Registered: 1442538912 Posts: 1,810
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The plot below is from the motor inboard horizontal position on a 350 HP VFD motor direct-coupled to a center-hung fan. Speed is 894 rpm. I have no information on the drive. A zoom spectra confirms the dominant peak is exactly 57X rpm. It is not unusual to see this, but I usually only see it at this level on the motor outboard vertical position when the reading is taken on the stator housing (as opposed to a more solid endbell). I don't recall ever seeing it at the MIH position before, at least not at anything near this level.
This peak is completely absent from the adjacent, identical motor/fan, though I am told the controllers are "different" which I understood to mean different types, and not just different manufacturers. It feels and sounds pretty intense on the motor, anywhere I touched/listened. The small sidebands are 1X and the larger upper sideband is 180 Hz. Is this something to be concerned about? This is the first time I've collected data on these motors (new CM program) and don't want to get bit in the butt the first time out. I know you can't make a definitive diagnosis, but just wondering what you think.
__________________ "The trend is your friend"
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Registered: 1442560369 Posts: 843
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Me, personally, at .09 in/sec or, in real speak, less than 2.5mm/sec :-) i wouldnt worry about it, unless your friend goes up in consecutive surveys. Pete mignt have more insight on the 3 x LF sideband.
Has the 1 x increased as a possible indicator of imbalance from a rotor bar sticking its head up a little?