Figure 1 - Inspecting the newly-replaced No. 4 motor
(Photo courtesy of Jim Hall, Ultra-Sound Technologies, Woodstock, GA)
Using the Ultraprobe 15000 instrument from UE Systems Inc., we noted what appeared to be an inner race defect. As you can see from the FFT report (Figure 2) of this motor, the obvious inner race defect was captured.
Figure 2 - Motor No. 4 Inboard bearing, June 2011, appears to have an inner race defect
(Photo courtesy of Jim Hall, Ultra-Sound Technologies)
What was really ground breaking about this particular inspection in June 2011 was the ability to see for the first time a defect with an ultrasonic instrument "in the field." Although recording a WAVE file with an ultrasonic instrument was not new, we had the ability with the same instrument to record a WAVE file, download the file and view it on a computer through spectrum analysis. Again, the big deal was the ability to see in-the-field FFT and/or time waveform in the field. This gives the user an unprecedented look at a motor bearing in the field and opens up a whole new practice of ultrasonic bearing inspections.
This particular waterpark, unfortunately, practices a "run-to-failure" maintenance strategy. Other than lubricating the bearings every month with three shots of grease, they have no predictive maintenance. However, allowing our presence to inspect the motors and pumps using airborne ultrasound, the park maintenance personnel are asking questions of upper management. They are requesting equipment to help them maintain the park's filtration pumps and motors.
This same waterpark, during the past year of June 2011 and June 2012, retrofitted all motors with variable frequency drives. To our surprise, we found out from the installers that four of the eight motors had burned up while in the process of installing and adjusting. The motor noted in the April/May 2012 Uptime article that we were to inspect as a follow-up was unfortunately one of those motors replaced.
During the summer season, the waterpark runs the filtration pump motors 24/7, supplying water to the various amusements within the park. In an effort to update the park on a limited budget and reduce downtime, the park decided to install variable speed drive (VSD) controls to vary the speed of the motors with load needs.
Inspection of these motors will now require more data for the ultrasonic inspector than before. The motors are direct drive (no coupling), so for proper inspection of these variable speed motors, a stroboscope or a non-contact tachometer will be required to retrieve and input the actual rpm at the time of the inspection. Since the speed will be determined by hertz, the hertz reading also should be entered into the notes. This is "must have information," especially when using the new bearing fault calculator of the instrument's spectra analysis software. With speeds/ hertz changing, care must be taken to assure good results by comparison.
The newly installed motor No. 4 showed no sign of any bearing defects (Figure 3). However, I have often had inquiries about inspecting motors equipped with VSD, and a major concern has always been the annoying whine. Yes, there is a very distinct whine when inspecting VSD motors. Even as I drove within 50 feet of the filtration pumps one afternoon, I had radio interference, lots of interference, through the AM station I was listening to.
Figure 3 - Motor No. 4 (newer motor) showing no bearing defects
Moving on to motor No. 3, its electrical display panel read 50Hz, 47.8 amp and 65.5Vdc. Shortly afterwards, the same motor's hertz reading was ramped up to 53Hz as it was programmed to do. Motor number three after the VSD retrofit is now reading six to eight decibels higher than before.
Four days later, this motor had to be replaced as well with a new motor due to armature problems.
Figure 4 - Outboard bearing, motor No. 3 photo of VSD control panel
Figure 5a - Motor No. 3 Inboard bearing, as it appeared on the instrument's display of the UE 15000
Figure 5b - Motor No. 3 Inboard bearing, four days after this reading the motor was replaced due to armature problems, no other specifics were given
Utilizing an instrument with recording capabilities is key to any inspection, regardless of whether the application is mechanical, electrical, or even leak locating. Having the ability to see defects in the field may reduce costly repairs and/or prevent a shut-down and lost production.
The readings during our follow-up visit were less than expected. We had hoped to reread motor No. 4's inboard bearing. Overall, the inspection did not yield anything of high interest, only that motor No. 3 possibly and motor No. 8 as well were high noise.
It's rather obvious that inspecting motors with VSD is going to be a challenge when utilizing the ultrasound instrument for this inspection. With more and more plants utilizing VSD, it would be helpful to the ultrasound community to discuss the challenges in available online forums, such as ReliabilityWeb.com's Maintenance Forum (online chat). Share with the community your suggestions, thoughts, as well as your experience with ultrasound inspection of motors equipped with VSD.
Figure 6 - Adrian Messer, UE Systems, inspecting motor No. 3 utilizing an Ultraprobe 15000
Figure 7 shows the data logged readings for several of the motors and pumps. Had the VSD not been retrofitted, maybe the decibels would have remained closer to baseline. Some technicians in the past have commented on the higher range of decibels on motors after having been converted to VSD. If you retrofit your existing motors with VSD, you want to take new baselines. Hopefully, a return trip to the waterpark in 90 days will reveal some real changes.
Figure 7 - The DMS software shows a number of the motors with higher decibels than the year before
Jim Hall is the President of Ultrasound Technologies Training Systems (USTTS). He has over 20 years experience and is a "vendor-neutral" company providing on-site ultrasonic training and consultation. USTTS provides an Associate Level, Level I and Level II Airborne Ultrasound Certification. He is also the author of a free, biweekly newsletter called "Ultrasonic War Stories" (visit www.ultra-soundtech.com to sign up).