Motor Current Signal Analysis

 

Motor Current Signal Analysis (MCSA) is a technique for diagnosing problems in mechanical equipment by monitoring nothing more than the input electrical signal. The induction motor acts as a bilateral transducer, converting mechanical vibrations into electrical signal perturbations. It provides a method for a non-invasive testing of mechanical systems. MCSA techniques can be used in conjunction with vibration and thermal analysis to confirm key machinery diagnostic decisions.

 

MCSA operates on the principle that induction motor circuits can, in essence, be viewed as a transducer. By clamping a Hall Effect Current sensor on either the primary or secondary circuit, fluctuations in motor current can be observed. Research has shown that when high resistance exists (for example due to broken rotor bars) harmonic fluxes are produced in the air gap. These fluxes induce current components in the stator winding that cause modulation of the supply current at ± the number of motor poles times slip. Advanced signal processing techniques extract the modulating frequency and clearly represent the amplitude relationship of modulating frequency to line frequency. Knowing this relationship allows you to estimate the presence and severity of the defect.

 

Using a standard accelerometer placed on the bearing cap, several unique mechanical vibration signals will be generated by electrical faults in the motor circuits. One of the more common is a signal at twice line frequency. If the line frequency is 50 Hz, the signal will be seen at 100 Hz. This two times line frequency signal will be created by any of the following faults: uneven air gap between the rotor and stator, and damage to the stator windings or insulation.

 

 
 

 

2X line frequency at 100 Hz

4-Point ODS shows vibration only at driver side at 100 Hz