Wikipedia

Axial flux motor

An axial flux motor (axial gap motor, or pancake motor) is a geometry of electric motor construction where the gap between the rotor and stator, and therefore the direction of magnetic flux between the two, is aligned parallel with the axis of rotation, rather than radially as with the concentric cylindrical geometry of the more common radial flux motor.[1][2] With axial flux geometry torque increases with the cube of the rotor diameter, whereas in a radial flux the increase is only quadratic. Axial flux motors have a larger magnetic surface and overall surface area (for cooling) than radial flux motors for a given volume.[3]

A miniature DC brushless axial motor used in a Digital Data Storage drive, showing the integration with PCB construction techniques. The rotor shown to the right is magnetized axially with alternating polarity.

Characteristics

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  • A motor can be built upon any flat structure, such as a PCB, by adding coils and a bearing.
  • The coil winding process and the process of joining the coil and core may be simpler.
  • Since the coils are flat, rectangular copper strips can more easily be used, simplifying high-current windings.
  • It is often possible to make the rotor significantly lighter.
  • Potentially shorter magnetic path length.
  • Most structural components are flat and can be produced without specialised casting or tooling.
  • Since the magnetic path through the windings is straight, grain-oriented electrical steel can be easily used, offering higher permeability and lower core losses.[4]
  • The rotor is typically much wider, causing increased rotational inertia, and the higher centrifugal forces can reduce the maximum rotational speed.
  • Uneven flux distribution due to wedge-shaped segments.
  • The segments narrow towards the centre, leaving less room to arrange windings and connections.

Design

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AFMs can use single or dual rotors or single or dual stators. The dual stator/single rotor design is more common in high power applications, although it requires a yoke (housing) with accompanying iron losses. Single stator/dual rotor designs can dispense with the yoke, saving its weight and increasing efficiency. In the latter, the rotors and their iron plates that close the flux move in the same direction/speed as the magnetic field.[5]

In one example, grain-oriented (30Q120) steel was used to make the stator tooth for an induction motor. It used 18 teeth between the two rotors. Each stator tooth was wound with coils connected in series, 6 for each phase. The magnetic potential adds the air gap magnetic potential, stator tooth magnetic potential and rotor yoke and tooth magnetic potential.[6][3]

Some AFMs can be easily stacked to provide higher power output in modular fashion.[3] YASA's 37 kg stackable 750 R motor delivers >5kw/kg with an axial length of 98 mm (3.9 in).[7]

Uses

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Although this geometry has been used since the first electromagnetic motors were developed, its usage was rare until the widespread availability of strong permanent magnets and the development of brushless DC motors, which could better exploit this geometry's advantages.

Axial geometry can be applied to almost any operating principle (e.g. brushed DC, induction, stepper, reluctance) that can be used in a radial motor. Even within the same electrical operating principle, different application and design considerations can make one geometry more suitable than the other. Axial geometries allow some magnetic topologies that would not be practical in a radial geometry. Axial motors are typically shorter and wider than an equivalent radial motor.

Axial motors have been commonly used for low-power applications, especially in tightly integrated electronics since the motor can be built directly upon a printed circuit board (PCB), and can use PCB traces as the stator windings. High-power, brushless axial motors are more recent, but are beginning to see usage in some electric vehicles.[8] One of the longest produced axial motors is the brushed DC Lynch motor, where the rotor is almost entirely composed of flat copper strips with small iron cores inserted, allowing power-dense operation.

Automotive

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Mercedes-Benz subsidiary YASA (Yokeless and Segmented Armature) makes AFMs that have powered various concept (Jaguar C-X75), prototype, and racing vehicles. It was also used in the Koenigsegg Regera, the Ferrari SF90 Stradale and S96GTB, Lamborghini Revuelto hybrid and the Lola-Drayson.[9] The company is investigating the potential for placing motors inside wheels, given that AFM's low mass does not excessively increase a vehicle's unsprung mass.[10] YASA is targeting motors that deliver 220 kw in a 7 kg package, or 31 kW/kg. By contrast, the state of the art EV motor from Lucid Motors offers a 500 kW, 31.4-kg motor, or 16 kW/kg.[11]

Off-Highway and Industrial Vehicles

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Turntide Technologies also develops axial flux motors aimed at the electrification of off-highway and industrial vehicles. The company's axial flux motor technology delivers high efficiency, compact design, and power density, making it suitable for demanding applications such as construction equipment and material handling vehicles. Turntide's axial flux motors are designed to address challenges in power output and energy efficiency while enabling greater system integration.

Turntide's motors are the first configurable, mass-produced axial flux motors at a lower price available to a multitude of different industries.

Aviation

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The Rolls-Royce ACCEL, holder of the current world speed record for an electric aircraft, uses three axial flux motors.[12]

YASA makes AFMs for the 3-motor Rolls Royce Spirit of Innovation. Their target is aircraft motors that deliver 50 kW/kg, to allow for the substantial weight reductions needed to enable electric-powered flight.[11]

General Purpose

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Emrax makes a line of axial flux motors: the Emrax 228 (power density 4.58 kw/kg), Emrax 268 (5.02 kw/kg), and Emrax 348 (4.87 kw/kg).[13]

Siemens offers a 5kw/kg motor.[14]

References

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  1. ^ Parviainen, Asko (April 2005). "Design of axial-flux permanent-magnet low-speed machines and performance comparison between radial-flux and axial-flux machines" (PDF). MIT.
  2. ^ EP2773023A1, Woolmer, Timothy; King, Charles & East, Mark et al., "Axial flux motor", issued 2014-09-03 
  3. ^ a b c "Axial Flux technology". AXYAL Propulsion. Retrieved 2024-04-03.
  4. ^ "Axial and Radial flux permanent magnet machines – What is the difference?". EMWorks Blog. 2020-10-12. Retrieved 2022-04-08.
  5. ^ "Double-rotor or Double-stator: a Matter of Efficiency". traxial.com. 2021-08-28. Retrieved 2024-03-31.
  6. ^ Huang, Pinglin; Li, Hang; Yang, Chen (February 2021). "A Yokeless Axial Flux Induction Motor for Electric Vehicles Based on Grain-oriented Silicon Steel". Journal of Physics: Conference Series. 1815 (1): 012042. Bibcode:2021JPhCS1815a2042H. doi:10.1088/1742-6596/1815/1/012042. ISSN 1742-6596.
  7. ^ "750 R Electric Motors Product Sheet" (PDF).
  8. ^ Moreels, Daan; Leijnen, Peter (30 Sep 2019). "This Inside-Out Motor for EVs Is Power Dense and (Finally) Practical". IEEE. Retrieved 2 August 2020.
  9. ^ "About YASA | The History Of YASA Axial Flux Motors | YASA Ltd". YASA Limited. Retrieved 2024-04-04.
  10. ^ "YASA & Mercedes Benz | A message from our Chairman | YASA Ltd". YASA Limited. Retrieved 2024-04-04.
  11. ^ a b Oliver, Ben. "An Innovative EV Motor Used by Lamborghini, McLaren, and Ferrari Is Being Mass-Produced by Mercedes". Wired. ISSN 1059-1028. Retrieved 2024-05-13.
  12. ^ "Electric Planes Are FINALLY Here and They're Breaking Records!". YouTube. 16 May 2023.
  13. ^ "348 (400kW | 1000Nm)". EMRAX. Retrieved 2024-03-31.
  14. ^ "Siemens and Emrax claim best power to weight ratio for electric motors in the 5 to 10 kilowatt per kg range | NextBigFuture.com". 2015-04-20. Retrieved 2024-03-31.
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