Monitoring of rotating unit shaft components
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Typical faults of the rotating shaft assembly include imbalance, misalignment, mechanical looseness, self-excited vibration, and excitation by electromagnetic forces. In the method of diagnosing faults in rotating shaft assemblies using vibration techniques, in addition to being able to use specialized instruments such as the Bentley system to directly observe the vibration of the shaft, we also need to understand how various excitation frequencies and vibration waveforms are measured and detected. I would appreciate your advice
For the Bently 3300 probe, Maxwell’s electromagnetic theory is used to derive a single-valued function of the excitation coil impedance in relation to the gap between the probe and the metal surface; a measurement conversion circuit (implemented in the preamplifier section) is then used to convert these impedance changes into voltage signals. The signal is then processed to carry out various measurements. For axial displacement, the DC component is used – the arithmetic average over the sampling period is divided by the sampling period to obtain the current signal, which represents a point on the curve. The curve itself is formed by repeated measurements and the resulting average voltage over time. Regarding radial displacement, it is known that it involves taking the peak-to-peak value of the AC component of the detected voltage; as for how exactly this should be understood, I need to consult experts. As for the understanding of the excitation frequency: starting from Maxwell’s electromagnetic theory, it refers to applying a direct current voltage to the excitation coil. When the compressor shaft moves away from or towards the probe due to faults, this creates obstacles to the magnetic field lines, resulting in regular changes in the impedance of the primary coil; ultimately, what we observe are regular changes in voltage. I’m not sure if my understanding is correct; this is for reference only, meant to spark further discussion.