Electrical defect found with Velocity data – Case Study

Has anyone found many electrical defects though vibration analysis? We know that VA will show the indications of electrical activity but not necessary the severity. This case study shows that the Velocity vibration data can indicate what the cause of the vibration problem is, this will enable the engineer to target the investigation.

Thanks to James Pearce for the data. linkedin.com/in/james-pearcevibrationanalysis

 

Background:

A routine client called after the operators noticed an increase in noise and vibration from a main plant drive motor. This is a DC motor and usually operates around 400-500 RPM. This is a rather old motor and drive system.

 

 

Initial Vibration Survey:

On attending site vibration data was collected, analysed and before leaving site recommendations were given.

Figure 1 is the Velocity Spectrum collected from the motor. This showed a 1 Order amplitude of 0.07mm/s RMS, with a dominant peak at 49.95Hz with an amplitude of 3.2 mm/s RMS with many harmonics. The motor was operating at 384 RPM during data collection.

Fig 1:

 

 

Figure 2 is the PeakVue Spectrum. This displayed a dominant peak at 149.86Hz, 3xLf. This was also sidebanded by running speed.

Fig 2:

 

The recommendations was to check all supply cable connections and inspect the variable speed drive components for condition.

 

 

Maintenance Inspection:

The site electrical engineer was dispatched to inspect the drive for this variable speed motor. Upon inspection 2 Thyristors were replaced and all electrical connections checked for security.

The operator then reported that the vibration magically disappeared.

 

 

Post Maintenance Vibration Survey:

Vibration data was then collected after maintenance. The motor was running at a higher speed of 456 RPM on the follow up survey.

Figure 3 is the Velocity overall trend from the initial survey and post maintenance survey. This trend shows the reduction on motion from 4.301 mm/s RMS to 1.162 mm/s RMS.

Fig 3:

 

 

Figure 4 compares the before and after maintenance Velocity Spectra’s. From this you can see the dominant 49.55Hz and harmonics have disappeared. The only activity left is a peak at 299.74Hz again sidebanded by 1 Order.

Fig 4:

 

 

 

Summary:

This again shows the benefits of sending a certified, experienced and correctly mentored Vibration Engineer and not a data dog to investigate vibration issues. James quickly pinpointed the cause of the excess vibration that enabled the client to efficiently target the area of concern and quickly rectify the issue saving time and money.

 

 

A reliable plant is a safe plant

…..an environmentally sound plant

….. a profitable plant

……a cost-effective plant