MOBIUS INSTITUTE - RELIABILITY IMPROVEMENT & VIBRATION ANALYSIS TRAINING AND CERTIFICATION PER ISO 18436

Mobius Institute - Vibration Analysis

Mobius - Induction motors

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Vibration Analysis Definitions


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  Induction motors: Loose rotor bars


Symptoms: 100/120 Hz sidebands around rotor bar pass frequency

If there are loose rotor bars there will be a peak at the rotor bar pass frequency (RBF - the number of rotor bars times the turning speed), with sidebands of twice line frequency (100 or 120 Hz). Even if you do not know the number of rotor bars, if you see a high frequency with twice line frequency sidebands, you can be somewhat confident that this fault condition exists.

Please note that it is quite common to see this pattern. You should only investigate further if these peaks change in amplitude.

Rotor bar pass frequency (RBF) = Number of rotor bars X Running speed

2xLF = Twice line frequency (100 or 120 Hz)

Spectrum analysis: Look for the peaks at RBF plus and minus 2xLF.  Note that sometimes you will see a high peak of RBF + 2xLF whereas the peak at RBF and RBF 2xLF may be quite small.

Please note that it is quite common to see this pattern.  You should only investigate further if these peaks change in amplitude.

Rotor bar pass frequency (RBF) = Number of rotor bars X Running speed

2xLF = Twice line frequency (100 or 120 Hz)