Electrical motors have the same mechanical faults that like other rotating machines, but there are also some specific faults for electrical motors.
Thermal bending of the rotor: Uneven electrical circuits in the rotor bars generates an uneven heat distribution in the rotor. This causes a deformation, bending, of the rotor which results in unbalance.
Eccentricity in the air gap: If the air gap is not uniform it generaties unbalance forces on the rotor which in turn induce a vibration at 120 Hz (2xLF).
Loose rotor: Sometimes the rotor skid on the axle, depending on the temperature. This generates a vibration 1xRPM and harmonics. Factors provoking this fenomenon are sudden changes in load or line frequency.
Eccentric rotor:If the rotor is not entirely round this will cause vibrations at 1xRPM. It will also generate unbalanced magnetic forces resulting in vibrations at a frequency corresponding to the slip frequency multiplied with the number of poles.
Loose windings: If the stator windings are slightly loose the vibration level at 120 Hz will increase. This causes scraped thread isolation which leads to short circuit between the windings. It can also be short circuit between the windings and earth, which leads to stator breakdown.
Problem with rotor bars: An important cause of problem in elctrical motors is that the rotor cracks. This mainly happens to motors often exposed to starts with load. During start the current in the rotor bars become very high as the speed of the rotor is much lower than the synchronous speed. The electric current causes heated rotor bars, causing them to expand in relation to the rotor itself. The difference i resistance between the rotor bars result in turn in uneven heating and uneven expansion. This leads to cracks in the joint between the bars and the shortning rings. When a crack appears the resistance in the bar increases, which in turn increases the heating and thereby enlarge the crack. The current also becomes larger in the other rotor bars as the current has been reduced in the broken bar.
|Source of vibration||Exciting frequency||Dominating direction||Amplitude||Spectral characteristic||Comments|
|Bad contact in commutator, (D.C Motors)||f = nCX/50 Hz||radial, transversal||narrow band||n=integer C=number of commutator segments X=speed (RPM)|
|Rotor bar||2 X slip X number of poles||radial, transversal||Sometimes pulsating||narrow band||Sometimes 2 x slip sideband around 120 Hz (Sv.100Hz)|
|Slot pass frequency||S = B/50 ± 120||radial, transversal||narrow band||S=Rotor slot pass frequency Hz B=number of rotor bars X=rotor speed (RPM) 120=two times mains frequency, Hz|