Dynamic charcteristics analysis and Design Modification for a Small-Sized Vertical Axis Wind Turbine

Abstract

The dynamic characteristics including natural frequencies and rotor excitation frequencies are evaluated for a 10 kW vertical axis wind turbine for investigating the root cause of excessive vibration of a structure near 30RPM of rotor speed. Acceleration responses are measured at 12 distributed locations from ambient vibration tests during non-operational and operational conditions. The natural frequencies for the lowest two vibration modes, i.e. f1 and f2, are estimated from both conditions and excitation frequencies such as 1P, 2P, 4P frequencies, are also identified according to the rotor rotational speed using the responses under operational conditions (i.e. power generation condition). It is verified thatthe excessive vibration is due to the resonance between the 4P excitation and the 1st vibration modes. To mitigate the resonance issue, dynamic design modification is carried out to increase the stiffness and the 1st natural frequency by filling concrete inside the steel tower. It is found that the 1st natural frequency could be maximally increased up to about 20.4% by filling inside the steel tower with concrete to 7 m by numerical simulation. After the concrete filling, the acceleration responses are measured again and it is observed that the 1st natural frequency is increased as amount of 21.3% by concrete filling and the increasing rate of 21.3% is very close to the one obtained from the numerical simulatio