Figure 1 - Velocity FFT Showing
Pattern Of Peaks Separated By 2xLine Frequency (Sidebands) In High Frequency
Range (30-90xRPM). This Will Be Accompanied
By The Symptom Seen In Figure 2:
Figure 2 - Envelope Plot Showing
2xLine Peak And Harmonics. This Indicates Impacts Occurring At 2xLine Frequency.
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| Looseness
in the winding slots- are detectable with
vibration analysis but
cannot be trended towards failure since the
problem does not worsen (vibration-wise) prior to winding failure. The
problem causes wear of the insluation on the windings and eventually a
ground short (catastrophic failure). Only winding testers (surge testing)
can trend this problem and assess the severity. It is commonly found and
should not be over-reacted to. The symptoms are very similar to loose rotor
bars on the velocity / acceleration spectra. Additionally, however, there
will be high amplitude peaks on your enveloping spectra (e.g. gSE spectrum)
at 2 x FLine and harmonics. Each time a rotor bar passes the loose winding,
it lifts and then drops back. The vibration frequency, therefore, is the
number of rotor bars x RPM (rotor bar pass
frequency = RBPF). Like WSPF, it will be surrounded
by 2 x FLine sidebands. Like the number of winding slots, we won't know
the number of rotor bars but it's not important - the pattern of peaks
separated by 2 x FLine is the clue we need. |
Looseness In The Windings Symptoms:
-
High amplitude
at a very high frequency (RBPF, but we don't know what it is) accompanied
by sidebands at 2 x FLine. This is not unusual and often means little.
In fact, it often has more to do with a potential winding problem than
rotor bar looseness (see the next page).
-
Amplitude peaks
on the enveloping spectra at 2 x FLine and harmonics.
Recommended
Actions:
-
Surge testing
to check insulation integrity and test for any wire to wire, turn to turn
and phase to phase shorts as well as the integrity of the ground wall insulation.
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