UNDERSTANDING THE VIBRATION FORCES IN INDUCTION MOTORS
By Michael J. Costello, President, Magnetic Products and Services, Incorporated Houston Texas, in PROCEEDINGS OF THE NINETEENTH TURBO MACHINERY SYMPOSIUM
There's a lot in there, but two things that interested me:
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1 – 1 times slip speed sidebands (instead of pole pass sidebands)
We know that dynamic eccentricity can cause pole pass sidebands (poles times slip speed) in vibration and static eccentricity can cause 2*LF. Apparently if you have both present, you can get sidebands at 1* slip speed. It makes some sense if you think about it, and he demonstrates the logic in figure 4. I’ve never seen it myself, but it is shown in his case study #5. There you see full load is 3567 rpm. Slip speed is (3600-3567)/60 = 0.55hz. Pole pass frequency would be poles times slip speed = 2*0.55hz = 1.1hz.
In figure 14 bottom plot, we see a familiar pattern of 120hz and 2x = 118.9hz, the difference being pole pass frequency 1.1 hz. BUT, there is a peak half way in between at 119.45hz - one times slip speed away from the other two.
In figure 14 middle plot we see running speed 59.45hz. Slip speed above that (+0.55hz) is a peak at 60hz. Another slip speed above that is 60.55hz. There are also similar spacings below running speed although they’re not labeled.
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2 - How the vibration transmission of 2*LF from stator core to frame can be tweaked to hide the 2*LF.
For 2-pole motors the stator core itself has a high 2*LF vibration due to the rotating elliptical wave. The motor OEM’s have a lot of sneaky ways to isolate the core vibration so it doesn’t get to the bearing housings where we measure vibration (There might be some benefit in reliability to protecting the bearings from that vib source, but based on conversations with OEM's it's done more to keep the customers from complaining about the 2*LF that shows on their housing vibration measurements).
In large sleeve bearing motors it’s easy. The frame is a large metal box of thin cheap walls with a lot of empty space inside The core is mounted to a broad area of the BOTTOM of the motor frame. The core is not in any way attached to the side or the ends of the motor frame, so it has very little influence on the bearing housing vibration. The ends of the frame where the housings are supported are also supported from below, not relying so much on their attachment to the center frame.
In smaller (NEMA frame) rolling element motors, it’s harder. The endbells are supported from the center frame rather then supported from below. Since the frame is made as small as possible, there is no empty space. If we tried to only attach the core at the bottom of the frame it’s not a broad enough support. The core has to be supported several places around the circumference by the frame. The interface between the core and the frame can be critical in influencing how the 2LF vibration is transmitted to the bearings. For motors with simple cylindrical interference fit between these components, I've heard they are more likely to have 2*LF in general and more susceptible to soft-foot effects due to twisting of the frame which changes the interference pressure / pattern between these two components. But I have a story about some small motors where the core was not set in the frame with a complete cylindrical interference fit, instead there were ribs on the frame supporting the core....
...In the early 2000's we bought seven 100hp 2-pole motors from a motor OEM factory that happens to be nearby, with a specification that they had to pass our vibration requirements during testing while rigidly bolted down. The OEM was used to testing motors on a flexible pad, but accepted our rigid spec. When we got to the factory test, they seemed very surprised to see high 2*LF (0.2ips+) in the horizontal direction on their motors bolted down during the test run. They spent several days trying to fix it, by getting the airgap perfect (even sleeved and rebored one of the endbells). They planed the motor feet. None of it worked, the 2*LF didn’t decrease very much. Then they had a phone call with an engineer at a remote location. They informed me they were going to cut some slots in the motor frame ribs supporting the core and that would reduce transmission of the 2*LF vibration from the core to the frame. They didn't permit me to watch exactly what they did, saying it was some kind of secret. I came back the next day for the run and the 2*LF was almost gone (<0.02 ips I think). Their modification had worked, but it’s always been just a mysterious story I can tell, with no proof or specifics of what they cut..... Until now! Figure 11 shows a modification where the frame ribs are notched to minimize transmission of 2*LF vibration from the core to the frame. I'm not sure if it's exactly the same as for our motors, but probably similar. It looks like the notches in the ribs are at each end of the ribs. Maybe by removing most of ribs at the end this transmits the core vibration efficiently to the center of the frame and keeps it away from the ends of the frame where the endbells are supported. Or mabye something much more subtle. Beats me.