# Higher energy yield with torque vectoring gears

##### February 23, 2011

Wind turbines have a problem: Depending on the wind's force, the rotational speed of the turbine and thus of the generator changes. However, alternating current must be fed into the grid with precisely 50 (or 60) hertz. Typically the generated alternating current is first rectified and then transformed back to alternating current of the required frequency. Scientists at the Technische Universitaet Muenchen, Germany, have now developed an active transmission that makes this double transformation superfluous.

Most large currently operate at variable speeds. When the wind is strong, the rotor turns fast; when it slows down, the rotor speed drops. Typically rotors complete 12 to 16 revolutions per minute. The generator is connected to the rotor via a gearbox. Here too, the speed of rotation varies with the speed of the wind.

Yet, a wind turbine may only feed alternating current with exactly the frequency of the . That is why the alternating current from generators is today transformed into direct current by way of giant rectifiers. In a second step the direct current is then transformed back into alternating current of the right frequency. This twofold conversion takes a loss of close to 5 percent.

In their research, scientists from the Chair of Machine Elements at the TU Muenchen took a closer look at the gears and generator system. To attain the grid frequency of 50 hertz, a generator with the usual two poles pairs must operate at a synchronous speed of exactly 1500 revolutions per minute. To fulfill this requirement in spite of the variable input rotational speed, the researchers developed a novel active torque-vectoring gear analogous to a controlled differential in motor vehicles.

As in conventional designs, planetary gears generate most of the transmission required. These are supplemented by a torque-vectoring gear with a supplemental electric motor that can be used as both as a drive and as a generator. This allows the power from the rotor to be either be boosted or diverted, leading to a constant rotational speed of the generator. Applying this concept to a 1.5 MW wind turbine, an electric motor of only about 80 KW is sufficient.

The advantage of this concept is a lighter power train that requires a much smaller nacelle for the wind turbine. Additionally a robust, low-maintenance synchronous generator can be used, which dispenses with the need for power electronics for frequency adjustment, thereby increasing the overall facility efficiency.

Originally this development, patented under the name of Torque Vectoring, was developed for cars. Within the Science Center for Electrical Mobility at the Technische Universitaet Muenchen the chair develops a torque-vectoring gear box for the electric vehicle concept "MUTE" which will be presented at the IAA 2011. Here the active control of force distribution improves driving safety, traction and provides for dynamic handling characteristics in curves.

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##### Nik_2213
not rated yet Feb 23, 2011

I suppose it is the same principle as negative feedback for an OpAmp. Still, I would have expected it would need more than +/- 5% power to keep the rotor's RPM steady...
##### kaasinees
2.3 / 5 (3) Feb 23, 2011
Finally a nice article that i like, with some real engineering and science. Germany has the highest ratio of renewable energy sources, we Dutch people surely could learn a few things from them.

Altough, about which wind turbine are we talking? A vertical or horizantal one? Or does this not matter? I have heard and read that horizontal wind turbines are more efficient then vertical ones...
##### PPihkala
3 / 5 (2) Feb 23, 2011
Altough, about which wind turbine are we talking? A vertical or horizantal one? Or does this not matter? I have heard and read that horizontal wind turbines are more efficient then vertical ones...

I think this is applicable to both types. I think that vertical models excel when there is more turbulent wind, one that changes direction more often. Horizontal models can not track those changes very well, while they perform best with winds that change direction slowly. Vertical ones also have lower point of mass, where that counts.
##### 3432682
1 / 5 (1) Feb 23, 2011
A video could explain this in about a minute.
##### that_guy
1 / 5 (1) Feb 23, 2011
I think in summary, it's sort of like a hybrid car. It saves some of the excess power and uses it when there is less than optimal power available. This way it can dispense with the transformers and power loss, reducing weight, and increasing conversion and size/weight efficiency.

An interesting result of this would be that instead of the windmill producing relatively continuous but variable power, it would have to produce at optimum power or not at all, so it would have to rev up before the power could be allowed to flow.

Mind you, they forgot to check what efficiency loss occurs by adding an electric motor and batteries to the mix.
##### astro_optics
Feb 23, 2011
This comment has been removed by a moderator.
##### Eikka
1 / 5 (1) Feb 24, 2011
Keeping the frequency steady isn't the only reason why wind turbines rectify and then invert the current back to AC outside of the generator.

The main reason is that gears in a windmill break very often, and these are million dollar machines that have to operate more or less flawlessly for 25 years to pay back their own investment and turn a modest profit. The trend is for no gears at all, as it lowers costs and reduces breakage.

I had a close vantage point of a Danish windmill built back in the 90's that could never turn a profit because you had to fix it every single year. Sometimes twice, and getting those guys to come over and replace whatever part was not cheap. They eventually sold it back after taking a decade of net loss.
##### Eikka
1 / 5 (1) Feb 24, 2011

Mind you, they forgot to check what efficiency loss occurs by adding an electric motor and batteries to the mix.

Between 20-30% for the power routed through that part of the system. It's a well tested concept, very similiar to a Toyota Prius transmission.

The problem is that at low wind speeds this thing is not going to be efficient at all, further compounding the natural on/off tendency of windmills in general. They naturally produce lots of energy in relatively short periods of time, and then all but stop.