The Dynamic Performance of a Composite Blade from a 5kW Wind Turbine. Part 1: Measured Blade Response

Abstract

This paper presents and considers dynamic structural measurements from the blade of an operating 5 kW wind turbine. One of the turbine's 2.5m long composite blades was instrumented with strain gauges at several locations along its length with signals acquired through a novel capacitive coupling device to a stationary computer. Data were acquired from the blade and from strain gauges measuring torque on the main shaft in azimuthal mode at 8 times per revolution, and otherwise in the time domain at a fixed rate of 200 Hz. Simultaneous with these measurements, analog signals from wind speed, wind direction, turbine direction, turbine speed, temperature and battery charging power were sampled at least once per revolution. The results from two situations, where the wind speed was approximately the same but the turbine operating conditions were measurably different, were considered in detail. A Fourier transform of the signals from the blade flapwise gauges show a dominant once per revolution load and smaller twice and three times per revolution loads. Fourier transform of signals from the torque gauges show a dominant once per revolution load for a high turbine speed case, but not for a lower speed case. Loads from a gearbox fault, with a frequency corresponding to the gear ratios from the dual stage gearbox are evident in both cases. Root flapwise bending moments and lead-lag bending moments show a skewed once per revolution sinusoidal response with some evidence of higher order components in the results from the low-speed case. The unusual twist section in the blade shape has lead to coupling between flapping and lead-lag motions, which has influenced the blades azimuthal response. A second paper, part II, is planned with further analysis.