Vibration Analysis

Vibration analysis is commonly used as condition monitoring tools of many rotating equipments. In this post, I want to explain the use of vibration analysis to determine the condition, priority for repair and location of defect of induction motor.

To determine the condition of induction motor, three parameters are used: displacement (mm), velocity (mm/s) and acceleration (mm2/s). Some manufacturer has also introduced measurement of acceleration using enveloping technique. The purpose of enveloping is to filter out the low frequency rotational vibration signals and to enhance the repetitive components of a bearing’s defect signals occurring in the bearing defect frequency range. Envelope detection is most common in rolling element bearing and gear mesh analysis where a low amplitude, repetitive vibration signal may be hidden by the machine’s rotational and structural vibration noise.

The measurement of those parameters are conducted on the DE-H (drive end horizontal), DE-V (drive end vertical), NDE-H (non-drive end horizontal), NDE- H (drive end-horizontal),  NDE – V( non drive end vertical) and axial side.

  • Horizontal measurements typically show the most vibration due to the machine being more flexible in the horizontal plane. Also, imbalance is one of the most common machinery problems and imbalance produces a radial vibration, that is, part vertical and part horizontal. Because the machine is usually more flexible in the horizontal plane, excessive horizontal vibration is a good indicator of imbalance.
  • Vertical measurements typically show less vibration than horizontal because of stiffness due to mounting and gravity.
  • Under ideal conditions, axial measurements should show very little vibration as most forces are generated perpendicular to the shaft. However, misalignment and bent shaft problems do create vibration in the axial plane.

To determine the condition of induction motor, three parameters are used: displacement (mm), velocity (mm/s) and acceleration (mm2/s). Some manufacturer has also introduced measurement of acceleration using enveloping technique. The purpose of enveloping is to filter out the low frequency rotational vibration signals and to enhance the repetitive components of a bearing’s defect signals occurring in the bearing defect frequency range. Envelope detection is most common in rolling element bearing and gear mesh analysis where a low amplitude, repetitive vibration signal may be hidden by the machine’s rotational and structural vibration noise.

vibration analysis

Vibration Measurement

To determine the condition based in the measured induction motor using vibration analysis, ISO 10816-3 ( Industrial machines with nominal power above 15 kW and nominal speeds between 120 r/min and 15 000 r/min when measured in situ) standard is used. Other standard such as ISO 20372 can also be use for determining machine condition based on vibration measurement. 

vibration analysis

ISO 10816 For Vibration Analysis

While for displacement parameter, the table below is shown :

vibration analysis

Displacement Vibration ISO 10816-3

For acceleration, ISO 10816 doesn’t show the acceptable criteria. But, some vibration monitoring manufacturers has given the standard for acceleration enveloping as guidance for vibration analysis. Alarm is when the acceleration is over 3 g and danger is when the acceleration is over 5g.

The measurement below will give example for the vibration measurement as a part of routine inspection for condition monitoring of a certain induction motor that drives pump

Example of Vibration Measurement on Motor X

The measurement of vibration is at class II machine. The measurement tells us that the machine is at unsatisfactory condition based in ISO 10816-3.

Example of Vibration Measurement on Motor X

The measurement of vibration is at class II machine. The measurement tells us that the machine is also at unsatisfactory condition based in ISO 10816-3.

Having determined the condition of induction motor, we can also analyze the location of damage. Thus, certain corrective action can be determined. The analysis is called spectrum analysis. Spectrum analysis is conducted by transforming time domain waveform to frequency domain waveform using fast fourier transformation (FFT). The spectrum analysis becomes important analysis tool. If a machinery problem exists, FFT spectra provide information to help determine the location of the problem, the cause of the problem and, with trending, how long until the problem becomes critical.

Before conducting, the FFT spectrum analysis, it is important to determine the speed of the induction motor. The speed of induction motor is easily identified from the nameplate. Just use the synchronous speed for induction motor. For example : for motor with 4 poles and 50 Hz has snch 1500 rpm speed while motor with 2 poles and 50 Hz has synch speed 3000 rpm.

Fast Fourier Transformaition

The fault diagnosis based on the frequency is explained below :

1. High 1 x amplitude shows unbalance problem. For example : rotor unbalace of induction motor. The corrective action is by conducting balancing of the rotor. The other example is related to soft foot of induction motor. Soft foot is damage of the foundation of induction motor. Experts say that almost half of machinery problems are caused by unbalance.

vibration analysis

Unbalance FFT

2. High 1x, 2x shows misalignment problem. The misalignment can be categorized into angular and parallel. I will explain the details about both those misalignment categorization on the next post. Angular misalignment causes axial vibration at the running speed frequency (1x). Parallel misalignment produces radial vibration at twice the running speed frequency (2x).

vibration analysis

Misalignment FFT

3. A long string of rotating frequency harmonics or 1/2 rotating frequency harmonics at abnormally high amplitudes shows mechanical looseness, or the improper fit between component parts, is generally characterized by

NOTE:
These harmonics may be sporadic. For example, looseness may display peaks at 2x, 3x, 4x, 5x, 6x, etc. or at 3x, 3.5x, 4x, 5.5x, 6x, etc

vibration analysis

Looseness FFT

4. With overall vibration analysis and spectral analysis, a bent shaft problem usually appears identical to a misalignment problem. Phase measurements are needed to distinguish between the two.

5. For bearing defect analysis, i will explain at my next post.

If you have question related to vibration analysis, just contact me

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