Case – Resonance Determination 

A frequency driven motorfan was measured throughout the entire control area. The goal of the measurement is establishing resonance areas and finding out what the possible action items are. 

Observation: 

A continuous measurement was arranged in three directions (horizontal, vertical and axial) on a bearing block at the free side. This enables continuous monitoring over the entire control area. To avoid irregularities during measurement the machine was driven in two directions (from operating speed to 50%, subsequently to 100%, back to 50% and finally back to operating speed). 

 Positioning of the sensors: 

1.  Ch1 = horizontal V4 
2.  Ch2 = vertical V4 
3.  Ch3 = axial V4 

 The following chart shows the trends of this measurement:

We can see that the rising side of the chart is perfectly symmetrical to the falling side. This means we haven’t encountered irregularities and we therefore have a representative measurement. 

1.  Operating speed at start of measurement 
2.  +/- 50% speed 
3.  Significant increase of horizontal and axial vibration 
4.  100% speed[IC1] In the vertical field we see an area where the vibrations are clearly rising. 

Spectral analysis shows the following chart:

To be able to gauge the frequency better, we look at these spectra from above. 

Horizontal

As the trend already shows, the bottom part of the control area reveals an increase in vibrations. The spectral analysis shows a provenance of 1x rpm. At this point the number of revolutions equals 12,5 Hz. That in turn corresponds to a speed of 50 %. That is less than operating speed (more than 60 %), so this area will not cause any problems during normal use. In zone 3 of the trend, there is also an increase. The following chart clarifies what happens at that time:

We see an increase of 2x rpm and to a lesser degree also of the other revolution harmonics. Dat shows instable behaviour, also visible in the axial measurement. Probably we are dealing with a tipping point of the resulting forces (mechanical versus process forces). This phenomenon has nothing to do with resonance and it occurs at +/- 19 Hz.

Vertical

As the trend already showed, at a certain point there is a strong increase in vibrations. This increase comes from 1x rpm at 16 Hz. That looks a lot like a resonance area and considering the amplitudes it reaches, it should be avoided. 

Axial

Axially we see a normal evolution of the vibrations. With the exception of the area that in the trend is described as zone 3. The following chart illustrates what happens here:

Just like in the horizontal field, we see an increase of the harmonics. Especially the 2x rpm spikes are on the rise. Measurement during normal operation: 

 Measurement during normal operation:&nbsp;</p><p>&nbsp;OVERALL LEVEL

•         M2H - Motor Coupling Side Horizontal              .927 mm/Sec
•         M2P - Motor Coupling Side Peakvue                 1.233 G-s
•         M2V - Motor Coupling Side Vertical               1.817 mm/Sec
•         M2A - Motor Coupling Side Axial                  1.986 mm/Sec
•         V3H - Vent Coupling Side Horizontal              1.185 mm/Sec
•         V3P - vent Coupling Side Peakvue                  1.505 G-s
•         V3V - Vent Coupling Side Vertical                1.065 mm/Sec
•         V4H - Vent Non Coupling Side Horizontal              5.252 mm/Sec
•         V4P - Vent Non Coupling Side Peakvue                  2.686 G-s
•         V4V - Vent Non Coupling Side Vertical                3.494 mm/Sec

This measurement was conducted around the area where the machine showed unstable behaviour before. In this measurement that doesn’t show. The vibrations are also less pronounced than in the trend in this area. That shows that the unstable behaviour changes with the conditions of the process. 

Summary

The measurement shows the vibration response throughout the entire control area. In the horizontal field we see higher vibrations during very limited load (+/- 750 rpm). Normally that is situated outside the machine’s operating zone. We can also see unstable behaviour around 1170 rpm. Probably that is a transitional area of the resulting forces. During normal operation on 28/11 the machine worked on 1150 rpm and then this phenomenon didn’t show. That shows that this is transitional phenomenon which doesn’t occur during normal operation or at least not in the same area. Vertically we see a resonance area around 950 rmp. That is around 65 % speed. Considering the height of the vibration peaks, this area should be avoided. Axially we see the same unstable behaviour as horizontally. 

Conclusion

This measurement has clearly depicted the vibration tendencies throughout the control area. In a simple way one can avoid excessive vibrations in the machine, that way lengthening its service life considerably.