SPM rotor assemblies feature diverse design variations to meet performance, manufacturing, and durability needs. The standard SPM design employs concentric arc magnets adhered directly to the rotor core surface with adhesives. However, ensuring the firm attachment of surface-mounted magnets during high-speed operation remains a significant challenge, often requiring innovative engineering solutions.
For small motors, robust adhesives are generally adequate to keep magnets secure. Conversely, high-speed or high-power applications demand extra retention methods to endure centrifugal forces. Common approaches include non-magnetic stainless steel sleeves for magnet security, while larger motors often use high-strength retention sleeves crafted from advanced materials like titanium alloys, Inconel alloys, carbon fiber, or glass fiber composites.
Non-concentric arc magnets offer an alternative design, with inner and outer magnet surfaces creating an uneven air gap between rotor and stator. This uneven gap shapes the back-EMF waveform closer to a sinusoidal form, thus enhancing motor efficiency and performance by optimizing the magnetic field interaction.
Another variation uses breadloaf magnets, distinguished by a flat bottom surface, simplifying the bonding process for easier attachment to the rotor core. Additionally, breadloaf magnets promote a more sinusoidal air gap flux distribution, leading to smoother motor operation and reduced harmonic distortion, improving overall performance.
In some SPM rotor assemblies, a ring magnet replaces individual magnets. Here, a continuous ring magnet is placed over the rotor core and magnetized afterward, streamlining assembly, removing the need for precise magnet alignment, and providing uniform magnetic flux distribution around the rotor for consistent performance.
Surface-inset permanent magnet (SIPM) structures maintain the benefits of standard rotor assemblies while adding rotor core protrusions between magnets. These protrusions produce extra reluctance torque, boosting overall torque density and improving motor performance, making SIPM designs a compelling option for advanced applications.
