Electrical Motor Defect found with Vibration Analysis

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This was a first for me in 17 years! It was one of those with the right experienced people at the right place at the right time beauties!

We were called to a site when the operators reported an unusual sound from a 4 Pole 1500RPM fan motor. We attended and through vibration analysis and temperature measurements recommended stopping the motor and checking the connections in the motor terminal connection box. This enabled the site electrical team to quickly pin point the defect.

The vibration data indicated a dominant 100Hz electrical noise, the motor felt like it was hunting or pulsing and in addition the whole motor was hot with the motor DE housing was over 100oC.

It would be interesting if anyone else has ever found a defect like this?

Vibration Data:

The overall velocity Increased from a normal level of 1mm/s RMS to 3.9mm/s RMS and then in two days increased to 8.88mm/s RMS.

The velocity spectrum below displayed a new peak at 100Hz:

This is a single frequency 100Hz Trend, and shows a marked increase at 100Hz:

The PeakVue spectrum displayed dominant 100Hz and harmonics.

The acceleration 10 KHz spectrum displayed two mounds of activity with 100Hz sidebands:

We then decided to look at the auto correlation data and found the following.

10 KHz Acceleration time waveform circle plot with four main peaks and inner lower peaks:


PeakVue Acceleration Time waveform circle plot showing 4 clear peaks:

And this is what was found.

We were surprised the motor was still running in this condition !!!!!

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