An argument in favor of IEEE-841's provision for motor bearing protection.
In June, 2007, a reader asked Electrical Apparatus magazine, "How long should motor bearings last?" The answer will astound you:
"If (and it's a big if) ball bearings are kept clean and properly lubricated, their average or L-10 life should be 3 ½ centuries-3,180,000 hours."
We did the math and came up with an L-10 life calculation of 3,175,926 hours or 362.30 years that ten percent of those bearings will fail because of metal fatigue. These numbers were assuming a 209 medium duty ball bearing with a dynamic load rating of 7,000-lbs and a rotor weight of 200 pounds at an operating speed of 1,800-rpm.
Why does the major cause (51 percent) of electrical motor failures originate in bearing failures? Let's investigate.
Mechanically, the electric motor is a simple machine: two securely mounted rolling element bearings support a large diameter shaft. Motor shafts are designed to transmit torque to driven equipment while resisting counteracting bending moments at the shaft extension. Bearings are sized to fit over the relatively large diameter shaft, so by definition they are considerably oversized to be able to carry the radial load of the rotor.
From an energy consumption standpoint, motor selection is of paramount importance since the motors in the United States consume approximately 75 percent of the electricity that goes to the industrial sector. Electrical costs can and do have a huge impact on the bottom line of most industries. Although incentive programs exist in the Power Generation sector and are also promoted by the U.S. and Canadian governments, the immediate monetary savings are real and tangible goals for motor selection in industry.
Motor electrical efficiency ratings are obviously important in motor selection. Both NEMA Premium and IEEE-841 motors have equal efficiency ratings. The motors are electrically identical, but certain structural enhancements of ‘841 make it an obvious choice for severe duty in the process industries. It is the only one with any kind of effective bearing protection as standard. Although it may or may not last 350 years, it will keep the bearings in a pristine state to accomplish maximum run-time-be it 10 or 30 years-and at least twice as long as the motors lacking adequate bearing protection.
If a NEMA premium motor has a 30 percent rate of return (ROI) due to energy savings over time, the IEEE-841's ROI will be more than twice as much as the non-bearing protected motor during its service life. Further savings are realized in the lack of downtime caused by the first unplanned outage due to bearing degradation of the non-bearing protected motor. Sometimes the focus on a minimum initial investment gives inadequate attention to life cycle costs.
The information presented here is not a matter of opinion by the writer, or a theoretical representation of accumulated, promotional data. At least one well-known major U.S. chemical company has been using the IEEE-841 motors since their inception in 1994 as an IEEE standard. Since then, they have banked an average energy reduction of 5.7 percent by replacing NEMA frame motors with the ‘841 as they come out of service. Additional benefits include:
- Reduced failure frequencies that have a direct impact on production capabilities and costs.
- Increased investment effectiveness due to better capital and life cycle cost guidelines.
- Corporate understanding of motor technology and energy-efficient considerations including demand charges, etc that amount to the total billing for energy.
- A calculated internal rate of return of more than 100 percent by means of motor energy conservation.
The entire industrial community needs to make a total commitment to take full advantage of energy conservation from motor efficiency enhancement. Management, Maintenance and Operations must be involved for best lasting results.
Motor Efficiency Mandates
In December, the Energy Independence and Security Act of 2007 was signed into law, mandating efficiency levels for electric motors manufactured or imported into the U.S. It ratchets EPAct efficiency up to levels to the NEMA Premium/IEEE - 841 ratings for motors of 1- to 200-hp. The only decision left for industrial plant personnel is whether to repair or replace a failed less efficient motor in service prior to the new efficiency rules of order. It will be a choice dictated by the magnitude of ROI evaluation.
