BISOPE series
51- How to set the appropriate frequency when measuring (setting the maximum
frequency for vibration measurement)
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When measuring vibration using an FFT vibration measuring
instrument, there are more things to consider in advance than measuring other
physical quantities (temperature, displacement, pressure, etc.). Above all, the
most difficult question at first is “How many Hz should I set it to?” However, if you
have a simple vibration system, you don't need to worry much. This is because
FFT processing is not possible in general vibration systems (the area that can
be measured using only the time waveform without setting the frequency itself
is cut out using appropriate filtering). However, FFT instruments are
different. If you set the frequency incorrectly, you may not be able to see the
desired signal at all, and it may be embarrassing because it is a basic
problem.
Frequency setting (maximum
frequency Fmax and sampling frequency)
When setting the frequency for
vibration measurement, you basically need to set the maximum frequency (Fmax). This means
the highest frequency on the horizontal axis of the spectrum graph. In
advanced FFT types of the past, this was expressed as sampling frequency (Sampling
rate), which is usually twice the Fmax in vibration measuring instruments. To
be exact, it is 2.56 times, but it is set at 2 times. Therefore, if you are
asking about the sampling frequency, just divide the maximum frequency by 2.
However, it is not important to enter the setting value, but for example, 'What can I see if I
set Fmax to 1000Hz?' The most important thing is to think about it in advance. See
below.
In other words, determine the
maximum frequency after determining the following matters. Above all, the most basic
principle is that the frequency group to be checked must be measured and placed
so that it is visible in the center of the screen.
1. Determine
the specifications of the machine
-Rotation speed of the prime mover
(drive component) (number of poles of the motor, electric frequency, number of
blades of the turbine, sleeve rolling bearing, and other rotating elements)
-Sleev
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