Types of Magnetic Gears – Based on their operations, magnetic gears can be classified into three types: linear magnetic gears (LMGs), coaxial magnetic gears (CMGs), and axial magnetic gears (AMGs). For LMGs and CMGs, the magnetic flux is typically generated radially inward or outward with reference to the shaft axis. However, for AMGs, magnetic flux lines are created parallel to the rotor axis. In this blog post, we will present examples for these three types of magnetic gears using Multiphysics.
Coaxial Magnetic Gear
The coaxial magnetic gear consists of three concentric rotors with different pole pairs, as shown in the figure below. The inner rotor is comprised of eight permanent magnets (PMs) and a soft iron yoke that forms an outward flux focusing 2 pole pairs on the rotor. The outer rotor includes 20 PMs and a soft iron yoke that forms an inward flux focusing 5 pole pairs on the rotor. In the inner and outer rotors, the permanent magnets are arranged in a Halbach array configuration. Seven pieces of steel are included in the middle stationary ring, and the same spacing between these pieces forms a stationary rotor with 7 pole pairs.
A coaxial magnetic bearing. Magnetic flux density, magnetic vector potential, radial magnetic flux density, and mesh plot for the coaxial gear.
Left: A schematic of a coaxial magnetic bearing showing the inner rotor, outer rotor, and stationary steel poles. Red arrows represent the magnetization direction of the permanent magnets. The permanent magnets are arranged such that the inner rotor is an outward flux focus and the outer rotor acts as the inward flux focus. Right: Magnetic flux density (norm), magnetic vector potential (Az), radial magnetic flux density (Br), and mesh plot in the 1st to 4th quadrant, respectively.
In the example here, the number of pole pairs are chosen to have the gear ratio of 5:2 and the minimum cogging torque as the cogging factor is unity. In Multiphysics, the 2D cross section of the coaxial magnetic gear is modeled using the Rotating Machinery, Magnetic interface in the AC/DC Module. Since the model consists of three separate parts, we must form a union of these parts and finalize the geometry using the Form Assembly so that two separate identity pairs are created in the air-gap regions.
We include the nonlinear material model in the soft iron domains using the BH/HB curves. However, the stationary steel pole pieces are modeled as a linear material with a relative permeability of . Rotating the inner and outer rotor is possible with the Prescribed Rotational Velocity feature. The axial torque on both rotors is calculated using the Maxwell’s stress tensor method via the Force Calculation feature.
Animation depicting the surface plot of magnetic flux density and the contour plot of magnetic vector potential, . The clockwise and anticlockwise rotation of the outer rotor and inner rotor are shown, respectively, along with the field interaction.
A plot showing the axial torque profile on the inner and outer rotors of a coaxial magnetic gear.
The axial torque profile on the inner and outer rotors. The torque ripple is higher on the inner rotor (less pole pairs).
You can download this example model file from our Application Gallery. Here, you will find the geometry sequence file, model file, and a PDF with step-by-step instructions.
Axial Magnetic Gear
When it comes to the axial magnetic gear, its operating principle is the same as that of the coaxial magnetic gears described above. In this design, the rotors are stacked in an axial direction on top of each other rather than in the radial direction, with a small air-gap separation. Since the surface for magnetic field interaction is large and the length of the three rotors is the same, AMGs provide higher torque densities compared to CMGs. The construction of a typical axial magnetic gear in 3D is depicted in the figure below on the left.
Construction of an axial magnetic gear. Magnetic flux density and mesh plot for the axial magnetic gear.
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