A useful spectral tool for detecting broken rotor bars is the swirl effect, which occurs just below the 5th harmonic of line frequency (300 Hz on a 60 Hz line frequency). Swirl peaks are a confirming tool for the pole-pass frequency sidebands around line frequency and occur at:
fswirl = [1 - (2/5)ks]5fLine
fswirl = location of the peaks just below the 5th harmonic of line frequency
k = harmonic index 1,2,3...
s = Slip
fLine = line frequency
For more information read Using a Six Fault Zone Approach for Predictive Maintenance on Motors at http://www.pdma.com/pdfs/Articles/Using_a_Six_Fault_Zone_Approach_for_Predictive_Maintenance_on_Motors.pdf.