Motor Current Signature Analysis (MCSA)
On-line Motor Monitoring
Electricians have been troubleshooting electric motor problems with only a megger for too many years.
This method is long out dated as several major problems cannot be “seen” by a megger. e.g.
(i) Turn to turn short
(ii) Insulation breakdown between individual turns in stator slot or at the end turn, resulting in complete isolation from ground
(iii) Phase to phase faults similar to (ii).
To overcome this shortcoming, the use of a dynamic testing method to evaluate electrical motor condition without shutting down and interrupting production is possible.
To effectively report on the condition of a motor, the following fault zones can be identified in this method:
- Power Analysis
- Stator Fault
- Rotor Fault
- Air Gap
1. Power Analysis
By analysing the power quality and power circuit condition, the health of the motor can be effectively determined.
Non-linear switching loads from variable Frequency Drives, computers and fluorescent lighting cause harmonic distortion in the system, resulting in excessive current and overheating. The additional current would lead to insulation failure of the windings.
The Power circuit refers to the incoming supply to the Motor Controller up to the motor windings. High resistance joints and contacts in this circuit are known to be the source of 46% of the faults which would reduce motor efficiency. This would cause problems like harmonics, voltage and current imbalance, resulting in reduced horsepower rating of the motor, to overheat the windings and eventually ending in insulation failure.
Stator faults are indicated by high impedance imbalance and rising negative sequence currents. Changes in start up current and time can be caused by stator or rotor faults.
Hence it is important to record the various, parameters and trend the data to obtain useful information for diagnosis.
2. Stator Fault
By measuring the AC inductance per phase, any extreme imbalance in the readings would indicate turn-to-turn stator fault.
A phase-to-phase fault is usually indicated by high voltage imbalance between the phases.
3. Rotor Fault
A rotor bar defect is identified by comparing the Pole Pass Frequency side band to the line frequency (50Hz) and analysing the data through an FFT Analyser.
The difference in amplitude would normally be 50dB or greater.
Broken rotor bars also cause skewed or erratic inductance patterns at the peak of the sinewave.
4. Air Gap
If the air gap between the stator and rotor is not evenly distributed around the 360 degrees of the motor, uneven magnetic fields will produce magnetic imbalances to cause movement of the stator windings, resulting in winding failure, vibration and bearing failure. This static eccentricity is usually caused by shaft misalignment or the shaft sitting too low in the bearing.
This defect can be detected through variation in the peaks of the sinewave from the inductance measurements.
Dynamic eccentricity happens when the rotor centre does not stay at one place but is allowed to move.
This defect results in the movement of the inductance of all three phases up or down together.