Stator-related failures account for a large portion of all electrical problems on AC motors. The cause of most stator faults is insulation breakdown that leads to winding failure.
Such failures are (Picture 1):
- Open Windings
- Turn-to-Turn Short
If the motor keeps running with stator faults, a high current flows through the windings where to the location of the problem. This high current generates heat that further degrades winding insulation and creates a short-circuit between the windings and eventually produces a phase-to-ground failure. The motor quickly fails after that.
- Voltage imbalance (imbalanced external loading)
- Loose connections in the windings
- Loose windings
- Restricted airflow
- Motor overload
- Damaged, contaminated, or old winding insulation
An electrical motor by definition produces magnetic flux. Any small unbalance in the magnetic or electrical circuit of motors is reflected in the axially transmitted fluxes. Electrical characteristics within a motor change due to asymmetries in the rotor or stator windings, as will the axially transmitted flux.
Broken rotor bars, as well as unbalanced phases in the stator windings such as turn-to-turn, phase-to-phase, and phase-to-ground shorts can produce electrical asymmetries.
Acquire axially transmitted flux measurements can be acquired with a flux coil mounted axially on a motor. A trend of certain magnetic flux measurement frequencies indicates electrical asymmetries associated with the rotor and stator windings.
The majority of the peaks in a flux coil spectrum occur at frequencies that have some relationship to running speed. Changes in these speed-related peaks provide strong indication of electrical faults such as broken rotor bars, unbalance voltages, and stator insulation breakdown.
If the windings are not firmly attached in the stator, this looseness may cause the windings to vibrate at 2 × LF (100 Hz in Europe or 120 Hz in the United States), or very rarely at 1×LF, especially during start-up. Loose laminations are another rare problem that occurs most often during start-up.
Uneven Air Gap (Static Eccentricity)
An uneven air gap occurs when the air gap between the stator and rotor is not uniform over the whole motor (360º).
One or more of the following conditions is the cause of an uneven gap:
- This problem is viewed at 2 × LF peak in vibration or at slot pass frequencies modulated by 2 × LF (vibration and current)
- A deformed stator (for example, caused by defective insulation
resulting in uneven heating)
- Soft-foot condition related to mounting
- An eccentric stator
Localized heating sometimes deforms the stator. Because of this deformation, a variation of distances in the air gap where the heating exists occurs. However, most stator deformation is due to soft foot or improper mounting.
Stator Fault Analysis
Stator fault and voltage unbalance is determined by trending certain frequencies obtained in a flux spectrum.
Provided a baseline flux spectrum is collected while the motor is in a good condition, stator-related faults can be estimated by comparing certain frequencies from flux spectra to that of the reference (baseline) flux spectrum. Analysis of flux measurements can be performed using spectra at two different frequency ranges:
- A low-frequency flux spectrum (0 – 200 Hz / 3200 lines)
- A high-frequency flux spectrum (0 – 3(6) KHz / 6400 (12800) lines)
Information courtesy of Emerson Controls.