The total number of CVTs in operation is still modest. The best known is Voith's hydraulic WinDrive product. It was first included in a 2MW DeWind geared prototype in late 2006 and is now a series product. Other CVTs developed for wind-turbine application include a mechanical system by IQgear of Israel and a fully hydraulic system by UK-based Artemis (now MHI).
As with the above models, VMT's W-10 RPM Equalizer enables variable rotor speed combined with a fixed-speed generator, which can be directly connected to a 50Hz or 60Hz grid network. One key benefit is the elimination of the need for a power electronic converter. If a medium-speed generator is selected, a medium-voltage transformer can also be eliminated. According to VMT, this represents a combined cost saving of around 11% for a 1.5MW wind turbine, assuming a similar cost level for the VMT W-10 RPM Equalizer gearbox and generator system.
The RPM Equalizer is incorporated into a geared drive system as the second stage of a three-stage gearbox. VMT inventor and founder Gary Lee explains that in the low-speed planetary gear stage, rotor revolutions are stepped up by about a factor of six. "As a next step, the RPM Equalizer continuously adjusts a variable input speed to a fixed 300rpm output speed and in the final step-up stage reaches the required generator speed," adds Lee. "The outer dimensions of our system are comparable to those of conventional gearboxes with similar power rating."
The RPM Equalizer comprises an input variator wheel equipped with three sprockets, each attached to a pivoting 'moon arm', which comes with a smaller fixed-diameter output wheel fitted to a separate shaft.
At minimum rotor revolutions a maximum RPM Equalizer step-up ratio is required to match the constant generator speed, which is achieved by a hydraulic-controlled system that moves the moon arms to a fully stretched position. At maximum rotor revolutions the sprocket moon arms are moved fully inward to a minimal radial position. Between the minimum and maximum points an infinite number of positions is possible.
One operational requirement is that the belt, instead of covering the shortest stretched path between two adjoining sprockets, moves along a circular path over the continuously engaging and disengaging moon wheels. Movable coning disks at each side of the belt, requiring only a modest axial compression force, enable this function. Finally a chain-tensioning sprocket and shock absorbers maintain chain tension and enable smooth operation at all times.
VMT has built and tested an initial prototype for technology validation followed by a larger scale prototype suitable for incorporating into a small wind turbine.