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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 3, Detuning and Proving Resonance

Section 11, Using Phase Change to Determine a Resonant Condition

Consider a rotor with changing rpm, such as when accelerating or decelerating. As it rotates through its resonant speed ranges, the vibration phase changes. When the rotor speed change is in a range that is not resonant, then vibration phase does not change (even though the rotor rpm is changing). When the rotor speed reaches the critical speed or resonant frequency range, the slightest change in speed will result in a phase change. For simplicity, assume the phase's angular position will be revealed by a strobelight.

With the rotor running at speeds well below the resonance frequencies of any parts of the support system or rotor, the rpm accelerates through the "below resonance" speed range. The phase remains at only one angular position. For example, as the strobe first starts to flash steadily, the phase will show at, say, 10:00 o'clock and remain at the same position through any speed increases until the rotor rpm reaches the end of the non-resonant "below resonance" speed range. As soon as the rotor speed enters the first resonance speed range, the phase will start changing with each increase in speed. The reference phase mark will start "rotating" or gradually move angularly to new clock positions until the rotor speed reaches the actual peak resonant speed. At the peak resonant speed, the phase should have shifted angularly about 90° (in this example, from 10:00 o'clock to 7:00 o'clock).

As the rotor continues to increase its rpm, the reference phase mark will continue to shift angularly (6:00 o'clock, 5:00 o'clock and so on) until it has shifted about 180° from when the rotor first entered the resonant speed range, to 4:00 o'clock. Now, as the rotor continues to increase its rpm, it enters the non-resonant "above resonance" speed range. Even though the rotor speed continues to increase, the phase mark will remain at approximately 4:00 o'clock, appearing as if it is standing still. It will remain so until another resonance speed range is entered. At that time, the phase mark will start shifting angularly again as the speed increases. While going through that subsequent resonance speed range, it will shift approximately 180° from the lowest speed of that range (phase mark at 4:00 o'clock) all the way up to the upper limit of that range (phase mark at 10:00 o'clock).

Throughout the non-resonance ranges, the phase will not change its angular or clock position, even though the rpm continues to increase. When the next resonance speed range is reached, the process starts all over again, and so on.

As mentioned, the usual shift is approximately 180° from entering to exiting any specific resonant speed range. When the phase shift reaches 90° from either the high frequency end or the low frequency end, it is an indication that the rotor is operating at the high point of a resonant speed. At this frequency, the amplitude should indicate a maximum "peak" reading.

Although the phase should theoretically shift 180° from below resonance to above resonance, sometimes the shift is less, such as only 150° or so. This is due to the fact that another part has started to resonate before the originally resonating part has completely gone through its resonant zone. In very rare, more complicated situations, several parts can resonate or partially resonate with overlapping frequency ranges. Each one affects the resultant phase. Sometimes this shows as phase shifts of considerably more or considerably less than 180°. In other situations while the rpm is still changing, the phase appears to rotate in one direction for a while and then rotate in the opposite direction. It seems that the composite effect on phase by two resonating parts can create a resultant phase that rotates in the opposite direction expected. It all finally reduces to a simple principle. That is, as a rotor accelerates or decelerates through the resonant range of a part, the phase will shift. When no resonance is present, as the rotor accelerates or decelerates, the phase will not shift. However, for some instruments or transducers, a slight complication can occur at slow speeds of well below 1000 rpm or at very high speeds, depending on the resonant frequencies of the transducer itself. Some vibration instruments have a phase shift of their own, especially at very low speeds.

Other than possible problems of excessive vibration that occur while the rotor is accelerating or decelerating through a resonance range, it is not usually of much concern if the observed resonances do not correspond to within 20 to 25 percent of operating speed. A high speed rotor may go through several resonant speed ranges as it accelerates to its operating speed, and yet operate well within a non-critical, non-resonant range. The source vibration, such as from unbalance, misalignment and so on, will then not be magnified.

 

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