Review of Flux-Concentrated Transverse Flux Permanent Magnet Machines

The machines that use permanent magnets are not prefabricated. The design's adaptability makes them useful in a variety of contexts, including wind turbines. Radial, axial, and transverse fluxes are the common ways to categorize them according to the direction of the flux.

For direct-drive energy-generating wind turbine applications, transverse flux permanent magnet (TFPM) machines are a good choice. Their torque and power densities are quite high.

There are two main types of TFPM machines: those that are surface-mounted and those that are flux-concentrated.

Ⅰ. Are flux-concentrated TFPM machines more effective than surface-mounted TFPM machines?

When using a flux-concentrated arrangement, magnetic flux is concentrated in a flux channel that has a smaller cross-section than the magnet, which enables the inset magnets to significantly improve the air gap flux density.

Comparing this design to a surface magnet design, the torque and power factor are increased by 20% to 50%. Additionally, by placing magnets inside the magnetic circuit, an additional significant kind of torque called reluctance torque is produced, allowing for a greater speed range to be operated at constant power.

Therefore, the sum of the PM and reluctance torque components equals the total torque. In addition, its structure is complex and its robustness is limited. This article introduces and analyzes a variety of topologies for flux-concentrated TFPM machines.

Based on their electromagnetic configuration, the TFPM machines can be further categorized as follows:

● Single-sided and double-sided

● Single-winding and dual-winding

● Inner rotor and outer rotor

● Stator type: C/U-core, Z-core, E-core, claw pole

Ⅱ. Single-Sided

Fig. 1 shows a single-sided flux-concentrated TFPM machine that only uses half of the magnets every time. Compared to surface-mounted configurations, this architecture improves performance while using less active material.

Fig. 1: Single-sided flux-concentrated TFPM machine Source: IEEE Access

Challenges

Nevertheless, the rotor assembly is more complicated. Mechanical stability can be achieved by utilizing the toothed rotor structure.

A comparative analysis of two 5 MW TFPM machines optimized for active mass reduction is conducted for the wind power generator. Compared to U-core, the C-core geometry uses 11% less active mass and a smaller PM volume.

Based on mass, cost, and copper loss, four distinct TFPM machines and a 5 MW RFPM machine have been analyzed. When these factors are taken into account, the C-core TFPM machine with a single winding and a double-sided air gap appears to be the ideal idea.

The outer rotor TFPM machine design approach for light-duty vehicles based on soft magnetic composite has been proposed.

The optimization procedure for the dual-rotor TFPM machine is reported.

The finite element method has been used to model two distinct TFPM machine topologies with scattered windings.

The TFPM machine prototype development with soft magnetic composite and formed lamination for aerospace applications is shown. Since this arrangement has a higher torque density and a greater power factor than a surface-mounted TFPM machine, it has also been extensively studied for wind power applications.

Ⅲ. Iron Bridge

In a traditional single-sided flux-concentrated TFPM machine, the stator bridges are put over the inactive magnets to seal the magnetic circuit and make sure that all of the magnets are used. This is shown in Fig. 2.

Fig. 2: Flux-concentrated TFPM machine with iron bridge Source: IEEE Access

Challenges

By changing the C-core stator into a horseshoe with a foot and the iron bridge into a trapezium, the leakage between the stator and iron bridge is reduced. Furthermore, the TFPM machine outperforms the PM synchronous machine in terms of torque and mass when its diameter is less than 1 m.

A new TFPM machine design with an E and I-shaped stator core is described for wind power. When compared to a cylindrical rotor, the disk-shaped rotor has a lower inductance and a higher power factor. While PM gets encased in the rotor, the coil is wound around the center arm of the E-shaped core.

Ⅳ. Double-Sided

The first double-sided TFPM machine with a U-core stator and dual winding was presented as shown in Fig. 3. More efficient use of magnets at any given moment results in higher performance.

Fig. 3: Double-sided flux-concentrated TFPM machine Source: IEEE Access

Challenges

However, because of its intricate design and rotating magnets, this structure is mechanically unstable.

Without changing the machine's torque rating, the same topology can be changed from double-winding to single-winding. A decrease in the outside diameter has eliminated the challenge of mounting top winding.

The E-core TFPM machine has a shorter rotor axial length and two distinct stator sections. The U-core is positioned in between two windings, and the C-core stator component surrounds the winding. The projected performance and disadvantages of this concept are the same as those of the traditional double-sided TFPM machine.

A quasi-U-shaped stator with ring winding in a modular double-sided TFPM machine for wind power is proposed. Ferrite magnets make up the rotor.

Furthermore, a double-sided TFPM machine's inter-polar leakage is reduced with the introduction of the Halbach array magnet arrangement. The flux linkage is increased by 90% with this configuration.

For wind power applications, a double-sided TFPM machine with an iron bridge is suggested. The machine is made up of an I-shaped magnetic shunt, a pancake-shaped rotor, and a C-core stator. The machine is designed with a lower volume and an improved power factor. On the other hand, the iron core is saturated and has a high PM volume.

A range of double-sided TFPM machines have been studied recently for wind power applications, utilizing their benefits in increased torque density, greater power factor, and better magnet use.

Ⅴ. Claw Pole

In Fig. 4, we can see a claw-pole TFPM machine topology. This topology has the benefits of both a single-sided and a double-sided TFPM machine: it is easy to use and has a higher power density.

Fig. 4: Flux concentrated claw pole TFPM machine. Source: IEEE Access

Challenges

On the other hand, the stator offers a short-circuit component for the saturated magnetic flux. SMC material is needed for manufacture due to its intricate design.

For use in wind power applications, a 5 MW TFPM machine was proposed. A comparison of different TFPM machine topologies was done. The V-type embedded magnets with a claw-pole stator were able to achieve the maximum torque density.

A single-phase TFPM machine is constructed to power a green mechanical press's driving system. The stator's rear iron and claws are laminated in different directions. Compared to a mechanical press, this equipment uses 30% less energy.

Ⅵ. Summarizing the Key Points

● Flux-concentrated transverse flux permanent magnet machines (TFPM) offer 20-50% increased torque and power factor compared to surface-mounted designs, enhancing overall performance and efficiency.

● The inset magnets in flux-concentrated designs enable a significant reluctance torque, expanding the operational speed range at constant power.

● Diverse TFPM machine topologies, including single-sided, double-sided, and claw-pole configurations, cater to different application requirements and performance goals.

● The use of flux-concentrated TFPM machines extends beyond wind turbines, with potential applications in aerospace, light-duty vehicles, and green mechanical press systems.

● The article provides insights into the optimization procedures and finite element method modeling for TFPM machines, contributing to ongoing advancements in this field.

Ⅶ. Reference

Kumar, Rajesh, Zi-Qiang Zhu, Alexander Duke, Arwyn Thomas, Richard Clark, Ziad Azar, and Zhan-Yuan Wu. “A Review on Transverse Flux Permanent Magnet Machines for Wind Power Applications.” IEEE Access 8 (2020): 216543–65. https://doi.org/10.1109/access.2020.3041217.