AC Induction Motor Problems: Elliptical Stator, Stator Weakness & Winding Shorts

Figure 1 - Typical Spectrum Showing Indications Of Variation In Air Gap, Winding Shorts, Stator Weakness

Air Gap Variation, Winding Shorts, Stator Weakness Symptoms: High amplitudes at 2 x FLine. Recommended Actions: Check for soft foot and repair. Check alignment and repair. Perform winding tests to assess insulation integrity of the windings. Live with it or buy a new motor. NOTE: It is important to realize that vibration at 2 x FLine is a normally occurring vibration. The effect on the bearings is no greater or less than the same amplitude due to unbalance. Do not over-react.

NOTE: Do not confuse the presence of a running speed harmonic with a pole pass frequency sideband. By definition, a running speed harmonic will always be separated from 2 x FLine by pole pass frequency.

Motor Construction

Winding Construction

The Stator- consists of the windings and the metal of the motor housing itself (i.e. the 'iron', or 'core'). The symptom we will see here is related to variation in the air gap between the windings and the rotor. That air gap is not perfectly even all the way around. Since the strength of a magnetic field - which causes the rotation of the rotor - is proportional to the gap (the smaller the gap, the stronger the force), variation in the gap produces vibration at (2 x FLine). The greater the variation, the higher the amplitude. The air gap can also be affected, however, by mechanical problems such as soft foot (which stresses & distorts the housing), stator looseness / weakness (allowing it to be influenced to a greater degree by those magnetic forces) and winding shorts (which cause localized heating and thermal distortion). The only one of the previous problems that is easily tested for and fixed is soft foot.