Torque step load case — Figure 15 illustrates the measurement results with an applied torque step. The two upper diagrams present the behavior of the hydrostatic transmission with constant motor displacement. The first graph plots the applied torque step and the resulting load torque measured for three different inertias of the turbine.

It can be seen that after the torque step occurs, the measured torque on the test bench also rises and oscillates to the new value. Due to the turbine’s increased inertia, the resonant frequency of the drivetrain decreases.6

The second plot from the top presents the effect on the rotational speed of the turbine. Rising pressure in the transmission increases the leakage, leading to slightly increasing turbine rotation speed. The higher the turbine’s inertia, the more energy that’s stored in the flywheel (with the slight change of speed).

The two lower plots display the same measurement, but in this case a torque-controlled transmission is employed. Here, the torque step leads to higher applied torque than braked by the transmission and, consequently, a rising rotational speed. The load torque on the test bench rises proportionally to the rotation speed until the equilibrium of torque is set again.

Gust-of-wind load case The measurements performed in the torque step load case also were used in the Mexican Hat load case, Figure 16. With a constant motor displacement, only a short delay occurs between applied and measured torque. The rotation speed can only rise slightly. Therefore, the peak power delivered by the wind must be transferred by the transmission. Due to the elasticity of the hydraulic drivetrain, the measured torque on the test bench is even higher than the one applied on the inertia.

As with the previous load case, torque was measured on the test bench using a torque-controlled transmission (lower portion of Figure 16). Here, the turbine can speed up about 2 rpm and, thereby, store most of the peak energy of the gust. The maximum torque measured on the test bench is 120 kNm lower than the applied load. When this torque decreases, rotational speed also decreases, extracting stored energy in the flywheel and reverting to constant operation.

Real wind conditions load case —In this load case, all usual effects such as the tower shadow effect, short gusts of wind, and a changing wind speed in the long term were considered. Figure 17 shows the measured torque and rotational speed behavior with the two different control strategies.

If the motor displacement is set constant, the measured torque follows the applied torque and smooths out torque peaks with a short delay. Activating the torque controller enhances the power smoothening thanks to a changing rotational speed. The short oscillations of this torque signal show that the transmission controller must be optimized.


A wind turbine using a hydrostatic transmission can be controlled in one of two ways. Of the two, the torque-based control strategy delivers a good compromise between ensuring optimal rotation speed and guaranteeing continuous power production.

Results obtained by IFAS engineers from dynamic-behavior test-bench measurements support implementation of the control strategy. In a simulated load from an extreme gust, an uncontrolled response resulted in an amplification of the rotor torque. The hydraulic drivetrain can cope and the response quickly dampens, but the scenario is undesirable. On the other hand, a controlled response severely reduced the effect of the gust. The turbulent wind load case shows, although less apparent, that the controlled response presents a more smooth response. The smoothness of the drivetrain's response to dynamic loads contributes significantly to the quality of power delivered by a wind turbine.

The next phase of the project, again funded by the Fluid Power Research Fund, will bring together the different controller modules (with all required safety functions) switching the components and controlling the rotation speed. In parallel, all needed peripheral systems, such as cooler and supply pumps, will be installed at the test bench. An improved aerodynamic model that can run in the real-time simulation at the test bench will make it possible to apply direct data from wind measurements. Thus, the transmission will act as if it’s installed in a turbine. At the end of this project, the transmission will be ready to be installed in a pilot plant.

Johannes Schmitz, Nils Vatheuer, and Hubertus Murrenhoff are with RWTH Aachen University, IFAS, Aachen, Germany. Niels Diepeveen, is with Delft University of Technology, Netherlands. For more information, visit

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