Tham khảo tài liệu 'rotating machinery vibration 2011 part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 258 Chapter 9 tion Sec. 4 Chapter 7 synchronized by the keyphaser signal Fig. 11 Chapter 7 to track the rotational speed frequency and thus provide a clean synchronous orbit picture which is comparable to the noise-free single-frequency elliptical orbits from a corresponding computational simulation. Figure 3 illustrates two typical measured synchronous rotor vibration orbits before filtering and after synchronous bandwidth filtering. When significant nonsynchronous orbit frequency components are present synchronous bandwidth filtering is ill advised in general troubleshooting because it removes the subsynchronous and higher harmonic components such as captured in the FFT illustration of Fig. 2. On the other hand for rotor balancing purposes since only synchronous vibration components are used in the balancing procedure synchronous bandwidth filtering is naturally applicable. Figure 4 illustrates an orbit with a period of two revolutions containing synchronous and one-half synchronous components the orbit being typical for cases where the one-half synchronous component is largest. Subsynchronous rotor vibrations are often associated with instability selfexcited rotor vibrations. The example shown in Fig. 4 is for the particular case where the subsynchronous component is exactly one-half the spin speed and thus being a periodic motion is easy to illustrate. In contrast subsynchronous rotor vibrations are more often not at an integer fraction of the spin speed and thus the motion is not strictly periodic. The real-time orbit display then may look similar to that in Fig. 4 but will have an additional unsteady bouncing motion to it due to its nonperiodic character. An additional unsteadiness is also common because Figure 3 Illustrated examples of measured rotor synchronous orbits a Forward whirl b backward whirl. Signal Analysis 259 Filtered Unfiltered Filtered FFT Figure 4 Measured orbit with synchronous and one-half synchronous components. instability self-excited .
