I was working on simulation of steam turbine vibration under unbalance condition. I wish to perform an FFT for the vibration velocity (mm/s) respond and plot the frequency on the x-axis and the real amplitude on the y-axis. The respond vibration velocity data can be downloaded here: https://drive.google.com/open?id=0B21C655TFYWJSVFjY1VPeVg0WHM
I am using the following MATLAB code:
Fs =1000; % Sample Frequency ti = 0:0.001:15; ti= ti'; data = signal; N = length(data); Y(1)=0; Y=fft(signal); freq = 0:Fs/N:Fs/2-Fs/N; freq = freq'; amplitude = abs(Y(1:floor(N/2)))/floor(N/2); subplot(211) plot(ti,data) xlabel('Time[s]') ylabel('Amplitude') title ('Time domain signal') grid on subplot (212) plot(freq,amplitude); xlabel('Frequency [Hz]'); ylabel ('Amplitude'); title('Amplitude Spectrum') grid on
when executed the code this is what I've got
The time in "time domain signal" was corresponding to increased the rotation load of the rotor . For example at time 0-2 second the rotor RPM was increasing , so the unbalance force will excite the system which is lead to greater amplitude. after the 2s the rotor has reached the maximum load , so the amplitude decreased because the presented of damping inside the system from the time domain the max peak was in 39.42 mm/s at time 1.248, and then the amplitude goes down and stable with average in 14 mm/s at the rest of the time. I Thought it will give me two spikes at frequency domain with exactly the same amplitude in y-axis which is given above
Why I didn't get the spectrum signal as I wanted, Can someone please help me? Thanks.