The old standard of testing a motor once a year with a megger and pulling the motor out of service to inspect the internals every three years can be eliminated with this new technology. But, with any new technology, there comes the challenge of change and education. Teaching this new technology to an already established maintenance program using the standards of testing that have been in place for many years through the Institute of Electrical and Electronics Engineers (IEEE) and the InterNational Electrical Testing Association (NETA) is an ongoing challenge for the technician. Although the standards are effective, there is a better and more cost saving way of performing maintenance.
As stated earlier, trending is one of the tools used in predicting how long a motor should be in use before being sent to a motor shop for reconditioning. With current standard practices, trending is not plausible, yet missing the chance to save a motor from failure also is not likely, thus the need for change. By trending, you can, with very good precision, determine how productive a motor will be by tracking and trending the changes in resistance to ground (RTG) and capacitance to ground (CTG).
By using the new technologies, the technician can observe and trend six fault zones - power quality, power circuit, rotor, stator, air gap and insulation - with not much more than a push of a button. Time is money and the time savings using this equipment not only saves the technician's time, but equipment as well, which is big money.
All of the fault zones are very important to test, but for the sake of this article, the focus will be on two areas of fault zones, insulation and rotor fault zone. Insulation integrity is one of the biggest issues concerning motor failure. Testing and trending insulation degradation should be done every year. The tests monitored very closely are the polarization index (PI) and the polarization index profile (PIP), along with the RTG.
In the area of PIP, there has not been very much attention paid, but a much closer look should be given to this. A PI of two is ideal; a little more is good, but less is not and is an indication of how clean the windings are. Or is it? There are many times when the PI is two or greater, which would indicate no issues with the windings. But by observing the PIP, you can see if this is a true statement. There are many times when the PI is good, but the PIP is not. In other words, you can have contaminated motor windings with a good PI.
Rotor faults are more common than one might believe. Yes, you can find rotor bar issues using vibration detection, but by using a specialized tester, you can see rotor bar issues before they become catastrophic.
Technology has come a long way and by using the testers of today, you can save your company from premature failures. Using this technology can save thousands of dollars per year in unnecessary motor repair and unplanned downtime.
Currently, Texas-based Energy Transfer is working toward this new way of maintenance by using these new testers, along with infrared and vibration. As with all maintenance programs, change does not come overnight, but Energy Transfer is leading the way with this type of electrical testing in the oil and gas industry to ensure a more reliable future for the company and to keep its competitive edge.
Figure 1 shows a 4160 volt motor at one of Energy Transfer's locations. The PI and PIP are shown. The windings are very contaminated with oil and dirt. This motor will be pulled and sent to a motor shop for reconditioning. If left in service, the motor would fail and cause interruption to the flow process of the natural gas.
Figure 1: 4160 volt motor showing both PI and PIP
Figure 2 is another 4160 volt motor at Energy Transfer showing a good PI, but the PIP is showing contamination of the windings. If the PI alone was considered, this motor would have been left in service and failed, but due to the PIP and low RTG, this motor is being sent out for reconditioning.
Figure 2: 4160 volt motor showing a good PI
Figure 3 is an example of one of Energy Transfer's 4160 volt motors showing unusual sidebands on the rotor evaluation. Although this is not a rotor bar issue, this is where you would find rotor bar signatures. The lower spectrum is showing sidebands at 55Hz and 65Hz, which is a false positive signal of the air ducts in the rotor.
Figure 3: 4160 volt motor showing unusual sidebands on the rotor evaluation
Figure 4 is an example of a rotor evaluation spectrum showing what a broken rotor bar would look like. It is confirmed by the swirl effect shown in Figure 5
Figure 4: A rotor evaluation spectrum showing an example of a broken rotor bar
Figure 5: A rotor evaluation spectrum confirming a broken rotor bar.
The reason for the swirl effect is due to the fact that a damage rotor will modulate the stator current at the same frequency at two different vectors. Simple to say, but difficult to understand without a very sound understanding of the mathematics involved in fast Fourier transforms (FFTs).
As the snapshots of motor insulation and rotor evaluation spectrums clearly indicate, there is no need to disassemble a motor every three years because with the tester, you can see contaminated windings or rotor issues without ever popping the end bell off a motor.
With Energy Transfer, as with most companies, the untimely failure of a critical motor will cost the company in lost production. The loss of production usually costs the company a large amount of money and, in some cases, the wait time to have another motor in place can be very lengthy. To avoid this at Energy Transfer, a move to a reliability/predictive maintenance program is being introduced.
Although preventive maintenance will always be part of the maintenance program at Energy Transfer, reactive maintenance will not. This change will allow the company to be very reliable and cost-effective, and ensure on time delivery of product to its customers.
Kelly Ballew is currently working on developing strategies for predictive maintenance and improving the reliability on high voltage equipment for Energy Transfer, a natural gas pipeline company. Mr. Ballew has worked in the high voltage field for over 30 years in the power distribution field. He has developed motor testing procedures along with cost savings through predictive maintenance for large companies. www.EnergyTransfer.com
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