Turbines are big sturdy beasts. They should be robust and able to stand a lot of what the elements can throw at them. But even the best of them needs maintenance from time-to-time. And then there’s the unexpected freak gust of wind.
An unexpected gust can stress the mechanical gearbox of the turbine to such an extent that the torque exceeds design parameters and crunch! A very heavy gearbox has to be detached from the nacelle, taken down by crane and then repaired, either in-situ or more likely at the manufacturer’s nearest plant. That’s expensive and will mean that the turbine will lose money by not operating, for days or even weeks.
If only there was a way to capture wind energy by a turbine without the need for a mechanical gearbox… A gearbox where there is no physical connection between the gear rotors. Magnetism? Yes, that’s possible- the gear rotors’ motion couples through a modulated interaction between the flux generated by magnets on input and output rotors. If the torque on the input exceeds maximum design levels, the magnetic gears simply slip; there are no broken teeth, ground-up gears or worse, a fire.
It may not surprise you to learn then, that magnetic gear research is one of the hottest areas of wind energy development. One such place is Texas A&M University’s Advanced Electric Machines and Power Electronics Lab. There Professor Hamid A. Toliyat directs a team working on various types of magnetic gears. Danish wind turbine giant Vestas have been supporting the work there.
Warning: Things get a bit technical here: The team has focused on three basic configurations of magnetic gear design- Axial, Radial and Trans-Rotary.
Axial
These designs consist of two disks each containing alternating positive and negative magnets with a modulator disk in between. The low-speed disk, or low-speed rotor, contains a magnetic pattern geometry that alternates more quickly than that of the high-speed rotor. The gear ratio is a function of the number of pairs on the two rotors and the air-gap fluxes they produce.Axial magnetic gears are probably the most challenging basic topology. As currently constructed, axial designs would be more difficult to manufacture than magnetic gears using radial or trans-rotary configurations.
Radial
These magnetic gears have a configuration similar to a synchronous motor. A typical topology uses a high-speed inner rotor concentric with a low-speed outer rotor. The two rotors are separated by a stator typically composed of electrical steel bars. As with axial magnetic gears, the stator bars modulate the flux between the permanent magnets on the input and output rotors. Manufacturing considerations are among the reasons why the industry seems to have a more initial interest in building radial magnetic gear designs than axial designs.
Trans-Rotary
It also looks as though trans-rotary designs are the promising technology. These operate in a way analogous to magnetic versions of mechanical ball screws, converting linear motion to rotational motion and vice-versa through the use of a shaft containing magnets oriented in a helical pattern like that of screw threads.
In practical applications, it can convert the low-speed, high-force linear motion of a translator (screw shaft) to the low-torque, high-speed rotation of a rotor (ball-screw nut). Both rotor and translator are typically made of ferromagnetic iron cores that are lined with alternating magnetic poles arranged in helical patterns. The magnetic helixes on both the rotor and translator have the same pitch. The gear ratio is the ratio of the rotor angular speed to the translator linear speed, which is affected by the number of poles and the pitch of each helical magnet arrangement. To boost the gear ratio, designers reduce the number of poles and make the magnets narrower. The number of poles is analogous to the number of starts in a mechanical threaded rod and the product of the number of pole pairs and their width is analogous to the lead of a mechanical lead screw system.
The Texas A&M group is progressing from studying magnetic gear topologies to addressing the practical issues of manufacturing magnetic gears and improving their performance. So far all their prototypes have all been in the sub-kilowatt range. However, the Texas A&M researchers think production versions of the radial magnetic gear designs have the potential to work in wind turbines at the megawatt scale without any major manufacturing issues.
Perhaps in the next few years? Watch this space!
