Vibration from electrical machinery such as motors, generators and alternators can cause mechanical or electrical problems. Some of the electrical defects can be investigated from the vibration spectrum, such as:
- Cracked / broken rotor bar
- Eccentric rotor
- Eccentric stator / soft foot
- Phase loss
- Unequal air gap between stator and rotor
Cracked/broken rotor bars
Cracked/broken rotor bars, shorted end rings or rotor blades, or loose rotor bar joints produce a series of harmonic peaks in rotation speed with Pole Passage Frequency (PPF) sidebands. Since the PPF has a very low frequency, it is necessary to use a high-resolution spectrum. Audible and visible beats also appear in the time curve. Another pattern can be observed for cracked / broken / loose rotor bars, of which there are two. Line frequency (2XLF) sidebands surrounding the rotor bar pass frequency (RBPF).
PPF = number of poles × slip frequency
Number of poles/strong> = 2 (3000 rpm) | 4 (1500 rpm) | 6 (1000 rpm)
Slip frequency = synchronized motor speed – actual speed
RBPF = Number of rotor bars × actual motor speed
Eccentric rotors create a rotating, variable air gap between the rotor and stator that induces pulsating vibrations. A key indicator is the presence of pole-pass sidebands around 1X and 2XLF.
Eccentric stator / soft foot
Stator eccentricity creates a non-uniform stationary air gap between the rotor and stator and can occur due to the soft-feet problem. This creates a very directional source of vibration generated from a two-line frequency (100 or 120 Hz)