and CPBL-WRSM models, respectively. These plots indicate that the machines are operating under the saturation level and are not saturated at any point of the operation.
Figure 10 presents both the induced harmonic and field currents of the studied machines. The harmonic current is induced in the harmonic winding of the rotor due to the harmonic component of the MMF in the air gap. When the harmonic current is rectified and injected into the field winding to excite the rotor, the main rotor field is produced. Figure 10(a) shows the rotor currents of the conventional BL-WRSM; however, the rotor currents of the proposed PMABL-WRSM and CPBL-WRSM topologies are presented in Figure 10(b) and 10(c).
Figure 11(a)–11(c) presents the torque profiles of the basic, PMABL-WRSM, and CPBL-WRSM machines. The average output torque of the basic model is 23.37 Nm. However, its magnitude for the proposed PMABL-WRSM and CPBL-WRSM models is 31.64 Nm and 33.88 Nm, respectively. The maximum torque of the basic model is 31.35 Nm, which is lower than the proposed PMABL-WRSM and CPBL-WRSM models. The maximum torque of the PMABL-WRSM is around 40 Nm, whereas its value for the CPBL-WRSM is 42 Nm. The torque ripple of the basic model, PM-assisted, and CPBL-WRSM models is 58.06%, 53.73%, and 44.27%, respectively. On the other hand, the PM volume used for the PMABL-WRSM and CPBL-WRSM models is around 7.562 cm3, and 5.507 cm3 respectively. These results indicate that even with the lower volume of PM, the proposed CPBL-WRSM performs better for average output torque, maximum torque, and torque ripple as compared to the basic and proposed PMABL-WRSM models. The minimum torque of the CPBL-WRSM is also higher than the basic and PMABL-WRSM models. These results are presented in Table 3.
One of the major disadvantages associated with the harmonic current induction-based BL-WRSM topologies is their low starting torque. This paper proposed PMABL-WRSM and CPBL-WRSM topologies for the high-efficient subharmonic-based BL-WRSM to improve its torque profiles. The simulation results of the basic and proposed models were compared using FEA in JMAG-Designer.
The obtained results show that the proposed CPHBL-WRSM performed well and showed better torque characteristics as compared to the basic and proposed PMABL-WRSM models. The average torque of the proposed CPBL-WRSM is 44.97% higher than the basic model and 7.07% higher than the proposed PMABL-WRSM model. On the other hand, the torque ripple of the proposed CPBL-WRSM is 13.79% and 9.46% lower than the basic and proposed PMABL-WRSM models, respectively. The proposed CPBL-WRSM also performed well in terms of maximum and minimum torques as compared to the investigated models. However, the starting torque of the proposed PMABL-WRSM is 70.80% higher than the CPBL-WRSM model. This is because the starting torque of the machines is dependent on the PM volume. As the PM volume of PMABL-WRSM topology is higher than the CPBL-WRSM model, its starting torque is also high. On the other hand, the basic model employs the conventional rotor structure without PMs and its starting torque is completely dependent on the harmonic induction of the rotor harmonic winding, it is therefore negative.
The effect of improving the torque characteristics of the proposed topologies suggests considerable room for further study of harmonic current induction-based BL-WRSMs.
No data were used to support this study.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This research was supported by the Human Resources Development (No.20204030200090) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy, National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2022R1A2C2004874), and the Brain Pool Program through the NRF of Korea funded by the Ministry of Science and ICT (2019H1D3A1A01102988).
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