Introduction
Permanent magnet machines are commonly used in various applications due to their high torque density and efficient performance. They are also considered as suitable options for continuous operations under faulty conditions [1]. Switched reluctance machines are also designed to be reliable in faulty conditions, but they are penalized by high torque ripples that generate vibration and noise, leading to instability in certain cases. Flux switching machines (FSMs), can be good alternatives for applications that require lightweight, small size, fault tolerance capability, and high-power density. It also represents a good solution to achieve a cost-effectiveness trade-off. In fact, FSMs are known by hybrid excitation as the armature sources combine PMs and coils. Indeed, instead of totally relying on magnets, FSMs investigate less magnet volume and compensate for the lack of field strength by winding energy. In this blog, two FSMs with different magnet positions are compared based on the flux production capability.
Motor Design Specifications
In the following analysis, we examine two Flux Switching Machines (FSMs). Both devices feature an equal number of slots per pole and per phase, set at 0.5, and integrate six neodymium magnets within the stator. Additionally, both motors share a rotor composed of eight specific segments constructed from M 36 material. All geometric parameters remain consistent, with the sole exception being the magnet location. Specifically, in FSM-1, magnets are positioned alternately in the stator tip tooth. Conversely, in FSM-2, the magnets adopt a radial placement, commonly referred to as the "in-between position" [2], as indicated in Figure 1.
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| Fig. 1. (a) 2D FSM Configuration of the FSM-1 | Fig. 1. (b) 2D FSM Configuration of the FSM-2 |
The motor specifications are listed in the table below.
| Items | FSM | |
| Number of Slots | 12 |  |
| Number of Poles | 8 | |
| Number of magnets | 6 | |
| Magnet area (mm*mm) | 22.44 | |
| Stator Outer Diameter (mm) | 150 | |
| Stator Inner Diameter (mm) | 90.6 | |
| Rotor Inner Diameter (mm) | 58 | |
| External rotor segment angle (mm) | 40 | |
| Airgap (mm) | 0.3 | |
| Speed (rpm) | 2000 | |
| Coil Excitation (A) | 30 | |
| Number of turns | 44 | |
| Stack Length (mm) | 15 | |
EMWorks Solution and Results
EMWorks2D is an electromagnetic software that enables users to design and simulate the model in a few minutes and obtain accurate results. It is a finite element analysis-based tool that provides different solution types such as magnetostatic and transient studies. It helps to gain time by working on the same platform. No need to import geometry and add more operations to fix the model structure.
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| Fig. 2. (a) Flux Density Plot of the FSM-1 | Fig. 2. (b) Flux Density Plot of the FSM-2 |
In Figure 2, the magnetic flux density mapping for both topologies is depicted under AC excitation at the same step time. In Fig (a), it is evident that the stator teeth encircled in the diagram exhibit a lower density of flux lines compared to the surrounding teeth. A similar observation is made in Fig (b), but in this case, the phenomenon is confined to just two teeth.
These specific regions highlight a phenomenon known as flux cancellation. Essentially, this occurs when the magnetic flux generated by a magnet opposes the flux produced by the winding. This phenomenon is facilitated by the alternating arrangement of magnets within the stator teeth, coupled with the presence of distinct north and south poles for each magnet.
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| Fig. 3. (a) Flux Cancellation in FSM-1 | Fig. 3. (b) Flux Cancellation in FSM-2 |
For (a) and (b) the same magnet volume is investigated but in different positions. In (b) the magnets have the same length as the slots, and their magnetization is radial, giving more guidance to the flux from the rotor to stator, then following the tooth path smoothly helps to gain more magnetic flux and make it active explains the reduction of the flux cancellation in the second topology.
Fig.4. Phase Flux Linkage Comparison
Figure 4 provides a comprehensive view of the magnetic flux linkage over time, comparing FSM-1 and FSM-2. Notably, the machine featuring "in between magnets" stands out for producing a more active flux, as discussed in earlier details. Additionally, the topography of the flux shape is notably influenced by the specific positioning of the magnets, adding a crucial dimension to the analysis.
In this blog, two flux-switching machines that differ by the magnet position, are designed and simulated using EMWorks2D software. Through finite element analysis, an electromagnetic comparison has been established showing the flux cancellation phenomena and its effect on the flux linkage. EMWorks2D is offering flexibility for users to design and optimize magnet shapes and volumes, ultimately enhancing overall machine performance.
References
[1] Raminosoa, T., Hamiti, T., Galea, M., et al.: ‘Feasibility and electromagnetic design of direct drive wheel actuator for green taxiing’. IEEE Energy Conversion Congress and Exposition (ECCE), USA, November 2011, pp.2798–2804
[2] Ali, Hassan, Erwan Sulaiman, and Takashi Kosaka. "Design and performance analysis of various high torque segmented rotor HE‐FSM topologies for aircraft applications." IET Electric Power Applications 14, no. 2 (2020): 297-304.