July 15, 2016
Shaft Distortion and Electric Motors
By Mac MacCormack
Although shaft distortion due to misalignment affects more than just the electric motor, here are a few things that are a telltale signs you may have an alignment issue.
In a perfectly straight shaft, the centers of each shaft cross-section from end-to-end of the shaft lie on a straight line. A shaft is bent if that is not the case.
In a bent shaft, the axis of the shaft is different than its axis of rotation. The range of the orbit or gyration caused by the shaft bend can increase with increased degree of misalignment.
Detection Using Vibration Analysis.
Symptoms: Twice line frequency (100 or 120 Hz) radial
Stator problems will generate high vibration at twice the line frequency (100 or 120 Hz). Stator eccentricity produces an uneven stationary air gap between the rotor and stator that produces a very directional source of vibration.
A stator may become eccentric due to soft foot.
Spectrum: The peak at 2xLF will be high. For a two-pole motor this peak will be close to 2xTS running speed – you will need sufficient resolution to separate them.
A live spectrum may reveal beating – the 2xLF and 2X peak may appear to rise and fall if you do not have sufficient resolution to separate the peaks.
The vibration will be strongest at the point(s) where the stator is closest to the rotor. Move the accelerometer around the motor housing to see if the peak is highest in one or two locations.
Waveform: The waveform will be a combination of 1xTS, possibly 2xTS and 2xLF and may therefore include a “wobble” or take on the “M” or “W” shape. Beating between 2xLF and 2xTS (2-pole motor) may be observed if the time waveform covers more than a few seconds.
TS = Turning Speed
LF = Line Frequency (50 or 60
Damage to the bearings, rotor and stator will also be evident due to the distortion of shafts due to misalignment.