A common misconception is that vibration analysis will always tell you when there’s an alignment problem with a motor installation. Although vibration analysis is essential to a good motor maintenance program and will reveal most alignment issues, it doesn’t tell the whole story.
If you own a car or truck, you’ve probably taken it in for an alignment. If you’ve done this a few times, you’ve probably been surprised at least once that the alignment was out of spec. “The car drives just fine! I never noticed any vibration.” Yet, you can read the alignment data for yourself. There’s a big difference between before and after. You drive home, and don’t notice any difference from the drive in there.
Couplings are typically designed to compensate for minor misalignments. They compensate by changing the angle of applied force. If you were to draw the vectors on paper, you’d see the mechanical equivalent of power factor.
In this case, it’s not capacitance or inductance changing the vector angle and thus reducing the “real power.” It’s mechanical redirection of the force, reducing the “real” torque. So the motor must turn a little harder to drive the same load, just as with power factor.
To borrow terms from power factor analysis, the difference between the “real” torque and the “apparent” torque is rotational energy that is converted to waste heat.
To ensure you don’t have an alignment problem, you need to test the alignment. Minor misalignments can go undetected for a very long time, all the while wasting energy and reducing overall reliability.
Unfortunately, alignments take time. If you want them done right, they also require an investment in test equipment and training (unless you outsource them).
The main appeal of using vibration testing or monitoring to avoid periodic alignment testing is you don’t have to take the equipment out of service. But how many times is that same equipment taken out of service for other reasons? Use those downtime windows to check alignment. You save energy and improve reliability by using vibration testing and vibration monitoring in addition to alignment testing, not instead of it.
A 200 HP motor may be correctly aligned upon installation, but it is unlikely to stay that way. Many factors contribute to misalignment, and you should expect it to occur over time. If you don’t know how often to realign specific installations, then track the alignment results in your computerized maintenance management system (CMMS) and trend them over time. Then adjust the testing frequency based on how long the trends show an alignment is likely to last before drifting out of spec.
