The present paper reassesses the dynamic instabilities of the KEWT ("Kosten Effectieve Wind Turbine") wind turbine prototype. The KEWT wind turbine was a two-bladed wind turbine designed in the 1980s. During the tests, violent vibrations were observed as the rotor angular velocity increased: 1) a first violent vibration appeared at a rotor angular velocity equal to half of the first tower torsion mode. The important deformations observed were tower torsion and blade edgewise modes and 2) after further increasing the rotor angular velocity, a second violent vibration appeared at an angular velocity equal to half of the second tower bending mode. The important deformations this time were tower second bending and blade edgewise modes. Investigating these instabilities, reference concluded that the strong vibrations are resonances of the rotor-tower coupled system, wherein the combination gravity force - constant rotor angular velocity forms a continuous input-energy condition for the vibration. The energy is supplied by the generator. The present paper further investigates the KEWT instabilities, concentrating on the case of tower second bending-blade edgewise instability. It is demonstrated that from the lag modes, the 'advancing lag mode' is the one responsible for the coupling between the lead-lag and the second tower bending mode. The source of energy is coming from the generator who supplies energy to control system in order to keep the rotor rpm constant. The new contributions of this paper to the study of the KEWT instabilities are the two approaches used to demonstrate this instability: first a prediction method which detected the critical distances in the complex plane responsible for the couplings and second a physical method which explained the mechanism of coupling in the system.
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