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Chapter 11
FFT (Option)
11.1 FFT Function
IM WT3001E-51EN
This function allows the power spectrum of the input signal to be displayed through FFT
(fast Fourier transform). This is useful when you wish to check the frequency distribution
of the input signal.
Waveform of current I1
Power spectrum
waveform of current I1
• FFT Source
You can select the source for taking the FFT from the following:
• Voltage, current, active power, and reactive power of each input element.
• Active power and reactive power of an Σ wiring unit.
• Torque and speed signals of motor input (option).
FFT can be performed simultaneously on two sources.
• FFT Window
You can select the FFT window from the following three types.
• Rectangular (Rect)
This window is effective on repetitive waveform of AC signals whose period is equal
to an integer multiple of the FFT measurement period. The FFT measurement
period is set to 1 s or 100 ms by setting the number of FFT points (200 k or 20 k).
For example, the rectangular window is effective on 50-Hz or 60-Hz signal with
repetitive waveform, because the above conditions are met.
• Hanning and FlatTop
These windows are effective on waveforms that do not meet the conditions for the
rectangular window. The Hanning and flattop windows allow continuity of the signal
by gradually attenuating the parts of the signal located near the ends of the time
window down to the 0 level.
For example, if the input frequency is 52.2 Hz, the integer multiple of the period is
not equal to the FFT measurement period. If the rectangular window is used in this
case, frequency components that do not actually exist are detected and appear as
side lobes.
Frequency components that do not actually exist are also detected using the
Hanning or flattop window, but the level of the non-existing frequency components
can be kept small as compared with the rectangular window. The figures on the
next page show a conceptual diagram of the result obtained by taking the FFT on a
52.2-Hz sine wave using each window.
With the Hanning window, the frequency resolution is high compared to the flattop
window. However, the flattop window has a higher level of accuracy.
When the FFT source is a continuous signal, select the proper window for the
application.
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Index
11-1
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