Probable Causes for 2 X Line Frequency in Motors

Can you please tell me the probable causes for 2 X line frequency in motors.
We had a problem of high vibration in one of our motors. Main contributor to vibration is 2 x line freq.. When we opened motor we found all things okay.
Can you tell me reasons for 2 x line frequency.


Answer :
The number one reason for any 2FL (twice line frequency) signature is electrical. Now, the type of electrical problem will be the question:
1) Stator eccentricity, shorted laminations, loose iron and a loose stator core will cause a high 2FL frequency, normally without sidebands.
This will also occur with unbalanced voltage incoming voltage.
2) Some 3600 RPM, and to a limited extent, some 1800 RPM, low voltage concentric-wound motors will show 2FL just because of the placement of the coils in relation to the stator core and rotor. This signature is more pronounced when the motor is connected for Delta.
3)Pole pass frequency (or twice slip frequency) sidebands around running speed and the 2FL peak indicate eccentric rotor conditions (usually dynamic eccentricity).
4) Broken rotor bars will show as multiples of running speed with pole pass frequency sidebands and may also show as rotor bar pass frequency
(RBPF) harmonics. 2FL sidebands around RBPF harmonics indicate looseness in the rotor bars.
5) Loose connections will appear as sidebands around a 2FL peak as 1/3 FL peaks with harmonic sidebands.
The good news is that motor current signature analysis is designed to quickly detect these issues using the motor current instead of mechanical vibration (the two technologies complement each other tremendously).