Rotor Bar Defect Sidebands Around the Line Frequency
Rotor bar defects cause sidebands in motor current spectra, appearing at the line frequency plus or minus the Pole Pass Frequency (PPF). These sidebands, indicative of rotor damage, can be detected using Electrical Signature Analysis (ESA).
Why Do Sidebands Exist in Rotor Defects? Let’s Decode the PPF Mystery
Ever spotted sidebands around the line frequency in your motor current spectrum? If they’re spaced at the Pole Pass Frequency (PPF), your rotor might be whispering a warning.
🧲 Rotor bar defects disrupt the magnetic symmetry, causing the stator current to modulate at the PPF. This modulation creates sidebands at:
Line Frequency ± PPF
📐 PPF = Slip × Number of Poles
For example, a 6-pole motor with a 20 RPM slip gives:
PPF = 20 × 6 = 120 RPM → 2 Hz
Sidebands appear at 58 Hz and 62 Hz (assuming 60 Hz supply).
🔧 These sidebands are more than just spectral features—they’re diagnostic gold. They help us detect rotor damage without opening the motor. That’s the power of Electrical Signature Analysis (ESA).
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