The most common way of performing fatigue testing of blades is to mount a rotating mass exciter onto the blade. Drawbacks of using this exciter type are that a large mass is placed on the blade and also that the forces from the exciter are not only introduced in the direction of the movement, but also along the blade.
As blade lengths increase, these drawbacks become more dominant and ground based exciters need to be introduced. Such exciters are typically based on hydraulic systems where you can easily control test frequency, force and speed of the exciter system. However, these types of systems require comprehensive investments in hydraulic power packs, and operational costs are also high because of energy losses in the hydraulics.
BLAEST has developed a new excitation system purely based on electro-mechanics, which requires less investment and is much more energy efficient. The energy saving for a flapwise fatigue test of a 70 m class blade is in the order of 100 – 200 MWh using our new exciter compared to using a hydraulic exciter. Another advantage of the system is that it operates very quietly being friendly for the workplace environment. The new exciter has successfully been used for commercial testing of a 7X m blade, and it will be our future preferred system when testing blades longer than about 60 m.
The system has been developed to include more exciters operating in parallel at different positions along the blade, thus improving the opportunities for achieving the target bending moment curve without placing a lot of dead load on the blade. A project including implementation of the exciter into a dual axis excitation system is under preparation.
The prototype exciter during trial operation
How it works
An electrical motor applies a reversing torque onto the input shaft of a gear box. The torque is controlled by an inverter connected to a PLC, from which the frequency and the amplitude of the torque can be set.
A gearbox converts the low torque and high speed from the input shaft into high torque and low speed on the output shaft. A torque arm is clamped to the output shaft. In operation the torque arm moves up and down by typically +/-30 deg. By means of flexible connections, such as swivel bearings, a push-pull rod is connected between the torque arm and the blade and thereby able to transfer the excitation forces from the exciter to the blade.
A tricky part of developing this new system is the control system. Like other exciters the new exciter operates near the natural frequency of the blade, so the motor reference torque must be strictly controlled, while taking test frequency and both blade and exciter inertia into consideration.