By Sumeet Singh |
17/03/2023
Introduction
Electrical Vehicles (EVs) are pivotal for eco-friendly transportation, with propulsion systems at their core, converting battery power into mechanical energy. Permanent magnet motors, vital for EVs, offer high power density and efficiency. However, due to the high cost and limited supply of magnets, Synchronous Reluctance Motors (SynRMs) emerge as a promising alternative, offering robustness, high power density, and efficiency, making them suitable for EV applications.
Operation Principle of SynRMs
The main idea behind the reluctance torque generation is described in Figure 1. In this figure, object (a) with an isotropic magnetic material has different reluctances (geometric) in the d-axis and the q-axis while the isotropic magnetic material geometry in object (b) has the same reluctance in all directions. A magnetic field (ψ) which is applied to the anisotropic object (a) produces torque if there is an angle difference between the d-axis and the field (δ ≠ 0) [1].
Fig. 1. An Object with Anisotropic Geometry (a) and Isotropic Geometry (b) in a Magnetic Field and Reluctance Torque Production Mechanism [1]
To efficiently use the presented idea, the SynRMs can adopt different structures. Figure 2 shows the available SynRM structures namely Salient Pole (Switched Reluctance), Transversally Laminated (TLA), and Axially Laminated (ALA).
Fig. 2. Main Structures of Reluctance Motors
The TLA-Structure is preferable, in practice, since it is suitable for industrial manufacturing compared to the ALA-Structure.
To understand and also describe the performance of the SynRMs, the phasor diagram consisting of the stator current and magnetization flux in DQ-coordinates can be very useful. Figure 3 illustrates the phasor diagram of the SynRMs.
Fig. 3. Phasor Diagram and dq-Circuits of SynRMs at Steady-State Operation
By using the equivalent dq-circuits, the relationship between the voltage, flux linkage, and currents can be easily described as follows:
Fig. 4. Dynamic and Steady-State Equations in dq-Coordinates
And finally, the torque generated by SynRM is the function of dq-current and flux linkages:
Fig. 5. Torque Equation
The equation above in Figure 5 introduces an important parameter called “Saliency Ratio” which determines the performance of the SynRMs with great significance. Therefore, it is necessary to increase the saliency ratio as much as possible to increase the torque density. Moreover, based on fixed values for dq-inductances and saliency ratio, the peak torque will be achieved in id=iq, where the current angle is theoretically equal to 45 electrical degrees.
Design Aspects and Methods
Since we are investigating the potential use of SynRMs in EV applications, it is important to consider the characteristics required for the traction motors. These characteristics include:
- High torque density to reduce weights and increase efficiency
- High overload capability to handle transient accelerations
- Low torque ripple to reduce vibrations and noise
- Large flux-weakening capability to operate in an extended speed range
- High efficiency to minimize losses
To achieve such an optimum design satisfying the requirements, the design equation of SynRM must be considered as the initial step. Figure 6 shows a flowchart describing the iterative process of the electric motor design.
Fig. 6. Design Flowchart of Electric Motors [2]
From an electromagnetic perspective, the rotor of SynRMs is the most intricate component of the motor's structure. As illustrated in Figure 7, the rotor cross-section is composed of various parameters.
Fig. 7. Design parameters located on machine cross section [2]
As demonstrated, the width of flux barriers and flux carriers, as well as their angular position, must be determined during the design process. Ideally, we would like to maximize the d-axis flux linkage by appropriately sizing the width and position of the flux carriers, while minimizing the q-axis flux linkage to increase the saliency ratio as much as possible.
Fig. 8. Torque/Power versus Speed for an Example SynRM [3]
Figure 8 illustrates the capability of an example SynRM through a wide range of speeds. The power of the machine starts to reduce after reaching the base speed. To compensate for this reduction, it is common to use Permanent Magnet (PM) material such as ferrite in the flux barriers. This structure is called “PM-Assisted SynRM" where the PMs are introduced into the flux barriers resulting in improved torque characteristics in a wide range of operation points.
Fig. 9. Torque/Power versus Speed for an Example PM-Assisted SynRM [3]
Figure 9 above shows how the magnets are inserted into the flux barriers. By this approach, the magnetic flux produced by the PMs will help reduce the q-axis flux linkage to lower values compared to SynRM and as a result, the apparent saliency ratio will increase.
The design of SynRMs necessitates a high level of technical knowledge and experience to identify an appropriate combination of design parameters that describe the geometry of the rotor's structure. Furthermore, magnetic saturation in the barrier bridges and barrier posts is a frequent occurrence. Consequently, the most effective way to examine and evaluate the performance of the SyRMs is through the use of finite element modeling (FEM) techniques.
EMWorks is a powerful provider of Finite Element Method (FEM)-based simulation software, which enables the analysis of various electromagnetic characteristics of SynRMs during the design process. Figures 10 and 11 below demonstrate the mesh generation and magnetostatic analysis of an example SynRM, using EMWorks' software product, EMWorks2D.
Fig. 10. Meshing of the Studied SynRM Geometry using EMWorks2D
Fig. 11. Flux Density Distribution Due to dq-Excitation Calculated by EMWorks2D
Conclusion
The demand for sustainable electric motors is rapidly increasing due to the growing popularity of electric vehicles (EVs). Synchronous Reluctance Motors (SynRMs) have demonstrated great potential for use in EV systems and propulsion units. Consequently, the fundamental principle of torque generation has been re-examined. A robust rotor structure is a key element, providing the motor with high efficiency and torque density, which are advantageous for EV applications. However, SynRMs experience power loss at higher speeds and cannot provide constant power across a wide range of speed variations. To address this issue, Permanent Magnet (PM) materials such as ferrite can be employed. To achieve the optimal design, it is necessary to utilize Finite Element Method (FEM) techniques to accurately predict the performance of the SynRM. EMWorks2D is a FEM-based solution that makes it possible to monitor the performance of the machine during the design process.
References
[1] R. R. Moghaddam, “Synchronous reluctance machine (SynRM) design,” KTH Electr. Eng., 2007.
[2] B. Ban, S. Stipetić, and M. Klanac, “Synchronous reluctance machines: Theory, design and the potential use in traction applications,” in 2019 International Conference on Electrical Drives \& Power Electronics (EDPE), 2019, pp. 177–188.
[3] N. Bianchi, S. Bolognani, E. Carraro, M. Castiello, and E. Fornasiero, “Electric vehicle traction based on synchronous reluctance motors,” IEEE Trans. Ind. Appl., vol. 52, no. 6, pp. 4762–4769, 2016.