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5 Tips for Detecting And Fixing Resonance Problems

Sept. 24, 2019
With an increase in variable frequency drives, resonance problems are becoming more common. Here's some advice on what you can do about it.

In the event of a breakdown, any number of issues may be the culprit, but there’s one in particular that’s particularly challenging to diagnose: resonance.

Resonance can be a problem in any machine, as physical structures have natural frequencies that can be excited. Resonance occurs when a forcing function excites the machine’s natural frequency, causing an excess in vibration. These excessive vibrations cause additional stress on a machine, resulting in poor reliability, premature failure, and greater cost in maintenance and parts. Correcting resonance early prevents secondary defects, such as bearing wear, and structural defects like cracked welds, loose bolts, and foundation damage.

Once you know that something is wrong with your asset, it can still be very difficult to diagnose resonance as it may only occur at certain points during the day—generally when demand requires the machine to run at the specific problematic speeds. When not excited, the machine may function normally.

The best way to detect and fully understand the effect of resonance is to continuously monitor the machine over time. With an increasing number of machines running on variable frequency drives (VFDs), resonance problems are becoming more common. Classic route-based maintenance isn’t terribly effective because there’s a good chance that the machine in question isn’t being excited at the moment of data collection. Depending on the operating conditions, you may never see resonance occurring in person.

Example of a resonance curve.

How do you effectively detect resonance?

1. Continuous diagnostics. Continuous diagnostics, includes full spectral data and machine speed information. The continuous information stream ensures detection of the machine’s excited state. Speed information allows tracking of the machine’s forcing functions. The residual unbalance forces associated with rotating shafts are the most likely inputs to excite resonance, but vibration energy associated with any machine component can serve as an input as well.

Gathering the full vibration spectrum enables precise tracking of all these features so that the machine’s response can be fully understood. As the forcing function approaches the resonant frequency, a dramatic increase in amplitude is observed, followed by a dramatic decrease as the frequency continues on through. Not only that, but the continuous data stream enables insight into the number of excitations and their duration as they occur throughout the day. Knowing both the nature and duration of the excited state enables the most accurate assessment of severity.

2. Sweep test. The sweep test is when you manually sweep the machine through a frequency range while taking readings at every step. This can map out vibration response against speed for easy detection of problematic frequencies. However, no insight is provided regarding how frequently the machine excites resonance during normal operation.

3. Run up or coast down test. This method involves collecting data over a period of time when a machine is either ramping up toward full speed or coming down to rest. This particular test uses the shaft vibration as a forcing function to provide energy input into the system. Doing this will excite resonance as the shaft vibration passes through the critical speed. A tachometer is used to measure the phase difference between the rotor heavy spot position and the vibration vector.  As a machine passes through resonance, the phase will shift 90 deg. Observing this shift provides proof of resonance.

4. The bump test. This involves measuring the response of a de-energized machine while it is bumped with a modal hammer or other source of broadband energy input. This bump input excites all frequencies simultaneously and the amplification due to resonance is easily observed.

Of these methods, only continuous diagnostics can assess the practical severity of a resonance condition. All machines have natural frequencies and understanding the nature of the response is only half the battle. Real-world operation under problematic conditions is required for damage to occur, and observation over time is the only way to truly understand the condition.

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Responding to Resonance

When choosing between adding a skip frequency to the VFD and changing the mass/stiffness of your structure, you need to be sensitive to the physical nature of the installation itself. Without doing a detailed engineering assessment ahead of time, it can be challenging to fully understand how a system will respond to any given change.

To ensure success, take some time before you implement your fix to determine what your asset is mounted on, the size and angle of the piping arrangements, and more, as these things will affect vibration response and need to be evaluated on a case-by-case basis.

1. Skip frequency. The quick fix is to employ a skip frequency on the VFD to move the machine through its problem frequency as fast as possible. While this technically isn’t a fix, it is a low-cost solution to the problem. Using a skip frequency means while that your machine will still have to pass through the negative frequency, it will do so faster. Process requirements may prohibit skip frequencies if those frequencies are required for proper equipment performance.

2. Altering mass or stiffness. Another solution is to change the mass or stiffness of the system so that you change the location of the natural frequency, thus shifting the frequency to a location outside of the machine’s natural running range. Increasing mass or decreasing stiffness results in a decrease in the natural frequency.  In order to change stiffness, you need to stabilize the structure by adding cross braces, reinforcements, or other structural modifications. The net result will be that the natural frequency will shift outside of the operating range.

In order to avoid resonance issues, continuous monitoring should be deployed from the time that your asset is commissioned. Doing so will give you regular snapshots of your machine’s health, and more specifically, can determine how much time it’s spending in resonance and how much amplitude it’s experiencing if it’s in distress.

It’s possible that even if a machine experiences resonance it may not reach damaging amplitude levels. This is one reason continuously monitoring for resonance is beneficial to determine if or when a fix is needed to maintain the assets usual functions.

Resonance can be a serious problem for asset reliability and often goes unnoticed due to its intermittent presence on an otherwise healthy machine.  By continuously monitoring machine health, you can avoid being blindsided by this “not so quiet” fault.

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