Overview
Permanent magnet synchronous motors are classified as inner-rotor or outer-rotor based on the rotor location. The distinction is simply whether the rotating component is on the inside or the outside. The rotating part is the rotor, which typically contains the embedded permanent magnets, while the stationary part with the windings is the stator. This article compares inner-rotor and outer-rotor permanent magnet motors and highlights their structural and application differences.
Key Differences
1. Rotor position. In an inner-rotor design the drive components rotate inside while the housing remains stationary. In an outer-rotor design the inner part remains stationary and the outer housing rotates. Typically the embedded permanent-magnet portion rotates and the coil portion does not; the rotor position and magnet mounting distinguish inner-rotor from outer-rotor designs.
2. Permanent magnet volume and axial size. Outer-rotor designs generally require smaller permanent magnet volume and have reduced axial dimension, which can improve system running stability. Inner-rotor designs usually require larger magnet volume and correspondingly larger axial dimension, producing a higher center of gravity and reduced running stability.
3. Speed. For the same brushless DC motor specification, inner-rotor motors typically run at significantly higher speeds than outer-rotor motors.
4. Applications. Inner-rotor configurations are commonly used in motors, generators, gas turbines, compressors, and other power machinery. Outer-rotor designs are often arranged around impellers and can assist with heat dissipation and cooling.
In an outer-rotor machine the stator is fixed near the axis and the rotor rotates around the outside of the stator. This is a radial air-gap flux structure where rotor and stator positions are swapped compared with the inner-rotor layout.
Direct-Drive Outer-Rotor Permanent Magnet Wind Turbine
The following describes the components and structure of a direct-drive outer-rotor permanent magnet wind turbine generator using a model of an outer-rotor permanent magnet generator.
The stator core is formed from laminated silicon steel with good magnetic permeability. The stator core outer circumference contains many slots where the windings are embedded; the windings are arranged according to a three-phase distribution. Large direct-drive outer-rotor wind generators typically have 30 to 40 pole pairs and stator slot counts around 180 to 240. For clarity, the model shown here has far fewer slots than a practical direct-drive machine.
The stator core is mounted on the stator frame. One end of the stator frame has a flange for attachment to the nacelle or hub, and the other end supports the outer-rotor shaft, which is also the main shaft of the wind turbine. The main shaft must bear the weight of the entire rotor and the wind wheel, so the shaft and flange require high strength.
The outer rotor is like a barrel that fits around the outside of the stator, made from magnetically permeable iron material. Permanent magnets are fixed to the inner circumference of the barrel; the barrel serves as the rotor yoke. One advantage of this structure is that the magnetic poles are relatively easy to secure, reducing the risk of magnets detaching under centrifugal forces. The outer-rotor yoke is mounted on the rotor sleeve.
The outer rotor is mounted on the generator main shaft to form the outer-rotor generator. The outer-rotor sleeve both secures the outer rotor and supports the entire wind wheel, bearing significant loads. Therefore the sleeve is mounted on the main shaft via two large bearings.
Advantages and Disadvantages
Inner-rotor advantages include lighter fan or rotor blades, easier manufacturing and assembly, and lower cost. However, inner-rotor machines can heat up quickly, consume more power, exhibit unstable voltages that may lead to damage, and have shorter service life.
Outer-rotor advantages include the option to implement a fully enclosed structure, rapid startup, low power consumption, lower-speed high-torque operation suitable for direct drive, high efficiency, and long service life. Drawbacks can include challenges with sealing, larger rotor inertia, higher noise, and stricter dynamic balancing requirements.
